Organize sky/framework/animation

sky/framework/animation/animation.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 'curves.dart';
+import 'dart:async';
+import 'generators.dart';
+
+class Animation {
+  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;
+    });
+  }
+}