Add initialDelay to AnimationGenerator and add Animation class

sky/framework/animation/generator.dart

Index: sky/framework/animation/generator.dart
diff --git a/sky/framework/animation/generator.dart b/sky/framework/animation/generator.dart
index b24c46227dff4e88f9f8ec82c97aaa2a6b70b85e..65c373f548a7029301c7b55d5f3ece344a7b6236 100644
--- a/sky/framework/animation/generator.dart
+++ b/sky/framework/animation/generator.dart
@@ -53,6 +53,7 @@ class FrameGenerator {
 
 class AnimationGenerator extends FrameGenerator {
   Stream<double> get onTick => _stream;
+  final double initialDelay;
   final double duration;
   final double begin;
   final double end;
@@ -61,6 +62,7 @@ class AnimationGenerator extends FrameGenerator {
   bool _done = false;
 
   AnimationGenerator(this.duration, {
+    this.initialDelay: 0.0,
     this.begin: 0.0,
     this.end: 1.0,
     this.curve: linear,
@@ -71,9 +73,12 @@ class AnimationGenerator extends FrameGenerator {
     _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);
   }
 
@@ -83,10 +88,55 @@ class AnimationGenerator extends FrameGenerator {
     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;
+    });
+  }
+}