Require that you pass transitions a performance.

This fixes #1103.

examples/stocks/lib/stock_home.dart

@@ -251,13 +251,13 @@ class StockHome extends StatefulComponent {
     });
   }
 
-  Anchor _snackBarAnchor = new Anchor();
+  GlobalKey snackBarKey = new GlobalKey(label: 'snackbar');
   Widget buildSnackBar() {
     if (_snackBarStatus == AnimationStatus.dismissed)
       return null;
     return new SnackBar(
+      transitionKey: snackBarKey,
       showing: _isSnackBarShowing,
-      anchor: _snackBarAnchor,
       content: new Text("Stock purchased!"),
       actions: [new SnackBarAction(label: "UNDO", onPressed: _handleUndo)],
       onDismissed: () { setState(() { _snackBarStatus = AnimationStatus.dismissed; }); }
@@ -272,12 +272,14 @@ class StockHome extends StatefulComponent {
   }
 
   Widget buildFloatingActionButton() {
-    return _snackBarAnchor.build(
-      new FloatingActionButton(
+    return new TransitionProxy(
+      transitionKey: snackBarKey,
+      child: new FloatingActionButton(
         child: new Icon(type: 'content/add', size: 24),
         backgroundColor: Colors.redAccent[200],
         onPressed: _handleStockPurchased
-      ));
+      )
+    );
   }
 
   void addMenuToOverlays(List<Widget> overlays) {

examples/stocks/lib/stock_list.dart

@@ -15,7 +15,7 @@ class Stocklist extends Component {
       child: new ScrollableList<Stock>(
         items: stocks,
         itemExtent: StockRow.kHeight,
-        itemBuilder: (stock) => new StockRow(stock: stock)
+        itemBuilder: (Stock stock) => new StockRow(stock: stock)
       )
     );
   }

examples/widgets/progress_indicator.dart

@@ -22,21 +22,28 @@ class ProgressIndicatorApp extends App {
         reverseCurve: ease,
         interval: new Interval(0.0, 0.9)
       );
+    valueAnimation.addStatusListener((AnimationStatus status) {
+      if (status == AnimationStatus.dismissed || status == AnimationStatus.completed)
+        reverseValueAnimationDirection();
+    });
+    valueAnimation.play(valueAnimationDirection);
   }
 
   void handleTap() {
-    if (valueAnimation.isAnimating)
-      valueAnimation.stop();
-    else
-      valueAnimation.resume();
+    setState(() {
+      // valueAnimation.isAnimating is part of our build state
+      if (valueAnimation.isAnimating)
+        valueAnimation.stop();
+      else
+        valueAnimation.resume();
+    });
   }
 
   void reverseValueAnimationDirection() {
-    setState(() {
-      valueAnimationDirection = (valueAnimationDirection == Direction.forward)
-        ? Direction.reverse
-        : Direction.forward;
-    });
+    valueAnimationDirection = (valueAnimationDirection == Direction.forward)
+      ? Direction.reverse
+      : Direction.forward;
+    valueAnimation.play(valueAnimationDirection);
   }
 
   Widget buildIndicators() {
@@ -58,11 +65,12 @@ class ProgressIndicatorApp extends App {
           width: 50.0,
           height: 30.0,
           child: new CircularProgressIndicator(value: valueAnimation.value)
-        )
+        ),
+        new Text("${(valueAnimation.value * 100.0).toStringAsFixed(1)}%" + (valueAnimation.isAnimating ? '' : ' (paused)'))
     ];
     return new Column(
       indicators
-        .map((c) => new Container(child: c, margin: const EdgeDims.symmetric(vertical: 20.0)))
+        .map((c) => new Container(child: c, margin: const EdgeDims.symmetric(vertical: 15.0, horizontal: 20.0)))
         .toList(),
       justifyContent: FlexJustifyContent.center
     );
@@ -76,10 +84,7 @@ class ProgressIndicatorApp extends App {
         decoration: new BoxDecoration(backgroundColor: Theme.of(this).cardColor),
         child: new BuilderTransition(
           variables: [valueAnimation.variable],
-          direction: valueAnimationDirection,
-          performance: valueAnimation,
-          onDismissed: reverseValueAnimationDirection,
-          onCompleted: reverseValueAnimationDirection,
+          performance: valueAnimation.view,
           builder: buildIndicators
         )
       )
@@ -94,10 +99,13 @@ class ProgressIndicatorApp extends App {
           accentColor: Colors.redAccent[200]
         ),
         child: new Title(
-          title: 'Cards',
+          title: 'Progress Indicators',
           child: new Scaffold(
             toolbar: new ToolBar(center: new Text('Progress Indicators')),
-            body: body
+            body: new DefaultTextStyle(
+              style: Theme.of(this).text.title,
+              child: body
+            )
           )
         )
       )

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

@@ -15,7 +15,10 @@ enum Direction {
   reverse
 }
 
-/// A variable that changes as an animation progresses
+/// An interface describing a variable that changes as an animation progresses.
+/// 
+/// AnimatedVariables, by convention, must be cheap to create. This allows them to be used in 
+/// build functions in Widgets.
 abstract class AnimatedVariable {
   /// Update the variable to a given time in an animation that is running in the given direction
   void setProgress(double t, Direction direction);

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

@@ -23,6 +23,26 @@ enum AnimationStatus {
   completed,
 }
 
+typedef void AnimationPerformanceListener();
+typedef void AnimationPerformanceStatusListener(AnimationStatus status);
+
+/// An interface that is implemented by [AnimationPerformance] that exposes a
+/// read-only view of the underlying performance. This is used by classes that
+/// want to watch a performance but should not be able to change the
+/// performance's state.
+abstract class WatchableAnimationPerformance {
+  /// Update the given variable according to the current progress of the performance
+  void updateVariable(AnimatedVariable variable);
+  /// Calls the listener every time the progress of the performance changes
+  void addListener(AnimationPerformanceListener listener);
+  /// Stop calling the listener every time the progress of the performance changes
+  void removeListener(AnimationPerformanceListener listener);
+  /// Calls listener every time the status of the performance changes
+  void addStatusListener(AnimationPerformanceStatusListener listener);
+  /// Stops calling the listener every time the status of the performance changes
+  void removeStatusListener(AnimationPerformanceStatusListener listener);
+}
+
 /// A collection of values that animated based on a timeline
 ///
 /// For example, a performance may handle an animation of a menu opening by
@@ -31,12 +51,17 @@ enum AnimationStatus {
 /// may also take direct control of the timeline by manipulating [progress], or
 /// [fling] the timeline causing a physics-based simulation to take over the
 /// progression.
-class AnimationPerformance {
+class AnimationPerformance implements WatchableAnimationPerformance {
   AnimationPerformance({ AnimatedVariable variable, this.duration }) :
     _variable = variable {
     _timeline = new Timeline(_tick);
   }
 
+  /// Returns a [WatchableAnimationPerformance] for this performance,
+  /// so that a pointer to this object can be passed around without
+  /// allowing users of that pointer to mutate the AnimationPerformance state.
+  WatchableAnimationPerformance get view => this;
+
   /// The length of time this performance should last
   Duration duration;
 
@@ -155,33 +180,33 @@ class AnimationPerformance {
     return _timeline.fling(force.release(progress, velocity));
   }
 
-  final List<Function> _listeners = new List<Function>();
+  final List<AnimationPerformanceListener> _listeners = new List<AnimationPerformanceListener>();
 
   /// Calls the listener every time the progress of this performance changes
-  void addListener(Function listener) {
+  void addListener(AnimationPerformanceListener listener) {
     _listeners.add(listener);
   }
 
   /// Stop calling the listener every time the progress of this performance changes
-  void removeListener(Function listener) {
+  void removeListener(AnimationPerformanceListener listener) {
     _listeners.remove(listener);
   }
 
   void _notifyListeners() {
-    List<Function> localListeners = new List<Function>.from(_listeners);
-    for (Function listener in localListeners)
+    List<AnimationPerformanceListener> localListeners = new List<AnimationPerformanceListener>.from(_listeners);
+    for (AnimationPerformanceListener listener in localListeners)
       listener();
   }
 
-  final List<Function> _statusListeners = new List<Function>();
+  final List<AnimationPerformanceStatusListener> _statusListeners = new List<AnimationPerformanceStatusListener>();
 
   /// Calls listener every time the status of this performance changes
-  void addStatusListener(Function listener) {
+  void addStatusListener(AnimationPerformanceStatusListener listener) {
     _statusListeners.add(listener);
   }
 
   /// Stops calling the listener every time the status of this performance changes
-  void removeStatusListener(Function listener) {
+  void removeStatusListener(AnimationPerformanceStatusListener listener) {
     _statusListeners.remove(listener);
   }
 
@@ -189,8 +214,8 @@ class AnimationPerformance {
   void _checkStatusChanged() {
     AnimationStatus currentStatus = status;
     if (currentStatus != _lastStatus) {
-      List<Function> localListeners = new List<Function>.from(_statusListeners);
-      for (Function listener in localListeners)
+      List<AnimationPerformanceStatusListener> localListeners = new List<AnimationPerformanceStatusListener>.from(_statusListeners);
+      for (AnimationPerformanceStatusListener listener in localListeners)
         listener(currentStatus);
     }
     _lastStatus = currentStatus;

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

@@ -7,47 +7,49 @@ import 'package:sky/src/widgets/framework.dart';
 
 abstract class AnimatedComponent extends StatefulComponent {
 
-  AnimatedComponent({ Key key }) : super(key: key);
-
-  void syncConstructorArguments(AnimatedComponent source) { }
-
-  final List<AnimationPerformance> _watchedPerformances = new List<AnimationPerformance>();
-
-  void _performanceChanged() {
-    setState(() {
-      // We don't actually have any state to change, per se,
-      // we just know that we have in fact changed state.
-    });
+  AnimatedComponent({ Key key, this.direction, this.duration }) : super(key: key);
+
+  Duration duration;
+  Direction direction;
+
+  void syncConstructorArguments(AnimatedComponent source) {
+    bool resumePerformance = false;
+    if (duration != source.duration) {
+      duration = source.duration;
+      resumePerformance = true;
+    }
+    if (direction != source.direction) {
+      direction = source.direction;
+      resumePerformance = true;
+    }
+    if (resumePerformance)
+      performance.play(direction);
   }
 
-  bool isWatching(AnimationPerformance performance) {
-    return _watchedPerformances.contains(performance);
-  }
+  AnimationPerformance get performance => _performance;
+  AnimationPerformance _performance;
 
-  void watch(AnimationPerformance performance) {
-    assert(!isWatching(performance));
-    _watchedPerformances.add(performance);
-    if (mounted)
-      performance.addListener(_performanceChanged);
-  }
-
-  void unwatch(AnimationPerformance performance) {
-    assert(isWatching(performance));
-    _watchedPerformances.remove(performance);
-    if (mounted)
-      performance.removeListener(_performanceChanged);
-  }
-
-  void didMount() {
-    for (AnimationPerformance performance in _watchedPerformances)
-      performance.addListener(_performanceChanged);
-    super.didMount();
+  void initState() {
+    _performance = new AnimationPerformance(duration: duration);
+    performance.addStatusListener((AnimationStatus status) {
+      if (status == AnimationStatus.completed)
+        handleCompleted();
+      else if (status == AnimationStatus.dismissed)
+        handleDismissed();
+    });
+    if (buildDependsOnPerformance) {
+      performance.addListener(() {
+        setState(() {
+          // We don't actually have any state to change, per se,
+          // we just know that we have in fact changed state.
+        });
+      });
+    }
+    performance.play(direction);
   }
 
-  void didUnmount() {
-    for (AnimationPerformance performance in _watchedPerformances)
-      performance.removeListener(_performanceChanged);
-    super.didUnmount();
-  }
+  bool get buildDependsOnPerformance => false;
+  void handleCompleted() { }
+  void handleDismissed() { }
 
 }

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

@@ -131,6 +131,8 @@ class Dialog extends Component {
   }
 }
 
+const Duration _kTransitionDuration = const Duration(milliseconds: 150);
+
 class DialogRoute extends RouteBase {
   DialogRoute({ this.completer, this.builder });
 
@@ -144,28 +146,10 @@ class DialogRoute extends RouteBase {
     completer.complete(result);
   }
 
-  TransitionBase buildTransition({ Key key }) => new DialogTransition(key: key);
-}
-
-const Duration _kTransitionDuration = const Duration(milliseconds: 150);
-class DialogTransition extends TransitionBase {
-  DialogTransition({
-    Key key,
-    Widget child,
-    Direction direction,
-    Function onDismissed,
-    Function onCompleted
-  }): super(key: key,
-            child: child,
-            duration: _kTransitionDuration,
-            direction: direction,
-            onDismissed: onDismissed,
-            onCompleted: onCompleted);
-
-  Widget buildWithChild(Widget child) {
+  Duration get transitionDuration => _kTransitionDuration;
+  TransitionBase buildTransition({ Key key, Widget child, WatchableAnimationPerformance performance }) {
     return new FadeTransition(
       performance: performance,
-      direction: direction,
       opacity: new AnimatedValue<double>(0.0, end: 1.0, curve: easeOut),
       child: child
     );

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

@@ -54,6 +54,10 @@ class Dismissable extends StatefulComponent {
 
   void initState() {
     _fadePerformance = new AnimationPerformance(duration: _kCardDismissFadeout);
+    _fadePerformance.addStatusListener((AnimationStatus status) {
+      if (status == AnimationStatus.completed)
+        _handleFadeCompleted();
+    });
   }
 
   void syncConstructorArguments(Dismissable source) {
@@ -99,6 +103,7 @@ class Dismissable extends StatefulComponent {
       _resizePerformance = new AnimationPerformance()
         ..duration = _kCardDismissResize
         ..addListener(_handleResizeProgressChanged);
+      _resizePerformance.play();
     });
   }
 
@@ -226,8 +231,7 @@ class Dismissable extends StatefulComponent {
       );
 
       return new SquashTransition(
-        performance: _resizePerformance,
-        direction: Direction.forward,
+        performance: _resizePerformance.view,
         width: _directionIsYAxis ? squashAxisExtent : null,
         height: !_directionIsYAxis ? squashAxisExtent : null
       );
@@ -243,11 +247,10 @@ class Dismissable extends StatefulComponent {
       child: new SizeObserver(
         callback: _handleSizeChanged,
         child: new FadeTransition(
-          performance: _fadePerformance,
-          onCompleted: _handleFadeCompleted,
+          performance: _fadePerformance.view,
           opacity: new AnimatedValue<double>(1.0, end: 0.0),
           child: new SlideTransition(
-            performance: _fadePerformance,
+            performance: _fadePerformance.view,
             position: new AnimatedValue<Point>(Point.origin, end: _activeCardDragEndPoint),
             child: child
           )

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

@@ -58,32 +58,41 @@ class Drawer extends StatefulComponent {
 
   void initState() {
     _performance = new AnimationPerformance(duration: _kBaseSettleDuration);
-
+    _performance.addStatusListener((AnimationStatus status) {
+      if (status == AnimationStatus.dismissed)
+        _handleDismissed();
+    });
     // Use a spring force for animating the drawer. We can't use curves for
     // this because we need a linear curve in order to track the user's finger
     // while dragging.
     _performance.attachedForce = kDefaultSpringForce;
-
     if (navigator != null) {
+      // TODO(ianh): This is crazy. We should convert drawer to use a pattern like openDialog().
+      // https://github.com/domokit/sky_engine/pull/1186
       scheduleMicrotask(() {
         navigator.pushState(this, (_) => _performance.reverse());
       });
     }
+    _performance.play(_direction);
   }
 
+  Direction get _direction => showing ? Direction.forward : Direction.reverse;
+
   void syncConstructorArguments(Drawer source) {
     children = source.children;
+    if (showing != source.showing) {
+      showing = source.showing;
+      _performance.play(_direction);
+    }
     level = source.level;
-    navigator = source.navigator;
-    showing = source.showing;
     onDismissed = source.onDismissed;
+    navigator = source.navigator;
   }
 
   Widget build() {
     var mask = new GestureDetector(
       child: new ColorTransition(
-        performance: _performance,
-        direction: showing ? Direction.forward : Direction.reverse,
+        performance: _performance.view,
         color: new AnimatedColorValue(Colors.transparent, end: const Color(0x7F000000)),
         child: new Container()
       ),
@@ -93,10 +102,8 @@ class Drawer extends StatefulComponent {
     );
 
     Widget content = new SlideTransition(
-      performance: _performance,
-      direction: showing ? Direction.forward : Direction.reverse,
+      performance: _performance.view,
       position: new AnimatedValue<Point>(_kClosedPosition, end: _kOpenPosition),
-      onDismissed: _onDismissed,
       child: new AnimatedContainer(
         behavior: implicitlyAnimate(const Duration(milliseconds: 200)),
         decoration: new BoxDecoration(
@@ -115,7 +122,7 @@ class Drawer extends StatefulComponent {
     );
   }
 
-  void _onDismissed() {
+  void _handleDismissed() {
     if (navigator != null &&
         navigator.currentRoute is RouteState &&
         (navigator.currentRoute as RouteState).owner == this) // TODO(ianh): remove cast once analyzer is cleverer

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

@@ -13,12 +13,14 @@ abstract class GlobalKeyWatcher extends StatefulComponent {
   GlobalKey watchedKey;
 
   void syncConstructorArguments(GlobalKeyWatcher source) {
-    if (source != source.watchedKey) {
-      _removeListeners();
+    if (watchedKey != source.watchedKey) {
+      if (watchedKey != null)
+        _removeListeners();
       watchedKey = source.watchedKey;
-      if (mounted)
+      if (mounted && watchedKey != null) {
         _setWatchedWidget(GlobalKey.getWidget(watchedKey));
-      _addListeners();
+        _addListeners();
+      }
     }
   }
 
@@ -29,7 +31,7 @@ abstract class GlobalKeyWatcher extends StatefulComponent {
     if (watchedWidget != value) {
       if (watchedWidget != null)
         stopWatching();
-      assert(debugValidateWatchedWidget(value));
+      assert(value == null || debugValidateWatchedWidget(value));
       setState(() {
         _watchedWidget = value;
       });
@@ -42,13 +44,16 @@ abstract class GlobalKeyWatcher extends StatefulComponent {
 
   void didMount() {
     super.didMount();
-    _setWatchedWidget(GlobalKey.getWidget(watchedKey));
-    _addListeners();
+    if (watchedKey != null) {
+      _setWatchedWidget(GlobalKey.getWidget(watchedKey));
+      _addListeners();
+    }
   }
 
   void didUnmount() {
     super.didUnmount();
-    _removeListeners();
+    if (watchedKey != null)
+      _removeListeners();
     _setWatchedWidget(null);
   }
 

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

@@ -10,13 +10,49 @@ import 'package:sky/src/widgets/transitions.dart';
 
 typedef Widget RouteBuilder(Navigator navigator, RouteBase route);
 
+typedef void NotificationCallback();
+
 abstract class RouteBase {
   Widget build(Navigator navigator, RouteBase route);
   bool get isOpaque;
   void popState([dynamic result]) { assert(result == null); }
-  TransitionBase buildTransition({ Key key });
+
+  AnimationPerformance _performance;
+  NotificationCallback onDismissed;
+  NotificationCallback onCompleted;
+  WatchableAnimationPerformance ensurePerformance({ Direction direction }) {
+    assert(direction != null);
+    if (_performance == null) {
+      _performance = new AnimationPerformance(duration: transitionDuration);
+      _performance.addStatusListener((AnimationStatus status) {
+        switch (status) {
+          case AnimationStatus.dismissed:
+            if (onDismissed != null)
+              onDismissed();
+            break;
+          case AnimationStatus.completed:
+            if (onCompleted != null)
+              onCompleted();
+            break;
+          default:
+            ;
+        }
+      });
+    }
+    AnimationStatus desiredStatus = direction == Direction.forward ? AnimationStatus.forward : AnimationStatus.reverse;
+    if (_performance.status != desiredStatus)
+      _performance.play(direction);
+    return _performance.view;
+  }
+
+  Duration get transitionDuration;
+  TransitionBase buildTransition({ Key key, Widget child, WatchableAnimationPerformance performance });
+
+  String toString() => '$runtimeType()';
 }
 
+const Duration _kTransitionDuration = const Duration(milliseconds: 150);
+const Point _kTransitionStartPoint = const Point(0.0, 75.0);
 class Route extends RouteBase {
   Route({ this.name, this.builder });
 
@@ -25,7 +61,24 @@ class Route extends RouteBase {
 
   Widget build(Navigator navigator, RouteBase route) => builder(navigator, route);
   bool get isOpaque => true;
-  TransitionBase buildTransition({ Key key }) => new SlideUpFadeTransition(key: key);
+
+  Duration get transitionDuration => _kTransitionDuration;
+  TransitionBase buildTransition({ Key key, Widget child, WatchableAnimationPerformance performance }) {
+    // TODO(jackson): Hit testing should ignore transform
+    // TODO(jackson): Block input unless content is interactive
+    return new SlideTransition(
+      key: key,
+      performance: performance,
+      position: new AnimatedValue<Point>(_kTransitionStartPoint, end: Point.origin, curve: easeOut),
+      child: new FadeTransition(
+        performance: performance,
+        opacity: new AnimatedValue<double>(0.0, end: 1.0, curve: easeOut),
+        child: child
+      )
+    );
+  }
+
+  String toString() => '$runtimeType(name="$name")';
 }
 
 class RouteState extends RouteBase {
@@ -44,44 +97,14 @@ class RouteState extends RouteBase {
       callback(this);
   }
 
-  TransitionBase buildTransition({ Key key }) {
-    // Custom state routes shouldn't be asked to construct a transition
+  // Custom state routes shouldn't be asked to construct a transition
+  Duration get transitionDuration {
     assert(false);
-    return null;
+    return const Duration();
   }
-}
-
-// TODO(jackson): Refactor this into its own file
-const Duration _kTransitionDuration = const Duration(milliseconds: 150);
-const Point _kTransitionStartPoint = const Point(0.0, 75.0);
-class SlideUpFadeTransition extends TransitionBase {
-  SlideUpFadeTransition({
-    Key key,
-    Widget child,
-    Direction direction,
-    Function onDismissed,
-    Function onCompleted
-  }): super(key: key,
-            child: child,
-            duration: _kTransitionDuration,
-            direction: direction,
-            onDismissed: onDismissed,
-            onCompleted: onCompleted);
-
-  Widget buildWithChild(Widget child) {
-    // TODO(jackson): Hit testing should ignore transform
-    // TODO(jackson): Block input unless content is interactive
-    return new SlideTransition(
-      performance: performance,
-      direction: direction,
-      position: new AnimatedValue<Point>(_kTransitionStartPoint, end: Point.origin, curve: easeOut),
-      child: new FadeTransition(
-        performance: performance,
-        direction: direction,
-        opacity: new AnimatedValue<double>(0.0, end: 1.0, curve: easeOut),
-        child: child
-      )
-    );
+  TransitionBase buildTransition({ Key key, Widget child, WatchableAnimationPerformance performance }) {
+    assert(false);
+    return null;
   }
 }
 
@@ -90,6 +113,7 @@ class HistoryEntry {
   final RouteBase route;
   bool fullyOpaque = false;
   // TODO(jackson): Keep track of the requested transition
+  String toString() => "HistoryEntry($route, hashCode=$hashCode)";
 }
 
 class NavigationState {
@@ -116,6 +140,7 @@ class NavigationState {
   }
 
   void push(RouteBase route) {
+    assert(!_debugCurrentlyHaveRoute(route));
     HistoryEntry historyEntry = new HistoryEntry(route: route);
     history.insert(historyIndex + 1, historyEntry);
     historyIndex++;
@@ -129,6 +154,10 @@ class NavigationState {
       historyIndex--;
     }
   }
+
+  bool _debugCurrentlyHaveRoute(RouteBase route) {
+    return history.any((entry) => entry.route == route);
+  }
 }
 
 class Navigator extends StatefulComponent {
@@ -184,19 +213,25 @@ class Navigator extends StatefulComponent {
       }
       if (child == null)
         continue;
-      TransitionBase transition = historyEntry.route.buildTransition(key: new ObjectKey(historyEntry))
-        ..child = child
-        ..direction = (i <= state.historyIndex) ? Direction.forward : Direction.reverse
-        ..onDismissed = () {
-          setState(() {
-            state.history.remove(historyEntry);
-          });
-        }
-        ..onCompleted = () {
-          setState(() {
-            historyEntry.fullyOpaque = historyEntry.route.isOpaque;
-          });
-        };
+      WatchableAnimationPerformance performance = historyEntry.route.ensurePerformance(
+        direction: (i <= state.historyIndex) ? Direction.forward : Direction.reverse
+      );
+      historyEntry.route.onDismissed = () {
+        setState(() {
+          assert(state.history.contains(historyEntry));
+          state.history.remove(historyEntry);
+        });
+      };
+      historyEntry.route.onCompleted = () {
+        setState(() {
+          historyEntry.fullyOpaque = historyEntry.route.isOpaque;
+        });
+      };
+      TransitionBase transition = historyEntry.route.buildTransition(
+        key: new ObjectKey(historyEntry),
+        child: child,
+        performance: performance
+      );
       visibleRoutes.add(transition);
     }
     return new Focus(child: new Stack(visibleRoutes));

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

@@ -45,10 +45,13 @@ class PopupMenu extends StatefulComponent {
   AnimationPerformance _performance;
 
   void initState() {
-    _performance = new AnimationPerformance()
-      ..duration = _kMenuDuration;
+    _performance = new AnimationPerformance(duration: _kMenuDuration);
     _performance.timing = new AnimationTiming()
-      ..reverseInterval = new Interval(0.0, _kMenuCloseIntervalEnd);
+                          ..reverseInterval = new Interval(0.0, _kMenuCloseIntervalEnd);
+    _performance.addStatusListener((AnimationStatus status) {
+      if (status == AnimationStatus.dismissed)
+        _handleDismissed();
+    });
     _updateBoxPainter();
 
     if (showing)
@@ -69,6 +72,7 @@ class PopupMenu extends StatefulComponent {
 
   void _open() {
     navigator.pushState(this, (_) => _close());
+    _performance.play();
   }
 
   void _close() {
@@ -82,7 +86,7 @@ class PopupMenu extends StatefulComponent {
       boxShadow: shadows[level]));
   }
 
-  void _onDismissed() {
+  void _handleDismissed() {
     if (navigator != null &&
         navigator.currentRoute is RouteState &&
         (navigator.currentRoute as RouteState).owner == this) // TODO(ianh): remove cast once analyzer is cleverer
@@ -100,24 +104,21 @@ class PopupMenu extends StatefulComponent {
       double start = (i + 1) * unit;
       double end = (start + 1.5 * unit).clamp(0.0, 1.0);
       children.add(new FadeTransition(
-        direction: showing ? Direction.forward : Direction.reverse,
-        performance: _performance,
+        performance: _performance.view,
         opacity: new AnimatedValue<double>(0.0, end: 1.0, interval: new Interval(start, end)),
-        child: items[i]));
+        child: items[i])
+      );
     }
 
     final width = new AnimatedValue<double>(0.0, end: 1.0, interval: new Interval(0.0, unit));
     final height = new AnimatedValue<double>(0.0, end: 1.0, interval: new Interval(0.0, unit * items.length));
     return new FadeTransition(
-      direction: showing ? Direction.forward : Direction.reverse,
-      performance: _performance,
-      onDismissed: _onDismissed,
+      performance: _performance.view,
       opacity: new AnimatedValue<double>(0.0, end: 1.0, interval: new Interval(0.0, 1.0 / 3.0)),
       child: new Container(
         margin: new EdgeDims.all(_kMenuMargin),
         child: new BuilderTransition(
-          direction: showing ? Direction.forward : Direction.reverse,
-          performance: _performance,
+          performance: _performance.view,
           variables: [width, height],
           builder: () {
             return new CustomPaint(

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

@@ -25,16 +25,21 @@ abstract class ProgressIndicator extends StatefulComponent {
   double value; // Null for non-determinate progress indicator.
   double bufferValue; // TODO(hansmuller) implement the support for this.
 
-  AnimationPerformance _animation;
-  double get _animationValue => (_animation.variable as AnimatedValue<double>).value;
+  AnimationPerformance _performance;
+  double get _performanceValue => (_performance.variable as AnimatedValue<double>).value;
   Color get _backgroundColor => Theme.of(this).primarySwatch[200];
   Color get _valueColor => Theme.of(this).primaryColor;
-  Object get _customPaintToken => value != null ? value : _animationValue;
+  Object get _customPaintToken => value != null ? value : _performanceValue;
 
   void initState() {
-    _animation = new AnimationPerformance()
+    _performance = new AnimationPerformance()
       ..duration = const Duration(milliseconds: 1500)
       ..variable = new AnimatedValue<double>(0.0, end: 1.0, curve: ease);
+    _performance.addStatusListener((AnimationStatus status) {
+      if (status == AnimationStatus.completed)
+        _restartAnimation();
+    });
+    _performance.play();
   }
 
   void syncConstructorArguments(ProgressIndicator source) {
@@ -43,8 +48,8 @@ abstract class ProgressIndicator extends StatefulComponent {
   }
 
   void _restartAnimation() {
-    _animation.progress = 0.0;
-    _animation.play();
+    _performance.progress = 0.0;
+    _performance.play();
   }
 
   Widget build() {
@@ -52,10 +57,8 @@ abstract class ProgressIndicator extends StatefulComponent {
       return _buildIndicator();
 
     return new BuilderTransition(
-      variables: [_animation.variable],
-      direction: Direction.forward,
-      performance: _animation,
-      onCompleted: _restartAnimation,
+      variables: [_performance.variable],
+      performance: _performance.view,
       builder: _buildIndicator
     );
   }
@@ -81,7 +84,7 @@ class LinearProgressIndicator extends ProgressIndicator {
       double width = value.clamp(0.0, 1.0) * size.width;
       canvas.drawRect(Point.origin & new Size(width, size.height), paint);
     } else {
-      double startX = size.width * (1.5 * _animationValue - 0.5);
+      double startX = size.width * (1.5 * _performanceValue - 0.5);
       double endX = startX + 0.5 * size.width;
       double x = startX.clamp(0.0, size.width);
       double width = endX.clamp(0.0, size.width) - x;
@@ -125,7 +128,7 @@ class CircularProgressIndicator extends ProgressIndicator {
         ..arcTo(Point.origin & size, _kStartAngle, angle, false);
       canvas.drawPath(path, paint);
     } else {
-      double startAngle = _kTwoPI * (1.75 * _animationValue - 0.75);
+      double startAngle = _kTwoPI * (1.75 * _performanceValue - 0.75);
       double endAngle = startAngle + _kTwoPI * 0.75;
       double arcAngle = startAngle.clamp(0.0, _kTwoPI);
       double arcSweep = endAngle.clamp(0.0, _kTwoPI) - arcAngle;

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

@@ -6,6 +6,7 @@
 import 'package:sky/animation.dart';
 import 'package:sky/painting.dart';
 import 'package:sky/material.dart';
+import 'package:sky/src/widgets/animated_component.dart';
 import 'package:sky/src/widgets/basic.dart';
 import 'package:sky/src/widgets/default_text_style.dart';
 import 'package:sky/src/widgets/framework.dart';
@@ -17,6 +18,7 @@ import 'package:sky/src/widgets/transitions.dart';
 typedef void SnackBarDismissedCallback();
 
 const Duration _kSlideInDuration = const Duration(milliseconds: 200);
+// TODO(ianh): factor out some of the constants below
 
 class SnackBarAction extends Component {
   SnackBarAction({Key key, this.label, this.onPressed }) : super(key: key) {
@@ -38,25 +40,37 @@ class SnackBarAction extends Component {
   }
 }
 
-class SnackBar extends Component {
+class SnackBar extends AnimatedComponent {
 
   SnackBar({
     Key key,
-    this.anchor,
+    this.transitionKey,
     this.content,
     this.actions,
-    this.showing,
+    bool showing,
     this.onDismissed
-  }) : super(key: key) {
+  }) : super(key: key, direction: showing ? Direction.forward : Direction.reverse, duration: _kSlideInDuration) {
     assert(content != null);
   }
 
-  Anchor anchor;
+  Key transitionKey;
   Widget content;
   List<SnackBarAction> actions;
-  bool showing;
   SnackBarDismissedCallback onDismissed;
 
+  void syncConstructorArguments(SnackBar source) {
+    transitionKey = source.transitionKey;
+    content = source.content;
+    actions = source.actions;
+    onDismissed = source.onDismissed;
+    super.syncConstructorArguments(source);
+  }
+
+  void handleDismissed() {
+    if (onDismissed != null)
+      onDismissed();
+  }
+
   Widget build() {
     List<Widget> children = [
       new Flexible(
@@ -71,15 +85,15 @@ class SnackBar extends Component {
     ];
     if (actions != null)
       children.addAll(actions);
-
     return new SlideTransition(
-      duration: _kSlideInDuration,
-      direction: showing ? Direction.forward : Direction.reverse,
-      position: new AnimatedValue<Point>(Point.origin,
-                                         end: const Point(0.0, -52.0),
-                                         curve: easeIn, reverseCurve: easeOut),
-      onDismissed: onDismissed,
-      anchor: anchor,
+      key: transitionKey,
+      performance: performance.view,
+      position: new AnimatedValue<Point>(
+        Point.origin,
+        end: const Point(0.0, -52.0),
+        curve: easeIn,
+        reverseCurve: easeOut
+      ),
       child: new Material(
         level: 2,
         color: const Color(0xFF323232),

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

@@ -542,8 +542,7 @@ class TabBar extends Scrollable {
         style: textStyle,
         child: new BuilderTransition(
           variables: [_indicatorRect],
-          direction: Direction.forward,
-          performance: _indicatorAnimation,
+          performance: _indicatorAnimation.view,
           builder: () {
             return new TabBarWrapper(
               children: tabs,

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

@@ -6,117 +6,107 @@ import 'package:sky/animation.dart';
 import 'package:sky/src/widgets/animated_component.dart';
 import 'package:sky/src/widgets/basic.dart';
 import 'package:sky/src/widgets/framework.dart';
+import 'package:sky/src/widgets/global_key_watcher.dart';
 import 'package:vector_math/vector_math.dart';
 
 export 'package:sky/animation.dart' show Direction;
 
-// A helper class to anchor widgets to one another. Pass an instance of this to
-// a Transition, then use the build() method to create a child with the same
-// transition applied.
-class Anchor {
-  Anchor();
+class TransitionProxy extends GlobalKeyWatcher {
 
-  TransitionBase transition;
-
-  Widget build(Widget child) {
-    return new _AnchorTransition(anchoredTo: this, child: child);
-  }
-}
-
-// Used with the Anchor class to apply a transition to multiple children.
-class _AnchorTransition extends AnimatedComponent {
-  _AnchorTransition({
+  TransitionProxy({
     Key key,
-    this.anchoredTo,
+    GlobalKey transitionKey,
     this.child
-  }) : super(key: key);
+  }) : super(key: key, watchedKey: transitionKey);
 
-  Anchor anchoredTo;
   Widget child;
-  TransitionBase get transition => anchoredTo.transition;
-
-  void initState() {
-    if (transition != null)
-      watch(transition.performance);
-  }
 
-  void syncConstructorArguments(_AnchorTransition source) {
-    if (transition != null && isWatching(transition.performance))
-      unwatch(transition.performance);
-    anchoredTo = source.anchoredTo;
-    if (transition != null)
-      watch(transition.performance);
+  void syncConstructorArguments(TransitionProxy source) {
     child = source.child;
     super.syncConstructorArguments(source);
   }
 
+  bool debugValidateWatchedWidget(Widget candidate) {
+    return candidate is TransitionBaseWithChild;
+  }
+
+  TransitionBaseWithChild get transition => this.watchedWidget;
+
+  void startWatching() {
+    transition.performance.addListener(_performanceChanged);
+  }
+
+  void stopWatching() {
+    transition.performance.removeListener(_performanceChanged);
+  }
+
+  void _performanceChanged() {
+    setState(() {
+      // The performance changed, so we probably need to ask the transition
+      // we're watching for a rebuild.
+    });
+  }
+
   Widget build() {
-    if (transition == null)
-      return child;
-    return transition.buildWithChild(child);
+    if (transition != null)
+      return transition.buildWithChild(child);
+    return child;
   }
+
 }
 
-abstract class TransitionBase extends AnimatedComponent {
+abstract class TransitionBase extends StatefulComponent {
+
   TransitionBase({
     Key key,
-    this.child,
-    this.anchor,
-    this.direction,
-    this.duration,
-    this.performance,
-    this.onDismissed,
-    this.onCompleted
-  }) : super(key: key);
+    this.performance
+  }) : super(key: key) {
+    assert(performance != null);
+  }
 
-  Widget child;
-  Anchor anchor;
-  Direction direction;
-  Duration duration;
-  AnimationPerformance performance;
-  Function onDismissed;
-  Function onCompleted;
-
-  void initState() {
-    if (anchor != null)
-      anchor.transition = this;
-
-    if (performance == null) {
-      assert(duration != null);
-      performance = new AnimationPerformance(duration: duration);
-      if (direction == Direction.reverse)
-        performance.progress = 1.0;
+  WatchableAnimationPerformance performance;
+
+  void syncConstructorArguments(TransitionBase source) {
+    if (performance != source.performance) {
+      if (mounted)
+        performance.removeListener(_performanceChanged);
+      performance = source.performance;
+      if (mounted)
+        performance.addListener(_performanceChanged);
     }
-    performance.addStatusListener(_checkStatusChanged);
+  }
 
-    watch(performance);
-    _start();
+  void _performanceChanged() {
+    setState(() {
+      // The performance's state is our build state, and it changed already.
+    });
   }
 
-  void syncConstructorArguments(TransitionBase source) {
-    child = source.child;
-    onCompleted = source.onCompleted;
-    onDismissed = source.onDismissed;
-    duration = source.duration;
-    if (direction != source.direction) {
-      direction = source.direction;
-      _start();
-    }
-    super.syncConstructorArguments(source);
+  void didMount() {
+    performance.addListener(_performanceChanged);
+    super.didMount();
   }
 
-  void _start() {
-    performance.play(direction);
+  void didUnmount() {
+    performance.removeListener(_performanceChanged);
+    super.didUnmount();
   }
 
-  void _checkStatusChanged(AnimationStatus status) {
-    if (performance.isDismissed) {
-      if (onDismissed != null)
-        onDismissed();
-    } else if (performance.isCompleted) {
-      if (onCompleted != null)
-        onCompleted();
-    }
+}
+
+abstract class TransitionBaseWithChild extends TransitionBase {
+
+  TransitionBaseWithChild({
+    Key key,
+    this.child,
+    WatchableAnimationPerformance performance
+  }) : super(key: key, performance: performance);
+
+  Widget child;
+
+  void syncConstructorArguments(TransitionBaseWithChild source) {
+    child = source.child;
+    super.syncConstructorArguments(source);
   }
 
   Widget build() {
@@ -124,26 +114,17 @@ abstract class TransitionBase extends AnimatedComponent {
   }
 
   Widget buildWithChild(Widget child);
+
 }
 
-class SlideTransition extends TransitionBase {
+class SlideTransition extends TransitionBaseWithChild {
   SlideTransition({
     Key key,
-    Anchor anchor,
     this.position,
-    Duration duration,
-    AnimationPerformance performance,
-    Direction direction,
-    Function onDismissed,
-    Function onCompleted,
+    WatchableAnimationPerformance performance,
     Widget child
   }) : super(key: key,
-             anchor: anchor,
-             duration: duration,
              performance: performance,
-             direction: direction,
-             onDismissed: onDismissed,
-             onCompleted: onCompleted,
              child: child);
 
   AnimatedValue<Point> position;
@@ -161,24 +142,14 @@ class SlideTransition extends TransitionBase {
   }
 }
 
-class FadeTransition extends TransitionBase {
+class FadeTransition extends TransitionBaseWithChild {
   FadeTransition({
     Key key,
-    Anchor anchor,
     this.opacity,
-    Duration duration,
-    AnimationPerformance performance,
-    Direction direction,
-    Function onDismissed,
-    Function onCompleted,
+    WatchableAnimationPerformance performance,
     Widget child
   }) : super(key: key,
-             anchor: anchor,
-             duration: duration,
              performance: performance,
-             direction: direction,
-             onDismissed: onDismissed,
-             onCompleted: onCompleted,
              child: child);
 
   AnimatedValue<double> opacity;
@@ -194,24 +165,14 @@ class FadeTransition extends TransitionBase {
   }
 }
 
-class ColorTransition extends TransitionBase {
+class ColorTransition extends TransitionBaseWithChild {
   ColorTransition({
     Key key,
-    Anchor anchor,
     this.color,
-    Duration duration,
-    AnimationPerformance performance,
-    Direction direction,
-    Function onDismissed,
-    Function onCompleted,
+    WatchableAnimationPerformance performance,
     Widget child
   }) : super(key: key,
-             anchor: anchor,
-             duration: duration,
              performance: performance,
-             direction: direction,
-             onDismissed: onDismissed,
-             onCompleted: onCompleted,
              child: child);
 
   AnimatedColorValue color;
@@ -230,25 +191,15 @@ class ColorTransition extends TransitionBase {
   }
 }
 
-class SquashTransition extends TransitionBase {
+class SquashTransition extends TransitionBaseWithChild {
   SquashTransition({
     Key key,
-    Anchor anchor,
     this.width,
     this.height,
-    Duration duration,
-    AnimationPerformance performance,
-    Direction direction,
-    Function onDismissed,
-    Function onCompleted,
+    WatchableAnimationPerformance performance,
     Widget child
   }) : super(key: key,
-             anchor: anchor,
-             duration: duration,
              performance: performance,
-             direction: direction,
-             onDismissed: onDismissed,
-             onCompleted: onCompleted,
              child: child);
 
   AnimatedValue<double> width;
@@ -274,23 +225,11 @@ typedef Widget BuilderFunction();
 class BuilderTransition extends TransitionBase {
   BuilderTransition({
     Key key,
-    Anchor anchor,
     this.variables,
     this.builder,
-    Duration duration,
-    AnimationPerformance performance,
-    Direction direction,
-    Function onDismissed,
-    Function onCompleted,
-    Widget child
+    WatchableAnimationPerformance performance
   }) : super(key: key,
-             anchor: anchor,
-             duration: duration,
-             performance: performance,
-             direction: direction,
-             onDismissed: onDismissed,
-             onCompleted: onCompleted,
-             child: child);
+             performance: performance);
 
   List<AnimatedValue> variables;
   BuilderFunction builder;
@@ -301,7 +240,7 @@ class BuilderTransition extends TransitionBase {
     super.syncConstructorArguments(source);
   }
 
-  Widget buildWithChild(Widget child) {
+  Widget build() {
     for (int i = 0; i < variables.length; ++i)
       performance.updateVariable(variables[i]);
     return builder();