nav_bar.dart

// Copyright 2014 The Flutter Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

/// @docImport 'refresh.dart';
library;

import 'dart:math' as math;
import 'dart:ui' show ImageFilter;

import 'package:flutter/foundation.dart';
import 'package:flutter/rendering.dart';
import 'package:flutter/services.dart';
import 'package:flutter/widgets.dart';

import 'button.dart';
import 'colors.dart';
import 'constants.dart';
import 'icons.dart';
import 'localizations.dart';
import 'page_scaffold.dart';
import 'route.dart';
import 'search_field.dart';
import 'sheet.dart';
import 'theme.dart';

/// Modes that determine how to display the navigation bar's bottom in relation to scroll events.
enum NavigationBarBottomMode {
  /// Enable hiding the bottom in response to scrolling.
  ///
  /// As scrolling starts, the large title stays pinned while the bottom resizes
  /// until it is completely consumed. Then, the large title scrolls under the
  /// persistent navigation bar.
  automatic,

  /// Always display the bottom regardless of the scroll activity.
  ///
  /// When scrolled, the bottom stays pinned while the large title scrolls under
  /// the persistent navigation bar.
  always,
}

/// Standard iOS navigation bar height without the status bar.
///
/// This height is constant and independent of accessibility as it is in iOS.
const double _kNavBarPersistentHeight = kMinInteractiveDimensionCupertino;

/// Size increase from expanding the navigation bar into an iOS-11-style large title
/// configuration in a [CustomScrollView].
const double _kNavBarLargeTitleHeightExtension = 52.0;

/// Number of logical pixels scrolled down before the title text is transferred
/// from the normal navigation bar to a big title below the navigation bar.
const double _kNavBarShowLargeTitleThreshold = 10.0;

/// Number of logical pixels scrolled during which the navigation bar's background
/// fades in or out.
///
/// Eyeballed on the native Settings app on an iPhone 15 simulator running iOS 17.4.
const double _kNavBarScrollUnderAnimationExtent = 10.0;

const double _kNavBarEdgePadding = 16.0;

const double _kNavBarBottomPadding = 8.0;

const double _kNavBarBackButtonTapWidth = 50.0;

// The minimum text scale to apply to contents of the nav bar which can scale to
// a size less than the default, such as the large title.
//
// Eyeballed on an iPhone 15 simulator running iOS 17.5.
const double _kMinScaleFactor = 0.9;

// The maximum text scale to apply to contents of the nav bar, except the large
// title which can grow larger but is damped.
//
// Calculated on an iPhone 15 simulator running iOS 17.5.
const double _kMaxScaleFactor = 1.235;

// The damping ratio applied to reduce the rate at which the large title scales.
//
// Eyeballed on an iPhone 15 simulator running iOS 17.5.
const double _kLargeTitleScaleDampingRatio = 3.0;

/// The width of the 'Cancel' button if the search field in a
/// [CupertinoSliverNavigationBar.search] is active.
///
/// Eyeballed on an iPhone 15 simulator running iOS 17.5.
const double _kSearchFieldCancelButtonWidth = 67.0;

/// The height of the unscaled search field used in
/// a [CupertinoSliverNavigationBar.search].
const double _kSearchFieldHeight = 36.0;

/// The duration of the animation when the search field in
/// [CupertinoSliverNavigationBar.search] is tapped.
const Duration _kNavBarSearchDuration = Duration(milliseconds: 300);

/// The curve of the animation when the search field in
/// [CupertinoSliverNavigationBar.search] is tapped.
const Curve _kNavBarSearchCurve = Curves.easeInOut;

/// Title text transfer fade.
const Duration _kNavBarTitleFadeDuration = Duration(milliseconds: 150);

const Color _kDefaultNavBarBorderColor = Color(0x4D000000);

const Border _kDefaultNavBarBorder = Border(
  bottom: BorderSide(
    color: _kDefaultNavBarBorderColor,
    width: 0.0, // 0.0 means one physical pixel
  ),
);

const Border _kTransparentNavBarBorder = Border(
  bottom: BorderSide(color: Color(0x00000000), width: 0.0),
);

/// The curve of the animation of the top nav bar regardless of push/pop
/// direction in the hero transition between two nav bars.
///
/// Eyeballed on an iPhone 15 Pro simulator running iOS 17.5.
const Curve _kTopNavBarHeaderTransitionCurve = Cubic(0.0, 0.45, 0.45, 0.98);

/// The curve of the animation of the bottom nav bar regardless of push/pop
/// direction in the hero transition between two nav bars.
///
/// Eyeballed on an iPhone 15 Pro simulator running iOS 17.5.
const Curve _kBottomNavBarHeaderTransitionCurve = Cubic(0.05, 0.90, 0.90, 0.95);

// There's a single tag for all instances of navigation bars because they can
// all transition between each other (per Navigator) via Hero transitions.
const _HeroTag _defaultHeroTag = _HeroTag(null);

@immutable
class _HeroTag {
  const _HeroTag(this.navigator);

  final NavigatorState? navigator;

  // Let the Hero tag be described in tree dumps.
  @override
  String toString() => 'Default Hero tag for Cupertino navigation bars with navigator $navigator';

  @override
  bool operator ==(Object other) {
    if (identical(this, other)) {
      return true;
    }
    if (other.runtimeType != runtimeType) {
      return false;
    }
    return other is _HeroTag && other.navigator == navigator;
  }

  @override
  int get hashCode => identityHashCode(navigator);
}

// An `AnimatedWidget` that imposes a fixed size on its child widget, and
// shifts the child widget in the parent stack, driven by its `offsetAnimation`
// property.
class _FixedSizeSlidingTransition extends AnimatedWidget {
  const _FixedSizeSlidingTransition({
    required this.isLTR,
    required this.offsetAnimation,
    required this.width,
    required this.height,
    required this.child,
  }) : super(listenable: offsetAnimation);

  // Whether the writing direction used in the navigation bar transition is
  // left-to-right.
  final bool isLTR;

  // The fixed width to impose on `child`.
  final double width;

  // The fixed height to impose on `child`.
  final double height;

  // The animated offset from the top-leading corner of the stack.
  //
  // When `isLTR` is true, the `Offset` is the position of the child widget in
  // the stack render box's regular coordinate space.
  //
  // When `isLTR` is false, the coordinate system is flipped around the
  // horizontal axis and the origin is set to the top right corner of the render
  // boxes. In other words, this parameter describes the offset from the top
  // right corner of the stack, to the top right corner of the child widget, and
  // the x-axis runs right to left.
  final Animation<Offset> offsetAnimation;

  final Widget child;

  @override
  Widget build(BuildContext context) {
    return Positioned(
      top: offsetAnimation.value.dy,
      left: isLTR ? offsetAnimation.value.dx : null,
      right: isLTR ? null : offsetAnimation.value.dx,
      width: width,
      height: height,
      child: child,
    );
  }
}

/// Returns `child` wrapped with background and a bottom border if background color
/// is opaque. Otherwise, also blur with [BackdropFilter].
///
/// When `updateSystemUiOverlay` is true, the nav bar will update the OS
/// status bar's color theme based on the background color of the nav bar.
Widget _wrapWithBackground({
  Border? border,
  required Color backgroundColor,
  Brightness? brightness,
  required Widget child,
  bool updateSystemUiOverlay = true,
  bool enableBackgroundFilterBlur = true,
}) {
  var result = child;
  if (updateSystemUiOverlay) {
    final bool isDark = backgroundColor.computeLuminance() < 0.179;
    final Brightness newBrightness = brightness ?? (isDark ? Brightness.dark : Brightness.light);
    final SystemUiOverlayStyle overlayStyle = switch (newBrightness) {
      Brightness.dark => SystemUiOverlayStyle.light,
      Brightness.light => SystemUiOverlayStyle.dark,
    };
    // [SystemUiOverlayStyle.light] and [SystemUiOverlayStyle.dark] set some system
    // navigation bar properties,
    // Before https://github.com/flutter/flutter/pull/104827 those properties
    // had no effect, now they are used if there is no AnnotatedRegion on the
    // bottom of the screen.
    // For backward compatibility, create a `SystemUiOverlayStyle` without the
    // system navigation bar properties.
    result = AnnotatedRegion<SystemUiOverlayStyle>(
      value: SystemUiOverlayStyle(
        statusBarColor: overlayStyle.statusBarColor,
        statusBarBrightness: overlayStyle.statusBarBrightness,
        statusBarIconBrightness: overlayStyle.statusBarIconBrightness,
        systemStatusBarContrastEnforced: overlayStyle.systemStatusBarContrastEnforced,
      ),
      child: result,
    );
  }
  final childWithBackground = DecoratedBox(
    decoration: BoxDecoration(border: border, color: backgroundColor),
    child: result,
  );

  return ClipRect(
    child: BackdropFilter(
      enabled: backgroundColor.alpha != 0xFF && enableBackgroundFilterBlur,
      filter: ImageFilter.blur(sigmaX: 10.0, sigmaY: 10.0),
      child: childWithBackground,
    ),
  );
}

double _dampScaleFactor(double scaledFontSize, double unscaledFontSize, double dampingRatio) {
  final double scaleFactor = scaledFontSize / unscaledFontSize;
  return scaleFactor < 1.0
      ? math.max(_kMinScaleFactor, scaleFactor)
      : 1.0 + ((scaleFactor - 1.0) / dampingRatio);
}

// Whether the current route supports nav bar hero transitions from or to.
bool _isTransitionable(BuildContext context) {
  final ModalRoute<dynamic>? route = ModalRoute.of(context);

  // Fullscreen dialogs never transitions their nav bar with other push-style
  // pages' nav bars or with other fullscreen dialog pages on the way in or on
  // the way out.
  return route is PageRoute &&
      !route.fullscreenDialog &&
      !CupertinoSheetRoute.hasParentSheet(context);
}

/// An iOS-styled navigation bar.
///
/// The navigation bar is a toolbar that minimally consists of a widget,
/// normally a page title.
///
/// It also supports [leading] and [trailing] widgets on either end of the
/// toolbar, typically for actions and navigation.
///
/// The [leading] widget will automatically be a back chevron icon button (or a
/// cancel button in case of a fullscreen dialog) to pop the current route if none
/// is provided and [automaticallyImplyLeading] is true (true by default).
///
/// This toolbar should be placed at top of the screen where it will
/// automatically account for the OS's status bar.
///
/// If the given [backgroundColor]'s opacity is not 1.0 (which is the case by
/// default), it will produce a blurring effect to the content behind it.
///
/// ### Layout options
///
/// While the [CupertinoSliverNavigationBar] can dynamically change size and
/// layout in response to scrolling, this static version can reflect the same
/// large (expanded) layout, or the small (collapsed) layout.
///
/// The default constructor will display the collapsed version of the
/// [CupertinoSliverNavigationBar]. The [middle] widget will automatically be a
/// title text from the current [CupertinoPageRoute] if none is provided and
/// [automaticallyImplyMiddle] is true (true by default).
///
/// Using the [CupertinoNavigationBar.large] constructor will display the
/// expanded version of [CupertinoSliverNavigationBar]. The [largeTitle] widget
/// will automatically be a title text from the current [CupertinoPageRoute] if
/// none is provided and `automaticallyImplyTitle` is true (true by default).
///
/// ### Transitions
///
/// When [transitionBetweenRoutes] is true, this navigation bar will transition
/// on top of the routes instead of inside them if the route being transitioned
/// to also has a [CupertinoNavigationBar] or a [CupertinoSliverNavigationBar]
/// with [transitionBetweenRoutes] set to true. If [transitionBetweenRoutes] is
/// true, none of the [Widget] parameters can contain a key in its subtree since
/// that widget will exist in multiple places in the tree simultaneously.
///
/// By default, only one [CupertinoNavigationBar] or [CupertinoSliverNavigationBar]
/// should be present in each [PageRoute] to support the default transitions.
/// Use [transitionBetweenRoutes] or [heroTag] to customize the transition
/// behavior for multiple navigation bars per route.
///
/// When used in a [CupertinoPageScaffold], [CupertinoPageScaffold.navigationBar]
/// disables text scaling to match the native iOS behavior. To override
/// this behavior, wrap each of the `navigationBar`'s components inside a
/// [MediaQuery] with the desired [TextScaler].
///
/// {@tool dartpad}
/// This example shows a [CupertinoNavigationBar] placed in a [CupertinoPageScaffold].
/// Since [backgroundColor]'s opacity is not 1.0, there is a blur effect and
/// content slides underneath.
///
/// ** See code in examples/api/lib/cupertino/nav_bar/cupertino_navigation_bar.0.dart **
/// {@end-tool}
///
/// {@tool dartpad}
/// This example shows the resulting layout from [CupertinoNavigationBar.large]
/// constructor, showing a large title similar to the expanded state of
/// [CupertinoSliverNavigationBar].
///
/// ** See code in examples/api/lib/cupertino/nav_bar/cupertino_navigation_bar.2.dart **
/// {@end-tool}
///
/// See also:
///
///  * [CupertinoPageScaffold], a page layout helper typically hosting the
///    [CupertinoNavigationBar].
///  * [CupertinoSliverNavigationBar] for a navigation bar to be placed in a
///    scrolling list and that supports iOS-11-style large titles.
///  * <https://developer.apple.com/design/human-interface-guidelines/ios/bars/navigation-bars/>
class CupertinoNavigationBar extends StatefulWidget implements ObstructingPreferredSizeWidget {
  /// Creates a static iOS style navigation bar, with a centered [middle] title.
  ///
  /// Similar to the collapsed state of [CupertinoSliverNavigationBar], which
  /// can dynamically change size in response to scrolling.
  ///
  /// See also:
  ///
  ///   * [CupertinoNavigationBar.large], which creates a static iOS style
  ///     navigation bar with a [largeTitle], similar to the expanded state of
  ///     [CupertinoSliverNavigationBar].
  const CupertinoNavigationBar({
    super.key,
    this.leading,
    this.automaticallyImplyLeading = true,
    this.automaticallyImplyMiddle = true,
    this.previousPageTitle,
    this.middle,
    this.trailing,
    this.border = _kDefaultNavBarBorder,
    this.backgroundColor,
    this.automaticBackgroundVisibility = true,
    this.enableBackgroundFilterBlur = true,
    this.brightness,
    this.padding,
    this.transitionBetweenRoutes = true,
    this.heroTag = _defaultHeroTag,
    this.bottom,
  }) : largeTitle = null,
       assert(
         !transitionBetweenRoutes || identical(heroTag, _defaultHeroTag),
         'Cannot specify a heroTag override if this navigation bar does not '
         'transition due to transitionBetweenRoutes = false.',
       );

  /// Creates a static iOS style navigation bar, with a left aligned [largeTitle].
  ///
  /// Similar to the expanded state of [CupertinoSliverNavigationBar], which
  /// can dynamically change size in response to scrolling.
  ///
  /// See also:
  ///
  ///   * [CupertinoNavigationBar]'s base constructor, which creates a static
  ///     iOS style navigation bar with [middle], similar to the collapsed state
  ///     of [CupertinoSliverNavigationBar].
  const CupertinoNavigationBar.large({
    super.key,
    this.largeTitle,
    this.leading,
    this.automaticallyImplyLeading = true,
    bool automaticallyImplyTitle = true,
    this.previousPageTitle,
    this.trailing,
    this.border = _kDefaultNavBarBorder,
    this.backgroundColor,
    this.automaticBackgroundVisibility = true,
    this.enableBackgroundFilterBlur = true,
    this.brightness,
    this.padding,
    this.transitionBetweenRoutes = true,
    this.heroTag = _defaultHeroTag,
    this.bottom,
  }) : middle = null,
       automaticallyImplyMiddle = automaticallyImplyTitle,
       assert(
         !transitionBetweenRoutes || identical(heroTag, _defaultHeroTag),
         'Cannot specify a heroTag override if this navigation bar does not '
         'transition due to transitionBetweenRoutes = false.',
       );

  /// The navigation bar's title, when using [CupertinoNavigationBar.large].
  ///
  /// If null and `automaticallyImplyTitle` is true, an appropriate [Text]
  /// title will be created if the current route is a [CupertinoPageRoute] and
  /// has a `title`.
  ///
  /// This property is null for the base [CupertinoNavigationBar] constructor,
  /// which shows a collapsed navigation bar and uses [middle] for the title
  /// instead.
  ///
  /// See also:
  ///
  ///   * [CupertinoSliverNavigationBar.largeTitle], a similar property
  ///     in the expanded state of [CupertinoSliverNavigationBar], which can
  ///     dynamically change size in response to scrolling.
  final Widget? largeTitle;

  /// {@template flutter.cupertino.CupertinoNavigationBar.leading}
  /// Widget to place at the start of the navigation bar. Normally a back button
  /// for a normal page or a cancel button for full page dialogs.
  ///
  /// If null and [automaticallyImplyLeading] is true, an appropriate button
  /// will be automatically created.
  /// {@endtemplate}
  final Widget? leading;

  /// {@template flutter.cupertino.CupertinoNavigationBar.automaticallyImplyLeading}
  /// Controls whether we should try to imply the leading widget if null.
  ///
  /// If true and [leading] is null, automatically try to deduce what the [leading]
  /// widget should be. If [leading] widget is not null, this parameter has no effect.
  ///
  /// Specifically this navigation bar will:
  ///
  /// 1. Show a 'Cancel' button if the current route is a `fullscreenDialog`.
  /// 2. Show a back chevron with [previousPageTitle] if [previousPageTitle] is
  ///    not null.
  /// 3. Show a back chevron with the previous route's `title` if the current
  ///    route is a [CupertinoPageRoute] and the previous route is also a
  ///    [CupertinoPageRoute].
  /// {@endtemplate}
  final bool automaticallyImplyLeading;

  /// Controls whether we should try to imply the middle widget if null.
  ///
  /// If true and [middle] is null, automatically fill in a [Text] widget with
  /// the current route's `title` if the route is a [CupertinoPageRoute].
  /// If [middle] widget is not null, this parameter has no effect.
  final bool automaticallyImplyMiddle;

  /// {@template flutter.cupertino.CupertinoNavigationBar.previousPageTitle}
  /// Manually specify the previous route's title when automatically implying
  /// the leading back button.
  ///
  /// Overrides the text shown with the back chevron instead of automatically
  /// showing the previous [CupertinoPageRoute]'s `title` when
  /// [automaticallyImplyLeading] is true.
  ///
  /// Has no effect when [leading] is not null or if [automaticallyImplyLeading]
  /// is false.
  /// {@endtemplate}
  final String? previousPageTitle;

  /// The navigation bar's default title.
  ///
  /// If null and [automaticallyImplyMiddle] is true, an appropriate [Text]
  /// title will be created if the current route is a [CupertinoPageRoute] and
  /// has a `title`.
  ///
  /// This property is null for the [CupertinoNavigationBar.large] constructor,
  /// which shows an expanded navigation bar and uses [largeTitle] instead.
  ///
  /// See also:
  ///
  /// * [CupertinoSliverNavigationBar.middle], a similar property
  ///    in the collapsed state of [CupertinoSliverNavigationBar], which can
  ///    dynamically change size in response to scrolling.
  final Widget? middle;

  /// {@template flutter.cupertino.CupertinoNavigationBar.trailing}
  /// Widget to place at the end of the navigation bar. Normally additional actions
  /// taken on the page such as a search or edit function.
  /// {@endtemplate}
  final Widget? trailing;

  /// {@template flutter.cupertino.CupertinoNavigationBar.backgroundColor}
  /// The background color of the navigation bar. If it contains transparency, the
  /// tab bar will automatically produce a blurring effect to the content
  /// behind it. This behavior can be disabled by setting [enableBackgroundFilterBlur]
  /// to false.
  ///
  /// By default, the navigation bar's background is visible only when scrolled under.
  /// This behavior can be controlled with [automaticBackgroundVisibility].
  ///
  /// Defaults to [CupertinoTheme]'s `barBackgroundColor` if null.
  /// {@endtemplate}
  final Color? backgroundColor;

  /// {@template flutter.cupertino.CupertinoNavigationBar.automaticBackgroundVisibility}
  /// Whether the navigation bar appears transparent when no content is scrolled under.
  ///
  /// If this is true, the navigation bar's background color will be transparent
  /// until the content scrolls under it. If false, the navigation bar will always
  /// use [backgroundColor] as its background color.
  ///
  /// If the navigation bar is not a child of a [CupertinoPageScaffold], this has no effect.
  ///
  /// This value defaults to true.
  /// {@endtemplate}
  final bool automaticBackgroundVisibility;

  /// {@template flutter.cupertino.CupertinoNavigationBar.brightness}
  /// The brightness of the specified [backgroundColor].
  ///
  /// Setting this value changes the style of the system status bar. Typically
  /// used to increase the contrast ratio of the system status bar over
  /// [backgroundColor].
  ///
  /// If set to null, the value of the property will be inferred from the relative
  /// luminance of [backgroundColor].
  /// {@endtemplate}
  final Brightness? brightness;

  /// {@template flutter.cupertino.CupertinoNavigationBar.padding}
  /// Padding for the contents of the navigation bar.
  ///
  /// If null, the navigation bar will adopt the following defaults:
  ///
  ///  * Vertically, contents will be sized to the same height as the navigation
  ///    bar itself minus the status bar.
  ///  * Horizontally, padding will be 16 pixels according to iOS specifications
  ///    unless the leading widget is an automatically inserted back button, in
  ///    which case the padding will be 0.
  ///
  /// Vertical padding won't change the height of the nav bar.
  /// {@endtemplate}
  final EdgeInsetsDirectional? padding;

  /// {@template flutter.cupertino.CupertinoNavigationBar.border}
  /// The border of the navigation bar. By default renders a single pixel bottom border side.
  ///
  /// If a border is null, the navigation bar will not display a border.
  /// {@endtemplate}
  final Border? border;

  /// {@template flutter.cupertino.CupertinoNavigationBar.transitionBetweenRoutes}
  /// Whether to transition between navigation bars.
  ///
  /// When [transitionBetweenRoutes] is true, this navigation bar will transition
  /// on top of the routes instead of inside it if the route being transitioned
  /// to also has a [CupertinoNavigationBar] or a [CupertinoSliverNavigationBar]
  /// with [transitionBetweenRoutes] set to true.
  ///
  /// This transition will also occur on edge back swipe gestures like on iOS
  /// but only if the previous page below has `maintainState` set to true on the
  /// [PageRoute].
  ///
  /// When set to true, only one navigation bar can be present per route unless
  /// [heroTag] is also set.
  ///
  /// This value defaults to true.
  /// {@endtemplate}
  final bool transitionBetweenRoutes;

  /// {@template flutter.cupertino.CupertinoNavigationBar.enableBackgroundFilterBlur}
  /// Whether to have a blur effect when a non-opaque background color is used.
  ///
  /// When [enableBackgroundFilterBlur] is set to false, the blur effect will be
  /// disabled. The behaviour of [enableBackgroundFilterBlur] will only be respected when
  /// [automaticBackgroundVisibility] is false or until content scrolls under the navbar.
  ///
  /// This value defaults to true.
  /// {@endtemplate}
  final bool enableBackgroundFilterBlur;

  /// {@template flutter.cupertino.CupertinoNavigationBar.heroTag}
  /// Tag for the navigation bar's Hero widget if [transitionBetweenRoutes] is true.
  ///
  /// Defaults to a common tag between all [CupertinoNavigationBar] and
  /// [CupertinoSliverNavigationBar] instances of the same [Navigator]. With the
  /// default tag, all navigation bars of the same navigator can transition
  /// between each other as long as there's only one navigation bar per route.
  ///
  /// This [heroTag] can be overridden to manually handle having multiple
  /// navigation bars per route or to transition between multiple
  /// [Navigator]s.
  ///
  /// To disable Hero transitions for this navigation bar, set
  /// [transitionBetweenRoutes] to false.
  /// {@endtemplate}
  final Object heroTag;

  /// A widget to place at the bottom of the navigation bar.
  ///
  /// Only widgets that implement [PreferredSizeWidget] can be used at the
  /// bottom of a navigation bar.
  ///
  /// {@tool dartpad}
  /// This example shows a [CupertinoSearchTextField] at the bottom of a
  /// [CupertinoNavigationBar].
  ///
  /// ** See code in examples/api/lib/cupertino/nav_bar/cupertino_navigation_bar.1.dart **
  /// {@end-tool}
  ///
  /// See also:
  ///
  ///  * [PreferredSize], which can be used to give an arbitrary widget a preferred size.
  final PreferredSizeWidget? bottom;

  /// True if the navigation bar's background color has no transparency.
  @override
  bool shouldFullyObstruct(BuildContext context) {
    final Color backgroundColor =
        CupertinoDynamicColor.maybeResolve(this.backgroundColor, context) ??
        CupertinoTheme.of(context).barBackgroundColor;
    return backgroundColor.alpha == 0xFF;
  }

  @override
  Size get preferredSize {
    final double bottomHeight = bottom?.preferredSize.height ?? 0.0;

    final double effectiveLargeHeight = largeTitle != null
        ? _kNavBarLargeTitleHeightExtension
        : 0.0;

    return Size.fromHeight(_kNavBarPersistentHeight + bottomHeight + effectiveLargeHeight);
  }

  @override
  State<CupertinoNavigationBar> createState() => _CupertinoNavigationBarState();
}

// A state class exists for the nav bar so that the keys of its sub-components
// don't change when rebuilding the nav bar, causing the sub-components to
// lose their own states.
class _CupertinoNavigationBarState extends State<CupertinoNavigationBar> {
  late _NavigationBarStaticComponentsKeys keys;

  ScrollNotificationObserverState? _scrollNotificationObserver;
  double _scrollAnimationValue = 0.0;

  @override
  void didChangeDependencies() {
    super.didChangeDependencies();
    _scrollNotificationObserver?.removeListener(_handleScrollNotification);
    _scrollNotificationObserver = ScrollNotificationObserver.maybeOf(context);
    _scrollNotificationObserver?.addListener(_handleScrollNotification);
  }

  @override
  void dispose() {
    if (_scrollNotificationObserver != null) {
      _scrollNotificationObserver!.removeListener(_handleScrollNotification);
      _scrollNotificationObserver = null;
    }
    super.dispose();
  }

  @override
  void initState() {
    super.initState();
    keys = _NavigationBarStaticComponentsKeys();
  }

  void _handleScrollNotification(ScrollNotification notification) {
    if (notification is ScrollUpdateNotification && notification.depth == 0) {
      final ScrollMetrics metrics = notification.metrics;
      final double oldScrollAnimationValue = _scrollAnimationValue;
      var scrollExtent = 0.0;
      switch (metrics.axisDirection) {
        case AxisDirection.up:
          // Scroll view is reversed
          scrollExtent = metrics.extentAfter;
        case AxisDirection.down:
          scrollExtent = metrics.extentBefore;
        case AxisDirection.right:
        case AxisDirection.left:
          // Scrolled under is only supported in the vertical axis, and should
          // not be altered based on horizontal notifications of the same
          // predicate since it could be a 2D scroller.
          break;
      }

      if (scrollExtent >= 0 && scrollExtent < _kNavBarScrollUnderAnimationExtent) {
        setState(() {
          _scrollAnimationValue = clampDouble(
            scrollExtent / _kNavBarScrollUnderAnimationExtent,
            0,
            1,
          );
        });
      } else if (scrollExtent > _kNavBarScrollUnderAnimationExtent &&
          oldScrollAnimationValue != 1.0) {
        setState(() {
          _scrollAnimationValue = 1.0;
        });
      } else if (scrollExtent <= 0 && oldScrollAnimationValue != 0.0) {
        setState(() {
          _scrollAnimationValue = 0.0;
        });
      }
    }
  }

  @override
  Widget build(BuildContext context) {
    // The static navigation bar does not expand or collapse (see CupertinoSliverNavigationBar),
    // it will either display the collapsed nav bar with middle, or the expanded with largeTitle.
    assert(widget.middle == null || widget.largeTitle == null);

    final Color backgroundColor =
        CupertinoDynamicColor.maybeResolve(widget.backgroundColor, context) ??
        CupertinoTheme.of(context).barBackgroundColor;

    final Color? parentPageScaffoldBackgroundColor = CupertinoPageScaffoldBackgroundColor.maybeOf(
      context,
    );

    final Border? initialBorder =
        widget.automaticBackgroundVisibility && parentPageScaffoldBackgroundColor != null
        ? _kTransparentNavBarBorder
        : widget.border;
    final Border? effectiveBorder = widget.border == null
        ? null
        : Border.lerp(initialBorder, widget.border, _scrollAnimationValue);

    final Color effectiveBackgroundColor =
        widget.automaticBackgroundVisibility && parentPageScaffoldBackgroundColor != null
        ? Color.lerp(parentPageScaffoldBackgroundColor, backgroundColor, _scrollAnimationValue) ??
              backgroundColor
        : backgroundColor;

    final double bottomHeight = widget.bottom?.preferredSize.height ?? 0.0;
    final double persistentHeight =
        _kNavBarPersistentHeight + bottomHeight + MediaQuery.paddingOf(context).top;
    final double largeHeight = persistentHeight + _kNavBarLargeTitleHeightExtension;

    final components = _NavigationBarStaticComponents(
      keys: keys,
      route: ModalRoute.of(context),
      userLeading: widget.leading,
      automaticallyImplyLeading: widget.automaticallyImplyLeading,
      automaticallyImplyTitle: widget.automaticallyImplyMiddle,
      previousPageTitle: widget.previousPageTitle,
      userMiddle: widget.middle,
      userTrailing: widget.trailing,
      padding: widget.padding,
      userLargeTitle: widget.largeTitle,
      userBottom: widget.bottom,
      large: widget.largeTitle != null,
      staticBar: true, // This one does not scroll
      context: context,
    );

    // Standard persistent components
    Widget navBar = _PersistentNavigationBar(
      components: components,
      padding: widget.padding,
      middleVisible: widget.largeTitle == null,
    );

    if (widget.largeTitle != null) {
      // Large nav bar
      navBar = ConstrainedBox(
        constraints: BoxConstraints(maxHeight: largeHeight),
        child: Column(
          children: <Widget>[
            navBar,
            Expanded(
              child: Padding(
                padding: const EdgeInsetsDirectional.only(
                  start: _kNavBarEdgePadding,
                  bottom: _kNavBarBottomPadding,
                ),
                child: Semantics(
                  header: true,
                  child: DefaultTextStyle(
                    style: CupertinoTheme.of(context).textTheme.navLargeTitleTextStyle,
                    maxLines: 1,
                    overflow: TextOverflow.ellipsis,
                    child: _LargeTitle(
                      height: _kNavBarLargeTitleHeightExtension,
                      child: components.largeTitle,
                    ),
                  ),
                ),
              ),
            ),
            if (widget.bottom != null)
              SizedBox(height: bottomHeight, child: components.navBarBottom),
          ],
        ),
      );
    } else {
      // Small nav bar
      navBar = ConstrainedBox(
        constraints: BoxConstraints(maxHeight: persistentHeight),
        child: Column(
          children: <Widget>[
            navBar,
            if (widget.bottom != null)
              SizedBox(height: bottomHeight, child: components.navBarBottom),
          ],
        ),
      );
    }

    navBar = _wrapWithBackground(
      border: effectiveBorder,
      backgroundColor: effectiveBackgroundColor,
      brightness: widget.brightness,
      enableBackgroundFilterBlur: widget.enableBackgroundFilterBlur,
      child: DefaultTextStyle(style: CupertinoTheme.of(context).textTheme.textStyle, child: navBar),
    );

    if (!widget.transitionBetweenRoutes || !_isTransitionable(context)) {
      // Lint ignore to maintain backward compatibility.
      return navBar;
    }

    return Builder(
      // Get the context that might have a possibly changed CupertinoTheme.
      builder: (BuildContext context) {
        return Hero(
          tag: widget.heroTag == _defaultHeroTag ? _HeroTag(Navigator.of(context)) : widget.heroTag,
          createRectTween: _linearTranslateWithLargestRectSizeTween,
          placeholderBuilder: _navBarHeroLaunchPadBuilder,
          flightShuttleBuilder: _navBarHeroFlightShuttleBuilder,
          transitionOnUserGestures: true,
          child: _TransitionableNavigationBar(
            componentsKeys: keys,
            backgroundColor: effectiveBackgroundColor,
            backButtonTextStyle: CupertinoTheme.of(context).textTheme.navActionTextStyle,
            titleTextStyle: CupertinoTheme.of(context).textTheme.navTitleTextStyle,
            largeTitleTextStyle: CupertinoTheme.of(context).textTheme.navLargeTitleTextStyle,
            border: effectiveBorder,
            hasUserMiddle: widget.middle != null,
            largeExpanded: widget.largeTitle != null,
            searchable: false,
            automaticBackgroundVisibility: widget.automaticBackgroundVisibility,
            child: navBar,
          ),
        );
      },
    );
  }
}

/// An iOS-styled navigation bar with iOS-11-style large titles using slivers.
///
/// The [CupertinoSliverNavigationBar] must be placed in a sliver group such
/// as the [CustomScrollView].
///
/// This navigation bar consists of two sections, a pinned static section on top
/// and a sliding section containing iOS-11-style large title below it.
///
/// It should be placed at top of the screen and automatically accounts for
/// the iOS status bar.
///
/// This navigation bar is expanded only in portrait orientation. In landscape
/// mode, the navigation bar remains permanently collapsed. The navigation bar
/// also collapses when scrolling in portrait mode.
///
/// Minimally, a [largeTitle] widget will appear in the middle of the app bar
/// when the sliver is collapsed and transfer to the area below in larger font
/// when the sliver is expanded. This expanded view will only trigger in
/// portrait orientation, while in landscape mode the bar will stay in its
/// collapsed view.
///
/// For advanced uses, an optional [middle] widget
/// can be supplied to show a different widget in the middle of the navigation
/// bar when the sliver is collapsed.
///
/// Like [CupertinoNavigationBar], it also supports a [leading] and [trailing]
/// widget on the static section on top that remains while scrolling.
///
/// The [leading] widget will automatically be a back chevron icon button (or a
/// cancel button in case of a fullscreen dialog) to pop the current route if none
/// is provided and [automaticallyImplyLeading] is true (true by default).
///
/// The [largeTitle] widget will automatically be a title text from the current
/// [CupertinoPageRoute] if none is provided and [automaticallyImplyTitle] is
/// true (true by default).
///
/// When [transitionBetweenRoutes] is true, this navigation bar will transition
/// on top of the routes instead of inside them if the route being transitioned
/// to also has a [CupertinoNavigationBar] or a [CupertinoSliverNavigationBar]
/// with [transitionBetweenRoutes] set to true. If [transitionBetweenRoutes] is
/// true, none of the [Widget] parameters can contain any [GlobalKey]s in their
/// subtrees since those widgets will exist in multiple places in the tree
/// simultaneously.
///
/// By default, only one [CupertinoNavigationBar] or [CupertinoSliverNavigationBar]
/// should be present in each [PageRoute] to support the default transitions.
/// Use [transitionBetweenRoutes] or [heroTag] to customize the transition
/// behavior for multiple navigation bars per route.
///
/// The [stretch] parameter determines whether the nav bar should stretch to
/// fill the over-scroll area. The nav bar can still expand and contract as the
/// user scrolls, but it will also stretch when the user over-scrolls if the
/// [stretch] value is `true`. Defaults to `false`.
///
/// {@tool dartpad}
/// This example shows [CupertinoSliverNavigationBar] in action inside a [CustomScrollView].
///
/// ** See code in examples/api/lib/cupertino/nav_bar/cupertino_sliver_nav_bar.0.dart **
/// {@end-tool}
///
/// {@tool dartpad}
/// To add a widget to the bottom of the nav bar, wrap it with [PreferredSize] and provide its fully extended size.
///
/// ** See code in examples/api/lib/cupertino/nav_bar/cupertino_sliver_nav_bar.2.dart **
/// {@end-tool}
///
/// See also:
///
///  * [CupertinoNavigationBar], an iOS navigation bar for use on non-scrolling
///    pages.
///  * [CustomScrollView], a ScrollView that creates custom scroll effects using slivers.
///  * <https://developer.apple.com/design/human-interface-guidelines/ios/bars/navigation-bars/>
class CupertinoSliverNavigationBar extends StatefulWidget {
  /// Creates a navigation bar for scrolling lists.
  ///
  /// If [automaticallyImplyTitle] is false, then the [largeTitle] argument is
  /// required.
  const CupertinoSliverNavigationBar({
    super.key,
    this.largeTitle,
    this.leading,
    this.automaticallyImplyLeading = true,
    this.automaticallyImplyTitle = true,
    this.alwaysShowMiddle = true,
    this.previousPageTitle,
    this.middle,
    this.trailing,
    this.border = _kDefaultNavBarBorder,
    this.backgroundColor,
    this.automaticBackgroundVisibility = true,
    this.enableBackgroundFilterBlur = true,
    this.brightness,
    this.padding,
    this.transitionBetweenRoutes = true,
    this.heroTag = _defaultHeroTag,
    this.stretch = false,
    this.bottom,
    this.bottomMode,
  }) : assert(
         automaticallyImplyTitle || largeTitle != null,
         'No largeTitle has been provided but automaticallyImplyTitle is also '
         'false. Either provide a largeTitle or set automaticallyImplyTitle to '
         'true.',
       ),
       assert(
         bottomMode == null || bottom != null,
         'A bottomMode was provided without a corresponding bottom.',
       ),
       onSearchableBottomTap = null,
       searchField = null,
       _searchable = false;

  /// A navigation bar for scrolling lists that integrates a provided search
  /// field directly into the navigation bar.
  ///
  /// This search-enabled navigation bar is functionally equivalent to
  /// the standard [CupertinoSliverNavigationBar] constructor, but with the
  /// addition of [searchField], which sits at the bottom of the navigation bar.
  ///
  /// When the search field is tapped, [leading], [trailing], [middle], and
  /// [largeTitle] all collapse, causing the search field to animate to the
  /// 'top' of the navigation bar. A 'Cancel' button is presented next to the
  /// active [searchField], which when tapped, closes the search view, bringing
  /// the navigation bar back to its initial state.
  ///
  /// If [automaticallyImplyTitle] is false, then the [largeTitle] argument is
  /// required.
  ///
  /// {@tool dartpad}
  /// This example demonstrates how to use a
  /// [CupertinoSliverNavigationBar.search] to manage a search view.
  ///
  /// ** See code in examples/api/lib/cupertino/nav_bar/cupertino_sliver_nav_bar.1.dart **
  /// {@end-tool}
  const CupertinoSliverNavigationBar.search({
    super.key,
    required Widget this.searchField,
    this.largeTitle,
    this.leading,
    this.automaticallyImplyLeading = true,
    this.automaticallyImplyTitle = true,
    this.alwaysShowMiddle = true,
    this.previousPageTitle,
    this.middle,
    this.trailing,
    this.border = _kDefaultNavBarBorder,
    this.backgroundColor,
    this.automaticBackgroundVisibility = true,
    this.enableBackgroundFilterBlur = true,
    this.brightness,
    this.padding,
    this.transitionBetweenRoutes = true,
    this.heroTag = _defaultHeroTag,
    this.stretch = false,
    this.bottomMode = NavigationBarBottomMode.automatic,
    this.onSearchableBottomTap,
  }) : assert(
         automaticallyImplyTitle || largeTitle != null,
         'No largeTitle has been provided but automaticallyImplyTitle is also '
         'false. Either provide a largeTitle or set automaticallyImplyTitle to '
         'true.',
       ),
       bottom = null,
       _searchable = true;

  /// The navigation bar's title.
  ///
  /// This text will appear in the top static navigation bar when collapsed and
  /// below the navigation bar, in a larger font, when expanded.
  ///
  /// A suitable [DefaultTextStyle] is provided around this widget as it is
  /// moved around, to change its font size.
  ///
  /// If [middle] is null, then the [largeTitle] widget will be inserted into
  /// the tree in two places when transitioning from the collapsed state to the
  /// expanded state. It is therefore imperative that this subtree not contain
  /// any [GlobalKey]s, and that it not rely on maintaining state (for example,
  /// animations will not survive the transition from one location to the other,
  /// and may in fact be visible in two places at once during the transition).
  ///
  /// If null and [automaticallyImplyTitle] is true, an appropriate [Text]
  /// title will be created if the current route is a [CupertinoPageRoute] and
  /// has a `title`.
  ///
  /// This parameter must either be non-null or the route must have a title
  /// ([CupertinoPageRoute.title]) and [automaticallyImplyTitle] must be true.
  final Widget? largeTitle;

  /// {@macro flutter.cupertino.CupertinoNavigationBar.leading}
  ///
  /// This widget is visible in both collapsed and expanded states.
  final Widget? leading;

  /// {@macro flutter.cupertino.CupertinoNavigationBar.automaticallyImplyLeading}
  final bool automaticallyImplyLeading;

  /// Controls whether we should try to imply the [largeTitle] widget if null.
  ///
  /// If true and [largeTitle] is null, automatically fill in a [Text] widget
  /// with the current route's `title` if the route is a [CupertinoPageRoute].
  /// If [largeTitle] widget is not null, this parameter has no effect.
  final bool automaticallyImplyTitle;

  /// Controls whether [middle] widget should always be visible (even in
  /// expanded state).
  ///
  /// If true (default) and [middle] is not null, [middle] widget is always
  /// visible. If false, [middle] widget is visible only in collapsed state if
  /// it is provided.
  ///
  /// This should be set to false if you only want to show [largeTitle] in
  /// expanded state and [middle] in collapsed state.
  final bool alwaysShowMiddle;

  /// {@macro flutter.cupertino.CupertinoNavigationBar.previousPageTitle}
  final String? previousPageTitle;

  /// A widget to place in the middle of the static navigation bar instead of
  /// the [largeTitle].
  ///
  /// If [alwaysShowMiddle] is true, this widget is visible in both the
  /// collapsed and expanded states of the navigation bar. Else, it is visible
  /// only in the collapsed state.
  ///
  /// If null, [largeTitle] will be displayed in the navigation bar's collapsed
  /// state.
  final Widget? middle;

  /// {@macro flutter.cupertino.CupertinoNavigationBar.trailing}
  ///
  /// This widget is visible in both collapsed and expanded states.
  final Widget? trailing;

  /// {@macro flutter.cupertino.CupertinoNavigationBar.backgroundColor}
  final Color? backgroundColor;

  /// {@macro flutter.cupertino.CupertinoNavigationBar.automaticBackgroundVisibility}
  final bool automaticBackgroundVisibility;

  /// {@macro flutter.cupertino.CupertinoNavigationBar.enableBackgroundFilterBlur}
  final bool enableBackgroundFilterBlur;

  /// {@macro flutter.cupertino.CupertinoNavigationBar.brightness}
  final Brightness? brightness;

  /// {@macro flutter.cupertino.CupertinoNavigationBar.padding}
  final EdgeInsetsDirectional? padding;

  /// {@macro flutter.cupertino.CupertinoNavigationBar.border}
  final Border? border;

  /// {@macro flutter.cupertino.CupertinoNavigationBar.transitionBetweenRoutes}
  final bool transitionBetweenRoutes;

  /// {@macro flutter.cupertino.CupertinoNavigationBar.heroTag}
  final Object heroTag;

  /// A widget to place at the bottom of the large title or static navigation
  /// bar if there is no large title.
  ///
  /// Only widgets that implement [PreferredSizeWidget] can be used at the
  /// bottom of a navigation bar.
  ///
  /// See also:
  ///
  ///  * [PreferredSize], which can be used to give an arbitrary widget a preferred size.
  final PreferredSizeWidget? bottom;

  /// Modes that determine how to display the navigation bar's [bottom], or the
  /// search field in a [CupertinoSliverNavigationBar.search].
  ///
  /// If null, defaults to [NavigationBarBottomMode.automatic] if either a
  /// [bottom] is provided or this is a [CupertinoSliverNavigationBar.search].
  final NavigationBarBottomMode? bottomMode;

  /// Called when the search field in [CupertinoSliverNavigationBar.search]
  /// is tapped, toggling between an active and an inactive search state.
  final ValueChanged<bool>? onSearchableBottomTap;

  /// True if the navigation bar's background color has no transparency.
  bool get opaque => backgroundColor?.alpha == 0xFF;

  /// Whether the nav bar should stretch to fill the over-scroll area.
  ///
  /// The nav bar can still expand and contract as the user scrolls, but it will
  /// also stretch when the user over-scrolls if the [stretch] value is `true`.
  ///
  /// When set to `true`, the nav bar will prevent subsequent slivers from
  /// accessing overscrolls. This may be undesirable for using overscroll-based
  /// widgets like the [CupertinoSliverRefreshControl].
  ///
  /// Defaults to `false`.
  final bool stretch;

  /// The search field used in [CupertinoSliverNavigationBar.search].
  ///
  /// The provided search field is constrained to a fixed height of 35 pixels in
  /// its inactive state, and [kMinInteractiveDimensionCupertino] pixels in its
  /// active state.
  ///
  /// Typically a [CupertinoSearchTextField].
  final Widget? searchField;

  /// True if the [CupertinoSliverNavigationBar.search] constructor is used.
  final bool _searchable;

  @override
  State<CupertinoSliverNavigationBar> createState() => _CupertinoSliverNavigationBarState();
}

// A state class exists for the nav bar so that the keys of its sub-components
// don't change when rebuilding the nav bar, causing the sub-components to
// lose their own states.
class _CupertinoSliverNavigationBarState extends State<CupertinoSliverNavigationBar>
    with TickerProviderStateMixin {
  late _NavigationBarStaticComponentsKeys keys;
  ScrollableState? _scrollableState;
  Widget? effectiveMiddle;
  late AnimationController _animationController;
  late CurvedAnimation _searchAnimation;
  late Animation<double> persistentHeightAnimation;
  late Animation<double> largeTitleHeightAnimation;
  late double scaledSearchFieldHeight;
  late double scaledLargeTitleHeight;
  bool searchIsActive = false;
  bool isPortrait = true;

  @override
  void initState() {
    super.initState();
    keys = _NavigationBarStaticComponentsKeys();
    _animationController = AnimationController(vsync: this, duration: _kNavBarSearchDuration);
    _searchAnimation = CurvedAnimation(parent: _animationController, curve: _kNavBarSearchCurve);
  }

  @override
  void didUpdateWidget(CupertinoSliverNavigationBar oldWidget) {
    super.didUpdateWidget(oldWidget);
    if (widget.middle != oldWidget.middle) {
      _updateEffectiveMiddle();
    }
  }

  @override
  void didChangeDependencies() {
    super.didChangeDependencies();
    isPortrait = MediaQuery.orientationOf(context) == Orientation.portrait;
    _updateEffectiveMiddle();
    _computeScaledHeights();
    _setupSearchableAnimation();
    _scrollableState?.position.isScrollingNotifier.removeListener(_handleScrollChange);
    _scrollableState = Scrollable.maybeOf(context);
    _scrollableState?.position.isScrollingNotifier.addListener(_handleScrollChange);
  }

  @override
  void dispose() {
    if (_scrollableState?.position != null) {
      _scrollableState?.position.isScrollingNotifier.removeListener(_handleScrollChange);
    }
    _searchAnimation.dispose();
    _animationController.dispose();
    super.dispose();
  }

  double get _bottomHeight {
    assert(!widget._searchable || widget.bottom == null);
    if (widget._searchable) {
      return scaledSearchFieldHeight + _kNavBarBottomPadding;
    } else if (widget.bottom != null) {
      return widget.bottom!.preferredSize.height;
    }
    return 0.0;
  }

  void _updateEffectiveMiddle() {
    effectiveMiddle = widget.middle ?? (isPortrait ? null : widget.largeTitle);
  }

  void _computeScaledHeights() {
    final TextScaler textScaler = MediaQuery.textScalerOf(context);
    scaledSearchFieldHeight =
        _kSearchFieldHeight *
        _dampScaleFactor(
          textScaler.scale(_kSearchFieldHeight),
          _kSearchFieldHeight,
          _kMaxScaleFactor,
        );
    scaledLargeTitleHeight = isPortrait
        ? _kNavBarLargeTitleHeightExtension *
              _dampScaleFactor(
                textScaler.scale(_kNavBarLargeTitleHeightExtension),
                _kNavBarLargeTitleHeightExtension,
                _kLargeTitleScaleDampingRatio,
              )
        : 0.0;
  }

  void _setupSearchableAnimation() {
    final persistentHeightTween = Tween<double>(begin: _kNavBarPersistentHeight, end: 0.0);
    persistentHeightAnimation = persistentHeightTween.animate(_animationController)
      ..addStatusListener(_handleSearchFieldStatusChanged);
    final largeTitleHeightTween = Tween<double>(begin: scaledLargeTitleHeight, end: 0.0);
    largeTitleHeightAnimation = largeTitleHeightTween.animate(_animationController);
  }

  void _handleScrollChange() {
    final ScrollPosition? position = _scrollableState?.position;
    if (position == null || !position.hasPixels || position.pixels <= 0.0) {
      return;
    }

    double? target;
    final double bottomScrollOffset = widget.bottomMode == NavigationBarBottomMode.always
        ? 0.0
        : _bottomHeight;
    final bool canScrollBottom =
        (widget._searchable || widget.bottom != null) && bottomScrollOffset > 0.0;

    // Snap the scroll view to a target determined by the navigation bar's
    // position.
    if (canScrollBottom && position.pixels < bottomScrollOffset) {
      target = position.pixels > bottomScrollOffset / 2 ? bottomScrollOffset : 0.0;
    } else if (position.pixels > bottomScrollOffset &&
        position.pixels < bottomScrollOffset + scaledLargeTitleHeight) {
      target = position.pixels > bottomScrollOffset + (scaledLargeTitleHeight / 2)
          ? bottomScrollOffset + scaledLargeTitleHeight
          : bottomScrollOffset;
    }

    // If the target is not null and within the scrollable range, animate to it.
    if (target != null && target <= position.maxScrollExtent) {
      position.animateTo(
        target,
        // Eyeballed on an iPhone 16 simulator running iOS 18.
        duration: const Duration(milliseconds: 300),
        curve: Curves.fastEaseInToSlowEaseOut,
      );
    }
  }

  void _handleSearchFieldStatusChanged(AnimationStatus status) {
    // If the search animation is stopped, rebuild so that the leading, middle,
    // and trailing widgets that were collapsed while the search field was
    // active are re-expanded. Otherwise, rebuild to update this widget with the
    // animation controller's values.
    setState(() {
      switch (status) {
        case AnimationStatus.forward:
          searchIsActive = true;
        case AnimationStatus.reverse:
          searchIsActive = false;
        case AnimationStatus.completed:
        case AnimationStatus.dismissed:
      }
    });
  }

  void _onSearchFieldTap() {
    if (widget.onSearchableBottomTap != null) {
      widget.onSearchableBottomTap!(!searchIsActive);
    }
    _animationController.toggle();
  }

  @override
  Widget build(BuildContext context) {
    final components = _NavigationBarStaticComponents(
      keys: keys,
      route: ModalRoute.of(context),
      userLeading: widget.leading != null
          ? Visibility(visible: !searchIsActive, child: widget.leading!)
          : null,
      automaticallyImplyLeading: widget.automaticallyImplyLeading,
      automaticallyImplyTitle: widget.automaticallyImplyTitle,
      previousPageTitle: widget.previousPageTitle,
      userMiddle: _animationController.isAnimating ? const Text('') : effectiveMiddle,
      userTrailing: widget.trailing != null
          ? Visibility(visible: !searchIsActive, child: widget.trailing!)
          : null,
      userLargeTitle: widget.largeTitle,
      userBottom:
          (widget._searchable
              ? searchIsActive
                    ? _ActiveSearchableBottom(
                        animationController: _animationController,
                        animation: persistentHeightAnimation,
                        searchField: widget.searchField,
                        searchFieldHeight: scaledSearchFieldHeight,
                        onSearchFieldTap: _onSearchFieldTap,
                      )
                    : _InactiveSearchableBottom(
                        animationController: _animationController,
                        animation: persistentHeightAnimation,
                        searchField: widget.searchField,
                        searchFieldHeight: scaledSearchFieldHeight,
                        onSearchFieldTap: _onSearchFieldTap,
                      )
              : widget.bottom) ??
          const SizedBox.shrink(),
      padding: widget.padding,
      large: isPortrait,
      staticBar: false, // This one scrolls.
      context: context,
    );

    return MediaQuery.withNoTextScaling(
      child: AnimatedBuilder(
        animation: _searchAnimation,
        builder: (BuildContext context, Widget? child) {
          return SliverPersistentHeader(
            pinned: true, // iOS navigation bars are always pinned.
            delegate: _LargeTitleNavigationBarSliverDelegate(
              keys: keys,
              components: components,
              userMiddle: effectiveMiddle,
              backgroundColor:
                  CupertinoDynamicColor.maybeResolve(widget.backgroundColor, context) ??
                  CupertinoTheme.of(context).barBackgroundColor,
              automaticBackgroundVisibility: widget.automaticBackgroundVisibility,
              brightness: widget.brightness,
              border: widget.border,
              padding: widget.padding,
              actionsForegroundColor: CupertinoTheme.of(context).primaryColor,
              transitionBetweenRoutes: widget.transitionBetweenRoutes,
              heroTag: widget.heroTag,
              persistentHeight: persistentHeightAnimation.value + MediaQuery.paddingOf(context).top,
              largeTitleHeight: largeTitleHeightAnimation.value,
              alwaysShowMiddle: widget.alwaysShowMiddle && effectiveMiddle != null,
              stretchConfiguration: widget.stretch && !searchIsActive
                  ? OverScrollHeaderStretchConfiguration()
                  : null,
              enableBackgroundFilterBlur: widget.enableBackgroundFilterBlur,
              bottomMode: searchIsActive
                  ? NavigationBarBottomMode.always
                  : widget.bottomMode ?? NavigationBarBottomMode.automatic,
              bottomHeight: _bottomHeight,
              controller: _animationController,
              searchable: widget._searchable,
            ),
          );
        },
      ),
    );
  }
}

class _LargeTitleNavigationBarSliverDelegate extends SliverPersistentHeaderDelegate
    with DiagnosticableTreeMixin {
  _LargeTitleNavigationBarSliverDelegate({
    required this.keys,
    required this.components,
    required this.userMiddle,
    required this.backgroundColor,
    required this.automaticBackgroundVisibility,
    required this.brightness,
    required this.border,
    required this.padding,
    required this.actionsForegroundColor,
    required this.transitionBetweenRoutes,
    required this.heroTag,
    required this.persistentHeight,
    required this.largeTitleHeight,
    required this.alwaysShowMiddle,
    required this.stretchConfiguration,
    required this.enableBackgroundFilterBlur,
    required this.bottomMode,
    required this.bottomHeight,
    required this.controller,
    required this.searchable,
  });

  final _NavigationBarStaticComponentsKeys keys;
  final _NavigationBarStaticComponents components;
  final Widget? userMiddle;
  final Color backgroundColor;
  final bool automaticBackgroundVisibility;
  final Brightness? brightness;
  final Border? border;
  final EdgeInsetsDirectional? padding;
  final Color actionsForegroundColor;
  final bool transitionBetweenRoutes;
  final Object heroTag;
  final double persistentHeight;
  final double largeTitleHeight;
  final bool alwaysShowMiddle;
  final bool enableBackgroundFilterBlur;
  final NavigationBarBottomMode bottomMode;
  final double bottomHeight;
  final AnimationController controller;
  final bool searchable;

  @override
  double get minExtent =>
      persistentHeight + (bottomMode == NavigationBarBottomMode.always ? bottomHeight : 0.0);

  @override
  double get maxExtent => persistentHeight + largeTitleHeight + bottomHeight;

  @override
  OverScrollHeaderStretchConfiguration? stretchConfiguration;

  @override
  Widget build(BuildContext context, double shrinkOffset, bool overlapsContent) {
    final double largeTitleThreshold = maxExtent - minExtent - _kNavBarShowLargeTitleThreshold;
    final bool showLargeTitle = shrinkOffset < largeTitleThreshold;

    // Calculate how much the bottom should shrink.
    final double bottomShrinkFactor = clampDouble(shrinkOffset / bottomHeight, 0, 1);

    final double shrinkAnimationValue = clampDouble(
      (shrinkOffset - largeTitleThreshold - _kNavBarScrollUnderAnimationExtent) /
          _kNavBarScrollUnderAnimationExtent,
      0,
      1,
    );

    final persistentNavigationBar = _PersistentNavigationBar(
      components: components,
      padding: padding,
      // If a user specified middle exists, always show it. Otherwise, show
      // title when sliver is collapsed.
      middleVisible: alwaysShowMiddle ? null : !showLargeTitle,
    );

    final Color? parentPageScaffoldBackgroundColor = CupertinoPageScaffoldBackgroundColor.maybeOf(
      context,
    );

    final Border? initialBorder =
        automaticBackgroundVisibility && parentPageScaffoldBackgroundColor != null
        ? _kTransparentNavBarBorder
        : border;
    final Border? effectiveBorder = border == null
        ? null
        : Border.lerp(initialBorder, border, shrinkAnimationValue);

    final Color effectiveBackgroundColor =
        automaticBackgroundVisibility && parentPageScaffoldBackgroundColor != null
        ? Color.lerp(parentPageScaffoldBackgroundColor, backgroundColor, shrinkAnimationValue) ??
              backgroundColor
        : backgroundColor;

    final Widget navBar = _wrapWithBackground(
      border: effectiveBorder,
      backgroundColor: effectiveBackgroundColor,
      brightness: brightness,
      enableBackgroundFilterBlur: enableBackgroundFilterBlur,
      child: DefaultTextStyle(
        style: CupertinoTheme.of(context).textTheme.textStyle,
        child: Column(
          children: <Widget>[
            Expanded(
              child: Stack(
                children: <Widget>[
                  Positioned(
                    top: persistentHeight,
                    left: 0.0,
                    right: 0.0,
                    bottom: bottomMode == NavigationBarBottomMode.automatic
                        ? bottomHeight * (1.0 - bottomShrinkFactor)
                        : 0.0,
                    child: ClipRect(
                      child: Padding(
                        padding: const EdgeInsetsDirectional.only(
                          start: _kNavBarEdgePadding,
                          bottom: _kNavBarBottomPadding,
                        ),
                        child: SafeArea(
                          top: false,
                          bottom: false,
                          child: AnimatedOpacity(
                            // Fade the large title as the search field animates from its expanded to its collapsed state.
                            opacity: showLargeTitle && !controller.isForwardOrCompleted ? 1.0 : 0.0,
                            duration: _kNavBarTitleFadeDuration,
                            child: Semantics(
                              header: true,
                              child: DefaultTextStyle(
                                style: CupertinoTheme.of(context).textTheme.navLargeTitleTextStyle,
                                maxLines: 1,
                                overflow: TextOverflow.ellipsis,
                                child: _LargeTitle(
                                  height: largeTitleHeight,
                                  child: components.largeTitle,
                                ),
                              ),
                            ),
                          ),
                        ),
                      ),
                    ),
                  ),
                  Positioned(left: 0.0, right: 0.0, top: 0.0, child: persistentNavigationBar),
                  if (bottomMode == NavigationBarBottomMode.automatic)
                    Positioned(
                      left: 0.0,
                      right: 0.0,
                      bottom: 0.0,
                      child: SizedBox(
                        height: bottomHeight * (1.0 - bottomShrinkFactor),
                        child: ClipRect(child: components.navBarBottom),
                      ),
                    ),
                ],
              ),
            ),
            if (bottomMode == NavigationBarBottomMode.always)
              SizedBox(height: bottomHeight, child: components.navBarBottom),
          ],
        ),
      ),
    );

    if (!transitionBetweenRoutes || !_isTransitionable(context)) {
      return navBar;
    }

    return Hero(
      tag: heroTag == _defaultHeroTag ? _HeroTag(Navigator.of(context)) : heroTag,
      createRectTween: _linearTranslateWithLargestRectSizeTween,
      flightShuttleBuilder: _navBarHeroFlightShuttleBuilder,
      placeholderBuilder: _navBarHeroLaunchPadBuilder,
      transitionOnUserGestures: true,
      // This is all the way down here instead of being at the top level of
      // CupertinoSliverNavigationBar like CupertinoNavigationBar because it
      // needs to wrap the top level RenderBox rather than a RenderSliver.
      child: _TransitionableNavigationBar(
        componentsKeys: keys,
        backgroundColor: effectiveBackgroundColor,
        backButtonTextStyle: CupertinoTheme.of(context).textTheme.navActionTextStyle,
        titleTextStyle: CupertinoTheme.of(context).textTheme.navTitleTextStyle,
        largeTitleTextStyle: CupertinoTheme.of(context).textTheme.navLargeTitleTextStyle,
        border: effectiveBorder,
        hasUserMiddle: userMiddle != null && (alwaysShowMiddle || !showLargeTitle),
        largeExpanded: showLargeTitle,
        searchable: searchable,
        automaticBackgroundVisibility: automaticBackgroundVisibility,
        child: navBar,
      ),
    );
  }

  @override
  bool shouldRebuild(_LargeTitleNavigationBarSliverDelegate oldDelegate) {
    return components != oldDelegate.components ||
        userMiddle != oldDelegate.userMiddle ||
        backgroundColor != oldDelegate.backgroundColor ||
        automaticBackgroundVisibility != oldDelegate.automaticBackgroundVisibility ||
        border != oldDelegate.border ||
        padding != oldDelegate.padding ||
        actionsForegroundColor != oldDelegate.actionsForegroundColor ||
        transitionBetweenRoutes != oldDelegate.transitionBetweenRoutes ||
        persistentHeight != oldDelegate.persistentHeight ||
        largeTitleHeight != oldDelegate.largeTitleHeight ||
        alwaysShowMiddle != oldDelegate.alwaysShowMiddle ||
        heroTag != oldDelegate.heroTag ||
        enableBackgroundFilterBlur != oldDelegate.enableBackgroundFilterBlur ||
        bottomMode != oldDelegate.bottomMode ||
        bottomHeight != oldDelegate.bottomHeight ||
        controller != oldDelegate.controller ||
        searchable != oldDelegate.searchable;
  }
}

/// The large title of the navigation bar.
///
/// Magnifies on over-scroll when [CupertinoSliverNavigationBar.stretch]
/// parameter is true.
class _LargeTitle extends SingleChildRenderObjectWidget {
  const _LargeTitle({super.child, required this.height});

  final double height;

  @override
  _RenderLargeTitle createRenderObject(BuildContext context) {
    return _RenderLargeTitle(
      alignment: AlignmentDirectional.bottomStart.resolve(Directionality.of(context)),
      height: height,
    );
  }

  @override
  void updateRenderObject(BuildContext context, _RenderLargeTitle renderObject) {
    renderObject
      ..alignment = AlignmentDirectional.bottomStart.resolve(Directionality.of(context))
      ..height = height;
  }
}

class _RenderLargeTitle extends RenderShiftedBox {
  _RenderLargeTitle({required Alignment alignment, required double height})
    : _alignment = alignment,
      _height = height,
      super(null);

  Alignment get alignment => _alignment;
  Alignment _alignment;
  set alignment(Alignment value) {
    if (_alignment == value) {
      return;
    }
    _alignment = value;

    markNeedsLayout();
  }

  double get height => _height;
  double _height;
  set height(double value) {
    if (_height == value) {
      return;
    }
    _height = value;

    markNeedsLayout();
  }

  double _scale = 1.0;

  static double _computeTitleScale(Size childSize, BoxConstraints constraints, double height) {
    final double maxHeight = height - _kNavBarBottomPadding;
    final double scale = 1.0 + 0.03 * (constraints.maxHeight - maxHeight) / maxHeight;
    final double maxScale = childSize.width != 0.0
        ? clampDouble(constraints.maxWidth / childSize.width, 1.0, 1.1)
        : 1.1;
    return clampDouble(scale, 1.0, maxScale);
  }

  @override
  double? computeDistanceToActualBaseline(TextBaseline baseline) {
    final double? distance = child?.getDistanceToActualBaseline(baseline);
    if (distance == null) {
      return null;
    }
    final childParentData = child!.parentData! as BoxParentData;
    return childParentData.offset.dy + distance * _scale;
  }

  @override
  double? computeDryBaseline(covariant BoxConstraints constraints, TextBaseline baseline) {
    final RenderBox? child = this.child;
    if (child == null) {
      return null;
    }
    final BoxConstraints childConstraints = constraints.widthConstraints().loosen();
    final double? result = child.getDryBaseline(childConstraints, baseline);
    if (result == null) {
      return null;
    }
    final Size childSize = child.getDryLayout(childConstraints);
    final double scale = _computeTitleScale(childSize, constraints, height);
    final Size scaledChildSize = childSize * scale;
    return result * scale +
        alignment.alongOffset(constraints.biggest - scaledChildSize as Offset).dy;
  }

  @override
  void performLayout() {
    final RenderBox? child = this.child;
    size = constraints.biggest;

    if (child == null) {
      return;
    }

    final BoxConstraints childConstraints = constraints.widthConstraints().loosen();
    child.layout(childConstraints, parentUsesSize: true);
    _scale = _computeTitleScale(child.size, constraints, height);
    final childParentData = child.parentData! as BoxParentData;
    childParentData.offset = alignment.alongOffset(size - (child.size * _scale) as Offset);
  }

  @override
  void applyPaintTransform(RenderBox child, Matrix4 transform) {
    assert(child == this.child);

    super.applyPaintTransform(child, transform);

    transform.scaleByDouble(_scale, _scale, _scale, 1);
  }

  @override
  void paint(PaintingContext context, Offset offset) {
    final RenderBox? child = this.child;

    if (child == null) {
      layer = null;
    } else {
      final childParentData = child.parentData! as BoxParentData;

      layer = context.pushTransform(
        needsCompositing,
        offset + childParentData.offset,
        Matrix4.diagonal3Values(_scale, _scale, 1.0),
        (PaintingContext context, Offset offset) => context.paintChild(child, offset),
        oldLayer: layer as TransformLayer?,
      );
    }
  }

  @override
  bool hitTestChildren(BoxHitTestResult result, {required Offset position}) {
    final RenderBox? child = this.child;

    if (child == null) {
      return false;
    }

    final Offset childOffset = (child.parentData! as BoxParentData).offset;

    final transform = Matrix4.identity()
      ..scaleByDouble(1.0 / _scale, 1.0 / _scale, 1.0, 1)
      ..translateByDouble(-childOffset.dx, -childOffset.dy, 0, 1);

    return result.addWithRawTransform(
      transform: transform,
      position: position,
      hitTest: (BoxHitTestResult result, Offset transformed) {
        return child.hitTest(result, position: transformed);
      },
    );
  }
}

/// The top part of the navigation bar that's never scrolled away.
///
/// Consists of the entire navigation bar without background and border when used
/// without large titles. With large titles, it's the top static half that
/// doesn't scroll.
class _PersistentNavigationBar extends StatelessWidget {
  const _PersistentNavigationBar({required this.components, this.padding, this.middleVisible});

  final _NavigationBarStaticComponents components;

  final EdgeInsetsDirectional? padding;

  /// Whether the middle widget has a visible animated opacity. A null value
  /// means the middle opacity will not be animated.
  final bool? middleVisible;

  @override
  Widget build(BuildContext context) {
    Widget? middle = components.middle;

    if (middle != null) {
      middle = DefaultTextStyle(
        style: CupertinoTheme.of(context).textTheme.navTitleTextStyle,
        child: Semantics(header: true, child: middle),
      );
      // When the middle's visibility can change on the fly like with large title
      // slivers, wrap with animated opacity.
      middle = middleVisible == null
          ? middle
          : AnimatedOpacity(
              opacity: middleVisible! ? 1.0 : 0.0,
              duration: _kNavBarTitleFadeDuration,
              child: middle,
            );
    }

    Widget? leading = components.leading;
    final Widget? backChevron = components.backChevron;
    final Widget? backLabel = components.backLabel;

    if (leading == null &&
        backChevron != null &&
        backLabel != null &&
        !CupertinoSheetRoute.hasParentSheet(context)) {
      leading = CupertinoNavigationBarBackButton._assemble(backChevron, backLabel);
    } else {
      leading = Align(widthFactor: 1.0, child: leading);
    }

    Widget paddedToolbar = NavigationToolbar(
      leading: leading,
      middle: middle,
      trailing: components.trailing,
      middleSpacing: 6.0,
    );

    if (padding != null) {
      paddedToolbar = Padding(
        padding: EdgeInsets.only(top: padding!.top, bottom: padding!.bottom),
        child: paddedToolbar,
      );
    }

    return SizedBox(
      height: _kNavBarPersistentHeight + MediaQuery.paddingOf(context).top,
      child: SafeArea(
        top: !CupertinoSheetRoute.hasParentSheet(context),
        bottom: false,
        child: paddedToolbar,
      ),
    );
  }
}

// A collection of keys always used when building static routes' nav bars's
// components with _NavigationBarStaticComponents and read in
// _NavigationBarTransition in Hero flights in order to reference the components'
// RenderBoxes for their positions.
//
// These keys should never re-appear inside the Hero flights.
@immutable
class _NavigationBarStaticComponentsKeys {
  _NavigationBarStaticComponentsKeys()
    : navBarBoxKey = GlobalKey(debugLabel: 'Navigation bar render box'),
      leadingKey = GlobalKey(debugLabel: 'Leading'),
      backChevronKey = GlobalKey(debugLabel: 'Back chevron'),
      backLabelKey = GlobalKey(debugLabel: 'Back label'),
      middleKey = GlobalKey(debugLabel: 'Middle'),
      trailingKey = GlobalKey(debugLabel: 'Trailing'),
      largeTitleKey = GlobalKey(debugLabel: 'Large title'),
      navBarBottomKey = GlobalKey(debugLabel: 'Navigation bar bottom');

  final GlobalKey navBarBoxKey;
  final GlobalKey leadingKey;
  final GlobalKey backChevronKey;
  final GlobalKey backLabelKey;
  final GlobalKey middleKey;
  final GlobalKey trailingKey;
  final GlobalKey largeTitleKey;
  final GlobalKey navBarBottomKey;
}

// Based on various user Widgets and other parameters, construct KeyedSubtree
// components that are used in common by the CupertinoNavigationBar and
// CupertinoSliverNavigationBar. The KeyedSubtrees are inserted into static
// routes and the KeyedSubtrees' child are reused in the Hero flights.
@immutable
class _NavigationBarStaticComponents {
  _NavigationBarStaticComponents({
    required _NavigationBarStaticComponentsKeys keys,
    required ModalRoute<dynamic>? route,
    required Widget? userLeading,
    required bool automaticallyImplyLeading,
    required bool automaticallyImplyTitle,
    required String? previousPageTitle,
    required Widget? userMiddle,
    required Widget? userTrailing,
    required Widget? userLargeTitle,
    required Widget? userBottom,
    required EdgeInsetsDirectional? padding,
    required bool large,
    required bool staticBar,
    required BuildContext context,
  }) : leading = createLeading(
         leadingKey: keys.leadingKey,
         userLeading: userLeading,
         route: route,
         automaticallyImplyLeading: automaticallyImplyLeading,
         padding: padding,
         context: context,
       ),
       backChevron = createBackChevron(
         backChevronKey: keys.backChevronKey,
         userLeading: userLeading,
         route: route,
         automaticallyImplyLeading: automaticallyImplyLeading,
         context: context,
       ),
       backLabel = createBackLabel(
         backLabelKey: keys.backLabelKey,
         userLeading: userLeading,
         route: route,
         previousPageTitle: previousPageTitle,
         automaticallyImplyLeading: automaticallyImplyLeading,
         context: context,
       ),
       middle = createMiddle(
         middleKey: keys.middleKey,
         userMiddle: userMiddle,
         userLargeTitle: userLargeTitle,
         route: route,
         automaticallyImplyTitle: automaticallyImplyTitle,
         large: large,
         staticBar: staticBar,
         context: context,
       ),
       trailing = createTrailing(
         trailingKey: keys.trailingKey,
         userTrailing: userTrailing,
         padding: padding,
         context: context,
       ),
       largeTitle = createLargeTitle(
         largeTitleKey: keys.largeTitleKey,
         userLargeTitle: userLargeTitle,
         route: route,
         automaticImplyTitle: automaticallyImplyTitle,
         large: large,
         context: context,
       ),
       navBarBottom = createNavBarBottom(
         navBarBottomKey: keys.navBarBottomKey,
         userBottom: userBottom,
         context: context,
       );

  static Widget? _derivedTitle({
    required bool automaticallyImplyTitle,
    ModalRoute<dynamic>? currentRoute,
  }) {
    // Auto use the CupertinoPageRoute's title if middle not provided.
    if (automaticallyImplyTitle &&
        currentRoute is CupertinoRouteTransitionMixin &&
        currentRoute.title != null) {
      return Text(currentRoute.title!);
    }

    return null;
  }

  final KeyedSubtree? leading;
  static KeyedSubtree? createLeading({
    required GlobalKey leadingKey,
    required Widget? userLeading,
    required ModalRoute<dynamic>? route,
    required bool automaticallyImplyLeading,
    required EdgeInsetsDirectional? padding,
    required BuildContext context,
  }) {
    Widget? leadingContent;

    if (userLeading != null) {
      leadingContent = userLeading;
    } else if (automaticallyImplyLeading &&
        route is PageRoute &&
        route.canPop &&
        route.fullscreenDialog) {
      leadingContent = CupertinoButton(
        padding: EdgeInsets.zero,
        onPressed: () {
          route.navigator!.maybePop();
        },
        child: Text(CupertinoLocalizations.of(context).cancelButtonLabel),
      );
    }

    if (leadingContent == null) {
      return null;
    }

    return KeyedSubtree(
      key: leadingKey,
      child: Padding(
        padding: EdgeInsetsDirectional.only(start: padding?.start ?? _kNavBarEdgePadding),
        child: MediaQuery(
          data: MediaQuery.of(context).copyWith(textScaler: _clampedTextScaler(context)),
          child: IconTheme.merge(data: const IconThemeData(size: 32.0), child: leadingContent),
        ),
      ),
    );
  }

  final KeyedSubtree? backChevron;
  static KeyedSubtree? createBackChevron({
    required GlobalKey backChevronKey,
    required Widget? userLeading,
    required ModalRoute<dynamic>? route,
    required bool automaticallyImplyLeading,
    required BuildContext context,
  }) {
    if (userLeading != null ||
        !automaticallyImplyLeading ||
        route == null ||
        !route.canPop ||
        (route is PageRoute && route.fullscreenDialog)) {
      return null;
    }

    return KeyedSubtree(
      key: backChevronKey,
      child: MediaQuery(
        data: MediaQuery.of(context).copyWith(textScaler: _clampedTextScaler(context)),
        child: const _BackChevron(),
      ),
    );
  }

  /// This widget is not decorated with a font since the font style could
  /// animate during transitions.
  final KeyedSubtree? backLabel;
  static KeyedSubtree? createBackLabel({
    required GlobalKey backLabelKey,
    required Widget? userLeading,
    required ModalRoute<dynamic>? route,
    required bool automaticallyImplyLeading,
    required String? previousPageTitle,
    required BuildContext context,
  }) {
    if (userLeading != null ||
        !automaticallyImplyLeading ||
        route == null ||
        !route.canPop ||
        (route is PageRoute && route.fullscreenDialog)) {
      return null;
    }

    return KeyedSubtree(
      key: backLabelKey,
      child: MediaQuery(
        data: MediaQuery.of(context).copyWith(textScaler: _clampedTextScaler(context)),
        child: _BackLabel(specifiedPreviousTitle: previousPageTitle, route: route),
      ),
    );
  }

  /// This widget is not decorated with a font since the font style could
  /// animate during transitions.
  final KeyedSubtree? middle;
  static KeyedSubtree? createMiddle({
    required GlobalKey middleKey,
    required Widget? userMiddle,
    required Widget? userLargeTitle,
    required bool large,
    required bool staticBar,
    required bool automaticallyImplyTitle,
    required ModalRoute<dynamic>? route,
    required BuildContext context,
  }) {
    var middleContent = userMiddle;

    if (large && staticBar) {
      // Static bar only displays the middle, or the large, not both.
      // A scrolling bar creates both middle and large to transition between.
      return null;
    }

    if (large) {
      middleContent ??= userLargeTitle;
    }

    middleContent ??= _derivedTitle(
      automaticallyImplyTitle: automaticallyImplyTitle,
      currentRoute: route,
    );

    if (middleContent == null) {
      return null;
    }

    return KeyedSubtree(
      key: middleKey,
      child: MediaQuery(
        data: MediaQuery.of(context).copyWith(textScaler: _clampedTextScaler(context)),
        child: middleContent,
      ),
    );
  }

  final KeyedSubtree? trailing;
  static KeyedSubtree? createTrailing({
    required GlobalKey trailingKey,
    required Widget? userTrailing,
    required EdgeInsetsDirectional? padding,
    required BuildContext context,
  }) {
    if (userTrailing == null) {
      return null;
    }

    return KeyedSubtree(
      key: trailingKey,
      child: Padding(
        padding: EdgeInsetsDirectional.only(end: padding?.end ?? _kNavBarEdgePadding),
        child: MediaQuery(
          data: MediaQuery.of(context).copyWith(textScaler: _clampedTextScaler(context)),
          child: IconTheme.merge(data: const IconThemeData(size: 32.0), child: userTrailing),
        ),
      ),
    );
  }

  /// This widget is not decorated with a font since the font style could
  /// animate during transitions.
  final KeyedSubtree? largeTitle;
  static KeyedSubtree? createLargeTitle({
    required GlobalKey largeTitleKey,
    required Widget? userLargeTitle,
    required bool large,
    required bool automaticImplyTitle,
    required ModalRoute<dynamic>? route,
    required BuildContext context,
  }) {
    if (!large) {
      return null;
    }

    final Widget? largeTitleContent =
        userLargeTitle ??
        _derivedTitle(automaticallyImplyTitle: automaticImplyTitle, currentRoute: route);

    assert(
      largeTitleContent != null,
      'largeTitle was not provided and there was no title from the route.',
    );

    return KeyedSubtree(
      key: largeTitleKey,
      child: MediaQuery(
        data: MediaQuery.of(context).copyWith(
          textScaler: TextScaler.linear(
            _dampScaleFactor(
              MediaQuery.textScalerOf(context).scale(_kNavBarLargeTitleHeightExtension),
              _kNavBarLargeTitleHeightExtension,
              _kLargeTitleScaleDampingRatio,
            ),
          ),
        ),
        child: largeTitleContent!,
      ),
    );
  }

  final KeyedSubtree? navBarBottom;
  static KeyedSubtree? createNavBarBottom({
    required GlobalKey navBarBottomKey,
    required Widget? userBottom,
    required BuildContext context,
  }) {
    return KeyedSubtree(
      key: navBarBottomKey,
      child: MediaQuery(
        data: MediaQuery.of(context).copyWith(textScaler: MediaQuery.textScalerOf(context)),
        child: userBottom ?? const SizedBox.shrink(),
      ),
    );
  }

  static TextScaler _clampedTextScaler(BuildContext context) {
    return MediaQuery.textScalerOf(
      context,
    ).clamp(minScaleFactor: 1.0, maxScaleFactor: _kMaxScaleFactor);
  }
}

/// A nav bar back button typically used in [CupertinoNavigationBar].
///
/// This is automatically inserted into [CupertinoNavigationBar] and
/// [CupertinoSliverNavigationBar]'s `leading` slot when
/// `automaticallyImplyLeading` is true.
///
/// When manually inserted, the [CupertinoNavigationBarBackButton] should only
/// be used in routes that can be popped unless a custom [onPressed] is
/// provided.
///
/// Shows a back chevron and the previous route's title when available from
/// the previous [CupertinoPageRoute.title]. If [previousPageTitle] is specified,
/// it will be shown instead.
class CupertinoNavigationBarBackButton extends StatelessWidget {
  /// Construct a [CupertinoNavigationBarBackButton] that can be used to pop
  /// the current route.
  const CupertinoNavigationBarBackButton({
    super.key,
    this.color,
    this.previousPageTitle,
    this.onPressed,
  }) : _backChevron = null,
       _backLabel = null;

  // Allow the back chevron and label to be separately created (and keyed)
  // because they animate separately during page transitions.
  const CupertinoNavigationBarBackButton._assemble(this._backChevron, this._backLabel)
    : previousPageTitle = null,
      color = null,
      onPressed = null;

  /// The [Color] of the back button.
  ///
  /// Can be used to override the color of the back button chevron and label.
  ///
  /// Defaults to [CupertinoTheme]'s `primaryColor` if null.
  final Color? color;

  /// An override for showing the previous route's title. If null, it will be
  /// automatically derived from [CupertinoPageRoute.title] if the current and
  /// previous routes are both [CupertinoPageRoute]s.
  final String? previousPageTitle;

  /// An override callback to perform instead of the default behavior which is
  /// to pop the [Navigator].
  ///
  /// It can, for instance, be used to pop the platform's navigation stack
  /// via [SystemNavigator] instead of Flutter's [Navigator] in add-to-app
  /// situations.
  ///
  /// Defaults to null.
  final VoidCallback? onPressed;

  final Widget? _backChevron;

  final Widget? _backLabel;

  @override
  Widget build(BuildContext context) {
    final ModalRoute<dynamic>? currentRoute = ModalRoute.of(context);
    if (onPressed == null) {
      assert(
        currentRoute?.canPop ?? false,
        'CupertinoNavigationBarBackButton should only be used in routes that can be popped',
      );
    }

    TextStyle actionTextStyle = CupertinoTheme.of(context).textTheme.navActionTextStyle;
    if (color != null) {
      actionTextStyle = actionTextStyle.copyWith(
        color: CupertinoDynamicColor.maybeResolve(color, context),
      );
    }

    final CupertinoLocalizations localizations = CupertinoLocalizations.of(context);
    return CupertinoButton(
      padding: EdgeInsets.zero,
      child: Semantics(
        container: true,
        excludeSemantics: true,
        label: localizations.backButtonLabel,
        button: true,
        child: DefaultTextStyle(
          style: actionTextStyle,
          child: ConstrainedBox(
            constraints: const BoxConstraints(minWidth: _kNavBarBackButtonTapWidth),
            child: Row(
              mainAxisSize: MainAxisSize.min,
              children: <Widget>[
                const Padding(padding: EdgeInsetsDirectional.only(start: 8.0)),
                _backChevron ?? const _BackChevron(),
                const Padding(padding: EdgeInsetsDirectional.only(start: 6.0)),
                Flexible(
                  child:
                      _backLabel ??
                      _BackLabel(specifiedPreviousTitle: previousPageTitle, route: currentRoute),
                ),
              ],
            ),
          ),
        ),
      ),
      onPressed: () {
        if (onPressed != null) {
          onPressed!();
        } else {
          Navigator.maybePop(context);
        }
      },
    );
  }
}

class _BackChevron extends StatelessWidget {
  const _BackChevron();

  @override
  Widget build(BuildContext context) {
    final TextDirection textDirection = Directionality.of(context);
    final TextStyle textStyle = DefaultTextStyle.of(context).style;

    // Replicate the Icon logic here to get a tightly sized icon and add
    // custom non-square padding.
    Widget iconWidget = Padding(
      padding: const EdgeInsetsDirectional.only(start: 6, end: 2),
      child: Text.rich(
        TextSpan(
          text: String.fromCharCode(CupertinoIcons.back.codePoint),
          style: TextStyle(
            inherit: false,
            color: textStyle.color,
            fontSize: 30.0,
            fontFamily: CupertinoIcons.back.fontFamily,
            package: CupertinoIcons.back.fontPackage,
          ),
        ),
      ),
    );
    switch (textDirection) {
      case TextDirection.rtl:
        iconWidget = Transform(
          transform: Matrix4.identity()..scaleByDouble(-1.0, 1.0, 1.0, 1),
          alignment: Alignment.center,
          transformHitTests: false,
          child: iconWidget,
        );
      case TextDirection.ltr:
        break;
    }

    return KeyedSubtree(key: StandardComponentType.backButton.key, child: iconWidget);
  }
}

/// A widget that shows next to the back chevron when `automaticallyImplyLeading`
/// is true.
class _BackLabel extends StatelessWidget {
  const _BackLabel({required this.specifiedPreviousTitle, required this.route});

  final String? specifiedPreviousTitle;
  final ModalRoute<dynamic>? route;

  // `child` is never passed in into ValueListenableBuilder so it's always
  // null here and unused.
  Widget _buildPreviousTitleWidget(BuildContext context, String? previousTitle, Widget? child) {
    if (previousTitle == null) {
      return const SizedBox.shrink();
    }

    var textWidget = Text(previousTitle, maxLines: 1, overflow: TextOverflow.ellipsis);

    if (previousTitle.length > 12) {
      textWidget = Text(CupertinoLocalizations.of(context).backButtonLabel);
    }

    return Align(alignment: AlignmentDirectional.centerStart, widthFactor: 1.0, child: textWidget);
  }

  @override
  Widget build(BuildContext context) {
    if (specifiedPreviousTitle != null) {
      return _buildPreviousTitleWidget(context, specifiedPreviousTitle, null);
    } else if (route is CupertinoRouteTransitionMixin<dynamic> && !route!.isFirst) {
      final cupertinoRoute = route! as CupertinoRouteTransitionMixin<dynamic>;
      // There is no timing issue because the previousTitle Listenable changes
      // happen during route modifications before the ValueListenableBuilder
      // is built.
      return ValueListenableBuilder<String?>(
        valueListenable: cupertinoRoute.previousTitle,
        builder: _buildPreviousTitleWidget,
      );
    } else {
      return const SizedBox.shrink();
    }
  }
}

/// The 'Cancel' button next to the search field in a
/// [CupertinoSliverNavigationBar.search].
class _CancelButton extends StatelessWidget {
  const _CancelButton({this.opacity = 1.0, required this.onPressed});

  final void Function()? onPressed;
  final double opacity;

  @override
  Widget build(BuildContext context) {
    final CupertinoLocalizations localizations = CupertinoLocalizations.of(context);
    return MediaQuery.withNoTextScaling(
      child: Align(
        alignment: Alignment.centerLeft,
        child: Opacity(
          opacity: opacity,
          child: CupertinoButton(
            padding: EdgeInsets.zero,
            onPressed: onPressed,
            child: Text(localizations.cancelButtonLabel, maxLines: 1, overflow: TextOverflow.clip),
          ),
        ),
      ),
    );
  }
}

/// The bottom of a [CupertinoSliverNavigationBar.search] when the search field
/// is inactive.
class _InactiveSearchableBottom extends StatelessWidget {
  const _InactiveSearchableBottom({
    required this.animationController,
    required this.searchField,
    required this.animation,
    required this.searchFieldHeight,
    required this.onSearchFieldTap,
  });

  final AnimationController animationController;
  final Widget? searchField;
  final Animation<double> animation;
  final double searchFieldHeight;
  final void Function()? onSearchFieldTap;

  @override
  Widget build(BuildContext context) {
    return AnimatedBuilder(
      animation: animation,
      child: GestureDetector(
        onTap: onSearchFieldTap,
        child: AbsorbPointer(
          child: FocusableActionDetector(
            descendantsAreFocusable: false,
            child: Padding(
              padding: const EdgeInsetsDirectional.only(
                start: _kNavBarEdgePadding,
                end: _kNavBarEdgePadding,
                bottom: _kNavBarBottomPadding,
              ),
              child: SizedBox(height: searchFieldHeight, child: searchField),
            ),
          ),
        ),
      ),
      builder: (BuildContext context, Widget? child) {
        return LayoutBuilder(
          builder: (BuildContext context, BoxConstraints constraints) {
            return Row(
              children: <Widget>[
                SizedBox(
                  width:
                      constraints.maxWidth -
                      (_kSearchFieldCancelButtonWidth * animationController.value),
                  child: child,
                ),
                // A decoy 'Cancel' button used in the collapsed-to-expanded animation.
                SizedBox(
                  width: animationController.value * _kSearchFieldCancelButtonWidth,
                  child: Padding(
                    padding: const EdgeInsets.only(bottom: _kNavBarBottomPadding),
                    child: _CancelButton(opacity: 0.4, onPressed: () {}),
                  ),
                ),
              ],
            );
          },
        );
      },
    );
  }
}

/// The bottom of a [CupertinoSliverNavigationBar.search] when the search field
/// is active.
class _ActiveSearchableBottom extends StatelessWidget {
  const _ActiveSearchableBottom({
    required this.animationController,
    required this.searchField,
    required this.animation,
    required this.searchFieldHeight,
    required this.onSearchFieldTap,
  });

  final AnimationController animationController;
  final Widget? searchField;
  final Animation<double> animation;
  final double searchFieldHeight;
  final void Function()? onSearchFieldTap;

  @override
  Widget build(BuildContext context) {
    return Padding(
      padding: const EdgeInsetsDirectional.only(
        start: _kNavBarEdgePadding,
        bottom: _kNavBarBottomPadding,
      ),
      child: Row(
        spacing: 12.0, // Eyeballed on an iPhone 15 simulator running iOS 17.5.
        children: <Widget>[
          Expanded(
            child: SizedBox(
              height: searchFieldHeight,
              child: searchField ?? const SizedBox.shrink(),
            ),
          ),
          AnimatedBuilder(
            animation: animation,
            child: FadeTransition(
              opacity: Tween<double>(begin: 0.0, end: 1.0).animate(animationController),
              child: _CancelButton(onPressed: onSearchFieldTap),
            ),
            builder: (BuildContext context, Widget? child) {
              return SizedBox(
                width: animationController.value * _kSearchFieldCancelButtonWidth,
                child: child,
              );
            },
          ),
        ],
      ),
    );
  }
}

/// This should always be the first child of Hero widgets.
///
/// This class helps each Hero transition obtain the start or end navigation
/// bar's box size and the inner components of the navigation bar that will
/// move around.
///
/// It should be wrapped around the biggest [RenderBox] of the static
/// navigation bar in each route.
class _TransitionableNavigationBar extends StatelessWidget {
  _TransitionableNavigationBar({
    required this.componentsKeys,
    required this.backgroundColor,
    required this.backButtonTextStyle,
    required this.titleTextStyle,
    required this.largeTitleTextStyle,
    required this.border,
    required this.hasUserMiddle,
    required this.largeExpanded,
    required this.searchable,
    required this.automaticBackgroundVisibility,
    required this.child,
  }) : assert(!largeExpanded || largeTitleTextStyle != null),
       super(key: componentsKeys.navBarBoxKey);

  final _NavigationBarStaticComponentsKeys componentsKeys;
  final Color? backgroundColor;
  final TextStyle backButtonTextStyle;
  final TextStyle titleTextStyle;
  final TextStyle? largeTitleTextStyle;
  final Border? border;
  final bool hasUserMiddle;
  final bool largeExpanded;
  final bool searchable;
  final bool automaticBackgroundVisibility;
  final Widget child;

  RenderBox get renderBox {
    final box = componentsKeys.navBarBoxKey.currentContext!.findRenderObject()! as RenderBox;
    assert(
      box.attached,
      '_TransitionableNavigationBar.renderBox should be called when building '
      'hero flight shuttles when the from and the to nav bar boxes are already '
      'laid out and painted.',
    );
    return box;
  }

  bool get userGestureInProgress {
    return Navigator.of(componentsKeys.navBarBoxKey.currentContext!).userGestureInProgress;
  }

  @override
  Widget build(BuildContext context) {
    assert(() {
      var inHero = false;
      context.visitAncestorElements((Element ancestor) {
        if (ancestor is ComponentElement) {
          assert(
            ancestor.widget.runtimeType != _NavigationBarTransition,
            '_TransitionableNavigationBar should never re-appear inside '
            '_NavigationBarTransition. Keyed _TransitionableNavigationBar should '
            'only serve as anchor points in routes rather than appearing inside '
            'Hero flights themselves.',
          );
          if (ancestor.widget.runtimeType == Hero) {
            inHero = true;
          }
        }
        return true;
      });
      assert(
        inHero,
        '_TransitionableNavigationBar should only be added as the immediate '
        'child of Hero widgets.',
      );
      return true;
    }());
    return child;
  }
}

/// This class represents the widget that will be in the Hero flight instead of
/// the 2 static navigation bars by taking inner components from both.
///
/// The `topNavBar` parameter is the nav bar that was on top regardless of
/// push/pop direction.
///
/// Similarly, the `bottomNavBar` parameter is the nav bar that was at the
/// bottom regardless of the push/pop direction.
///
/// If [MediaQueryData.padding] is still present in this widget's
/// [BuildContext], that padding will become part of the transitional navigation
/// bar as well.
///
/// [MediaQueryData.padding] should be consistent between the from/to routes and
/// the Hero overlay. Inconsistent [MediaQueryData.padding] will produce
/// undetermined results.
class _NavigationBarTransition extends StatelessWidget {
  _NavigationBarTransition({
    required this.animation,
    required this.topNavBar,
    required this.bottomNavBar,
  }) : heightTween = Tween<double>(
         begin: bottomNavBar.renderBox.size.height,
         end: topNavBar.renderBox.size.height,
       );

  final Animation<double> animation;
  final _TransitionableNavigationBar topNavBar;
  final _TransitionableNavigationBar bottomNavBar;

  final Tween<double> heightTween;

  @override
  Widget build(BuildContext context) {
    final componentsTransition = _NavigationBarComponentsTransition(
      animation: animation,
      bottomNavBar: bottomNavBar,
      topNavBar: topNavBar,
      directionality: Directionality.of(context),
    );

    final children = <Widget>[
      ?componentsTransition.bottomNavBarBackground,
      ?componentsTransition.bottomBackChevron,
      ?componentsTransition.bottomBackLabel,
      ?componentsTransition.bottomLeading,
      ?componentsTransition.bottomMiddle,
      ?componentsTransition.bottomLargeTitle,
      ?componentsTransition.bottomTrailing,
      ?componentsTransition.bottomNavBarBottom,
      // Draw top components on top of the bottom components.
      ?componentsTransition.topNavBarBackground,
      ?componentsTransition.topLeading,
      ?componentsTransition.topBackChevron,
      ?componentsTransition.topBackLabel,
      ?componentsTransition.topMiddle,
      ?componentsTransition.topLargeTitle,
      ?componentsTransition.topTrailing,
      ?componentsTransition.topNavBarBottom,
    ];

    // The text scaling is disabled to avoid odd transitions between pages.
    return MediaQuery.withNoTextScaling(
      child: SizedBox(
        height: math.max(heightTween.begin!, heightTween.end!) + MediaQuery.paddingOf(context).top,
        width: double.infinity,
        child: Stack(children: children),
      ),
    );
  }
}

/// This class helps create widgets that are in transition based on static
/// components from the bottom and top navigation bars.
///
/// It animates these transitional components both in terms of position and
/// their appearance.
///
/// Instead of running the transitional components through their normal static
/// navigation bar layout logic, this creates transitional widgets that are based
/// on these widgets' existing render objects' layout and position.
///
/// This is possible because this widget is only used during Hero transitions
/// where both the from and to routes are already built and laid out.
///
/// The components' existing layout constraints and positions are then
/// replicated using [Positioned] or [PositionedTransition] wrappers.
///
/// This class should never return [KeyedSubtree]s created by
/// _NavigationBarStaticComponents directly. Since widgets from
/// _NavigationBarStaticComponents are still present in the widget tree during the
/// hero transitions, it would cause global key duplications. Instead, return
/// only the [KeyedSubtree]s' child.
@immutable
class _NavigationBarComponentsTransition {
  _NavigationBarComponentsTransition({
    required this.animation,
    required _TransitionableNavigationBar bottomNavBar,
    required _TransitionableNavigationBar topNavBar,
    required TextDirection directionality,
  }) : bottomComponents = bottomNavBar.componentsKeys,
       topComponents = topNavBar.componentsKeys,
       bottomNavBarBox = bottomNavBar.renderBox,
       topNavBarBox = topNavBar.renderBox,
       bottomBackButtonTextStyle = bottomNavBar.backButtonTextStyle,
       topBackButtonTextStyle = topNavBar.backButtonTextStyle,
       bottomTitleTextStyle = bottomNavBar.titleTextStyle,
       topTitleTextStyle = topNavBar.titleTextStyle,
       bottomLargeTitleTextStyle = bottomNavBar.largeTitleTextStyle,
       topLargeTitleTextStyle = topNavBar.largeTitleTextStyle,
       bottomHasUserMiddle = bottomNavBar.hasUserMiddle,
       topHasUserMiddle = topNavBar.hasUserMiddle,
       bottomLargeExpanded = bottomNavBar.largeExpanded,
       topLargeExpanded = topNavBar.largeExpanded,
       bottomBackgroundColor = bottomNavBar.backgroundColor,
       topBackgroundColor = topNavBar.backgroundColor,
       bottomBorder = bottomNavBar.border,
       topBorder = topNavBar.border,
       bottomAutomaticBackgroundVisibility = bottomNavBar.automaticBackgroundVisibility,
       userGestureInProgress =
           topNavBar.userGestureInProgress || bottomNavBar.userGestureInProgress,
       searchable = topNavBar.searchable && bottomNavBar.searchable,
       transitionBox =
           // paintBounds are based on offset zero so it's ok to expand the Rects.
           bottomNavBar.renderBox.paintBounds.expandToInclude(topNavBar.renderBox.paintBounds),
       forwardDirection = directionality == TextDirection.ltr ? 1.0 : -1.0;

  static final Animatable<double> fadeOut = Tween<double>(begin: 1.0, end: 0.0);
  static final Animatable<double> fadeIn = Tween<double>(begin: 0.0, end: 1.0);

  final Animation<double> animation;
  final _NavigationBarStaticComponentsKeys bottomComponents;
  final _NavigationBarStaticComponentsKeys topComponents;

  // These render boxes that are the ancestors of all the bottom and top
  // components are used to determine the components' relative positions inside
  // their respective navigation bars.
  final RenderBox bottomNavBarBox;
  final RenderBox topNavBarBox;

  final TextStyle bottomBackButtonTextStyle;
  final TextStyle topBackButtonTextStyle;
  final TextStyle bottomTitleTextStyle;
  final TextStyle topTitleTextStyle;
  final TextStyle? bottomLargeTitleTextStyle;
  final TextStyle? topLargeTitleTextStyle;

  final bool bottomHasUserMiddle;
  final bool topHasUserMiddle;
  final bool bottomLargeExpanded;
  final bool topLargeExpanded;
  final bool userGestureInProgress;
  final bool searchable;
  final bool bottomAutomaticBackgroundVisibility;

  final Color? bottomBackgroundColor;
  final Color? topBackgroundColor;
  final Border? bottomBorder;
  final Border? topBorder;

  // This is the outer box in which all the components will be fitted. The
  // sizing component of RelativeRects will be based on this rect's size.
  final Rect transitionBox;

  // x-axis unity number representing the direction of growth for text.
  final double forwardDirection;

  // Take a widget in its original ancestor navigation bar render box and
  // translate it into a RelativeBox in the transition navigation bar box.
  RelativeRect positionInTransitionBox(GlobalKey key, {required RenderBox from}) {
    final componentBox = key.currentContext!.findRenderObject()! as RenderBox;
    assert(componentBox.attached);

    return RelativeRect.fromRect(
      componentBox.localToGlobal(Offset.zero, ancestor: from) & componentBox.size,
      transitionBox,
    );
  }

  // Create an animated widget that moves the given child widget between its
  // original position in its ancestor navigation bar to another widget's
  // position in that widget's navigation bar.
  //
  // Anchor their positions based on the vertical middle of their respective
  // render boxes' leading edge.
  //
  // This method assumes there's no other transforms other than translations
  // when converting a rect from the original navigation bar's coordinate space
  // to the other navigation bar's coordinate space, to avoid performing
  // floating point operations on the size of the child widget, so that the
  // incoming constraints used for sizing the child widget will be exactly the
  // same.
  _FixedSizeSlidingTransition slideFromLeadingEdge({
    required GlobalKey fromKey,
    required RenderBox fromNavBarBox,
    required GlobalKey toKey,
    required RenderBox toNavBarBox,
    Curve curve = const Interval(0.0, 1.0),
    required Widget child,
  }) {
    final fromBox = fromKey.currentContext!.findRenderObject()! as RenderBox;
    final toBox = toKey.currentContext!.findRenderObject()! as RenderBox;

    final bool isLTR = forwardDirection > 0;

    // The animation moves the fromBox so its anchor (left-center or right-center
    // depending on the writing direction) aligns with toBox's anchor.
    final fromAnchorLocal = Offset(isLTR ? 0 : fromBox.size.width, fromBox.size.height / 2);
    final toAnchorLocal = Offset(isLTR ? 0 : toBox.size.width, toBox.size.height / 2);
    final Offset fromAnchorInFromBox = fromBox.localToGlobal(
      fromAnchorLocal,
      ancestor: fromNavBarBox,
    );
    final Offset toAnchorInToBox = toBox.localToGlobal(toAnchorLocal, ancestor: toNavBarBox);

    // We can't get ahold of the render box of the stack (i.e., `transitionBox`)
    // we place components on yet, but we know the stack needs to be top-leading
    // aligned with both fromNavBarBox and toNavBarBox to make the transition
    // look smooth. Also use the top-leading point as the origin for ease of
    // calculation.

    // The offset to move fromAnchor to toAnchor, in transitionBox's top-leading
    // coordinates.
    final Offset translation = isLTR
        ? toAnchorInToBox - fromAnchorInFromBox
        : Offset(toNavBarBox.size.width - toAnchorInToBox.dx, toAnchorInToBox.dy) -
              Offset(fromNavBarBox.size.width - fromAnchorInFromBox.dx, fromAnchorInFromBox.dy);

    final RelativeRect fromBoxMargin = positionInTransitionBox(fromKey, from: fromNavBarBox);
    final fromOriginInTransitionBox = Offset(
      isLTR ? fromBoxMargin.left : fromBoxMargin.right,
      fromBoxMargin.top,
    );

    final anchorMovementInTransitionBox = Tween<Offset>(
      begin: fromOriginInTransitionBox,
      end: fromOriginInTransitionBox + translation,
    );

    return _FixedSizeSlidingTransition(
      isLTR: isLTR,
      offsetAnimation: animation
          .drive(CurveTween(curve: curve))
          .drive(anchorMovementInTransitionBox),
      width: fromNavBarBox.size.width,
      height: fromBox.size.height,
      child: child,
    );
  }

  Animation<double> fadeInFrom(double t, {Curve curve = Curves.easeIn}) {
    return animation.drive(fadeIn.chain(CurveTween(curve: Interval(t, 1.0, curve: curve))));
  }

  Animation<double> fadeOutBy(double t, {Curve curve = Curves.easeOut}) {
    return animation.drive(fadeOut.chain(CurveTween(curve: Interval(0.0, t, curve: curve))));
  }

  // The parent of the hero animation, which is the route animation.
  Animation<double> get routeAnimation {
    // The hero animation is a CurvedAnimation.
    assert(animation is CurvedAnimation);
    return (animation as CurvedAnimation).parent;
  }

  Widget? get bottomNavBarBackground {
    if (bottomBackgroundColor == null ||
        (bottomLargeExpanded && bottomAutomaticBackgroundVisibility)) {
      return null;
    }
    final Curve animationCurve = animation.status == AnimationStatus.forward
        ? Curves.fastEaseInToSlowEaseOut
        : Curves.fastEaseInToSlowEaseOut.flipped;

    final Animation<double> pageTransitionAnimation = routeAnimation.drive(
      CurveTween(curve: userGestureInProgress ? Curves.linear : animationCurve),
    );

    final RelativeRect from = positionInTransitionBox(
      bottomComponents.navBarBoxKey,
      from: bottomNavBarBox,
    );

    final positionTween = RelativeRectTween(
      end: from.shift(Offset(forwardDirection * -bottomNavBarBox.size.width, 0.0)),
      begin: from,
    );

    return PositionedTransition(
      rect: pageTransitionAnimation.drive(positionTween),
      child: _wrapWithBackground(
        // Don't update the system status bar color mid-flight.
        updateSystemUiOverlay: false,
        backgroundColor: bottomBackgroundColor!,
        border: topBorder,
        child: SizedBox(height: bottomNavBarBox.size.height, width: double.infinity),
      ),
    );
  }

  Widget? get bottomLeading {
    final bottomLeading = bottomComponents.leadingKey.currentWidget as KeyedSubtree?;

    if (bottomLeading == null) {
      return null;
    }

    return Positioned.fromRelativeRect(
      rect: positionInTransitionBox(bottomComponents.leadingKey, from: bottomNavBarBox),
      child: FadeTransition(opacity: fadeOutBy(0.4), child: bottomLeading.child),
    );
  }

  Widget? get bottomBackChevron {
    final bottomBackChevron = bottomComponents.backChevronKey.currentWidget as KeyedSubtree?;

    if (bottomBackChevron == null) {
      return null;
    }

    return Positioned.fromRelativeRect(
      rect: positionInTransitionBox(bottomComponents.backChevronKey, from: bottomNavBarBox),
      child: FadeTransition(
        opacity: fadeOutBy(0.6),
        child: DefaultTextStyle(style: bottomBackButtonTextStyle, child: bottomBackChevron.child),
      ),
    );
  }

  Widget? get bottomBackLabel {
    final bottomBackLabel = bottomComponents.backLabelKey.currentWidget as KeyedSubtree?;

    if (bottomBackLabel == null) {
      return null;
    }

    final RelativeRect from = positionInTransitionBox(
      bottomComponents.backLabelKey,
      from: bottomNavBarBox,
    );

    // Transition away by sliding horizontally to the leading edge off of the screen.
    final positionTween = RelativeRectTween(
      begin: from,
      end: from.shift(Offset(forwardDirection * (-bottomNavBarBox.size.width / 2.0), 0.0)),
    );

    return PositionedTransition(
      rect: animation.drive(positionTween),
      child: FadeTransition(
        opacity: fadeOutBy(0.2),
        child: DefaultTextStyle(style: bottomBackButtonTextStyle, child: bottomBackLabel.child),
      ),
    );
  }

  Widget? get bottomMiddle {
    final bottomMiddle = bottomComponents.middleKey.currentWidget as KeyedSubtree?;
    final topBackLabel = topComponents.backLabelKey.currentWidget as KeyedSubtree?;
    final topLeading = topComponents.leadingKey.currentWidget as KeyedSubtree?;

    // The middle component is non-null when the nav bar is a large title
    // nav bar but would be invisible when expanded, therefore don't show it here.
    if (!bottomHasUserMiddle && bottomLargeExpanded) {
      return null;
    }

    if (bottomMiddle != null && topBackLabel != null) {
      // Move from current position to the top page's back label position.
      return slideFromLeadingEdge(
        fromKey: bottomComponents.middleKey,
        fromNavBarBox: bottomNavBarBox,
        toKey: topComponents.backLabelKey,
        toNavBarBox: topNavBarBox,
        child: FadeTransition(
          // A custom middle widget like a segmented control fades away faster.
          opacity: fadeOutBy(bottomHasUserMiddle ? 0.4 : 0.7),
          child: Align(
            // As the text shrinks, make sure it's still anchored to the leading
            // edge of a constantly sized outer box.
            alignment: AlignmentDirectional.centerStart,
            child: DefaultTextStyleTransition(
              style: animation.drive(
                TextStyleTween(begin: bottomTitleTextStyle, end: topBackButtonTextStyle),
              ),
              child: bottomMiddle.child,
            ),
          ),
        ),
      );
    }

    // When the top page has a leading widget override (one of the few ways to
    // not have a top back label), don't move the bottom middle widget and just
    // fade.
    if (bottomMiddle != null && topLeading != null) {
      return Positioned.fromRelativeRect(
        rect: positionInTransitionBox(bottomComponents.middleKey, from: bottomNavBarBox),
        child: FadeTransition(
          opacity: fadeOutBy(bottomHasUserMiddle ? 0.4 : 0.7),
          // Keep the font when transitioning into a non-back label leading.
          child: DefaultTextStyle(style: bottomTitleTextStyle, child: bottomMiddle.child),
        ),
      );
    }

    return null;
  }

  Widget? get bottomLargeTitle {
    final bottomLargeTitle = bottomComponents.largeTitleKey.currentWidget as KeyedSubtree?;
    final topBackLabel = topComponents.backLabelKey.currentWidget as KeyedSubtree?;

    if (bottomLargeTitle == null || !bottomLargeExpanded) {
      return null;
    }

    if (topBackLabel != null) {
      // Move from current position to the top page's back label position.
      return slideFromLeadingEdge(
        fromKey: bottomComponents.largeTitleKey,
        fromNavBarBox: bottomNavBarBox,
        toKey: topComponents.backLabelKey,
        toNavBarBox: topNavBarBox,
        curve: Interval(0.0, animation.status == AnimationStatus.forward ? 0.7 : 1.0),
        child: FadeTransition(
          opacity: fadeOutBy(0.6),
          child: Align(
            // As the text shrinks, make sure it's still anchored to the leading
            // edge of a constantly sized outer box.
            alignment: AlignmentDirectional.centerStart,
            child: DefaultTextStyleTransition(
              style: animation.drive(
                TextStyleTween(begin: bottomLargeTitleTextStyle, end: topBackButtonTextStyle),
              ),
              maxLines: 1,
              overflow: TextOverflow.ellipsis,
              child: bottomLargeTitle.child,
            ),
          ),
        ),
      );
    }

    // Unlike bottom middle, the bottom large title moves when it can't
    // transition to the top back label position.
    final RelativeRect from = positionInTransitionBox(
      bottomComponents.largeTitleKey,
      from: bottomNavBarBox,
    );

    final positionTween = RelativeRectTween(
      begin: from,
      end: from.shift(Offset(forwardDirection * bottomNavBarBox.size.width / 4.0, 0.0)),
    );

    // Just shift slightly towards the trailing edge instead of moving to the
    // back label position.
    return PositionedTransition(
      rect: animation.drive(positionTween),
      child: FadeTransition(
        opacity: fadeOutBy(0.4),
        // Keep the font when transitioning into a non-back-label leading.
        child: DefaultTextStyle(style: bottomLargeTitleTextStyle!, child: bottomLargeTitle.child),
      ),
    );
  }

  Widget? get bottomTrailing {
    final bottomTrailing = bottomComponents.trailingKey.currentWidget as KeyedSubtree?;

    if (bottomTrailing == null) {
      return null;
    }

    return Positioned.fromRelativeRect(
      rect: positionInTransitionBox(bottomComponents.trailingKey, from: bottomNavBarBox),
      child: FadeTransition(opacity: fadeOutBy(0.6), child: bottomTrailing.child),
    );
  }

  Widget? get bottomNavBarBottom {
    final bottomNavBarBottom = bottomComponents.navBarBottomKey.currentWidget as KeyedSubtree?;

    if (bottomNavBarBottom == null) {
      return null;
    }

    final RelativeRect from = positionInTransitionBox(
      bottomComponents.navBarBottomKey,
      from: bottomNavBarBox,
    );
    // Shift in from the leading edge of the screen.
    final positionTween = RelativeRectTween(
      begin: from,
      end: from.shift(Offset(forwardDirection * -bottomNavBarBox.size.width, 0.0)),
    );

    Widget child = bottomNavBarBottom.child;
    final Curve animationCurve = animation.status == AnimationStatus.forward
        ? _kBottomNavBarHeaderTransitionCurve
        : _kBottomNavBarHeaderTransitionCurve.flipped;

    // Fade out only if this is not a CupertinoSliverNavigationBar.search to
    // CupertinoSliverNavigationBar.search transition.
    if (!searchable) {
      child = FadeTransition(
        opacity: fadeOutBy(0.8, curve: animationCurve),
        child: child,
      );
    }

    return PositionedTransition(
      rect:
          // The bottom widget animates linearly during a backswipe by a user gesture.
          userGestureInProgress
          ? routeAnimation.drive(CurveTween(curve: Curves.linear)).drive(positionTween)
          : animation.drive(CurveTween(curve: animationCurve)).drive(positionTween),

      child: ClipRect(child: child),
    );
  }

  Widget? get topNavBarBackground {
    if (topBackgroundColor == null) {
      return null;
    }
    final Curve animationCurve = animation.status == AnimationStatus.forward
        ? Curves.fastEaseInToSlowEaseOut
        : Curves.fastEaseInToSlowEaseOut.flipped;

    final Animation<double> pageTransitionAnimation = routeAnimation.drive(
      CurveTween(curve: userGestureInProgress ? Curves.linear : animationCurve),
    );

    final RelativeRect to = positionInTransitionBox(topComponents.navBarBoxKey, from: topNavBarBox);

    final positionTween = RelativeRectTween(
      begin: to.shift(Offset(forwardDirection * topNavBarBox.size.width, 0.0)),
      end: to,
    );

    return PositionedTransition(
      rect: pageTransitionAnimation.drive(positionTween),
      child: _wrapWithBackground(
        // Don't update the system status bar color mid-flight.
        updateSystemUiOverlay: false,
        backgroundColor: topBackgroundColor!,
        border: topBorder,
        child: SizedBox(height: topNavBarBox.size.height, width: double.infinity),
      ),
    );
  }

  Widget? get topLeading {
    final topLeading = topComponents.leadingKey.currentWidget as KeyedSubtree?;

    if (topLeading == null) {
      return null;
    }

    return Positioned.fromRelativeRect(
      rect: positionInTransitionBox(topComponents.leadingKey, from: topNavBarBox),
      child: FadeTransition(opacity: fadeInFrom(0.6), child: topLeading.child),
    );
  }

  Widget? get topBackChevron {
    final topBackChevron = topComponents.backChevronKey.currentWidget as KeyedSubtree?;
    final bottomBackChevron = bottomComponents.backChevronKey.currentWidget as KeyedSubtree?;

    if (topBackChevron == null) {
      return null;
    }

    final RelativeRect to = positionInTransitionBox(
      topComponents.backChevronKey,
      from: topNavBarBox,
    );
    var from = to;

    Widget child = topBackChevron.child;
    // Values eyeballed from an iPhone 15 simulator running iOS 17.5.
    const Curve forwardScaleCurve = Interval(0.0, 0.2);
    const Curve backwardScaleCurve = Interval(0.8, 1.0);
    const Curve forwardPositionCurve = Interval(0.0, 0.5);
    const Curve backwardPositionCurve = Interval(0.5, 1.0);
    final Curve effectiveScaleCurve;
    final Curve effectivePositionCurve;

    if (animation.status == AnimationStatus.forward) {
      effectiveScaleCurve = forwardScaleCurve;
      effectivePositionCurve = forwardPositionCurve;
    } else {
      effectiveScaleCurve = backwardScaleCurve;
      effectivePositionCurve = backwardPositionCurve;
    }

    // If it's the first page with a back chevron, shrink and shift in slightly
    // from the right.
    if (bottomBackChevron == null) {
      final topBackChevronBox =
          topComponents.backChevronKey.currentContext!.findRenderObject()! as RenderBox;
      from = to.shift(Offset(forwardDirection * topBackChevronBox.size.width * 2.0, 0.0));
      child = ScaleTransition(
        scale: routeAnimation.drive(CurveTween(curve: effectiveScaleCurve)),
        child: child,
      );
    }

    final positionTween = RelativeRectTween(begin: from, end: to);

    return PositionedTransition(
      rect: routeAnimation.drive(CurveTween(curve: effectivePositionCurve)).drive(positionTween),
      child: FadeTransition(
        opacity: routeAnimation.drive(
          CurveTween(
            curve: Interval(
              // Fades faster going back from the first page with a back chevron.
              bottomBackChevron == null && animation.status != AnimationStatus.forward ? 0.9 : 0.4,
              1.0,
            ),
          ),
        ),
        child: DefaultTextStyle(style: topBackButtonTextStyle, child: child),
      ),
    );
  }

  Widget? get topBackLabel {
    final bottomMiddle = bottomComponents.middleKey.currentWidget as KeyedSubtree?;
    final bottomLargeTitle = bottomComponents.largeTitleKey.currentWidget as KeyedSubtree?;
    final topBackLabel = topComponents.backLabelKey.currentWidget as KeyedSubtree?;

    if (topBackLabel == null) {
      return null;
    }

    final RenderAnimatedOpacity? topBackLabelOpacity = topComponents.backLabelKey.currentContext
        ?.findAncestorRenderObjectOfType<RenderAnimatedOpacity>();

    Animation<double>? midClickOpacity;
    if (topBackLabelOpacity != null && topBackLabelOpacity.opacity.value < 1.0) {
      midClickOpacity = animation.drive(
        Tween<double>(begin: 0.0, end: topBackLabelOpacity.opacity.value),
      );
    }

    // Pick up from an incoming transition from the large title. This is
    // duplicated here from the bottomLargeTitle transition widget because the
    // content text might be different. For instance, if the bottomLargeTitle
    // text is too long, the topBackLabel will say 'Back' instead of the original
    // text.
    if (bottomLargeTitle != null && bottomLargeExpanded) {
      return slideFromLeadingEdge(
        fromKey: bottomComponents.largeTitleKey,
        fromNavBarBox: bottomNavBarBox,
        toKey: topComponents.backLabelKey,
        toNavBarBox: topNavBarBox,
        curve: Interval(0.0, animation.status == AnimationStatus.forward ? 0.7 : 1.0),
        child: FadeTransition(
          opacity: midClickOpacity ?? fadeInFrom(0.4),
          child: DefaultTextStyleTransition(
            style: animation.drive(
              TextStyleTween(begin: bottomLargeTitleTextStyle, end: topBackButtonTextStyle),
            ),
            maxLines: 1,
            overflow: TextOverflow.ellipsis,
            child: topBackLabel.child,
          ),
        ),
      );
    }

    // The topBackLabel always comes from the large title first if available
    // and expanded instead of middle.
    if (bottomMiddle != null) {
      return slideFromLeadingEdge(
        fromKey: bottomComponents.middleKey,
        fromNavBarBox: bottomNavBarBox,
        toKey: topComponents.backLabelKey,
        toNavBarBox: topNavBarBox,
        child: FadeTransition(
          opacity: midClickOpacity ?? fadeInFrom(0.3),
          child: DefaultTextStyleTransition(
            style: animation.drive(
              TextStyleTween(begin: bottomTitleTextStyle, end: topBackButtonTextStyle),
            ),
            child: topBackLabel.child,
          ),
        ),
      );
    }

    return null;
  }

  Widget? get topMiddle {
    final topMiddle = topComponents.middleKey.currentWidget as KeyedSubtree?;

    if (topMiddle == null) {
      return null;
    }

    // The middle component is non-null when the nav bar is a large title
    // nav bar but would be invisible when expanded, therefore don't show it here.
    if (!topHasUserMiddle && topLargeExpanded) {
      return null;
    }

    final RelativeRect to = positionInTransitionBox(topComponents.middleKey, from: topNavBarBox);
    final toBox = topComponents.middleKey.currentContext!.findRenderObject()! as RenderBox;

    final bool isLTR = forwardDirection > 0;

    // Anchor is the top-leading point of toBox, in transition box's top-leading
    // coordinate space.
    final toAnchorInTransitionBox = Offset(isLTR ? to.left : to.right, to.top);

    // Shift in from the trailing edge of the screen.
    final anchorMovementInTransitionBox = Tween<Offset>(
      begin: Offset(
        // the "width / 2" here makes the middle widget's horizontal center on
        // the trailing edge of the top nav bar.
        topNavBarBox.size.width - toBox.size.width / 2,
        to.top,
      ),
      end: toAnchorInTransitionBox,
    );

    return _FixedSizeSlidingTransition(
      isLTR: isLTR,
      offsetAnimation: animation.drive(anchorMovementInTransitionBox),
      width: toBox.size.width,
      height: toBox.size.height,
      child: FadeTransition(
        opacity: fadeInFrom(0.25),
        child: DefaultTextStyle(style: topTitleTextStyle, child: topMiddle.child),
      ),
    );
  }

  Widget? get topTrailing {
    final topTrailing = topComponents.trailingKey.currentWidget as KeyedSubtree?;

    if (topTrailing == null) {
      return null;
    }

    return Positioned.fromRelativeRect(
      rect: positionInTransitionBox(topComponents.trailingKey, from: topNavBarBox),
      child: FadeTransition(opacity: fadeInFrom(0.4), child: topTrailing.child),
    );
  }

  Widget? get topLargeTitle {
    final topLargeTitle = topComponents.largeTitleKey.currentWidget as KeyedSubtree?;

    if (topLargeTitle == null || !topLargeExpanded) {
      return null;
    }

    final RelativeRect to = positionInTransitionBox(
      topComponents.largeTitleKey,
      from: topNavBarBox,
    );

    // Shift in from the trailing edge of the screen.
    final positionTween = RelativeRectTween(
      begin: to.shift(Offset(forwardDirection * topNavBarBox.size.width, 0.0)),
      end: to,
    );

    final Curve animationCurve = animation.status == AnimationStatus.forward
        ? _kTopNavBarHeaderTransitionCurve
        : _kTopNavBarHeaderTransitionCurve.flipped;

    return PositionedTransition(
      rect:
          // The large title animates linearly during a backswipe by a user gesture.
          userGestureInProgress
          ? routeAnimation.drive(CurveTween(curve: Curves.linear)).drive(positionTween)
          : animation.drive(CurveTween(curve: animationCurve)).drive(positionTween),
      child: FadeTransition(
        opacity: fadeInFrom(0.0, curve: animationCurve),
        child: DefaultTextStyle(
          style: topLargeTitleTextStyle!,
          maxLines: 1,
          overflow: TextOverflow.ellipsis,
          child: topLargeTitle.child,
        ),
      ),
    );
  }

  Widget? get topNavBarBottom {
    final topNavBarBottom = topComponents.navBarBottomKey.currentWidget as KeyedSubtree?;

    if (topNavBarBottom == null) {
      return null;
    }

    final RelativeRect to = positionInTransitionBox(
      topComponents.navBarBottomKey,
      from: topNavBarBox,
    );
    // Shift in from the trailing edge of the screen.
    final positionTween = RelativeRectTween(
      begin: to.shift(Offset(forwardDirection * topNavBarBox.size.width, 0.0)),
      end: to,
    );

    Widget child = topNavBarBottom.child;

    final Curve animationCurve = animation.status == AnimationStatus.forward
        ? _kTopNavBarHeaderTransitionCurve
        : _kTopNavBarHeaderTransitionCurve.flipped;

    // Fade in only if this is not a CupertinoSliverNavigationBar.search to
    // CupertinoSliverNavigationBar.search transition.
    if (!searchable) {
      child = FadeTransition(
        opacity: fadeInFrom(0.0, curve: animationCurve),
        child: child,
      );
    }

    return PositionedTransition(
      rect:
          // The bottom widget animates linearly during a backswipe by a user gesture.
          userGestureInProgress
          ? routeAnimation.drive(CurveTween(curve: Curves.linear)).drive(positionTween)
          : animation.drive(CurveTween(curve: animationCurve)).drive(positionTween),
      child: ClipRect(child: child),
    );
  }
}

/// Navigation bars' hero rect tween that will move between the static bars
/// but keep a constant size that's the bigger of both navigation bars.
RectTween _linearTranslateWithLargestRectSizeTween(Rect? begin, Rect? end) {
  final largestSize = Size(
    math.max(begin!.size.width, end!.size.width),
    math.max(begin.size.height, end.size.height),
  );
  return RectTween(begin: begin.topLeft & largestSize, end: end.topLeft & largestSize);
}

Widget _navBarHeroLaunchPadBuilder(BuildContext context, Size heroSize, Widget child) {
  assert(child is _TransitionableNavigationBar);
  // Tree reshaping is fine here because the Heroes' child is always a
  // _TransitionableNavigationBar which has a GlobalKey.

  // Keeping the Hero subtree here is needed (instead of just swapping out the
  // anchor nav bars for fixed size boxes during flights) because the nav bar
  // and their specific component children may serve as anchor points again if
  // another mid-transition flight diversion is triggered.

  // This is ok performance-wise because static nav bars are generally cheap to
  // build and layout but expensive to GPU render (due to clips and blurs) which
  // we're skipping here.
  return Visibility(
    maintainSize: true,
    maintainAnimation: true,
    maintainState: true,
    visible: false,
    child: child,
  );
}

/// Navigation bars' hero flight shuttle builder.
Widget _navBarHeroFlightShuttleBuilder(
  BuildContext flightContext,
  Animation<double> animation,
  HeroFlightDirection flightDirection,
  BuildContext fromHeroContext,
  BuildContext toHeroContext,
) {
  assert(fromHeroContext.widget is Hero);
  assert(toHeroContext.widget is Hero);

  final fromHeroWidget = fromHeroContext.widget as Hero;
  final toHeroWidget = toHeroContext.widget as Hero;

  assert(fromHeroWidget.child is _TransitionableNavigationBar);
  assert(toHeroWidget.child is _TransitionableNavigationBar);

  final fromNavBar = fromHeroWidget.child as _TransitionableNavigationBar;
  final toNavBar = toHeroWidget.child as _TransitionableNavigationBar;

  assert(
    fromNavBar.componentsKeys.navBarBoxKey.currentContext!.owner != null,
    'The from nav bar to Hero must have been mounted in the previous frame',
  );
  assert(
    toNavBar.componentsKeys.navBarBoxKey.currentContext!.owner != null,
    'The to nav bar to Hero must have been mounted in the previous frame',
  );

  switch (flightDirection) {
    case HeroFlightDirection.push:
      return _NavigationBarTransition(
        animation: animation,
        bottomNavBar: fromNavBar,
        topNavBar: toNavBar,
      );
    case HeroFlightDirection.pop:
      return _NavigationBarTransition(
        animation: animation,
        bottomNavBar: toNavBar,
        topNavBar: fromNavBar,
      );
  }
}