Remove value listener from semantics bindings (#21296)

packages/flutter/lib/semantics.dart

@@ -14,5 +14,6 @@
 library semantics;
 
 export 'src/semantics/binding.dart';
+export 'src/semantics/debug.dart';
 export 'src/semantics/semantics.dart';
 export 'src/semantics/semantics_service.dart';

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

@@ -401,7 +401,7 @@ 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 (_ticker.disableAnimations) {
+    if (SemanticsBinding.instance.disableAnimations) {
       switch (behavior) {
         case AnimationBehavior.normal:
           // Since the framework cannot handle zero duration animations, we run it at 5% of the normal
@@ -496,7 +496,7 @@ class AnimationController extends Animation<double>
                                          : upperBound + _kFlingTolerance.distance;
     double scale = 1.0;
     final AnimationBehavior behavior = animationBehavior ?? this.animationBehavior;
-    if (_ticker.disableAnimations) {
+    if (SemanticsBinding.instance.disableAnimations) {
       switch (behavior) {
         case AnimationBehavior.normal:
           // TODO(jonahwilliams): determine a better process for setting velocity.

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

@@ -68,12 +68,6 @@ 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
@@ -279,7 +273,6 @@ 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

@@ -6,6 +6,8 @@ import 'dart:ui' as ui show AccessibilityFeatures, window;
 
 import 'package:flutter/foundation.dart';
 
+import 'debug.dart';
+
 export 'dart:ui' show AccessibilityFeatures;
 
 /// The glue between the semantics layer and the Flutter engine.
@@ -23,7 +25,7 @@ class SemanticsBinding extends BindingBase {
   void initInstances() {
     super.initInstances();
     _instance = this;
-    _accessibilityFeatures = new ValueNotifier<ui.AccessibilityFeatures>(ui.window.accessibilityFeatures);
+    _accessibilityFeatures = ui.window.accessibilityFeatures;
   }
 
   /// Called when the platform accessibility features change.
@@ -31,7 +33,7 @@ class SemanticsBinding extends BindingBase {
   /// See [Window.onAccessibilityFeaturesChanged].
   @protected
   void handleAccessibilityFeaturesChanged() {
-    _accessibilityFeatures.value = ui.window.accessibilityFeatures;
+    _accessibilityFeatures = ui.window.accessibilityFeatures;
   }
 
   /// The currently active set of [AccessibilityFeatures].
@@ -41,6 +43,20 @@ class SemanticsBinding extends BindingBase {
   ///
   /// To listen to changes to accessibility features, create a
   /// [WidgetsBindingObserver] and listen to [didChangeAccessibilityFeatures].
-  ValueListenable<ui.AccessibilityFeatures> get accessibilityFeatures => _accessibilityFeatures;
-  ValueNotifier<ui.AccessibilityFeatures> _accessibilityFeatures;
+  ui.AccessibilityFeatures get accessibilityFeatures => _accessibilityFeatures;
+  ui.AccessibilityFeatures _accessibilityFeatures;
+
+  /// The platform is requesting that animations be disabled or simplified.
+  ///
+  /// This setting can be overriden for testing or debugging by setting
+  /// [debugSemanticsDisableAnimations].
+  bool get disableAnimations {
+    bool value = _accessibilityFeatures.disableAnimations;
+    assert(() {
+      if (debugSemanticsDisableAnimations != null)
+          value = debugSemanticsDisableAnimations;
+      return true;
+    }());
+    return value;
+  }
 }

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

+// Copyright 2018 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.
+
+/// Overrides the setting of [SemanticsBinding.disableAnimations] for debugging
+/// and testing.
+///
+/// This value is ignored in non-debug builds.
+bool debugSemanticsDisableAnimations;

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

@@ -4,7 +4,6 @@
 
 import 'package:flutter/foundation.dart';
 import 'package:flutter/scheduler.dart';
-import 'package:flutter/semantics.dart';
 
 import 'framework.dart';
 
@@ -95,10 +94,7 @@ abstract class SingleTickerProviderStateMixin<T extends StatefulWidget> extends
         'mixing in a SingleTickerProviderStateMixin, use a regular TickerProviderStateMixin.'
       );
     }());
-    final ValueListenable<AccessibilityFeatures> accessibilityFeatures = SemanticsBinding.instance.accessibilityFeatures;
-    _ticker = new Ticker(onTick, debugLabel: 'created by $this')
-      ..disableAnimations = accessibilityFeatures.value.disableAnimations;
-    accessibilityFeatures.addListener(_handleAccessibilityFeaturesChanged);
+    _ticker = new Ticker(onTick, debugLabel: 'created by $this');
     // 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
@@ -121,8 +117,6 @@ 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();
   }
 
@@ -149,13 +143,6 @@ 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
@@ -179,11 +166,8 @@ abstract class TickerProviderStateMixin<T extends StatefulWidget> extends State<
   @override
   Ticker createTicker(TickerCallback onTick) {
     _tickers ??= new Set<_WidgetTicker>();
-    final ValueListenable<AccessibilityFeatures> accessibilityFeatures = SemanticsBinding.instance.accessibilityFeatures;
-    final _WidgetTicker result = new _WidgetTicker(onTick, this, debugLabel: 'created by $this')
-      ..disableAnimations = accessibilityFeatures.value.disableAnimations;
+    final _WidgetTicker result = new _WidgetTicker(onTick, this, debugLabel: 'created by $this');
     _tickers.add(result);
-    accessibilityFeatures.addListener(_handleAccessibilityFeaturesChanged);
     return result;
   }
 
@@ -213,8 +197,6 @@ abstract class TickerProviderStateMixin<T extends StatefulWidget> extends State<
       }
       return true;
     }());
-    final ValueListenable<AccessibilityFeatures> accessibilityFeatures = SemanticsBinding.instance.accessibilityFeatures;
-    accessibilityFeatures.removeListener(_handleAccessibilityFeaturesChanged);
     super.dispose();
   }
 
@@ -241,15 +223,6 @@ abstract class TickerProviderStateMixin<T extends StatefulWidget> extends State<
       defaultValue: null,
     ));
   }
-
-  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

@@ -4,6 +4,7 @@
 
 import 'dart:ui' as ui;
 
+import 'package:flutter/semantics.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:flutter/animation.dart';
 import 'package:flutter/scheduler.dart';
@@ -581,16 +582,17 @@ void main() {
 
   group('AnimationBehavior', () {
     test('Default values for constructor', () {
-      final AnimationController controller = new AnimationController(vsync: const TestVSync(disableAnimations: true));
+      final AnimationController controller = new AnimationController(vsync: const TestVSync());
       expect(controller.animationBehavior, AnimationBehavior.normal);
 
-      final AnimationController repeating = new AnimationController.unbounded(vsync: const TestVSync(disableAnimations: true));
+      final AnimationController repeating = new AnimationController.unbounded(vsync: const TestVSync());
       expect(repeating.animationBehavior, AnimationBehavior.preserve);
     });
 
     test('AnimationBehavior.preserve runs at normal speed when animatingTo', () async {
+      debugSemanticsDisableAnimations = true;
       final AnimationController controller = new AnimationController(
-        vsync: const TestVSync(disableAnimations: true),
+        vsync: const TestVSync(),
         animationBehavior: AnimationBehavior.preserve,
       );
 
@@ -609,11 +611,13 @@ void main() {
 
       expect(controller.value, 1.0);
       expect(controller.status, AnimationStatus.completed);
+      debugSemanticsDisableAnimations = false;
     });
 
     test('AnimationBehavior.normal runs at 20x speed when animatingTo', () async {
+      debugSemanticsDisableAnimations = true;
       final AnimationController controller = new AnimationController(
-        vsync: const TestVSync(disableAnimations: true),
+        vsync: const TestVSync(),
         animationBehavior: AnimationBehavior.normal,
       );
 
@@ -632,14 +636,16 @@ void main() {
 
       expect(controller.value, 1.0);
       expect(controller.status, AnimationStatus.completed);
+      debugSemanticsDisableAnimations = null;
     });
 
     test('AnimationBehavior.normal runs "faster" whan AnimationBehavior.preserve', () {
+      debugSemanticsDisableAnimations = true;
       final AnimationController controller = new AnimationController(
-        vsync: const TestVSync(disableAnimations: true),
+        vsync: const TestVSync(),
       );
       final AnimationController fastController = new AnimationController(
-        vsync: const TestVSync(disableAnimations: true),
+        vsync: const TestVSync(),
       );
 
       controller.fling(velocity: 1.0, animationBehavior: AnimationBehavior.preserve);
@@ -649,6 +655,7 @@ void main() {
 
       // We don't assert a specific faction that normal animation.
       expect(controller.value < fastController.value, true);
+      debugSemanticsDisableAnimations = null;
     });
   });
 }

packages/flutter/test/rendering/proxy_box_test.dart

@@ -273,9 +273,6 @@ class _FakeTicker implements Ticker {
   @override
   bool get shouldScheduleTick => null;
 
-  @override
-  bool disableAnimations = false;
-
   @override
   void dispose() {}
 

packages/flutter_test/lib/src/test_vsync.dart

@@ -10,16 +10,8 @@ import 'package:flutter/scheduler.dart';
 /// tree.
 class TestVSync implements TickerProvider {
   /// Creates a ticker provider that creates standalone tickers.
-  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;
+  const TestVSync();
 
   @override
-  Ticker createTicker(TickerCallback onTick) => new Ticker(onTick)..disableAnimations = disableAnimations;
+  Ticker createTicker(TickerCallback onTick) => new Ticker(onTick);
 }