[new feature]introduce ScrollMetricsNotification

packages/flutter/lib/src/widgets/scroll_position.dart

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'dart:async';
+
 import 'package:flutter/foundation.dart';
 import 'package:flutter/gestures.dart';
 import 'package:flutter/physics.dart';
@@ -10,6 +12,7 @@ import 'package:flutter/scheduler.dart';
 
 import 'basic.dart';
 import 'framework.dart';
+import 'notification_listener.dart';
 import 'page_storage.dart';
 import 'scroll_activity.dart';
 import 'scroll_context.dart';
@@ -508,6 +511,17 @@ abstract class ScrollPosition extends ViewportOffset with ScrollMetrics {
   ScrollMetrics? _lastMetrics;
   Axis? _lastAxis;
 
+  bool _isMetricsChanged() {
+    assert(haveDimensions);
+    final ScrollMetrics currentMetrics = copyWith();
+
+    return _lastMetrics == null ||
+      !(currentMetrics.extentBefore == _lastMetrics!.extentBefore
+      && currentMetrics.extentInside == _lastMetrics!.extentInside
+      && currentMetrics.extentAfter == _lastMetrics!.extentAfter
+      && currentMetrics.axisDirection == _lastMetrics!.axisDirection);
+  }
+
   @override
   bool applyContentDimensions(double minScrollExtent, double maxScrollExtent) {
     assert(minScrollExtent != null);
@@ -537,7 +551,14 @@ abstract class ScrollPosition extends ViewportOffset with ScrollMetrics {
       _pendingDimensions = false;
     }
     assert(!_didChangeViewportDimensionOrReceiveCorrection, 'Use correctForNewDimensions() (and return true) to change the scroll offset during applyContentDimensions().');
-    _lastMetrics = copyWith();
+
+    if (_isMetricsChanged()) {
+      // It isn't safe to trigger the ScrollMetricsNotification if we are in
+      // the middle of rendering the frame, the developer is likely to schedule
+      // a new frame(build scheduled during frame is illegal).
+      scheduleMicrotask(didUpdateScrollMetrics);
+      _lastMetrics = copyWith();
+    }
     return true;
   }
 
@@ -898,6 +919,13 @@ abstract class ScrollPosition extends ViewportOffset with ScrollMetrics {
     UserScrollNotification(metrics: copyWith(), context: context.notificationContext!, direction: direction).dispatch(context.notificationContext);
   }
 
+  /// Dispatches a notification that the [ScrollMetrics] has changed.
+  void didUpdateScrollMetrics() {
+    assert(SchedulerBinding.instance!.schedulerPhase != SchedulerPhase.persistentCallbacks);
+    if (context.notificationContext != null)
+      ScrollMetricsNotification(metrics: copyWith(), context: context.notificationContext!).dispatch(context.notificationContext);
+  }
+
   /// Provides a heuristic to determine if expensive frame-bound tasks should be
   /// deferred.
   ///
@@ -942,3 +970,99 @@ abstract class ScrollPosition extends ViewportOffset with ScrollMetrics {
     description.add('viewport: ${_viewportDimension?.toStringAsFixed(1)}');
   }
 }
+
+/// A notification that a scrollable widget's [ScrollMetrics] have changed.
+///
+/// For example, when the content of a scrollable is altered, making it larger
+/// or smaller, this notification will be dispatched. Similarly, if the size
+/// of the window or parent changes, the scrollable can notify of these
+/// changes in dimensions.
+///
+/// The above behaviors usually do not trigger [ScrollNotification] events,
+/// so this is useful for listening to [ScrollMetrics] changes that are not
+/// caused by the user scrolling.
+///
+/// {@tool dartpad --template=freeform}
+/// This sample shows how a [ScrollMetricsNotification] is dispatched when
+/// the `windowSize` is changed. Press the floating action button to increase
+/// the scrollable window's size.
+///
+/// ```dart main
+/// import 'package:flutter/material.dart';
+///
+/// void main() => runApp(const ScrollMetricsDemo());
+///
+/// class ScrollMetricsDemo extends StatefulWidget {
+///   const ScrollMetricsDemo({Key? key}) : super(key: key);
+///
+///   @override
+///   State<ScrollMetricsDemo> createState() => ScrollMetricsDemoState();
+/// }
+///
+/// class ScrollMetricsDemoState extends State<ScrollMetricsDemo> {
+///   double windowSize = 200.0;
+///
+///   @override
+///   Widget build(BuildContext context) {
+///     return MaterialApp(
+///       home: Scaffold(
+///         appBar: AppBar(
+///           title: const Text('ScrollMetrics Demo'),
+///         ),
+///         floatingActionButton: FloatingActionButton(
+///           child: const Icon(Icons.add),
+///           onPressed: () => setState(() {
+///             windowSize += 10.0;
+///           }),
+///         ),
+///         body: NotificationListener<ScrollMetricsNotification>(
+///           onNotification: (ScrollMetricsNotification notification) {
+///             ScaffoldMessenger.of(notification.context).showSnackBar(
+///               const SnackBar(
+///                 content: Text('Scroll metrics changed!'),
+///               ),
+///             );
+///             return false;
+///           },
+///           child: Scrollbar(
+///             isAlwaysShown: true,
+///             child: SizedBox(
+///               height: windowSize,
+///               width: double.infinity,
+///               child: const SingleChildScrollView(
+///                 child: FlutterLogo(
+///                   size: 300.0,
+///                 ),
+///               ),
+///             ),
+///           ),
+///         ),
+///       ),
+///     );
+///   }
+/// }
+/// ```
+/// {@end-tool}
+class ScrollMetricsNotification extends LayoutChangedNotification with ViewportNotificationMixin {
+  /// Creates a notification that the scrollable widget's [ScrollMetrics] have
+  /// changed.
+  ScrollMetricsNotification({
+    required this.metrics,
+    required this.context,
+  });
+
+  /// Description of a scrollable widget's [ScrollMetrics].
+  final ScrollMetrics metrics;
+
+  /// The build context of the widget that fired this notification.
+  ///
+  /// This can be used to find the scrollable widget's render objects to
+  /// determine the size of the viewport, for instance.
+  final BuildContext context;
+
+  @override
+  void debugFillDescription(List<String> description) {
+    super.debugFillDescription(description);
+    description.add('$metrics');
+  }
+}

packages/flutter/test/widgets/scroll_notification_test.dart

@@ -7,6 +7,64 @@ import 'package:flutter/material.dart';
 import 'package:flutter_test/flutter_test.dart';
 
 void main() {
+  testWidgets('ScrollMetricsNotification test', (WidgetTester tester) async {
+    final List<LayoutChangedNotification> events = <LayoutChangedNotification>[];
+    Widget buildFrame(double height) {
+      return NotificationListener<LayoutChangedNotification>(
+        onNotification: (LayoutChangedNotification value) {
+          events.add(value);
+          return false;
+        },
+        child: SingleChildScrollView(
+          child: SizedBox(height: height),
+        ),
+      );
+    }
+    await tester.pumpWidget(buildFrame(1200.0));
+    // Initial metrics notification.
+    expect(events.length, 1);
+    ScrollMetricsNotification event = events[0] as ScrollMetricsNotification;
+    expect(event.metrics.extentBefore, 0.0);
+    expect(event.metrics.extentInside, 600.0);
+    expect(event.metrics.extentAfter, 600.0);
+
+    events.clear();
+    await tester.pumpWidget(buildFrame(1000.0));
+    // Change the content dimensions will trigger a new event.
+    expect(events.length, 1);
+    event = events[0] as ScrollMetricsNotification;
+    expect(event.metrics.extentBefore, 0.0);
+    expect(event.metrics.extentInside, 600.0);
+    expect(event.metrics.extentAfter, 400.0);
+
+    events.clear();
+    final TestGesture gesture = await tester.startGesture(const Offset(100.0, 100.0));
+    expect(events.length, 1);
+    // user scroll do not trigger the ScrollContentMetricsNotification.
+    expect(events[0] is ScrollStartNotification, true);
+
+    events.clear();
+    await gesture.moveBy(const Offset(-10.0, -10.0));
+    expect(events.length, 2);
+    // User scroll do not trigger the ScrollContentMetricsNotification.
+    expect(events[0] is UserScrollNotification, true);
+    expect(events[1] is ScrollUpdateNotification, true);
+
+    events.clear();
+    // Change the content dimensions again.
+    await tester.pumpWidget(buildFrame(500.0));
+    expect(events.length, 1);
+    event = events[0] as ScrollMetricsNotification;
+    expect(event.metrics.extentBefore, 10.0);
+    expect(event.metrics.extentInside, 590.0);
+    expect(event.metrics.extentAfter, 0.0);
+
+    events.clear();
+    // The content dimensions does not change.
+    await tester.pumpWidget(buildFrame(500.0));
+    expect(events.length, 0);
+  });
+
   testWidgets('Scroll notification basics', (WidgetTester tester) async {
     late ScrollNotification notification;