Begin work on the PopupMenu entrance animation

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 Animation {
+class AnimatedValue {
+  StreamController _controller = new StreamController(sync: true);
+  AnimationGenerator _animation;
+  double _value;
+
+  AnimatedValue(double initial) {
+    value = initial;
+  }
+
   Stream<double> get onValueChanged => _controller.stream;
 
   double get value => _value;
 
   void set value(double value) {
     stop();
    _setValue(value);
   }
 
   bool get isAnimating => _animation != null;
 
-  StreamController _controller = new StreamController(sync: true);
-
-  AnimationGenerator _animation;
-
-  double _value;
-
   void _setValue(double value) {
     _value = value;
     _controller.add(_value);
   }
 
   void stop() {
     if (_animation != null) {
       _animation.cancel();
       _animation = null;
     }
   }
 
   void animateTo(double newValue, double duration,
       { Curve curve: linear, double initialDelay: 0.0 }) {
     stop();
 
     _animation = new AnimationGenerator(
         duration: duration,
         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;
+  }
+}