Animate the snack bar in the stocks app.

sky/sdk/lib/widgets/animated_component.dart

 // Copyright 2015 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 'dart:async';
 
 import '../animation/animated_value.dart';
 import '../animation/animation_performance.dart';
 import 'basic.dart';
 
 class _AnimationEntry {
   _AnimationEntry(this.value);
   final AnimatedValue value;
   StreamSubscription<double> subscription;
 }
 
 abstract class AnimatedComponent extends StatefulComponent {
 
   AnimatedComponent({ String key }) : super(key: key);
 
   void syncFields(AnimatedComponent source) { }
 
   final List<_AnimationEntry> _animatedFields = new List<_AnimationEntry>();
   final List<AnimationPerformance> _watchedPerformances = new List<AnimationPerformance>();
 
   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
     // expensive, since we'll be doing it for every copy of the object
     // even though only the first one will use it (since we're
     // stateful, the others will all be discarded early).
+    assert(!mounted);
     _animatedFields.add(new _AnimationEntry(value));
   }
 
   void watchPerformance(AnimationPerformance performance) {
-    assert(!mounted);
     assert(!_watchedPerformances.contains(performance));
     _watchedPerformances.add(performance);
+    if (mounted)
+      performance.addListener(scheduleBuild);
   }
 
   void didMount() {
     for (_AnimationEntry entry in _animatedFields) {
       entry.subscription = entry.value.onValueChanged.listen((_) {
         scheduleBuild();
       });
     }
 
     for (AnimationPerformance performance in _watchedPerformances)
       performance.addListener(scheduleBuild);
 
     super.didMount();
   }
 
   void didUnmount() {
     for (_AnimationEntry entry in _animatedFields) {
       assert(entry.subscription != null);
       entry.subscription.cancel();
       entry.subscription = null;
     }
 
     for (AnimationPerformance performance in _watchedPerformances)
       performance.removeListener(scheduleBuild);
 
     super.didUnmount();
   }
 
 }