Merge pull request #1286 from HansMuller/limit_scrolling_animation

Currently, fling scrolls animate far too long after the display has effectively stopped moving. Use the ScrollSimulation's tolerance property to stop the scrolling roughly when the display stops changing.

packages/flutter/lib/src/animation/scroll_behavior.dart

@@ -3,6 +3,7 @@
 // found in the LICENSE file.
 
 import 'dart:math' as math;
+import 'dart:sky' as sky;
 
 import 'package:newton/newton.dart';
 
@@ -95,10 +96,23 @@ class UnboundedBehavior extends ExtentScrollBehavior {
 }
 
 Simulation _createDefaultScrollSimulation(double position, double velocity, double minScrollOffset, double maxScrollOffset) {
-  double velocityPerSecond = velocity * _kSecondsPerMillisecond;
-  SpringDescription spring = new SpringDescription.withDampingRatio(
-      mass: 1.0, springConstant: 170.0, ratio: 1.1);
-  return new ScrollSimulation(position, velocityPerSecond, minScrollOffset, maxScrollOffset, spring, _kScrollDrag);
+  double startVelocity = velocity * _kSecondsPerMillisecond;
+
+  // Assume that we're rendering at atleast 15 FPS. Stop when we're
+  // scrolling less than one logical pixel per frame. We're essentially
+  // normalizing by the devicePixelRatio so that the threshold has the
+  // same effect independent of the device's pixel density.
+  double endVelocity = 15.0 * sky.view.devicePixelRatio;
+
+  // Similar to endVelocity. Stop scrolling when we're this close to
+  // destiniation scroll offset.
+  double endDistance = 0.5 * sky.view.devicePixelRatio;
+
+  SpringDescription spring = new SpringDescription.withDampingRatio(mass: 1.0, springConstant: 170.0, ratio: 1.1);
+  ScrollSimulation simulation =
+      new ScrollSimulation(position, startVelocity, minScrollOffset, maxScrollOffset, spring, _kScrollDrag)
+    ..tolerance = new Tolerance(velocity: endVelocity, distance: endDistance);
+  return simulation;
 }
 
 /// A scroll behavior that lets the user scroll beyond the scroll bounds with some resistance