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

sky/framework/components/fixed_height_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/scroll_behavior.dart';
 import '../debug/tracing.dart';
 import '../fn.dart';
 import 'dart:math' as math;
 import 'dart:sky' as sky;
 import 'dart:async';
 import 'scrollable.dart';
 import '../layouts/block.dart';
 
 abstract class FixedHeightScrollable extends Scrollable {
   static final Style _style = new Style('''
     overflow: hidden;
     position: relative;
     will-change: transform;'''
   );
 
   static final Style _scrollAreaStyle = new Style('''
     position:relative;
     will-change: transform;'''
   );
 
+  FixedHeightScrollable({
+    Object key
+  }) : super(key: key);
+
+  ScrollBehavior createScrollBehavior() => new OverscrollBehavior();
+
   double _height = 0.0;
   double _itemHeight;
 
-  FixedHeightScrollable({
-    Object key,
-    ScrollBehavior scrollBehavior
-  }) : super(key: key, scrollBehavior: scrollBehavior);
+  int _itemCount = 0;
+  int get itemCount => _itemCount;
+  void set itemCount (int value) {
+    if (_itemCount != value) {
+      _itemCount = value;
+      if (_itemHeight != null)
+        scrollBehavior.contentsHeight = _itemHeight * _itemCount;
+    }
+  }
 
   void _measureHeights() {
     trace('FixedHeightScrollable::_measureHeights', () {
       if (_itemHeight != null)
         return;
       var root = getRoot();
       if (root == null)
         return;
       var item = root.firstChild.firstChild;
       if (item == null)
         return;
       sky.ClientRect scrollRect = root.getBoundingClientRect();
       sky.ClientRect itemRect = item.getBoundingClientRect();
       assert(scrollRect.height > 0);
       assert(itemRect.height > 0);
 
       setState(() {
         _height = scrollRect.height;
         _itemHeight = itemRect.height;
+        scrollBehavior.containerHeight = _height;
+        scrollBehavior.contentsHeight = _itemHeight * _itemCount;
       });
     });
   }
 
   UINode buildContent() {
     var itemNumber = 0;
     var drawCount = 1;
     var transformStyle = '';
 
     if (_itemHeight == null)
@@ skipping 25 matching lines @@
       children: [
         new BlockLayout(
           style: _scrollAreaStyle,
           inlineStyle: transformStyle,
           children: buildItems(itemNumber, drawCount)
         )
       ]
     );
   }
 }