[Effen] Add AnimatedComponent base class

sky/framework/animation/animated_value.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 '../fn.dart';
 import 'curves.dart';
 import 'dart:async';
 import 'generators.dart';
 
 class AnimatedValue {
   StreamController _controller = new StreamController(sync: true);
   AnimationGenerator _animation;
   double _value;
 
   AnimatedValue(double initial) {
-    value = initial;
+    _value = initial;

[rafaelw, 2015/03/20 05:36:00]

Note: this prevents the initial value from being broadcast in the stream.
Consumers are expected to retrieve the initial value via the accessor.

[abarth-chromium, 2015/03/20 05:41:49]

Maybe add a comment explaining this subtle point?

[rafaelw, 2015/03/20 05:52:31]

Done.

   }
 
   Stream<double> get onValueChanged => _controller.stream;
 
   double get value => _value;
 
   void set value(double value) {
     stop();
    _setValue(value);
   }
@@ skipping 21 matching lines @@
         begin: _value,
         end: newValue,
         curve: curve,
         initialDelay: initialDelay);
 
     _animation.onTick.listen(_setValue, onDone: () {
       _animation = null;
     });
   }
 }
-
-class AnimatedValueListener {
-  final Component _component;
-  final AnimatedValue _value;
-  StreamSubscription<double> _subscription;
-
-  AnimatedValueListener(this._component, this._value);
-
-  double get value => _value == null ? null : _value.value;
-
-  void ensureListening() {
-    if (_subscription != null || _value == null)
-      return;
-    _subscription = _value.onValueChanged.listen((_) {
-      _component.scheduleBuild();
-    });
-  }
-
-  void stopListening() {
-    if (_subscription == null)
-      return;
-    _subscription.cancel();
-    _subscription = null;
-  }
-}