[Effen] Prevent scrolling past the bottom of a scrollable list.

sky/framework/animation/scroll_behavior.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;
 import 'mechanics.dart';
 import 'generators.dart';
 
 const double _kScrollFriction = 0.005;
 const double _kOverscrollFriction = 0.075;
@@ skipping 16 matching lines @@
     double newScrollOffset = scrollOffset + scrollDelta;
     if (minOffset != null)
       newScrollOffset = math.max(minOffset, newScrollOffset);
     if (maxOffset != null)
       newScrollOffset = math.min(maxOffset, newScrollOffset);
     return newScrollOffset;
   }
 }
 
 class OverscrollBehavior extends ScrollBehavior {
+
+  double _contentsHeight;
+  double get contentsHeight => _contentsHeight;
+  void set contentsHeight (double value) {
+    if (_contentsHeight != value) {
+      _contentsHeight = value;
+      // TODO(ianh) now what? what if we have a simulation ongoing?
+    }
+  }
+
+  double _containerHeight;
+  double get containerHeight => _containerHeight;
+  void set containerHeight (double value) {
+    if (_containerHeight != value) {
+      _containerHeight = value;
+      // TODO(ianh) now what? what if we have a simulation ongoing?
+    }
+  }
+
+  OverscrollBehavior({double contentsHeight: 0.0, double containerHeight: 0.0})
+    : _contentsHeight = contentsHeight,
+      _containerHeight = containerHeight;
+
+  double get maxScroll => math.max(0.0, _contentsHeight - _containerHeight);
+
   Simulation release(Particle particle) {
     System system;
-    if (particle.position >= 0.0) {
+    if ((particle.position >= 0.0) && (particle.position < maxScroll)) {
       if (particle.velocity == 0.0)
         return null;
       System slowdownSystem = new ParticleInBoxWithFriction(
         particle: particle,
         friction: _kScrollFriction,
-        box: new GeofenceBox(min: 0.0, onEscape: () {
+        box: new GeofenceBox(min: 0.0, max: maxScroll, onEscape: () {
           (system as Multisystem).transitionToSystem(new ParticleInBoxWithFriction(
             particle: particle,
             friction: _kOverscrollFriction,
             box: new ClosedBox(),
             onStop: () => (system as Multisystem).transitionToSystem(getBounceBackSystem(particle))
           ));
         }));
       system = new Multisystem(particle: particle, system: slowdownSystem);
     } else {
       system = getBounceBackSystem(particle);
     }
     return new Simulation(system, terminationCondition: () => particle.position == 0.0);
   }
 
   System getBounceBackSystem(Particle particle) {
-    return new ParticleClimbingRamp(
+    if (particle.position < 0.0)
+      return new ParticleClimbingRamp(
         particle: particle,
         box: new ClosedBox(max: 0.0),
         theta: _kBounceSlopeAngle,
         targetPosition: 0.0);
+    return new ParticleClimbingRamp(
+      particle: particle,
+      box: new ClosedBox(min: maxScroll),
+      theta: _kBounceSlopeAngle,
+      targetPosition: maxScroll);
   }
 
   double applyCurve(double scrollOffset, double scrollDelta) {
     double newScrollOffset = scrollOffset + scrollDelta;
+    // 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. We
+    // do similar things for overscroll in the other direction.
     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;
+    } else if (newScrollOffset > maxScroll) {
+      newScrollOffset -= (newScrollOffset - math.max(maxScroll, scrollOffset)) / 2.0;
     }
     return newScrollOffset;
   }
 }