Make it harder to forget itemCount for FixedHeightScrollable

Previously, if a subclass of FixedHeightScrollable didn't set itemCount,
everything would silently work except that you wouldn't actully be able to
scroll. This CL makes it harder to forget to provide the itemCount by making it
an abstract getter. Now the analyzer will flag it as missing in subclasses and
we'll throw an exception at runtime if you forget it.

R=ianh@google.com, tonyg@google.com

Review URL: https://codereview.chromium.org/1219933004 .

sdk/example/demo_launcher/lib/main.dart

@@ -129,9 +129,9 @@ const double kCardHeight = 120.0;
 const EdgeDims kListPadding = const EdgeDims.all(4.0);
 
 class DemoList extends FixedHeightScrollable {
-  DemoList({ String key }) : super(key: key, itemHeight: kCardHeight, padding: kListPadding) {
-    itemCount = demos.length;
-  }
+  DemoList({ String key }) : super(key: key, itemHeight: kCardHeight, padding: kListPadding);
+
+  int get itemCount => demos.length;
 
   Widget buildDemo(SkyDemo demo) {
     return new Listener(

sdk/example/stocks/lib/stock_home.dart

@@ -164,7 +164,7 @@ class StockHome extends StatefulComponent {
       _drawerController.close();
     });
   }
-  
+
   Widget buildToolBar() {
     return new ToolBar(
         left: new IconButton(
@@ -185,18 +185,30 @@ class StockHome extends StatefulComponent {
   int selectedTabIndex = 0;
   List<String> portfolioSymbols = ["AAPL","FIZZ", "FIVE", "FLAT", "ZINC", "ZNGA"];
 
+  Iterable<Stock> _filterByPortfolio(Iterable<Stock> stocks) {
+    return stocks.where((stock) => portfolioSymbols.contains(stock.symbol));
+  }
+
+  Iterable<Stock> _filterBySearchQuery(Iterable<Stock> stocks) {
+    if (_searchQuery == null)
+      return stocks;
+    RegExp regexp = new RegExp(_searchQuery, caseSensitive: false);
+    return stocks.where((stock) => stock.symbol.contains(regexp));
+  }
+
+  Widget buildMarketStockList() {
+    return new Stocklist(stocks: _filterBySearchQuery(stocks).toList());
+  }
+
   Widget buildPortfolioStocklist() {
-    return new Stocklist(
-     stocks: stocks.where((s) => portfolioSymbols.contains(s.symbol)).toList(), 
-     query: _searchQuery
-   );
+    return new Stocklist(stocks: _filterBySearchQuery(_filterByPortfolio(stocks)).toList());
   }
 
   Widget buildTabNavigator() {
     List<TabNavigatorView> views = <TabNavigatorView>[
       new TabNavigatorView(
         label: const TabLabel(text: 'MARKET'),
-        builder: () => new Stocklist(stocks: stocks, query: _searchQuery)
+        builder: buildMarketStockList
       ),
       new TabNavigatorView(
         label: const TabLabel(text: 'PORTFOLIO'),

sdk/example/stocks/lib/stock_list.dart

@@ -10,28 +10,20 @@ import 'stock_row.dart';
 
 class Stocklist extends FixedHeightScrollable {
 
-  Stocklist({
-    String key,
-    this.stocks,
-    this.query
-  }) : super(itemHeight: StockRow.kHeight, key: key);
+  Stocklist({ String key, this.stocks })
+    : super(itemHeight: StockRow.kHeight, key: key);
 
-  String query;
   List<Stock> stocks;
 
+  int get itemCount => stocks.length;
+
   void syncFields(Stocklist source) {
-    query = source.query;
     stocks = source.stocks;
     super.syncFields(source);
   }
 
   List<Widget> buildItems(int start, int count) {
-    var filteredStocks = stocks.where((stock) {
-      return query == null ||
-             stock.symbol.contains(new RegExp(query, caseSensitive: false));
-    });
-    itemCount = filteredStocks.length;
-    return filteredStocks
+    return stocks
       .skip(start)
       .take(count)
       .map((stock) => new StockRow(stock: stock))

sdk/lib/widgets/fixed_height_scrollable.dart

@@ -12,16 +12,20 @@ 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);
   }
@@ -29,18 +33,6 @@ abstract class FixedHeightScrollable extends Scrollable {
   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(() {
@@ -57,10 +49,18 @@ abstract class FixedHeightScrollable extends Scrollable {
     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) {