Address follow up to https://github.com/flutter/flutter/pull/20354 (#21181)

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

@@ -7,8 +7,8 @@ import 'dart:ui' as ui show lerpDouble;
 
 import 'package:flutter/foundation.dart';
 import 'package:flutter/physics.dart';
-import 'package:flutter/semantics.dart';
 import 'package:flutter/scheduler.dart';
+import 'package:flutter/semantics.dart';
 
 import 'animation.dart';
 import 'curves.dart';
@@ -44,7 +44,7 @@ const Tolerance _kFlingTolerance = Tolerance(
 /// When [AccessibilityFeatures.disableAnimations] is true, the device is asking
 /// flutter to reduce or disable animations as much as possible. To honor this,
 /// we reduce the duration and the corresponding number of frames for animations.
-/// This enum is used to allow certain [AnimationControllers] to opt out of this
+/// This enum is used to allow certain [AnimationController]s to opt out of this
 /// behavior.
 ///
 /// For example, the [AnimationController] which controls the physics simulation
@@ -200,9 +200,9 @@ class AnimationController extends Animation<double>
   /// The behavior of the controller when [AccessibilityFeatures.disableAnimations]
   /// is true.
   ///
-  /// Defaults to [AnimationBehavior.normal] for the [new AnimationBehavior]
+  /// Defaults to [AnimationBehavior.normal] for the [new AnimationController]
   /// constructor, and [AnimationBehavior.preserve] for the
-  /// [new AnimationBehavior.unbounded] constructor.
+  /// [new AnimationController.unbounded] constructor.
   final AnimationBehavior animationBehavior;
 
   /// Returns an [Animation<double>] for this animation controller, so that a
@@ -401,9 +401,12 @@ class AnimationController extends Animation<double>
   TickerFuture _animateToInternal(double target, { Duration duration, Curve curve = Curves.linear, AnimationBehavior animationBehavior }) {
     final AnimationBehavior behavior = animationBehavior ?? this.animationBehavior;
     double scale = 1.0;
-    if (SemanticsBinding.instance.disableAnimations) {
+    if (_ticker.disableAnimations) {
       switch (behavior) {
         case AnimationBehavior.normal:
+          // Since the framework cannot handle zero duration animations, we run it at 5% of the normal
+          // duration to limit most animations to a single frame.
+          // TODO(jonahwilliams): determine a better process for setting duration.
           scale = 0.05;
           break;
         case AnimationBehavior.preserve:
@@ -487,15 +490,17 @@ class AnimationController extends Animation<double>
   /// The most recently returned [TickerFuture], if any, is marked as having been
   /// canceled, meaning the future never completes and its [TickerFuture.orCancel]
   /// derivative future completes with a [TickerCanceled] error.
-  TickerFuture fling({ double velocity = 1.0, AnimationBehavior animationBehavior}) {
+  TickerFuture fling({ double velocity = 1.0, AnimationBehavior animationBehavior }) {
     _direction = velocity < 0.0 ? _AnimationDirection.reverse : _AnimationDirection.forward;
     final double target = velocity < 0.0 ? lowerBound - _kFlingTolerance.distance
                                          : upperBound + _kFlingTolerance.distance;
     double scale = 1.0;
     final AnimationBehavior behavior = animationBehavior ?? this.animationBehavior;
-    if (SemanticsBinding.instance.disableAnimations) {
+    if (_ticker.disableAnimations) {
       switch (behavior) {
         case AnimationBehavior.normal:
+          // TODO(jonahwilliams): determine a better process for setting velocity.
+          // the value below was arbitrarily chosen because it worked for the drawer widget.
           scale = 200.0;
           break;
         case AnimationBehavior.preserve:

packages/flutter/lib/src/scheduler/ticker.dart

@@ -68,6 +68,12 @@ class Ticker {
 
   TickerFuture _future;
 
+  /// Whether or not the platform is requesting that animations be disabled.
+  ///
+  /// See also:
+  ///   * [AccessibilityFeatures.disableAnimations], for the setting this value comes from.
+  bool disableAnimations = false;
+
   /// Whether this ticker has been silenced.
   ///
   /// While silenced, a ticker's clock can still run, but the callback will not
@@ -273,6 +279,7 @@ class Ticker {
     assert(_startTime == null);
     assert(_animationId == null);
     assert((originalTicker._future == null) == (originalTicker._startTime == null), 'Cannot absorb Ticker after it has been disposed.');
+    disableAnimations = originalTicker.disableAnimations;
     if (originalTicker._future != null) {
       _future = originalTicker._future;
       _startTime = originalTicker._startTime;

packages/flutter/lib/src/semantics/binding.dart

@@ -5,12 +5,12 @@
 import 'dart:ui' as ui show AccessibilityFeatures, window;
 
 import 'package:flutter/foundation.dart';
-import 'package:flutter/services.dart';
 
+export 'dart:ui' show AccessibilityFeatures;
 
 /// The glue between the semantics layer and the Flutter engine.
 // TODO(jonahwilliams): move the remaining semantic related bindings here.
-class SemanticsBinding extends BindingBase with ServicesBinding {
+class SemanticsBinding extends BindingBase {
   // This class is intended to be used as a mixin, and should not be
   // extended directly.
   factory SemanticsBinding._() => null;
@@ -23,7 +23,7 @@ class SemanticsBinding extends BindingBase with ServicesBinding {
   void initInstances() {
     super.initInstances();
     _instance = this;
-    _accessibilityFeatures = ui.window.accessibilityFeatures;
+    _accessibilityFeatures = new ValueNotifier<ui.AccessibilityFeatures>(ui.window.accessibilityFeatures);
   }
 
   /// Called when the platform accessibility features change.
@@ -31,7 +31,7 @@ class SemanticsBinding extends BindingBase with ServicesBinding {
   /// See [Window.onAccessibilityFeaturesChanged].
   @protected
   void handleAccessibilityFeaturesChanged() {
-    _accessibilityFeatures = ui.window.accessibilityFeatures;
+    _accessibilityFeatures.value = ui.window.accessibilityFeatures;
   }
 
   /// The currently active set of [AccessibilityFeatures].
@@ -41,9 +41,6 @@ class SemanticsBinding extends BindingBase with ServicesBinding {
   ///
   /// To listen to changes to accessibility features, create a
   /// [WidgetsBindingObserver] and listen to [didChangeAccessibilityFeatures].
-  ui.AccessibilityFeatures get accessibilityFeatures => _accessibilityFeatures;
-  ui.AccessibilityFeatures _accessibilityFeatures;
-
-  /// Whether the device is requesting that animations be disabled.
-  bool get disableAnimations => accessibilityFeatures.disableAnimations;
+  ValueListenable<ui.AccessibilityFeatures> get accessibilityFeatures => _accessibilityFeatures;
+  ValueNotifier<ui.AccessibilityFeatures> _accessibilityFeatures;
 }
\ No newline at end of file

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

@@ -4,6 +4,7 @@
 
 import 'package:flutter/foundation.dart';
 import 'package:flutter/scheduler.dart';
+import 'package:flutter/semantics.dart';
 
 import 'framework.dart';
 
@@ -94,7 +95,10 @@ abstract class SingleTickerProviderStateMixin<T extends StatefulWidget> extends
         'mixing in a SingleTickerProviderStateMixin, use a regular TickerProviderStateMixin.'
       );
     }());
-    _ticker = new Ticker(onTick, debugLabel: 'created by $this');
+    final ValueListenable<AccessibilityFeatures> accessibilityFeatures = SemanticsBinding.instance.accessibilityFeatures;
+    _ticker = new Ticker(onTick, debugLabel: 'created by $this')
+      ..disableAnimations = accessibilityFeatures.value.disableAnimations;
+    accessibilityFeatures.addListener(_handleAccessibilityFeaturesChanged);
     // We assume that this is called from initState, build, or some sort of
     // event handler, and that thus TickerMode.of(context) would return true. We
     // can't actually check that here because if we're in initState then we're
@@ -117,6 +121,8 @@ abstract class SingleTickerProviderStateMixin<T extends StatefulWidget> extends
         'The offending ticker was: ${_ticker.toString(debugIncludeStack: true)}'
       );
     }());
+    final ValueListenable<AccessibilityFeatures> accessibilityFeatures = SemanticsBinding.instance.accessibilityFeatures;
+    accessibilityFeatures.removeListener(_handleAccessibilityFeaturesChanged);
     super.dispose();
   }
 
@@ -144,6 +150,12 @@ abstract class SingleTickerProviderStateMixin<T extends StatefulWidget> extends
     properties.add(new DiagnosticsProperty<Ticker>('ticker', _ticker, description: tickerDescription, showSeparator: false, defaultValue: null));
   }
 
+  void _handleAccessibilityFeaturesChanged() {
+    final ValueListenable<AccessibilityFeatures> accessibilityFeatures = SemanticsBinding.instance.accessibilityFeatures;
+    if (_ticker != null) {
+      _ticker.disableAnimations = accessibilityFeatures.value.disableAnimations;
+    }
+  }
 }
 
 /// Provides [Ticker] objects that are configured to only tick while the current
@@ -167,8 +179,11 @@ abstract class TickerProviderStateMixin<T extends StatefulWidget> extends State<
   @override
   Ticker createTicker(TickerCallback onTick) {
     _tickers ??= new Set<_WidgetTicker>();
-    final _WidgetTicker result = new _WidgetTicker(onTick, this, debugLabel: 'created by $this');
+    final ValueListenable<AccessibilityFeatures> accessibilityFeatures = SemanticsBinding.instance.accessibilityFeatures;
+    final _WidgetTicker result = new _WidgetTicker(onTick, this, debugLabel: 'created by $this')
+      ..disableAnimations = accessibilityFeatures.value.disableAnimations;
     _tickers.add(result);
+    accessibilityFeatures.addListener(_handleAccessibilityFeaturesChanged);
     return result;
   }
 
@@ -198,6 +213,8 @@ abstract class TickerProviderStateMixin<T extends StatefulWidget> extends State<
       }
       return true;
     }());
+    final ValueListenable<AccessibilityFeatures> accessibilityFeatures = SemanticsBinding.instance.accessibilityFeatures;
+    accessibilityFeatures.removeListener(_handleAccessibilityFeaturesChanged);
     super.dispose();
   }
 
@@ -205,8 +222,9 @@ abstract class TickerProviderStateMixin<T extends StatefulWidget> extends State<
   void didChangeDependencies() {
     final bool muted = !TickerMode.of(context);
     if (_tickers != null) {
-      for (Ticker ticker in _tickers)
+      for (Ticker ticker in _tickers) {
         ticker.muted = muted;
+      }
     }
     super.didChangeDependencies();
   }
@@ -224,6 +242,14 @@ abstract class TickerProviderStateMixin<T extends StatefulWidget> extends State<
     ));
   }
 
+  void _handleAccessibilityFeaturesChanged() {
+    final ValueListenable<AccessibilityFeatures> accessibilityFeatures = SemanticsBinding.instance.accessibilityFeatures;
+    if (_tickers != null) {
+      for (Ticker ticker in _tickers) {
+        ticker.disableAnimations = accessibilityFeatures.value.disableAnimations;
+      }
+    }
+  }
 }
 
 // This class should really be called _DisposingTicker or some such, but this

packages/flutter/test/animation/animation_controller_test.dart

@@ -581,17 +581,16 @@ void main() {
 
   group('AnimationBehavior', () {
     test('Default values for constructor', () {
-      final AnimationController controller = new AnimationController(vsync: const TestVSync());
+      final AnimationController controller = new AnimationController(vsync: const TestVSync(disableAnimations: true));
       expect(controller.animationBehavior, AnimationBehavior.normal);
 
-      final AnimationController repeating = new AnimationController.unbounded(vsync: const TestVSync());
+      final AnimationController repeating = new AnimationController.unbounded(vsync: const TestVSync(disableAnimations: true));
       expect(repeating.animationBehavior, AnimationBehavior.preserve);
     });
 
-    testWidgets('AnimationBehavior.preserve runs at normal speed when animatingTo', (WidgetTester tester) async {
-      tester.binding.disableAnimations = true;
+    test('AnimationBehavior.preserve runs at normal speed when animatingTo', () async {
       final AnimationController controller = new AnimationController(
-        vsync: const TestVSync(),
+        vsync: const TestVSync(disableAnimations: true),
         animationBehavior: AnimationBehavior.preserve,
       );
 
@@ -610,13 +609,11 @@ void main() {
 
       expect(controller.value, 1.0);
       expect(controller.status, AnimationStatus.completed);
-      tester.binding.disableAnimations = false;
     });
 
-    testWidgets('AnimationBehavior.normal runs at 20x speed when animatingTo', (WidgetTester tester) async {
-      tester.binding.disableAnimations = true;
+    test('AnimationBehavior.normal runs at 20x speed when animatingTo', () async {
       final AnimationController controller = new AnimationController(
-        vsync: const TestVSync(),
+        vsync: const TestVSync(disableAnimations: true),
         animationBehavior: AnimationBehavior.normal,
       );
 
@@ -635,17 +632,14 @@ void main() {
 
       expect(controller.value, 1.0);
       expect(controller.status, AnimationStatus.completed);
-
-      tester.binding.disableAnimations = false;
     });
 
-    testWidgets('AnimationBehavior.normal runs "faster" whan AnimationBehavior.preserve', (WidgetTester tester) async {
-      tester.binding.disableAnimations = true;
+    test('AnimationBehavior.normal runs "faster" whan AnimationBehavior.preserve', () {
       final AnimationController controller = new AnimationController(
-        vsync: const TestVSync(),
+        vsync: const TestVSync(disableAnimations: true),
       );
       final AnimationController fastController = new AnimationController(
-        vsync: const TestVSync(),
+        vsync: const TestVSync(disableAnimations: true),
       );
 
       controller.fling(velocity: 1.0, animationBehavior: AnimationBehavior.preserve);
@@ -655,7 +649,6 @@ void main() {
 
       // We don't assert a specific faction that normal animation.
       expect(controller.value < fastController.value, true);
-      tester.binding.disableAnimations = false;
     });
   });
 }

packages/flutter/test/rendering/proxy_box_test.dart

@@ -246,7 +246,7 @@ void main() {
 
 class _FakeTickerProvider implements TickerProvider {
   @override
-  Ticker createTicker(TickerCallback onTick) {
+  Ticker createTicker(TickerCallback onTick, [bool disableAnimations = false]) {
     return new _FakeTicker();
   }
 }
@@ -273,6 +273,9 @@ class _FakeTicker implements Ticker {
   @override
   bool get shouldScheduleTick => null;
 
+  @override
+  bool disableAnimations = false;
+
   @override
   void dispose() {}
 

packages/flutter_test/lib/src/binding.dart

@@ -128,9 +128,6 @@ abstract class TestWidgetsFlutterBinding extends BindingBase
   @protected
   bool get checkIntrinsicSizes => false;
 
-  @override
-  bool disableAnimations = false;
-
   /// Creates and initializes the binding. This function is
   /// idempotent; calling it a second time will just return the
   /// previously-created instance.

packages/flutter_test/lib/src/test_vsync.dart

@@ -10,8 +10,16 @@ import 'package:flutter/scheduler.dart';
 /// tree.
 class TestVSync implements TickerProvider {
   /// Creates a ticker provider that creates standalone tickers.
-  const TestVSync();
+  const TestVSync({this.disableAnimations = false});
+
+  /// Whether to disable the animations of tickers create from this picker.
+  ///
+  /// See also:
+  ///
+  ///   * [AccessibilityFeatures.disableAnimations], for the setting that controls this flag.
+  ///   * [AnimationBehavior], for how animation controllers change when created from tickers with this flag.
+  final bool disableAnimations;
 
   @override
-  Ticker createTicker(TickerCallback onTick) => new Ticker(onTick);
+  Ticker createTicker(TickerCallback onTick) => new Ticker(onTick)..disableAnimations = disableAnimations;
 }