Implement an OverscrollCurve for Scrollable

sky/framework/animation/scroll_curve.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 'dart:math' as math;
 
 abstract class ScrollCurve {
   // Returns the new scroll offset.
   double apply(double scrollOffset, double scrollDelta);
 }
 
 class BoundedScrollCurve extends ScrollCurve {
   double minOffset;
   double maxOffset;
 
   BoundedScrollCurve({this.minOffset: 0.0, this.maxOffset});
 
   double apply(double scrollOffset, double scrollDelta) {
     double newScrollOffset = scrollOffset + scrollDelta;
     if (minOffset != null)
       newScrollOffset = math.max(minOffset, newScrollOffset);
     if (maxOffset != null)
       newScrollOffset = math.min(maxOffset, newScrollOffset);
     return newScrollOffset;
   }
 }
+
+class OverscrollCurve extends ScrollCurve {
+  double apply(double scrollOffset, double scrollDelta) {
+    double newScrollOffset = scrollOffset + scrollDelta;
+    if (newScrollOffset < 0.0) {
+      // If we're overscrolling, we want move the scroll offset 2x slower than
+      // we would otherwise. Therefore, we "rewind" the newScrollOffset by half
+      // the amount that we moved it above. Notice that we clap the "old" value
+      // to 0.0 so that we only reduce the portion of scrollDelta that's applied
+      // beyond 0.0.
+      newScrollOffset -= (newScrollOffset - math.min(0.0, scrollOffset)) / 2.0;
+    }
+    return newScrollOffset;
+  }
+}