Drive overscroll animations with a physics simulation

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 33 matching lines @@
 
   void _tick(double timeStamp) {
     _animationId = 0;
     _controller.add(timeStamp);
     if (!_cancelled) {
       _scheduleTick();
     }
   }
 }
 
-class AnimationGenerator extends FrameGenerator {
+class AnimationGenerator {
   Stream<double> get onTick => _stream;
   final double initialDelay;
   final double duration;
   final double begin;
   final double end;
   final Curve curve;
+
+  FrameGenerator _generator;
   Stream<double> _stream;
   bool _done = false;
 
   AnimationGenerator({
     this.initialDelay: 0.0,
     this.duration,
     this.begin: 0.0,
     this.end: 1.0,
     this.curve: linear,
     Function onDone
-  }):super(onDone: onDone) {
+  }) {
     assert(duration != null && duration > 0.0);
+    _generator = new FrameGenerator(onDone: onDone);
+
     double startTime = 0.0;
-    _stream = super.onTick.map((timeStamp) {
+    _stream = _generator.onTick.map((timeStamp) {
       if (startTime == 0.0)
         startTime = timeStamp;
 
       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);
   }
 
+  void cancel() {
+    _generator.cancel();
+  }
+
   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;
@@ skipping 42 matching lines @@
         begin: _value,
         end: newValue,
         curve: curve,
         initialDelay: initialDelay);
 
     _animation.onTick.listen(_setValue, onDone: () {
       _animation = null;
     });
   }
 }