Make FadeTransition more efficient (#14155)

* Make FadeTransition more efficient

* Update fade_transition_test.dart

* Update proxy_box.dart

examples/flutter_gallery/lib/demo/material/bottom_navigation_demo.dart

@@ -152,8 +152,8 @@ class _BottomNavigationDemoState extends State<BottomNavigationDemo>
 
     // We want to have the newly animating (fading in) views on top.
     transitions.sort((FadeTransition a, FadeTransition b) {
-      final Animation<double> aAnimation = a.listenable;
-      final Animation<double> bAnimation = b.listenable;
+      final Animation<double> aAnimation = a.opacity;
+      final Animation<double> bAnimation = b.opacity;
       final double aValue = aAnimation.value;
       final double bValue = bAnimation.value;
       return aValue.compareTo(bValue);

packages/flutter/lib/src/rendering/proxy_box.dart

@@ -4,6 +4,7 @@
 
 import 'dart:ui' as ui show ImageFilter, Gradient;
 
+import 'package:flutter/animation.dart';
 import 'package:flutter/foundation.dart';
 import 'package:flutter/gestures.dart';
 import 'package:flutter/painting.dart';
@@ -776,6 +777,99 @@ class RenderOpacity extends RenderProxyBox {
   }
 }
 
+/// Makes its child partially transparent, driven from an [Animation].
+///
+/// This is a variant of [RenderOpacity] that uses an [Animation<double>] rather
+/// than a [double] to control the opacity.
+class RenderAnimatedOpacity extends RenderProxyBox {
+  /// Creates a partially transparent render object.
+  ///
+  /// The [opacity] argument must not be null.
+  RenderAnimatedOpacity({ @required Animation<double> opacity, RenderBox child }) : super(child) {
+    this.opacity = opacity;
+  }
+
+  int _alpha;
+
+  @override
+  bool get alwaysNeedsCompositing => child != null && _currentlyNeedsCompositing;
+  bool _currentlyNeedsCompositing;
+
+  /// The animation that drives this render object's opacity.
+  ///
+  /// An opacity of 1.0 is fully opaque. An opacity of 0.0 is fully transparent
+  /// (i.e., invisible).
+  ///
+  /// To change the opacity of a child in a static manner, not animated,
+  /// consider [RenderOpacity] instead.
+  Animation<double> get opacity => _opacity;
+  Animation<double> _opacity;
+  set opacity(Animation<double> value) {
+    assert(value != null);
+    if (_opacity == value)
+      return;
+    if (attached && _opacity != null)
+      _opacity.removeListener(_updateOpacity);
+    _opacity = value;
+    if (attached)
+      _opacity.addListener(_updateOpacity);
+    _updateOpacity();
+  }
+
+  @override
+  void attach(PipelineOwner owner) {
+    super.attach(owner);
+    _opacity.addListener(_updateOpacity);
+    _updateOpacity(); // in case it changed while we weren't listening
+ }
+
+  @override
+  void detach() {
+    _opacity.removeListener(_updateOpacity);
+    super.detach();
+  }
+
+  void _updateOpacity() {
+    final int oldAlpha = _alpha;
+    _alpha = _getAlphaFromOpacity(_opacity.value.clamp(0.0, 1.0));
+    if (oldAlpha != _alpha) {
+      final bool didNeedCompositing = _currentlyNeedsCompositing;
+      _currentlyNeedsCompositing = _alpha > 0 || _alpha < 255;
+      if (child != null && didNeedCompositing != _currentlyNeedsCompositing)
+        markNeedsCompositingBitsUpdate();
+      markNeedsPaint();
+      if (oldAlpha == 0 || _alpha == 0)
+        markNeedsSemanticsUpdate();
+    }
+  }
+
+  @override
+  void paint(PaintingContext context, Offset offset) {
+    if (child != null) {
+      if (_alpha == 0)
+        return;
+      if (_alpha == 255) {
+        context.paintChild(child, offset);
+        return;
+      }
+      assert(needsCompositing);
+      context.pushOpacity(offset, _alpha, super.paint);
+    }
+  }
+
+  @override
+  void visitChildrenForSemantics(RenderObjectVisitor visitor) {
+    if (child != null && _alpha != 0)
+      visitor(child);
+  }
+
+  @override
+  void debugFillProperties(DiagnosticPropertiesBuilder description) {
+    super.debugFillProperties(description);
+    description.add(new DiagnosticsProperty<Animation<double>>('opacity', opacity));
+  }
+}
+
 /// Signature for a function that creates a [Shader] for a given [Rect].
 ///
 /// Used by [RenderShaderMask] and the [ShaderMask] widget.
@@ -969,7 +1063,7 @@ class ShapeBorderClipper extends CustomClipper<Path> {
     this.textDirection,
   }) : assert(shapeBorder != null);
 
-  // The shape border whose outer path this clipper clips to.
+  /// The shape border whose outer path this clipper clips to.
   final ShapeBorder shapeBorder;
 
   /// The text direction to use for getting the outer path for [shapeBorder].

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

@@ -5,6 +5,7 @@
 import 'dart:math' as math;
 
 import 'package:flutter/foundation.dart';
+import 'package:flutter/rendering.dart';
 import 'package:vector_math/vector_math_64.dart' show Matrix4;
 
 import 'basic.dart';
@@ -301,15 +302,15 @@ class SizeTransition extends AnimatedWidget {
 ///
 /// For a widget that automatically animates between the sizes of two children,
 /// fading between them, see [AnimatedCrossFade].
-class FadeTransition extends AnimatedWidget {
+class FadeTransition extends SingleChildRenderObjectWidget {
   /// Creates an opacity transition.
   ///
   /// The [opacity] argument must not be null.
   const FadeTransition({
     Key key,
-    @required Animation<double> opacity,
-    this.child,
-  }) : super(key: key, listenable: opacity);
+    @required this.opacity,
+    Widget child,
+  }) : super(key: key, child: child);
 
   /// The animation that controls the opacity of the child.
   ///
@@ -317,16 +318,19 @@ class FadeTransition extends AnimatedWidget {
   /// painted with an opacity of v. For example, if v is 0.5, the child will be
   /// blended 50% with its background. Similarly, if v is 0.0, the child will be
   /// completely transparent.
-  Animation<double> get opacity => listenable;
+  final Animation<double> opacity;
 
-  /// The widget below this widget in the tree.
-  ///
-  /// {@macro flutter.widgets.child}
-  final Widget child;
+  @override
+  RenderAnimatedOpacity createRenderObject(BuildContext context) {
+    return new RenderAnimatedOpacity(
+      opacity: opacity,
+    );
+  }
 
   @override
-  Widget build(BuildContext context) {
-    return new Opacity(opacity: opacity.value, child: child);
+  void updateRenderObject(BuildContext context, RenderAnimatedOpacity renderObject) {
+    renderObject
+      ..opacity = opacity;
   }
 }
 

packages/flutter/test/cupertino/button_test.dart

@@ -142,11 +142,11 @@ void main() {
     await tester.pumpAndSettle();
 
     // Check opacity
-    final Opacity opacity = tester.widget(find.descendant(
+    final FadeTransition opacity = tester.widget(find.descendant(
       of: find.byType(CupertinoButton),
-      matching: find.byType(Opacity),
+      matching: find.byType(FadeTransition),
     ));
-    expect(opacity.opacity, 0.1);
+    expect(opacity.opacity.value, 0.1);
   });
 
   testWidgets('pressedOpacity parameter', (WidgetTester tester) async {
@@ -163,11 +163,11 @@ void main() {
     await tester.pumpAndSettle();
 
     // Check opacity
-    final Opacity opacity = tester.widget(find.descendant(
+    final FadeTransition opacity = tester.widget(find.descendant(
       of: find.byType(CupertinoButton),
-      matching: find.byType(Opacity),
+      matching: find.byType(FadeTransition),
     ));
-    expect(opacity.opacity, pressedOpacity);
+    expect(opacity.opacity.value, pressedOpacity);
   });
 
   testWidgets('Cupertino button is semantically a button', (WidgetTester tester) async {

packages/flutter/test/material/page_test.dart

@@ -28,8 +28,8 @@ void main() {
     await tester.pump();
     await tester.pump(const Duration(milliseconds: 1));
 
-    Opacity widget2Opacity =
-        tester.element(find.text('Page 2')).ancestorWidgetOfExactType(Opacity);
+    FadeTransition widget2Opacity =
+        tester.element(find.text('Page 2')).ancestorWidgetOfExactType(FadeTransition);
     Offset widget2TopLeft = tester.getTopLeft(find.text('Page 2'));
     final Size widget2Size = tester.getSize(find.text('Page 2'));
 
@@ -40,7 +40,7 @@ void main() {
     // Animation begins 3/4 of the way up the page.
     expect(widget2TopLeft.dy < widget2Size.height / 4.0, true);
     // Animation starts with page 2 being near transparent.
-    expect(widget2Opacity.opacity < 0.01, MaterialPageRoute.debugEnableFadingRoutes); // ignore: deprecated_member_use
+    expect(widget2Opacity.opacity.value < 0.01, MaterialPageRoute.debugEnableFadingRoutes); // ignore: deprecated_member_use
 
     await tester.pump(const Duration(milliseconds: 300));
 
@@ -53,13 +53,13 @@ void main() {
     await tester.pump(const Duration(milliseconds: 1));
 
     widget2Opacity =
-        tester.element(find.text('Page 2')).ancestorWidgetOfExactType(Opacity);
+        tester.element(find.text('Page 2')).ancestorWidgetOfExactType(FadeTransition);
     widget2TopLeft = tester.getTopLeft(find.text('Page 2'));
 
     // Page 2 starts to move down.
     expect(widget1TopLeft.dy < widget2TopLeft.dy, true);
     // Page 2 starts to lose opacity.
-    expect(widget2Opacity.opacity < 1.0, MaterialPageRoute.debugEnableFadingRoutes); // ignore: deprecated_member_use
+    expect(widget2Opacity.opacity.value < 1.0, MaterialPageRoute.debugEnableFadingRoutes); // ignore: deprecated_member_use
 
     await tester.pump(const Duration(milliseconds: 300));
 

packages/flutter/test/material/text_field_test.dart

@@ -491,15 +491,16 @@ void main() {
 
     // Toolbar should fade in. Starting at 0% opacity.
     final Element target = tester.element(find.text('SELECT ALL'));
-    Opacity opacity = target.ancestorWidgetOfExactType(Opacity);
+    final FadeTransition opacity = target.ancestorWidgetOfExactType(FadeTransition);
     expect(opacity, isNotNull);
-    expect(opacity.opacity, equals(0.0));
+    expect(opacity.opacity.value, equals(0.0));
 
     // Still fading in.
     await tester.pump(const Duration(milliseconds: 50));
-    opacity = target.ancestorWidgetOfExactType(Opacity);
-    expect(opacity.opacity, greaterThan(0.0));
-    expect(opacity.opacity, lessThan(1.0));
+    final FadeTransition opacity2 = target.ancestorWidgetOfExactType(FadeTransition);
+    expect(opacity, same(opacity2));
+    expect(opacity.opacity.value, greaterThan(0.0));
+    expect(opacity.opacity.value, lessThan(1.0));
 
     // End the test here to ensure the animation is properly disposed of.
   });

packages/flutter/test/rendering/semantics_and_children_test.dart

@@ -4,8 +4,11 @@
 
 import 'package:flutter/material.dart';
 import 'package:flutter/rendering.dart';
+import 'package:flutter_test/flutter_test.dart' show TestVSync;
 import 'package:test/test.dart';
 
+import 'rendering_tester.dart';
+
 int countSemanticsChildren(RenderObject object) {
   int count = 0;
   object.visitChildrenForSemantics((RenderObject child) {
@@ -36,4 +39,36 @@ void main() {
     box.opacity = 0.0;
     expect(countSemanticsChildren(box), 0);
   });
+
+  test('RenderOpacity and children and semantics', () {
+    final AnimationController controller = new AnimationController(vsync: const TestVSync());
+    final RenderAnimatedOpacity box = new RenderAnimatedOpacity(
+      opacity: controller,
+      child: new RenderParagraph(
+        const TextSpan(),
+        textDirection: TextDirection.ltr,
+      ),
+    );
+    expect(countSemanticsChildren(box), 0); // controller defaults to 0.0
+    controller.value = 0.2; // has no effect, box isn't subscribed yet
+    expect(countSemanticsChildren(box), 0);
+    controller.value = 1.0; // ditto
+    expect(countSemanticsChildren(box), 0); // alpha is still 0
+    layout(box); // this causes the box to attach, which makes it subscribe
+    expect(countSemanticsChildren(box), 1);
+    controller.value = 1.0;
+    expect(countSemanticsChildren(box), 1);
+    controller.value = 0.5;
+    expect(countSemanticsChildren(box), 1);
+    controller.value = 0.25;
+    expect(countSemanticsChildren(box), 1);
+    controller.value = 0.125;
+    expect(countSemanticsChildren(box), 1);
+    controller.value = 0.0;
+    expect(countSemanticsChildren(box), 0);
+    controller.value = 0.125;
+    expect(countSemanticsChildren(box), 1);
+    controller.value = 0.0;
+    expect(countSemanticsChildren(box), 0);
+  });
 }

packages/flutter/test/widgets/fade_transition_test.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.
+
+import 'package:flutter/foundation.dart';
+import 'package:flutter/widgets.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('FadeTransition', (WidgetTester tester) async {
+    final DebugPrintCallback oldPrint = debugPrint;
+    final List<String> log = <String>[];
+    debugPrint = (String message, { int wrapWidth }) {
+      log.add(message);
+    };
+    debugPrintBuildScope = true;
+    final AnimationController controller = new AnimationController(
+      vsync: const TestVSync(),
+      duration: const Duration(seconds: 2),
+    );
+    await tester.pumpWidget(new FadeTransition(
+      opacity: controller,
+      child: const Placeholder(),
+    ));
+    expect(log, hasLength(2));
+    expect(log.last, 'buildScope finished');
+    await tester.pump();
+    expect(log, hasLength(2));
+    controller.forward();
+    await tester.pumpAndSettle();
+    expect(log, hasLength(2));
+    debugPrint = oldPrint;
+    debugPrintBuildScope = false;
+  });
+}