Add initialDelay to AnimationGenerator and add Animation class

sky/framework/animation/generator.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 'dart:math' as math;
 import 'dart:sky' as sky;
 
 class FrameGenerator {
@@ skipping 35 matching lines @@
     _animationId = 0;
     _controller.add(timeStamp);
     if (!_cancelled) {
       _scheduleTick();
     }
   }
 }
 
 class AnimationGenerator extends FrameGenerator {
   Stream<double> get onTick => _stream;
+  final double initialDelay;
   final double duration;
   final double begin;
   final double end;
   final Curve curve;
   Stream<double> _stream;
   bool _done = false;
 
   AnimationGenerator(this.duration, {
+    this.initialDelay: 0.0,
     this.begin: 0.0,
     this.end: 1.0,
     this.curve: linear,
     Function onDone
   }):super(onDone: onDone) {
     assert(duration > 0);
     double startTime = 0.0;
     _stream = super.onTick.map((timeStamp) {
       if (startTime == 0.0)
         startTime = timeStamp;
-      return math.min((timeStamp - startTime) / duration, 1.0);
+
+      double t = (timeStamp - (startTime + initialDelay)) / duration;
+      return math.max(0.0, math.min(t, 1.0));
     })
-    .takeWhile(_checkForCompletion)
+    .takeWhile(_checkForCompletion) //
+    .where((t) => t >= 0.0)
     .map(_transform);
   }
 
   double _transform(double t) {
     if (_done)
       return end;
     return begin + (end - begin) * curve.transform(t);
   }
 
+  // This is required because Dart Streams don't have takeUntil (inclusive).
   bool _checkForCompletion(double t) {
     if (_done)
       return false;
+
     _done = t >= 1;
     return true;
   }
 }
+
+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, begin: _value, end: newValue,
+        curve: curve, initialDelay: initialDelay);
+
+    _animation.onTick.listen(_setValue, onDone: () {
+      _animation = null;
+    });
+  }
+}