AnimatedSize: state machine, tests, animate only when needed (#11305)

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

@@ -10,6 +10,41 @@ import 'box.dart';
 import 'object.dart';
 import 'shifted_box.dart';
 
+/// A [RenderAnimatedSize] can be in exactly one of these states.
+@visibleForTesting
+enum RenderAnimatedSizeState {
+  /// The initial state, when we do not yet know what the starting and target
+  /// sizes are to animate.
+  ///
+  /// Next possible state is [stable].
+  start,
+
+  /// At this state the child's size is assumed to be stable and we are either
+  /// animating, or waiting for the child's size to change.
+  ///
+  /// Next possible state is [changed].
+  stable,
+
+  /// At this state we know that the child has changed once after being assumed
+  /// [stable].
+  ///
+  /// Next possible states are:
+  ///
+  /// - [stable] - if the child's size stabilized immediately, this is a signal
+  /// for us to begin animating the size towards the child's new size.
+  /// - [unstable] - if the child's size continues to change, we assume it is
+  /// not stable and enter the [unstable] state.
+  changed,
+
+  /// At this state the child's size is assumed to be unstable.
+  ///
+  /// Instead of chasing the child's size in this state we tightly track the
+  /// child's size until it stabilizes.
+  ///
+  /// Next possible state is [stable].
+  unstable,
+}
+
 /// A render object that animates its size to its child's size over a given
 /// [duration] and with a given [curve]. If the child's size itself animates
 /// (i.e. if it changes size two frames in a row, as opposed to abruptly
@@ -60,10 +95,16 @@ class RenderAnimatedSize extends RenderAligningShiftedBox {
   AnimationController _controller;
   CurvedAnimation _animation;
   final SizeTween _sizeTween = new SizeTween();
-  bool _didChangeTargetSizeLastFrame = false;
   bool _hasVisualOverflow;
   double _lastValue;
 
+  /// The state this size animation is in.
+  ///
+  /// See [RenderAnimatedSizeState] for possible states.
+  @visibleForTesting
+  RenderAnimatedSizeState get state => _state;
+  RenderAnimatedSizeState _state = RenderAnimatedSizeState.start;
+
   /// The duration of the animation.
   Duration get duration => _controller.duration;
   set duration(Duration value) {
@@ -82,6 +123,12 @@ class RenderAnimatedSize extends RenderAligningShiftedBox {
     _animation.curve = value;
   }
 
+  /// Whether the size is being currently animated towards the child's size.
+  ///
+  /// See [RenderAnimatedSizeState] for situations when we may not be animating
+  /// the size.
+  bool get isAnimating => _controller.isAnimating;
+
   /// The [TickerProvider] for the [AnimationController] that runs the animation.
   TickerProvider get vsync => _vsync;
   TickerProvider _vsync;
@@ -93,16 +140,10 @@ class RenderAnimatedSize extends RenderAligningShiftedBox {
     _controller.resync(vsync);
   }
 
-  @override
-  void attach(PipelineOwner owner) {
-    super.attach(owner);
-    if (_animatedSize != _sizeTween.end && !_controller.isAnimating)
-      _controller.forward();
-  }
-
   @override
   void detach() {
     _controller.stop();
+    _state = RenderAnimatedSizeState.start;
     super.detach();
   }
 
@@ -121,34 +162,93 @@ class RenderAnimatedSize extends RenderAligningShiftedBox {
     }
 
     child.layout(constraints, parentUsesSize: true);
-    if (_sizeTween.end != child.size) {
-      _sizeTween.begin = _animatedSize ?? child.size;
-      _sizeTween.end = child.size;
 
-      if (_didChangeTargetSizeLastFrame) {
-        size = child.size;
-        _controller.stop();
-      } else {
-        // Don't register first change as a last-frame change.
-        if (_sizeTween.end != _sizeTween.begin)
-          _didChangeTargetSizeLastFrame = true;
+    switch(_state) {
+      case RenderAnimatedSizeState.start:
+        _layoutStart();
+        break;
+      case RenderAnimatedSizeState.stable:
+        _layoutStable();
+        break;
+      case RenderAnimatedSizeState.changed:
+        _layoutChanged();
+        break;
+      case RenderAnimatedSizeState.unstable:
+        _layoutUnstable();
+        break;
+      default:
+        throw new StateError('$runtimeType is in an invalid state $_state');
+    }
+
+    size = constraints.constrain(_animatedSize);
+    alignChild();
+
+    if (size.width < _sizeTween.end.width ||
+        size.height < _sizeTween.end.height)
+      _hasVisualOverflow = true;
+  }
 
+  void _restartAnimation() {
     _lastValue = 0.0;
     _controller.forward(from: 0.0);
+  }
 
-        size = constraints.constrain(_animatedSize);
+  /// Laying out the child for the first time.
+  ///
+  /// We have the initial size to animate from, but we do not have the target
+  /// size to animate to, so we set both ends to child's size.
+  void _layoutStart() {
+    _sizeTween.begin = _sizeTween.end = child.size;
+    _state = RenderAnimatedSizeState.stable;
   }
-    } else {
-      _didChangeTargetSizeLastFrame = false;
 
-      size = constraints.constrain(_animatedSize);
+  /// At this state we're assuming the child size is stable and letting the
+  /// animation run its course.
+  ///
+  /// If during animation the size of the child changes we restart the
+  /// animation.
+  void _layoutStable() {
+    if (_sizeTween.end != child.size) {
+      _sizeTween.end = child.size;
+      _restartAnimation();
+      _state = RenderAnimatedSizeState.changed;
+    } else if (_controller.value == _controller.upperBound) {
+      // Animation finished. Reset target sizes.
+      _sizeTween.begin = _sizeTween.end = child.size;
+    }
   }
 
-    alignChild();
+  /// This state indicates that the size of the child changed once after being
+  /// considered stable.
+  ///
+  /// If the child stabilizes immediately, we go back to stable state. If it
+  /// changes again, we match the child's size, restart animation and go to
+  /// unstable state.
+  void _layoutChanged() {
+    if (_sizeTween.end != child.size) {
+      // Child size changed again. Match the child's size and restart animation.
+      _sizeTween.begin = _sizeTween.end = child.size;
+      _restartAnimation();
+      _state = RenderAnimatedSizeState.unstable;
+    } else {
+      // Child size stabilized.
+      _state = RenderAnimatedSizeState.stable;
+    }
+  }
 
-    if (size.width < _sizeTween.end.width ||
-        size.height < _sizeTween.end.height)
-      _hasVisualOverflow = true;
+  /// The child's size is not stable.
+  ///
+  /// Continue tracking the child's size until is stabilizes.
+  void _layoutUnstable() {
+    if (_sizeTween.end != child.size) {
+      // Still unstable. Continue tracking the child.
+      _sizeTween.begin = _sizeTween.end = child.size;
+      _restartAnimation();
+    } else {
+      // Child size stabilized.
+      _controller.stop();
+      _state = RenderAnimatedSizeState.stable;
+    }
   }
 
   @override

packages/flutter/test/widgets/animated_size_test.dart

@@ -16,7 +16,8 @@ class TestPaintingContext implements PaintingContext {
 }
 
 void main() {
-  testWidgets('AnimatedSize test', (WidgetTester tester) async {
+  group('AnimatedSize', () {
+    testWidgets('animates forwards then backwards with stable-sized children', (WidgetTester tester) async {
       await tester.pumpWidget(
         new Center(
           child: new AnimatedSize(
@@ -89,7 +90,7 @@ void main() {
       expect(box.size.height, equals(100.0));
     });
 
-  testWidgets('AnimatedSize constrained test', (WidgetTester tester) async {
+    testWidgets('clamps animated size to constraints', (WidgetTester tester) async {
       await tester.pumpWidget(
         new Center(
           child: new SizedBox (
@@ -111,6 +112,7 @@ void main() {
       expect(box.size.width, equals(100.0));
       expect(box.size.height, equals(100.0));
 
+      // Attempt to animate beyond the outer SizedBox.
       await tester.pumpWidget(
         new Center(
           child: new SizedBox (
@@ -128,13 +130,30 @@ void main() {
         ),
       );
 
+      // Verify that animated size is the same as the outer SizedBox.
       await tester.pump(const Duration(milliseconds: 100));
       box = tester.renderObject(find.byType(AnimatedSize));
       expect(box.size.width, equals(100.0));
       expect(box.size.height, equals(100.0));
     });
 
-  testWidgets('AnimatedSize with AnimatedContainer', (WidgetTester tester) async {
+    testWidgets('tracks unstable child, then resumes animation when child stabilizes', (WidgetTester tester) async {
+      Future<Null> pumpMillis(int millis) async {
+        await tester.pump(new Duration(milliseconds: millis));
+      }
+
+      void verify({double size, RenderAnimatedSizeState state}) {
+        assert(size != null || state != null);
+        final RenderAnimatedSize box = tester.renderObject(find.byType(AnimatedSize));
+        if (size != null) {
+          expect(box.size.width, size);
+          expect(box.size.height, size);
+        }
+        if (state != null) {
+          expect(box.state, state);
+        }
+      }
+
       await tester.pumpWidget(
         new Center(
           child: new AnimatedSize(
@@ -149,10 +168,9 @@ void main() {
         ),
       );
 
-    RenderBox box = tester.renderObject(find.byType(AnimatedSize));
-    expect(box.size.width, equals(100.0));
-    expect(box.size.height, equals(100.0));
+      verify(size: 100.0, state: RenderAnimatedSizeState.stable);
 
+      // Animate child size from 100 to 200 slowly (100ms).
       await tester.pumpWidget(
         new Center(
           child: new AnimatedSize(
@@ -167,17 +185,45 @@ void main() {
         ),
       );
 
-    await tester.pump(const Duration(milliseconds: 1)); // register change
-    await tester.pump(const Duration(milliseconds: 49));
-    expect(box.size.width, equals(150.0));
-    expect(box.size.height, equals(150.0));
-    await tester.pump(const Duration(milliseconds: 50));
-    box = tester.renderObject(find.byType(AnimatedSize));
-    expect(box.size.width, equals(200.0));
-    expect(box.size.height, equals(200.0));
+      // Make sure animation proceeds at child's pace, with AnimatedSize
+      // tightly tracking the child's size.
+      verify(state: RenderAnimatedSizeState.stable);
+      await pumpMillis(1);  // register change
+      verify(state: RenderAnimatedSizeState.changed);
+      await pumpMillis(49);
+      verify(size: 150.0, state: RenderAnimatedSizeState.unstable);
+      await pumpMillis(50);
+      verify(size: 200.0, state: RenderAnimatedSizeState.unstable);
+
+      // Stabilize size
+      await pumpMillis(50);
+      verify(size: 200.0, state: RenderAnimatedSizeState.stable);
+
+      // Quickly (in 1ms) change size back to 100
+      await tester.pumpWidget(
+        new Center(
+          child: new AnimatedSize(
+            duration: const Duration(milliseconds: 200),
+            vsync: tester,
+            child: new AnimatedContainer(
+              duration: const Duration(milliseconds: 1),
+              width: 100.0,
+              height: 100.0,
+            ),
+          ),
+        ),
+      );
+
+      verify(size: 200.0, state: RenderAnimatedSizeState.stable);
+      await pumpMillis(1);  // register change
+      verify(state: RenderAnimatedSizeState.changed);
+      await pumpMillis(100);
+      verify(size: 150.0, state: RenderAnimatedSizeState.stable);
+      await pumpMillis(100);
+      verify(size: 100.0, state: RenderAnimatedSizeState.stable);
     });
 
-  testWidgets('AnimatedSize resync', (WidgetTester tester) async {
+    testWidgets('resyncs its animation controller', (WidgetTester tester) async {
       await tester.pumpWidget(
         const Center(
           child: const AnimatedSize(
@@ -209,4 +255,29 @@ void main() {
       final RenderBox box = tester.renderObject(find.byType(AnimatedSize));
       expect(box.size.width, equals(150.0));
     });
+
+    testWidgets('does not run animation unnecessarily', (WidgetTester tester) async {
+      await tester.pumpWidget(
+        new Center(
+          child: new AnimatedSize(
+            duration: const Duration(milliseconds: 200),
+            vsync: tester,
+            child: const SizedBox(
+              width: 100.0,
+              height: 100.0,
+            ),
+          ),
+        ),
+      );
+
+      for (int i = 0; i < 20; i++) {
+        final RenderAnimatedSize box = tester.renderObject(find.byType(AnimatedSize));
+        expect(box.size.width, 100.0);
+        expect(box.size.height, 100.0);
+        expect(box.state, RenderAnimatedSizeState.stable);
+        expect(box.isAnimating, false);
+        await tester.pump(const Duration(milliseconds: 10));
+      }
+    });
+  });
 }