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();
   }
 
@@ -121,29 +162,25 @@ 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;
-
-        _lastValue = 0.0;
-        _controller.forward(from: 0.0);
-
-        size = constraints.constrain(_animatedSize);
-      }
-    } else {
-      _didChangeTargetSizeLastFrame = false;
 
-      size = constraints.constrain(_animatedSize);
+    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 ||
@@ -151,6 +188,69 @@ class RenderAnimatedSize extends RenderAligningShiftedBox {
       _hasVisualOverflow = true;
   }
 
+  void _restartAnimation() {
+    _lastValue = 0.0;
+    _controller.forward(from: 0.0);
+  }
+
+  /// 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;
+  }
+
+  /// 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;
+    }
+  }
+
+  /// 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;
+    }
+  }
+
+  /// 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
   void paint(PaintingContext context, Offset offset) {
     if (child != null && _hasVisualOverflow) {