Make it harder to forget itemCount for FixedHeightScrollable

sky/sdk/lib/widgets/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 'dart:math' as math;
 
 import 'package:vector_math/vector_math.dart';
 
 import '../animation/scroll_behavior.dart';
 import 'basic.dart';
 import 'scrollable.dart';
 
 abstract class FixedHeightScrollable extends Scrollable {
 
-  final EdgeDims padding;
-
   FixedHeightScrollable({ String key, this.itemHeight, Color backgroundColor, this.padding })
       : super(key: key, backgroundColor: backgroundColor) {
     assert(itemHeight != null);
   }
 
+  EdgeDims padding;
   double itemHeight;
 
+  /// Subclasses must implement `get itemCount` to tell FixedHeightScrollable
+  /// how many items there are in the list.
+  int get itemCount;
+
   void syncFields(FixedHeightScrollable source) {
+    padding = source.padding;
     itemHeight = source.itemHeight;
     super.syncFields(source);
   }
 
   ScrollBehavior createScrollBehavior() => new OverscrollBehavior();
   OverscrollBehavior get scrollBehavior => super.scrollBehavior;
 
-  int _itemCount = 0;
-  int get itemCount => _itemCount;
-  void set itemCount (int value) {
-    if (_itemCount != value) {
-      _itemCount = value;
-      double contentsHeight = itemHeight * _itemCount;
-      if (padding != null)
-         contentsHeight += padding.top + padding.bottom;
-      scrollBehavior.contentsHeight = contentsHeight;
-    }
-  }
-
   double _height;
   void _handleSizeChanged(Size newSize) {
     setState(() {
       _height = newSize.height;
       scrollBehavior.containerHeight = _height;
     });
   }
 
   bool scrollTo(double newScrollOffset) {
     if (_height != null && _height > 0.0) {
       double maxScrollOffset = math.max(0.0, itemCount * itemHeight - _height);
       newScrollOffset = math.min(newScrollOffset, maxScrollOffset);
     }
     return super.scrollTo(newScrollOffset);
   }
 
+  void _updateContentsHeight() {
+    double contentsHeight = itemHeight * itemCount;
+    if (padding != null)
+      contentsHeight += padding.top + padding.bottom;
+    scrollBehavior.contentsHeight = contentsHeight;
+  }
+
   Widget buildContent() {
+    _updateContentsHeight();
+
     var itemShowIndex = 0;
     var itemShowCount = 0;
-
     Matrix4 transform = new Matrix4.identity();
 
     if (_height != null && _height > 0.0) {
       if (scrollOffset < 0.0) {
         double visibleHeight = _height + scrollOffset;
         itemShowCount = (visibleHeight / itemHeight).round() + 1;
         transform.translate(0.0, -scrollOffset);
       } else {
         itemShowCount = (_height / itemHeight).ceil() + 1;
         double alignmentDelta = -scrollOffset % itemHeight;
@@ skipping 20 matching lines @@
             child: new Block(items)
           )
         )
       )
     );
   }
 
   List<Widget> buildItems(int start, int count);
 
 }