Restored change (#48529)

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

@@ -1293,6 +1293,8 @@ class _AnimatedPositionedDirectionalState extends AnimatedWidgetBaseState<Animat
 ///  * [AnimatedSwitcher], for fading between many children in sequence.
 ///  * [FadeTransition], an explicitly animated version of this widget, where
 ///    an [Animation] is provided by the caller instead of being built in.
+///  * [SliverAnimatedOpacity], for automatically transitioning a sliver's
+///    opacity over a given duration whenever the given opacity changes.
 class AnimatedOpacity extends ImplicitlyAnimatedWidget {
   /// Creates a widget that animates its opacity implicitly.
   ///
@@ -1366,6 +1368,141 @@ class _AnimatedOpacityState extends ImplicitlyAnimatedWidgetState<AnimatedOpacit
   }
 }
 
+/// Animated version of [SliverOpacity] which automatically transitions the
+/// sliver child's opacity over a given duration whenever the given opacity
+/// changes.
+///
+/// Animating an opacity is relatively expensive because it requires painting
+/// the sliver child into an intermediate buffer.
+///
+/// Here's an illustration of what using this widget looks like, using a [curve]
+/// of [Curves.fastOutSlowIn].
+///
+/// {@animation 250 266 https://flutter.github.io/assets-for-api-docs/assets/widgets/animated_opacity.mp4}
+///
+/// {@tool sample --template=stateful_widget_scaffold_center_freeform_state}
+/// Creates a [CustomScrollView] with a [SliverFixedExtentList] and a
+/// [FloatingActionButton]. Pressing the button animates the lists' opacity.
+///
+/// ```dart
+/// class _MyStatefulWidgetState extends State<MyStatefulWidget> with SingleTickerProviderStateMixin {
+///   bool _visible = true;
+///
+///   Widget build(BuildContext context) {
+///     return CustomScrollView(
+///       slivers: <Widget>[
+///         SliverAnimatedOpacity(
+///           opacity: _visible ? 1.0 : 0.0,
+///           duration: Duration(milliseconds: 500),
+///           sliver: SliverFixedExtentList(
+///             itemExtent: 100.0,
+///             delegate: SliverChildBuilderDelegate(
+///               (BuildContext context, int index) {
+///                 return Container(
+///                   color: index % 2 == 0
+///                     ? Colors.indigo[200]
+///                     : Colors.orange[200],
+///                 );
+///               },
+///               childCount: 5,
+///             ),
+///           ),
+///         ),
+///         SliverToBoxAdapter(
+///           child: FloatingActionButton(
+///             onPressed: () {
+///               setState(() {
+///                 _visible = !_visible;
+///               });
+///             },
+///             tooltip: 'Toggle opacity',
+///             child: Icon(Icons.flip),
+///           )
+///         ),
+///       ]
+///     );
+///   }
+/// }
+/// ```
+/// {@end-tool}
+///
+/// See also:
+///
+///  * [SliverFadeTransition], an explicitly animated version of this widget, where
+///    an [Animation] is provided by the caller instead of being built in.
+///  * [AnimatedOpacity], for automatically transitioning a box child's
+///    opacity over a given duration whenever the given opacity changes.
+class SliverAnimatedOpacity extends ImplicitlyAnimatedWidget {
+  /// Creates a widget that animates its opacity implicitly.
+  ///
+  /// The [opacity] argument must not be null and must be between 0.0 and 1.0,
+  /// inclusive. The [curve] and [duration] arguments must not be null.
+  const SliverAnimatedOpacity({
+    Key key,
+    this.sliver,
+    @required this.opacity,
+    Curve curve = Curves.linear,
+    @required Duration duration,
+    VoidCallback onEnd,
+    this.alwaysIncludeSemantics = false,
+  }) : assert(opacity != null && opacity >= 0.0 && opacity <= 1.0),
+      super(key: key, curve: curve, duration: duration, onEnd: onEnd);
+
+  /// The sliver below this widget in the tree.
+  final Widget sliver;
+
+  /// The target opacity.
+  ///
+  /// An opacity of 1.0 is fully opaque. An opacity of 0.0 is fully transparent
+  /// (i.e., invisible).
+  ///
+  /// The opacity must not be null.
+  final double opacity;
+
+  /// Whether the semantic information of the children is always included.
+  ///
+  /// Defaults to false.
+  ///
+  /// When true, regardless of the opacity settings the sliver child's semantic
+  /// information is exposed as if the widget were fully visible. This is
+  /// useful in cases where labels may be hidden during animations that
+  /// would otherwise contribute relevant semantics.
+  final bool alwaysIncludeSemantics;
+
+  @override
+  _SliverAnimatedOpacityState createState() => _SliverAnimatedOpacityState();
+
+  @override
+  void debugFillProperties(DiagnosticPropertiesBuilder properties) {
+    super.debugFillProperties(properties);
+    properties.add(DoubleProperty('opacity', opacity));
+  }
+}
+
+class _SliverAnimatedOpacityState extends ImplicitlyAnimatedWidgetState<SliverAnimatedOpacity> {
+  Tween<double> _opacity;
+  Animation<double> _opacityAnimation;
+
+  @override
+  void forEachTween(TweenVisitor<dynamic> visitor) {
+    _opacity = visitor(_opacity, widget.opacity, (dynamic value) => Tween<double>(begin: value as double)) as Tween<double>;
+  }
+
+  @override
+  void didUpdateTweens() {
+    _opacityAnimation = animation.drive(_opacity);
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return SliverFadeTransition(
+      opacity: _opacityAnimation,
+      sliver: widget.sliver,
+      alwaysIncludeSemantics: widget.alwaysIncludeSemantics,
+    );
+  }
+}
+
 /// Animated version of [DefaultTextStyle] which automatically transitions the
 /// default text style (the text style to apply to descendant [Text] widgets
 /// without explicit style) over a given duration whenever the given style

packages/flutter/test/widgets/implicit_animations_test.dart

@@ -69,7 +69,11 @@ void main() {
 
   testWidgets('AnimatedContainer onEnd callback test', (WidgetTester tester) async {
     await tester.pumpWidget(wrap(
-        child: TestAnimatedWidget(callback: mockOnEndFunction, switchKey: switchKey, state: _TestAnimatedContainerWidgetState(),)
+      child: TestAnimatedWidget(
+        callback: mockOnEndFunction,
+        switchKey: switchKey,
+        state: _TestAnimatedContainerWidgetState(),
+      )
     ));
 
     final Finder widgetFinder = find.byKey(switchKey);
@@ -86,7 +90,11 @@ void main() {
 
   testWidgets('AnimatedPadding onEnd callback test', (WidgetTester tester) async {
     await tester.pumpWidget(wrap(
-        child: TestAnimatedWidget(callback: mockOnEndFunction, switchKey: switchKey, state: _TestAnimatedPaddingWidgetState(),)
+      child: TestAnimatedWidget(
+        callback: mockOnEndFunction,
+        switchKey: switchKey,
+        state: _TestAnimatedPaddingWidgetState(),
+      )
     ));
 
     final Finder widgetFinder = find.byKey(switchKey);
@@ -103,7 +111,11 @@ void main() {
 
   testWidgets('AnimatedAlign onEnd callback test', (WidgetTester tester) async {
     await tester.pumpWidget(wrap(
-        child: TestAnimatedWidget(callback: mockOnEndFunction, switchKey: switchKey, state: _TestAnimatedAlignWidgetState(),)
+      child: TestAnimatedWidget(
+        callback: mockOnEndFunction,
+        switchKey: switchKey,
+        state: _TestAnimatedAlignWidgetState(),
+      )
     ));
 
     final Finder widgetFinder = find.byKey(switchKey);
@@ -120,7 +132,11 @@ void main() {
 
   testWidgets('AnimatedPositioned onEnd callback test', (WidgetTester tester) async {
     await tester.pumpWidget(wrap(
-        child: TestAnimatedWidget(callback: mockOnEndFunction, switchKey: switchKey, state: _TestAnimatedPositionedWidgetState(),)
+      child: TestAnimatedWidget(
+        callback: mockOnEndFunction,
+        switchKey: switchKey,
+        state: _TestAnimatedPositionedWidgetState(),
+      )
     ));
 
     final Finder widgetFinder = find.byKey(switchKey);
@@ -137,7 +153,11 @@ void main() {
 
   testWidgets('AnimatedPositionedDirectional onEnd callback test', (WidgetTester tester) async {
     await tester.pumpWidget(wrap(
-        child: TestAnimatedWidget(callback: mockOnEndFunction, switchKey: switchKey, state: _TestAnimatedPositionedDirectionalWidgetState(),)
+      child: TestAnimatedWidget(
+        callback: mockOnEndFunction,
+        switchKey: switchKey,
+        state: _TestAnimatedPositionedDirectionalWidgetState(),
+      )
     ));
 
     final Finder widgetFinder = find.byKey(switchKey);
@@ -154,7 +174,58 @@ void main() {
 
   testWidgets('AnimatedOpacity onEnd callback test', (WidgetTester tester) async {
     await tester.pumpWidget(wrap(
-        child: TestAnimatedWidget(callback: mockOnEndFunction, switchKey: switchKey, state: _TestAnimatedOpacityWidgetState(),)
+      child: TestAnimatedWidget(
+        callback: mockOnEndFunction,
+        switchKey: switchKey,
+        state: _TestAnimatedOpacityWidgetState(),
+      )
+    ));
+
+    final Finder widgetFinder = find.byKey(switchKey);
+
+    await tester.tap(widgetFinder);
+    await tester.pump();
+    expect(mockOnEndFunction.called, 0);
+    await tester.pump(animationDuration);
+    expect(mockOnEndFunction.called, 0);
+    await tester.pump(additionalDelay);
+    expect(mockOnEndFunction.called, 1);
+  });
+
+  testWidgets('AnimatedOpacity transition test', (WidgetTester tester) async {
+    await tester.pumpWidget(wrap(
+      child: TestAnimatedWidget(
+        switchKey: switchKey,
+        state: _TestAnimatedOpacityWidgetState(),
+      )
+    ));
+
+    final Finder switchFinder = find.byKey(switchKey);
+    final FadeTransition opacityWidget = tester.widget<FadeTransition>(
+      find.ancestor(
+        of: find.byType(Placeholder),
+        matching: find.byType(FadeTransition),
+      ).first,
+    );
+
+    await tester.tap(switchFinder);
+    await tester.pump();
+    expect(opacityWidget.opacity.value, equals(0.0));
+
+    await tester.pump(const Duration(milliseconds: 500));
+    expect(opacityWidget.opacity.value, equals(0.5));
+    await tester.pump(const Duration(milliseconds: 250));
+    expect(opacityWidget.opacity.value, equals(0.75));
+    await tester.pump(const Duration(milliseconds: 250));
+    expect(opacityWidget.opacity.value, equals(1.0));
+  });
+
+
+  testWidgets('SliverAnimatedOpacity onEnd callback test', (WidgetTester tester) async {
+    await tester.pumpWidget(TestAnimatedWidget(
+      callback: mockOnEndFunction,
+      switchKey: switchKey,
+      state: _TestSliverAnimatedOpacityWidgetState(),
     ));
 
     final Finder widgetFinder = find.byKey(switchKey);
@@ -169,9 +240,41 @@ void main() {
     expect(mockOnEndFunction.called, 1);
   });
 
+  testWidgets('SliverAnimatedOpacity transition test', (WidgetTester tester) async {
+    await tester.pumpWidget(wrap(
+      child: TestAnimatedWidget(
+        switchKey: switchKey,
+        state: _TestSliverAnimatedOpacityWidgetState(),
+      )
+    ));
+
+    final Finder switchFinder = find.byKey(switchKey);
+    final SliverFadeTransition opacityWidget = tester.widget<SliverFadeTransition>(
+      find.ancestor(
+        of: find.byType(Placeholder),
+        matching: find.byType(SliverFadeTransition),
+      ).first,
+    );
+
+    await tester.tap(switchFinder);
+    await tester.pump();
+    expect(opacityWidget.opacity.value, equals(0.0));
+
+    await tester.pump(const Duration(milliseconds: 500));
+    expect(opacityWidget.opacity.value, equals(0.5));
+    await tester.pump(const Duration(milliseconds: 250));
+    expect(opacityWidget.opacity.value, equals(0.75));
+    await tester.pump(const Duration(milliseconds: 250));
+    expect(opacityWidget.opacity.value, equals(1.0));
+  });
+
   testWidgets('AnimatedDefaultTextStyle onEnd callback test', (WidgetTester tester) async {
     await tester.pumpWidget(wrap(
-        child: TestAnimatedWidget(callback: mockOnEndFunction, switchKey: switchKey, state: _TestAnimatedDefaultTextStyleWidgetState(),)
+      child: TestAnimatedWidget(
+        callback: mockOnEndFunction,
+        switchKey: switchKey,
+        state: _TestAnimatedDefaultTextStyleWidgetState(),
+      )
     ));
 
     final Finder widgetFinder = find.byKey(switchKey);
@@ -188,7 +291,11 @@ void main() {
 
   testWidgets('AnimatedPhysicalModel onEnd callback test', (WidgetTester tester) async {
     await tester.pumpWidget(wrap(
-        child: TestAnimatedWidget(callback: mockOnEndFunction, switchKey: switchKey, state: _TestAnimatedPhysicalModelWidgetState(),)
+      child: TestAnimatedWidget(
+        callback: mockOnEndFunction,
+        switchKey: switchKey,
+        state: _TestAnimatedPhysicalModelWidgetState(),
+      )
     ));
 
     final Finder widgetFinder = find.byKey(switchKey);
@@ -205,7 +312,11 @@ void main() {
 
   testWidgets('TweenAnimationBuilder onEnd callback test', (WidgetTester tester) async {
     await tester.pumpWidget(wrap(
-        child: TestAnimatedWidget(callback: mockOnEndFunction, switchKey: switchKey, state: _TestTweenAnimationBuilderWidgetState(),)
+      child: TestAnimatedWidget(
+        callback: mockOnEndFunction,
+        switchKey: switchKey,
+        state: _TestTweenAnimationBuilderWidgetState(),
+      )
     ));
 
     final Finder widgetFinder = find.byKey(switchKey);
@@ -222,7 +333,11 @@ void main() {
 
   testWidgets('AnimatedTheme onEnd callback test', (WidgetTester tester) async {
     await tester.pumpWidget(wrap(
-        child: TestAnimatedWidget(callback: mockOnEndFunction, switchKey: switchKey, state: _TestAnimatedThemeWidgetState(),)
+      child: TestAnimatedWidget(
+        callback: mockOnEndFunction,
+        switchKey: switchKey,
+        state: _TestAnimatedThemeWidgetState(),
+      )
     ));
 
     final Finder widgetFinder = find.byKey(switchKey);
@@ -360,7 +475,42 @@ class _TestAnimatedOpacityWidgetState extends _TestAnimatedWidgetState {
       child: child,
       duration: duration,
       onEnd: widget.callback,
-      opacity: toggle ? 0.1 : 0.9,
+      opacity: toggle ? 1.0 : 0.0,
+    );
+  }
+}
+
+class _TestSliverAnimatedOpacityWidgetState extends _TestAnimatedWidgetState {
+  @override
+  Widget getAnimatedWidget() {
+    return SliverAnimatedOpacity(
+      sliver: SliverToBoxAdapter(child: child),
+      duration: duration,
+      onEnd: widget.callback,
+      opacity: toggle ? 1.0 : 0.0,
+    );
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    final Widget animatedWidget = getAnimatedWidget();
+
+    return Material(
+      child: Directionality(
+        textDirection: TextDirection.ltr,
+        child: CustomScrollView(
+          slivers: <Widget>[
+            animatedWidget,
+            SliverToBoxAdapter(
+              child: Switch(
+                key: widget.switchKey,
+                value: toggle,
+                onChanged: onChanged,
+              ),
+            ),
+          ],
+        ),
+      ),
     );
   }
 }