Notify a Scrollable that the associated animation has stopped (#5209)

This also required changing the AnimationController state transition
logic to signal completion of the animation during the tick that
finishes the simulation.

Fixes https://github.com/flutter/flutter/issues/3675

packages/flutter/lib/src/animation/animation_controller.dart

@@ -53,8 +53,9 @@ class AnimationController extends Animation<double>
     this.upperBound: 1.0
   }) {
     assert(upperBound >= lowerBound);
-    _value = (value ?? lowerBound).clamp(lowerBound, upperBound);
+    _direction = _AnimationDirection.forward;
     _ticker = new Ticker(_tick);
+    _internalSetValue(value ?? lowerBound);
   }
 
   /// Creates an animation controller with no upper or lower bound for its value.
@@ -71,10 +72,11 @@ class AnimationController extends Animation<double>
     this.duration,
     this.debugLabel
   }) : lowerBound = double.NEGATIVE_INFINITY,
-       upperBound = double.INFINITY,
-       _value = value {
+       upperBound = double.INFINITY {
     assert(value != null);
+    _direction = _AnimationDirection.forward;
     _ticker = new Ticker(_tick);
+    _internalSetValue(value);
   }
 
   /// The value at which this animation is deemed to be dismissed.
@@ -119,11 +121,23 @@ class AnimationController extends Animation<double>
   set value(double newValue) {
     assert(newValue != null);
     stop();
-    _value = newValue.clamp(lowerBound, upperBound);
+    _internalSetValue(newValue);
     notifyListeners();
     _checkStatusChanged();
   }
 
+  void _internalSetValue(double newValue) {
+    _value = newValue.clamp(lowerBound, upperBound);
+    if (_value == lowerBound) {
+      _status = AnimationStatus.dismissed;
+    } else if (_value == upperBound) {
+      _status = AnimationStatus.completed;
+    } else
+      _status = (_direction == _AnimationDirection.forward) ?
+        AnimationStatus.forward :
+        AnimationStatus.reverse;
+  }
+
   /// The amount of time that has passed between the time the animation started and the most recent tick of the animation.
   ///
   /// If the controller is not animating, the last elapsed duration is null.
@@ -136,29 +150,22 @@ class AnimationController extends Animation<double>
   _AnimationDirection _direction;
 
   @override
-  AnimationStatus get status {
-    if (!isAnimating && value == upperBound)
-      return AnimationStatus.completed;
-    if (!isAnimating && value == lowerBound)
-      return AnimationStatus.dismissed;
-    return _direction == _AnimationDirection.forward ?
-        AnimationStatus.forward :
-        AnimationStatus.reverse;
-  }
+  AnimationStatus get status => _status;
+  AnimationStatus _status;
 
   /// Starts running this animation forwards (towards the end).
   Future<Null> forward({ double from }) {
+    _direction = _AnimationDirection.forward;
     if (from != null)
       value = from;
-    _direction = _AnimationDirection.forward;
     return animateTo(upperBound);
   }
 
   /// Starts running this animation in reverse (towards the beginning).
   Future<Null> reverse({ double from }) {
+    _direction = _AnimationDirection.reverse;
     if (from != null)
       value = from;
-    _direction = _AnimationDirection.reverse;
     return animateTo(lowerBound);
   }
 
@@ -174,6 +181,9 @@ class AnimationController extends Animation<double>
     stop();
     if (simulationDuration == Duration.ZERO) {
       assert(value == target);
+      _status = (_direction == _AnimationDirection.forward) ?
+        AnimationStatus.completed :
+        AnimationStatus.dismissed;
       _checkStatusChanged();
       return new Future<Null>.value();
     }
@@ -215,6 +225,9 @@ class AnimationController extends Animation<double>
     _lastElapsedDuration = Duration.ZERO;
     _value = simulation.x(0.0).clamp(lowerBound, upperBound);
     Future<Null> result = _ticker.start();
+    _status = (_direction == _AnimationDirection.forward) ?
+      AnimationStatus.forward :
+      AnimationStatus.reverse;
     _checkStatusChanged();
     return result;
   }
@@ -245,8 +258,12 @@ class AnimationController extends Animation<double>
     _lastElapsedDuration = elapsed;
     double elapsedInSeconds = elapsed.inMicroseconds.toDouble() / Duration.MICROSECONDS_PER_SECOND;
     _value = _simulation.x(elapsedInSeconds).clamp(lowerBound, upperBound);
-    if (_simulation.isDone(elapsedInSeconds))
+    if (_simulation.isDone(elapsedInSeconds)) {
+      _status = (_direction == _AnimationDirection.forward) ?
+        AnimationStatus.completed :
+        AnimationStatus.dismissed;
       stop();
+    }
     notifyListeners();
     _checkStatusChanged();
   }

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

@@ -249,7 +249,8 @@ class ScrollableState<T extends Scrollable> extends State<T> {
   void initState() {
     super.initState();
     _controller = new AnimationController.unbounded()
-      ..addListener(_handleAnimationChanged);
+      ..addListener(_handleAnimationChanged)
+      ..addStatusListener(_handleAnimationStatusChanged);
     _scrollOffset = PageStorage.of(context)?.readState(context) ?? config.initialScrollOffset ?? 0.0;
   }
 
@@ -365,6 +366,11 @@ class ScrollableState<T extends Scrollable> extends State<T> {
     _setScrollOffset(_controller.value);
   }
 
+  void _handleAnimationStatusChanged(AnimationStatus status) {
+    if (!_controller.isAnimating)
+      _simulation = null;
+  }
+
   void _setScrollOffset(double newScrollOffset, { DragUpdateDetails details }) {
     if (_scrollOffset == newScrollOffset)
       return;

packages/flutter/test/animation/animation_controller_test.dart

@@ -132,6 +132,26 @@ void main() {
     expect(controller.value, equals(0.0));
   });
 
+  test('Forward only from value', () {
+    AnimationController controller = new AnimationController(
+      duration: const Duration(milliseconds: 100)
+    );
+    List<double> valueLog = <double>[];
+    List<AnimationStatus> statusLog = <AnimationStatus>[];
+    controller
+      ..addStatusListener((AnimationStatus status) {
+        statusLog.add(status);
+      })
+      ..addListener(() {
+        valueLog.add(controller.value);
+      });
+
+    controller.forward(from: 0.2);
+    expect(statusLog, equals(<AnimationStatus>[ AnimationStatus.forward ]));
+    expect(valueLog, equals(<double>[ 0.2 ]));
+    expect(controller.value, equals(0.2));
+  });
+
   test('Can fling to upper and lower bounds', () {
     AnimationController controller = new AnimationController(
       duration: const Duration(milliseconds: 100)

packages/flutter/test/widget/scrollable_dispose_test.dart

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter_test/flutter_test.dart';
+import 'package:flutter/widgets.dart';
+
+import 'test_widgets.dart';
+
+void main() {
+  testWidgets('simultaneously dispose a widget and end the scroll animation', (WidgetTester tester) async {
+    List<Widget> textWidgets = <Widget>[];
+    for (int i = 0; i < 250; i++)
+      textWidgets.add(new Text('$i'));
+    await tester.pumpWidget(new FlipWidget(
+      left: new Block(children: textWidgets),
+      right: new Container()
+    ));
+
+    await tester.fling(find.byType(Scrollable), new Offset(0.0, -200.0), 1000.0);
+    await tester.pump();
+
+    FlipWidgetState flipWidget = tester.state(find.byType(FlipWidget));
+    flipWidget.flip();
+    await tester.pump(new Duration(hours: 5));
+  });
+}