Break dependency of AnimationPerformance on AnimatedValue

sky/sdk/lib/widgets/animated_component.dart

Index: sky/sdk/lib/widgets/animated_component.dart
diff --git a/sky/sdk/lib/widgets/animated_component.dart b/sky/sdk/lib/widgets/animated_component.dart
index af088f846434d415523be11d13d2aaef0ef8bb14..526c2ba6a5ef1c7bcd6123162a68a664c4562a87 100644
--- a/sky/sdk/lib/widgets/animated_component.dart
+++ b/sky/sdk/lib/widgets/animated_component.dart
@@ -23,9 +23,10 @@ abstract class AnimatedComponent extends StatefulComponent {
 
   void syncFields(AnimatedComponent source) { }
 
-  List<_AnimationEntry> _animatedFields = new List<_AnimationEntry>();
+  final List<_AnimationEntry> _animatedFields = new List<_AnimationEntry>();
+  final List<AnimationPerformance> _watchedPerformances = new List<AnimationPerformance>();
 
-  watch(AnimatedValue value) {
+  void watch(AnimatedValue value) {
     assert(!mounted);
     // TODO(ianh): we really should assert that we're not doing this
     // in the constructor since doing it there is pointless and
@@ -35,12 +36,22 @@ abstract class AnimatedComponent extends StatefulComponent {
     _animatedFields.add(new _AnimationEntry(value));
   }
 
+  void watchPerformance(AnimationPerformance performance) {
+    assert(!mounted);
+    assert(!_watchedPerformances.contains(performance));
+    _watchedPerformances.add(performance);
+  }
+
   void didMount() {
     for (_AnimationEntry entry in _animatedFields) {
       entry.subscription = entry.value.onValueChanged.listen((_) {
         scheduleBuild();
       });
     }
+
+    for (AnimationPerformance performance in _watchedPerformances)
+      performance.addListener(scheduleBuild);
+
     super.didMount();
   }
 
@@ -50,6 +61,10 @@ abstract class AnimatedComponent extends StatefulComponent {
       entry.subscription.cancel();
       entry.subscription = null;
     }
+
+    for (AnimationPerformance performance in _watchedPerformances)
+      performance.removeListener(scheduleBuild);
+
     super.didUnmount();
   }