Drive overscroll animations with a physics simulation

sky/framework/components/scrollable.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 '../animation/curves.dart';
 import '../animation/fling_curve.dart';
 import '../animation/generator.dart';
 import '../animation/scroll_curve.dart';
+import '../animation/kinematics.dart';
+import '../animation/simulation.dart';
 import '../fn.dart';
 import 'dart:sky' as sky;
 
 abstract class Scrollable extends Component {
   ScrollCurve scrollCurve;
   double get scrollOffset => _scrollOffset;
 
   double _scrollOffset = 0.0;
   FlingCurve _flingCurve;
   int _flingAnimationId;
-  AnimationGenerator _scrollAnimation;
+  Simulation _simulation;
 
   Scrollable({Object key, this.scrollCurve}) : super(key: key) {
     events.listen('pointerdown', _handlePointerDown);
     events.listen('pointerup', _handlePointerUpOrCancel);
     events.listen('pointercancel', _handlePointerUpOrCancel);
     events.listen('gestureflingstart', _handleFlingStart);
     events.listen('gestureflingcancel', _handleFlingCancel);
     events.listen('gesturescrollupdate', _handleScrollUpdate);
     events.listen('wheel', _handleWheel);
   }
 
   void didUnmount() {
     super.didUnmount();
     _stopFling();
-    _stopScrollAnimation();
+    _stopSimulation();
   }
 
   bool scrollBy(double scrollDelta) {
     var newScrollOffset = scrollCurve.apply(_scrollOffset, scrollDelta);
     if (newScrollOffset == _scrollOffset)
       return false;
     setState(() {
       _scrollOffset = newScrollOffset;
     });
     return true;
   }
 
-  void animateScrollTo(double targetScrollOffset, {
-      double initialDelay: 0.0,
-      double duration: 0.0,
-      Curve curve: linear}) {
-    _stopScrollAnimation();
-    _scrollAnimation = new AnimationGenerator(
-        duration: duration,
-        begin: _scrollOffset,
-        end: targetScrollOffset,
-        initialDelay: initialDelay,
-        curve: curve);
-    _scrollAnimation.onTick.listen((newScrollOffset) {
-      if (!scrollBy(newScrollOffset - _scrollOffset))
-        _stopScrollAnimation();
-    }, onDone: () {
-      _scrollAnimation = null;
-    });
-  }
-
   void _scheduleFlingUpdate() {
     _flingAnimationId = sky.window.requestAnimationFrame(_updateFling);
   }
 
   void _stopFling() {
     if (_flingAnimationId == null)
       return;
     sky.window.cancelAnimationFrame(_flingAnimationId);
     _flingCurve = null;
     _flingAnimationId = null;
   }
 
-  void _stopScrollAnimation() {
-    if (_scrollAnimation == null)
+  void _stopSimulation() {
+    if (_simulation == null)
       return;
-    _scrollAnimation.cancel();
-    _scrollAnimation = null;
+    _simulation.cancel();
+    _simulation = null;
   }
 
   void _updateFling(double timeStamp) {
     double scrollDelta = _flingCurve.update(timeStamp);
     if (!scrollBy(scrollDelta))
       return _settle();
     _scheduleFlingUpdate();
   }
 
   void _settle() {
     _stopFling();
-    if (_scrollOffset < 0.0)
-      animateScrollTo(0.0, duration: 200.0, curve: easeOut);
+    Particle particle = new Particle(position: scrollOffset);
+    _simulation = scrollCurve.release(particle);
+    if (_simulation == null)
+      return;
+    _simulation.onTick.listen((_) {
+      setState(() {
+        _scrollOffset = particle.position;
+      });
+    });
   }
 
   void _handlePointerDown(_) {
     _stopFling();
-    _stopScrollAnimation();
+    _stopSimulation();
   }
 
   void _handlePointerUpOrCancel(_) {
     if (_flingCurve == null)
       _settle();
   }
 
   void _handleScrollUpdate(sky.GestureEvent event) {
     scrollBy(-event.dy);
   }
 
   void _handleFlingStart(sky.GestureEvent event) {
-    _stopScrollAnimation();
+    _stopSimulation();
     _flingCurve = new FlingCurve(-event.velocityY, event.timeStamp);
     _scheduleFlingUpdate();
   }
 
   void _handleFlingCancel(sky.GestureEvent event) {
     _settle();
   }
 
   void _handleWheel(sky.WheelEvent event) {
     scrollBy(-event.offsetY);
   }
 }