page_view.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.

// @dart = 2.8

import 'dart:async';
import 'dart:math' as math;

import 'package:flutter/physics.dart';
import 'package:flutter/rendering.dart';
import 'package:flutter/gestures.dart' show DragStartBehavior;
import 'package:flutter/foundation.dart' show precisionErrorTolerance;

import 'basic.dart';
import 'debug.dart';
import 'framework.dart';
import 'notification_listener.dart';
import 'page_storage.dart';
import 'scroll_context.dart';
import 'scroll_controller.dart';
import 'scroll_metrics.dart';
import 'scroll_notification.dart';
import 'scroll_physics.dart';
import 'scroll_position.dart';
import 'scroll_position_with_single_context.dart';
import 'scroll_view.dart';
import 'scrollable.dart';
import 'sliver.dart';
import 'sliver_fill.dart';
import 'viewport.dart';

/// A controller for [PageView].
///
/// A page controller lets you manipulate which page is visible in a [PageView].
/// In addition to being able to control the pixel offset of the content inside
/// the [PageView], a [PageController] also lets you control the offset in terms
/// of pages, which are increments of the viewport size.
///
/// See also:
///
///  * [PageView], which is the widget this object controls.
///
/// {@tool snippet}
///
/// This widget introduces a [MaterialApp], [Scaffold] and [PageView] with two pages
/// using the default constructor. Both pages contain an [ElevatedButton] allowing you
/// to animate the [PageView] using a [PageController].
///
/// ```dart
/// class MyPageView extends StatefulWidget {
///   MyPageView({Key key}) : super(key: key);
///
///   _MyPageViewState createState() => _MyPageViewState();
/// }
///
/// class _MyPageViewState extends State<MyPageView> {
///   PageController _pageController;
///
///   @override
///   void initState() {
///     super.initState();
///     _pageController = PageController();
///   }
///
///   @override
///   void dispose() {
///     _pageController.dispose();
///     super.dispose();
///   }
///
///   @override
///   Widget build(BuildContext context) {
///     return MaterialApp(
///       home: Scaffold(
///         body: PageView(
///           controller: _pageController,
///           children: [
///             Container(
///               color: Colors.red,
///               child: Center(
///                 child: ElevatedButton(
///                   onPressed: () {
///                     if (_pageController.hasClients) {
///                       _pageController.animateToPage(
///                         1,
///                         duration: const Duration(milliseconds: 400),
///                         curve: Curves.easeInOut,
///                       );
///                     }
///                   },
///                   child: Text('Next'),
///                 ),
///               ),
///             ),
///             Container(
///               color: Colors.blue,
///               child: Center(
///                 child: ElevatedButton(
///                   onPressed: () {
///                     if (_pageController.hasClients) {
///                       _pageController.animateToPage(
///                         0,
///                         duration: const Duration(milliseconds: 400),
///                         curve: Curves.easeInOut,
///                       );
///                     }
///                   },
///                   child: Text('Previous'),
///                 ),
///               ),
///             ),
///           ],
///         ),
///       ),
///     );
///   }
/// }
///
/// ```
/// {@end-tool}
class PageController extends ScrollController {
  /// Creates a page controller.
  ///
  /// The [initialPage], [keepPage], and [viewportFraction] arguments must not be null.
  PageController({
    this.initialPage = 0,
    this.keepPage = true,
    this.viewportFraction = 1.0,
  }) : assert(initialPage != null),
       assert(keepPage != null),
       assert(viewportFraction != null),
       assert(viewportFraction > 0.0);

  /// The page to show when first creating the [PageView].
  final int initialPage;

  /// Save the current [page] with [PageStorage] and restore it if
  /// this controller's scrollable is recreated.
  ///
  /// If this property is set to false, the current [page] is never saved
  /// and [initialPage] is always used to initialize the scroll offset.
  /// If true (the default), the initial page is used the first time the
  /// controller's scrollable is created, since there's isn't a page to
  /// restore yet. Subsequently the saved page is restored and
  /// [initialPage] is ignored.
  ///
  /// See also:
  ///
  ///  * [PageStorageKey], which should be used when more than one
  ///    scrollable appears in the same route, to distinguish the [PageStorage]
  ///    locations used to save scroll offsets.
  final bool keepPage;

  /// The fraction of the viewport that each page should occupy.
  ///
  /// Defaults to 1.0, which means each page fills the viewport in the scrolling
  /// direction.
  final double viewportFraction;

  /// The current page displayed in the controlled [PageView].
  ///
  /// There are circumstances that this [PageController] can't know the current
  /// page. Reading [page] will throw an [AssertionError] in the following cases:
  ///
  /// 1. No [PageView] is currently using this [PageController]. Once a
  /// [PageView] starts using this [PageController], the new [page]
  /// position will be derived:
  ///
  ///   * First, based on the attached [PageView]'s [BuildContext] and the
  ///     position saved at that context's [PageStorage] if [keepPage] is true.
  ///   * Second, from the [PageController]'s [initialPage].
  ///
  /// 2. More than one [PageView] using the same [PageController].
  ///
  /// The [hasClients] property can be used to check if a [PageView] is attached
  /// prior to accessing [page].
  double get page {
    assert(
      positions.isNotEmpty,
      'PageController.page cannot be accessed before a PageView is built with it.',
    );
    assert(
      positions.length == 1,
      'The page property cannot be read when multiple PageViews are attached to '
      'the same PageController.',
    );
    final _PagePosition position = this.position as _PagePosition;
    return position.page;
  }

  /// Animates the controlled [PageView] from the current page to the given page.
  ///
  /// The animation lasts for the given duration and follows the given curve.
  /// The returned [Future] resolves when the animation completes.
  ///
  /// The `duration` and `curve` arguments must not be null.
  Future<void> animateToPage(
    int page, {
    @required Duration duration,
    @required Curve curve,
  }) {
    final _PagePosition position = this.position as _PagePosition;
    return position.animateTo(
      position.getPixelsFromPage(page.toDouble()),
      duration: duration,
      curve: curve,
    );
  }

  /// Changes which page is displayed in the controlled [PageView].
  ///
  /// Jumps the page position from its current value to the given value,
  /// without animation, and without checking if the new value is in range.
  void jumpToPage(int page) {
    final _PagePosition position = this.position as _PagePosition;
    position.jumpTo(position.getPixelsFromPage(page.toDouble()));
  }

  /// Animates the controlled [PageView] to the next page.
  ///
  /// The animation lasts for the given duration and follows the given curve.
  /// The returned [Future] resolves when the animation completes.
  ///
  /// The `duration` and `curve` arguments must not be null.
  Future<void> nextPage({ @required Duration duration, @required Curve curve }) {
    return animateToPage(page.round() + 1, duration: duration, curve: curve);
  }

  /// Animates the controlled [PageView] to the previous page.
  ///
  /// The animation lasts for the given duration and follows the given curve.
  /// The returned [Future] resolves when the animation completes.
  ///
  /// The `duration` and `curve` arguments must not be null.
  Future<void> previousPage({ @required Duration duration, @required Curve curve }) {
    return animateToPage(page.round() - 1, duration: duration, curve: curve);
  }

  @override
  ScrollPosition createScrollPosition(ScrollPhysics physics, ScrollContext context, ScrollPosition oldPosition) {
    return _PagePosition(
      physics: physics,
      context: context,
      initialPage: initialPage,
      keepPage: keepPage,
      viewportFraction: viewportFraction,
      oldPosition: oldPosition,
    );
  }

  @override
  void attach(ScrollPosition position) {
    super.attach(position);
    final _PagePosition pagePosition = position as _PagePosition;
    pagePosition.viewportFraction = viewportFraction;
  }
}

/// Metrics for a [PageView].
///
/// The metrics are available on [ScrollNotification]s generated from
/// [PageView]s.
class PageMetrics extends FixedScrollMetrics {
  /// Creates an immutable snapshot of values associated with a [PageView].
  PageMetrics({
    @required double minScrollExtent,
    @required double maxScrollExtent,
    @required double pixels,
    @required double viewportDimension,
    @required AxisDirection axisDirection,
    @required this.viewportFraction,
  }) : super(
         minScrollExtent: minScrollExtent,
         maxScrollExtent: maxScrollExtent,
         pixels: pixels,
         viewportDimension: viewportDimension,
         axisDirection: axisDirection,
       );

  @override
  PageMetrics copyWith({
    double minScrollExtent,
    double maxScrollExtent,
    double pixels,
    double viewportDimension,
    AxisDirection axisDirection,
    double viewportFraction,
  }) {
    return PageMetrics(
      minScrollExtent: minScrollExtent ?? this.minScrollExtent,
      maxScrollExtent: maxScrollExtent ?? this.maxScrollExtent,
      pixels: pixels ?? this.pixels,
      viewportDimension: viewportDimension ?? this.viewportDimension,
      axisDirection: axisDirection ?? this.axisDirection,
      viewportFraction: viewportFraction ?? this.viewportFraction,
    );
  }

  /// The current page displayed in the [PageView].
  double get page {
    return math.max(0.0, pixels.clamp(minScrollExtent, maxScrollExtent)) /
           math.max(1.0, viewportDimension * viewportFraction);
  }

  /// The fraction of the viewport that each page occupies.
  ///
  /// Used to compute [page] from the current [pixels].
  final double viewportFraction;
}

class _PagePosition extends ScrollPositionWithSingleContext implements PageMetrics {
  _PagePosition({
    ScrollPhysics physics,
    ScrollContext context,
    this.initialPage = 0,
    bool keepPage = true,
    double viewportFraction = 1.0,
    ScrollPosition oldPosition,
  }) : assert(initialPage != null),
       assert(keepPage != null),
       assert(viewportFraction != null),
       assert(viewportFraction > 0.0),
       _viewportFraction = viewportFraction,
       _pageToUseOnStartup = initialPage.toDouble(),
       super(
         physics: physics,
         context: context,
         initialPixels: null,
         keepScrollOffset: keepPage,
         oldPosition: oldPosition,
       );

  final int initialPage;
  double _pageToUseOnStartup;

  @override
  double get viewportFraction => _viewportFraction;
  double _viewportFraction;
  set viewportFraction(double value) {
    if (_viewportFraction == value)
      return;
    final double oldPage = page;
    _viewportFraction = value;
    if (oldPage != null)
      forcePixels(getPixelsFromPage(oldPage));
  }

  // The amount of offset that will be added to [minScrollExtent] and subtracted
  // from [maxScrollExtent], such that every page will properly snap to the center
  // of the viewport when viewportFraction is greater than 1.
  //
  // The value is 0 if viewportFraction is less than or equal to 1, larger than 0
  // otherwise.
  double get _initialPageOffset => math.max(0, viewportDimension * (viewportFraction - 1) / 2);

  double getPageFromPixels(double pixels, double viewportDimension) {
    final double actual = math.max(0.0, pixels - _initialPageOffset) / math.max(1.0, viewportDimension * viewportFraction);
    final double round = actual.roundToDouble();
    if ((actual - round).abs() < precisionErrorTolerance) {
      return round;
    }
    return actual;
  }

  double getPixelsFromPage(double page) {
    return page * viewportDimension * viewportFraction + _initialPageOffset;
  }

  @override
  double get page {
    assert(
      pixels == null || (minScrollExtent != null && maxScrollExtent != null),
      'Page value is only available after content dimensions are established.',
    );
    return pixels == null ? null : getPageFromPixels(pixels.clamp(minScrollExtent, maxScrollExtent) as double, viewportDimension);
  }

  @override
  void saveScrollOffset() {
    PageStorage.of(context.storageContext)?.writeState(context.storageContext, getPageFromPixels(pixels, viewportDimension));
  }

  @override
  void restoreScrollOffset() {
    if (pixels == null) {
      final double value = PageStorage.of(context.storageContext)?.readState(context.storageContext) as double;
      if (value != null)
        _pageToUseOnStartup = value;
    }
  }

  @override
  void saveOffset() {
    context.saveOffset(getPageFromPixels(pixels, viewportDimension));
  }

  @override
  void restoreOffset(double offset, {bool initialRestore = false}) {
    assert(initialRestore != null);
    assert(offset != null);
    if (initialRestore) {
      _pageToUseOnStartup = offset;
    } else {
      jumpTo(getPixelsFromPage(offset));
    }
  }

  @override
  bool applyViewportDimension(double viewportDimension) {
    final double oldViewportDimensions = this.viewportDimension;
    if (viewportDimension == oldViewportDimensions) {
      return true;
    }
    final bool result = super.applyViewportDimension(viewportDimension);
    final double oldPixels = pixels;
    final double page = (oldPixels == null || oldViewportDimensions == 0.0) ? _pageToUseOnStartup : getPageFromPixels(oldPixels, oldViewportDimensions);
    final double newPixels = getPixelsFromPage(page);

    if (newPixels != oldPixels) {
      correctPixels(newPixels);
      return false;
    }
    return result;
  }

  @override
  bool applyContentDimensions(double minScrollExtent, double maxScrollExtent) {
    final double newMinScrollExtent = minScrollExtent + _initialPageOffset;
    return super.applyContentDimensions(
      newMinScrollExtent,
      math.max(newMinScrollExtent, maxScrollExtent - _initialPageOffset),
    );
  }

  @override
  PageMetrics copyWith({
    double minScrollExtent,
    double maxScrollExtent,
    double pixels,
    double viewportDimension,
    AxisDirection axisDirection,
    double viewportFraction,
  }) {
    return PageMetrics(
      minScrollExtent: minScrollExtent ?? this.minScrollExtent,
      maxScrollExtent: maxScrollExtent ?? this.maxScrollExtent,
      pixels: pixels ?? this.pixels,
      viewportDimension: viewportDimension ?? this.viewportDimension,
      axisDirection: axisDirection ?? this.axisDirection,
      viewportFraction: viewportFraction ?? this.viewportFraction,
    );
  }
}

class _ForceImplicitScrollPhysics extends ScrollPhysics {
  const _ForceImplicitScrollPhysics({
    @required this.allowImplicitScrolling,
    ScrollPhysics parent,
  }) : assert(allowImplicitScrolling != null),
       super(parent: parent);

  @override
  _ForceImplicitScrollPhysics applyTo(ScrollPhysics ancestor) {
    return _ForceImplicitScrollPhysics(
      allowImplicitScrolling: allowImplicitScrolling,
      parent: buildParent(ancestor),
    );
  }

  @override
  final bool allowImplicitScrolling;
}

/// Scroll physics used by a [PageView].
///
/// These physics cause the page view to snap to page boundaries.
///
/// See also:
///
///  * [ScrollPhysics], the base class which defines the API for scrolling
///    physics.
///  * [PageView.physics], which can override the physics used by a page view.
class PageScrollPhysics extends ScrollPhysics {
  /// Creates physics for a [PageView].
  const PageScrollPhysics({ ScrollPhysics parent }) : super(parent: parent);

  @override
  PageScrollPhysics applyTo(ScrollPhysics ancestor) {
    return PageScrollPhysics(parent: buildParent(ancestor));
  }

  double _getPage(ScrollMetrics position) {
    if (position is _PagePosition)
      return position.page;
    return position.pixels / position.viewportDimension;
  }

  double _getPixels(ScrollMetrics position, double page) {
    if (position is _PagePosition)
      return position.getPixelsFromPage(page);
    return page * position.viewportDimension;
  }

  double _getTargetPixels(ScrollMetrics position, Tolerance tolerance, double velocity) {
    double page = _getPage(position);
    if (velocity < -tolerance.velocity)
      page -= 0.5;
    else if (velocity > tolerance.velocity)
      page += 0.5;
    return _getPixels(position, page.roundToDouble());
  }

  @override
  Simulation createBallisticSimulation(ScrollMetrics position, double velocity) {
    // If we're out of range and not headed back in range, defer to the parent
    // ballistics, which should put us back in range at a page boundary.
    if ((velocity <= 0.0 && position.pixels <= position.minScrollExtent) ||
        (velocity >= 0.0 && position.pixels >= position.maxScrollExtent))
      return super.createBallisticSimulation(position, velocity);
    final Tolerance tolerance = this.tolerance;
    final double target = _getTargetPixels(position, tolerance, velocity);
    if (target != position.pixels)
      return ScrollSpringSimulation(spring, position.pixels, target, velocity, tolerance: tolerance);
    return null;
  }

  @override
  bool get allowImplicitScrolling => false;
}

// Having this global (mutable) page controller is a bit of a hack. We need it
// to plumb in the factory for _PagePosition, but it will end up accumulating
// a large list of scroll positions. As long as you don't try to actually
// control the scroll positions, everything should be fine.
final PageController _defaultPageController = PageController();
const PageScrollPhysics _kPagePhysics = PageScrollPhysics();

/// A scrollable list that works page by page.
///
/// Each child of a page view is forced to be the same size as the viewport.
///
/// You can use a [PageController] to control which page is visible in the view.
/// In addition to being able to control the pixel offset of the content inside
/// the [PageView], a [PageController] also lets you control the offset in terms
/// of pages, which are increments of the viewport size.
///
/// The [PageController] can also be used to control the
/// [PageController.initialPage], which determines which page is shown when the
/// [PageView] is first constructed, and the [PageController.viewportFraction],
/// which determines the size of the pages as a fraction of the viewport size.
///
/// {@youtube 560 315 https://www.youtube.com/watch?v=J1gE9xvph-A}
///
/// See also:
///
///  * [PageController], which controls which page is visible in the view.
///  * [SingleChildScrollView], when you need to make a single child scrollable.
///  * [ListView], for a scrollable list of boxes.
///  * [GridView], for a scrollable grid of boxes.
///  * [ScrollNotification] and [NotificationListener], which can be used to watch
///    the scroll position without using a [ScrollController].
class PageView extends StatefulWidget {
  /// Creates a scrollable list that works page by page from an explicit [List]
  /// of widgets.
  ///
  /// This constructor is appropriate for page views with a small number of
  /// children because constructing the [List] requires doing work for every
  /// child that could possibly be displayed in the page view, instead of just
  /// those children that are actually visible.
  ///
  /// {@template flutter.widgets.pageView.allowImplicitScrolling}
  /// The [allowImplicitScrolling] parameter must not be null. If true, the
  /// [PageView] will participate in accessibility scrolling more like a
  /// [ListView], where implicit scroll actions will move to the next page
  /// rather than into the contents of the [PageView].
  /// {@endtemplate}
  PageView({
    Key key,
    this.scrollDirection = Axis.horizontal,
    this.reverse = false,
    PageController controller,
    this.physics,
    this.pageSnapping = true,
    this.onPageChanged,
    List<Widget> children = const <Widget>[],
    this.dragStartBehavior = DragStartBehavior.start,
    this.allowImplicitScrolling = false,
    this.restorationId,
    this.clipBehavior = Clip.hardEdge,
  }) : assert(allowImplicitScrolling != null),
       assert(clipBehavior != null),
       controller = controller ?? _defaultPageController,
       childrenDelegate = SliverChildListDelegate(children),
       super(key: key);

  /// Creates a scrollable list that works page by page using widgets that are
  /// created on demand.
  ///
  /// This constructor is appropriate for page views with a large (or infinite)
  /// number of children because the builder is called only for those children
  /// that are actually visible.
  ///
  /// Providing a non-null [itemCount] lets the [PageView] compute the maximum
  /// scroll extent.
  ///
  /// [itemBuilder] will be called only with indices greater than or equal to
  /// zero and less than [itemCount].
  ///
  /// [PageView.builder] by default does not support child reordering. If
  /// you are planning to change child order at a later time, consider using
  /// [PageView] or [PageView.custom].
  ///
  /// {@macro flutter.widgets.pageView.allowImplicitScrolling}
  PageView.builder({
    Key key,
    this.scrollDirection = Axis.horizontal,
    this.reverse = false,
    PageController controller,
    this.physics,
    this.pageSnapping = true,
    this.onPageChanged,
    @required IndexedWidgetBuilder itemBuilder,
    int itemCount,
    this.dragStartBehavior = DragStartBehavior.start,
    this.allowImplicitScrolling = false,
    this.restorationId,
    this.clipBehavior = Clip.hardEdge,
  }) : assert(allowImplicitScrolling != null),
       assert(clipBehavior != null),
       controller = controller ?? _defaultPageController,
       childrenDelegate = SliverChildBuilderDelegate(itemBuilder, childCount: itemCount),
       super(key: key);

  /// Creates a scrollable list that works page by page with a custom child
  /// model.
  ///
  /// {@tool snippet}
  ///
  /// This [PageView] uses a custom [SliverChildBuilderDelegate] to support child
  /// reordering.
  ///
  /// ```dart
  /// class MyPageView extends StatefulWidget {
  ///   @override
  ///   _MyPageViewState createState() => _MyPageViewState();
  /// }
  ///
  /// class _MyPageViewState extends State<MyPageView> {
  ///   List<String> items = <String>['1', '2', '3', '4', '5'];
  ///
  ///   void _reverse() {
  ///     setState(() {
  ///       items = items.reversed.toList();
  ///     });
  ///   }
  ///
  ///   @override
  ///   Widget build(BuildContext context) {
  ///     return Scaffold(
  ///       body: SafeArea(
  ///         child: PageView.custom(
  ///           childrenDelegate: SliverChildBuilderDelegate(
  ///             (BuildContext context, int index) {
  ///               return KeepAlive(
  ///                 data: items[index],
  ///                 key: ValueKey<String>(items[index]),
  ///               );
  ///             },
  ///             childCount: items.length,
  ///             findChildIndexCallback: (Key key) {
  ///               final ValueKey valueKey = key;
  ///               final String data = valueKey.value;
  ///               return items.indexOf(data);
  ///             }
  ///           ),
  ///         ),
  ///       ),
  ///       bottomNavigationBar: BottomAppBar(
  ///         child: Row(
  ///           mainAxisAlignment: MainAxisAlignment.center,
  ///           children: <Widget>[
  ///             TextButton(
  ///               onPressed: () => _reverse(),
  ///               child: Text('Reverse items'),
  ///             ),
  ///           ],
  ///         ),
  ///       ),
  ///     );
  ///   }
  /// }
  ///
  /// class KeepAlive extends StatefulWidget {
  ///   const KeepAlive({Key key, this.data}) : super(key: key);
  ///
  ///   final String data;
  ///
  ///   @override
  ///   _KeepAliveState createState() => _KeepAliveState();
  /// }
  ///
  /// class _KeepAliveState extends State<KeepAlive> with AutomaticKeepAliveClientMixin{
  ///   @override
  ///   bool get wantKeepAlive => true;
  ///
  ///   @override
  ///   Widget build(BuildContext context) {
  ///     super.build(context);
  ///     return Text(widget.data);
  ///   }
  /// }
  /// ```
  /// {@end-tool}
  ///
  /// {@macro flutter.widgets.pageView.allowImplicitScrolling}
  PageView.custom({
    Key key,
    this.scrollDirection = Axis.horizontal,
    this.reverse = false,
    PageController controller,
    this.physics,
    this.pageSnapping = true,
    this.onPageChanged,
    @required this.childrenDelegate,
    this.dragStartBehavior = DragStartBehavior.start,
    this.allowImplicitScrolling = false,
    this.restorationId,
    this.clipBehavior = Clip.hardEdge,
  }) : assert(childrenDelegate != null),
       assert(allowImplicitScrolling != null),
       assert(clipBehavior != null),
       controller = controller ?? _defaultPageController,
       super(key: key);

  /// Controls whether the widget's pages will respond to
  /// [RenderObject.showOnScreen], which will allow for implicit accessibility
  /// scrolling.
  ///
  /// With this flag set to false, when accessibility focus reaches the end of
  /// the current page and the user attempts to move it to the next element, the
  /// focus will traverse to the next widget outside of the page view.
  ///
  /// With this flag set to true, when accessibility focus reaches the end of
  /// the current page and user attempts to move it to the next element, focus
  /// will traverse to the next page in the page view.
  final bool allowImplicitScrolling;

  /// {@macro flutter.widgets.scrollable.restorationId}
  final String restorationId;

  /// The axis along which the page view scrolls.
  ///
  /// Defaults to [Axis.horizontal].
  final Axis scrollDirection;

  /// Whether the page view scrolls in the reading direction.
  ///
  /// For example, if the reading direction is left-to-right and
  /// [scrollDirection] is [Axis.horizontal], then the page view scrolls from
  /// left to right when [reverse] is false and from right to left when
  /// [reverse] is true.
  ///
  /// Similarly, if [scrollDirection] is [Axis.vertical], then the page view
  /// scrolls from top to bottom when [reverse] is false and from bottom to top
  /// when [reverse] is true.
  ///
  /// Defaults to false.
  final bool reverse;

  /// An object that can be used to control the position to which this page
  /// view is scrolled.
  final PageController controller;

  /// How the page view should respond to user input.
  ///
  /// For example, determines how the page view continues to animate after the
  /// user stops dragging the page view.
  ///
  /// The physics are modified to snap to page boundaries using
  /// [PageScrollPhysics] prior to being used.
  ///
  /// Defaults to matching platform conventions.
  final ScrollPhysics physics;

  /// Set to false to disable page snapping, useful for custom scroll behavior.
  final bool pageSnapping;

  /// Called whenever the page in the center of the viewport changes.
  final ValueChanged<int> onPageChanged;

  /// A delegate that provides the children for the [PageView].
  ///
  /// The [PageView.custom] constructor lets you specify this delegate
  /// explicitly. The [PageView] and [PageView.builder] constructors create a
  /// [childrenDelegate] that wraps the given [List] and [IndexedWidgetBuilder],
  /// respectively.
  final SliverChildDelegate childrenDelegate;

  /// {@macro flutter.widgets.scrollable.dragStartBehavior}
  final DragStartBehavior dragStartBehavior;

  /// {@macro flutter.widgets.Clip}
  ///
  /// Defaults to [Clip.hardEdge].
  final Clip clipBehavior;

  @override
  _PageViewState createState() => _PageViewState();
}

class _PageViewState extends State<PageView> {
  int _lastReportedPage = 0;

  @override
  void initState() {
    super.initState();
    _lastReportedPage = widget.controller.initialPage;
  }

  AxisDirection _getDirection(BuildContext context) {
    switch (widget.scrollDirection) {
      case Axis.horizontal:
        assert(debugCheckHasDirectionality(context));
        final TextDirection textDirection = Directionality.of(context);
        final AxisDirection axisDirection = textDirectionToAxisDirection(textDirection);
        return widget.reverse ? flipAxisDirection(axisDirection) : axisDirection;
      case Axis.vertical:
        return widget.reverse ? AxisDirection.up : AxisDirection.down;
    }
    return null;
  }

  @override
  Widget build(BuildContext context) {
    final AxisDirection axisDirection = _getDirection(context);
    final ScrollPhysics physics = _ForceImplicitScrollPhysics(
      allowImplicitScrolling: widget.allowImplicitScrolling,
    ).applyTo(widget.pageSnapping
        ? _kPagePhysics.applyTo(widget.physics)
        : widget.physics);

    return NotificationListener<ScrollNotification>(
      onNotification: (ScrollNotification notification) {
        if (notification.depth == 0 && widget.onPageChanged != null && notification is ScrollUpdateNotification) {
          final PageMetrics metrics = notification.metrics as PageMetrics;
          final int currentPage = metrics.page.round();
          if (currentPage != _lastReportedPage) {
            _lastReportedPage = currentPage;
            widget.onPageChanged(currentPage);
          }
        }
        return false;
      },
      child: Scrollable(
        dragStartBehavior: widget.dragStartBehavior,
        axisDirection: axisDirection,
        controller: widget.controller,
        physics: physics,
        restorationId: widget.restorationId,
        viewportBuilder: (BuildContext context, ViewportOffset position) {
          return Viewport(
            // TODO(dnfield): we should provide a way to set cacheExtent
            // independent of implicit scrolling:
            // https://github.com/flutter/flutter/issues/45632
            cacheExtent: widget.allowImplicitScrolling ? 1.0 : 0.0,
            cacheExtentStyle: CacheExtentStyle.viewport,
            axisDirection: axisDirection,
            offset: position,
            clipBehavior: widget.clipBehavior,
            slivers: <Widget>[
              SliverFillViewport(
                viewportFraction: widget.controller.viewportFraction,
                delegate: widget.childrenDelegate,
              ),
            ],
          );
        },
      ),
    );
  }

  @override
  void debugFillProperties(DiagnosticPropertiesBuilder description) {
    super.debugFillProperties(description);
    description.add(EnumProperty<Axis>('scrollDirection', widget.scrollDirection));
    description.add(FlagProperty('reverse', value: widget.reverse, ifTrue: 'reversed'));
    description.add(DiagnosticsProperty<PageController>('controller', widget.controller, showName: false));
    description.add(DiagnosticsProperty<ScrollPhysics>('physics', widget.physics, showName: false));
    description.add(FlagProperty('pageSnapping', value: widget.pageSnapping, ifFalse: 'snapping disabled'));
    description.add(FlagProperty('allowImplicitScrolling', value: widget.allowImplicitScrolling, ifTrue: 'allow implicit scrolling'));
  }
}