Drive overscroll animations with a physics simulation

sky/framework/animation/simulation.dart

Index: sky/framework/animation/simulation.dart
diff --git a/sky/framework/animation/simulation.dart b/sky/framework/animation/simulation.dart
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+// 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 'dart:async';
+import 'generator.dart';
+import 'kinematics.dart';
+
+class Simulation {
+  Stream<double> get onTick => _stream;
+  final System system;
+
+  FrameGenerator _generator;
+  Stream<double> _stream;
+  double _previousTime = 0.0;
+
+  Simulation(this.system, {Function terminationCondition, Function onDone}) {
+    _generator = new FrameGenerator(onDone: onDone);
+    _stream = _generator.onTick.map(_update);
+
+    if (terminationCondition != null) {
+      bool done = false;
+      _stream = _stream.takeWhile((_) {
+        if (done)
+          return false;
+        done = terminationCondition();
+        return true;
+      });
+    }
+  }
+
+  void cancel() {
+    _generator.cancel();
+  }
+
+  double _update(double timeStamp) {
+    double previousTime = _previousTime;
+    _previousTime = timeStamp;
+    if (previousTime == 0.0)
+      return timeStamp;
+    double deltaT = timeStamp - previousTime;
+    system.update(deltaT);
+    return timeStamp;
+  }
+}