Merge pull request #1098 from abarth/pageable_list2

Switch PageableList over to using RenderList

examples/material_gallery/lib/demo/tabs_demo.dart

@@ -29,7 +29,7 @@ class _TabsDemoState extends State<TabsDemo> {
   Widget build(_) {
     return new TabBarView<String>(
       items: _iconNames,
-      itemBuilder: (BuildContext context, String iconName, int index) {
+      itemBuilder: (String iconName) {
         return new Container(
           key: new ValueKey<String>(iconName),
           padding: const EdgeDims.all(12.0),

examples/stocks/lib/stock_home.dart

@@ -273,7 +273,7 @@ class StockHomeState extends State<StockHome> {
         drawer: _buildDrawer(context),
         body: new TabBarView<StockHomeTab>(
           items: <StockHomeTab>[StockHomeTab.market, StockHomeTab.portfolio],
-          itemBuilder: (BuildContext context, StockHomeTab tab, _) {
+          itemBuilder: (StockHomeTab tab) {
             switch (tab) {
               case StockHomeTab.market:
                 return _buildStockTab(context, tab, config.symbols);

examples/widgets/pageable_list.dart

@@ -41,7 +41,7 @@ class PageableListAppState extends State<PageableListApp> {
   ScrollDirection scrollDirection = ScrollDirection.horizontal;
   bool itemsWrap = false;
 
-  Widget buildCard(BuildContext context, CardModel cardModel, int index) {
+  Widget buildCard(CardModel cardModel) {
     Widget card = new Card(
       color: cardModel.color,
       child: new Container(
@@ -114,10 +114,9 @@ class PageableListAppState extends State<PageableListApp> {
   }
 
   Widget _buildBody(BuildContext context) {
-    return new PageableList<CardModel>(
-      items: cardModels,
+    return new PageableList(
+      children: cardModels.map(buildCard),
       itemsWrap: itemsWrap,
-      itemBuilder: buildCard,
       scrollDirection: scrollDirection
     );
   }

packages/flutter/lib/src/material/tabs.dart

@@ -772,26 +772,30 @@ class _TabBarState<T> extends ScrollableState<TabBar<T>> implements TabBarSelect
   }
 }
 
-class TabBarView<T> extends PageableList<T> {
+typedef Widget TabItemBuilder<T>(T item);
+
+class TabBarView<T> extends PageableList {
   TabBarView({
     Key key,
     List<T> items,
-    ItemBuilder<T> itemBuilder
-  }) : super(
+    TabItemBuilder<T> itemBuilder
+  }) : items = items, itemBuilder = itemBuilder, super(
     key: key,
     scrollDirection: ScrollDirection.horizontal,
-    items: items,
-    itemBuilder: itemBuilder,
+    children: items.map((T item) => itemBuilder(item)),
     itemsWrap: false
   ) {
     assert(items != null);
     assert(items.length > 1);
   }
 
+  final List<T> items;
+  final TabItemBuilder<T> itemBuilder;
+
   _TabBarViewState createState() => new _TabBarViewState<T>();
 }
 
-class _TabBarViewState<T> extends PageableListState<T, TabBarView<T>> implements TabBarSelectionPerformanceListener {
+class _TabBarViewState<T> extends PageableListState<TabBarView<T>> implements TabBarSelectionPerformanceListener {
 
   TabBarSelectionState _selection;
   List<int> _itemIndices = [0, 1];
@@ -916,14 +920,10 @@ class _TabBarViewState<T> extends PageableListState<T, TabBarView<T>> implements
     return settleScrollOffset();
   }
 
-  List<Widget> buildItems(BuildContext context, int start, int count) {
+  Widget buildContent(BuildContext context) {
     TabBarSelectionState<T> newSelection = TabBarSelection.of(context);
     if (_selection != newSelection)
       _initSelection(newSelection);
-    return _itemIndices
-      .skip(start)
-      .take(count)
-      .map((int i) => config.itemBuilder(context, config.items[i], i))
-      .toList();
+    return super.buildContent(context);
   }
 }

packages/flutter/lib/src/rendering/list.dart

@@ -28,7 +28,6 @@ class RenderList extends RenderVirtualViewport<ListParentData> implements HasScr
          paintOffset: paintOffset,
          callback: callback
        ) {
-    assert(itemExtent != null);
     addAll(children);
   }
 
@@ -75,6 +74,8 @@ class RenderList extends RenderVirtualViewport<ListParentData> implements HasScr
   }
 
   double get _preferredExtent {
+    if (itemExtent == null)
+      return double.INFINITY;
     double extent = itemExtent * virtualChildCount;
     if (padding != null)
       extent += _scrollAxisPadding;
@@ -118,8 +119,16 @@ class RenderList extends RenderVirtualViewport<ListParentData> implements HasScr
   }
 
   void performLayout() {
-    size = new Size(constraints.maxWidth,
-                    constraints.constrainHeight(_preferredExtent));
+    switch (scrollDirection) {
+      case ScrollDirection.vertical:
+        size = new Size(constraints.maxWidth,
+                        constraints.constrainHeight(_preferredExtent));
+        break;
+      case ScrollDirection.horizontal:
+        size = new Size(constraints.constrainWidth(_preferredExtent),
+                        constraints.maxHeight);
+        break;
+    }
 
     if (callback != null)
       invokeLayoutCallback(callback);
@@ -136,15 +145,15 @@ class RenderList extends RenderVirtualViewport<ListParentData> implements HasScr
     switch (scrollDirection) {
       case ScrollDirection.vertical:
         itemWidth = math.max(0, size.width - (padding == null ? 0.0 : padding.horizontal));
-        itemHeight = itemExtent;
+        itemHeight = itemExtent ?? size.height;
         y = padding != null ? padding.top : 0.0;
-        dy = itemExtent;
+        dy = itemHeight;
         break;
       case ScrollDirection.horizontal:
-        itemWidth = itemExtent;
+        itemWidth = itemExtent ?? size.width;
         itemHeight = math.max(0, size.height - (padding == null ? 0.0 : padding.vertical));
         x = padding != null ? padding.left : 0.0;
-        dx = itemExtent;
+        dx = itemWidth;
         break;
     }
 

packages/flutter/lib/src/widgets/homogeneous_viewport.dart

@@ -220,80 +220,3 @@ class _HomogeneousViewportElement extends _ViewportBaseElement<HomogeneousViewpo
     return widget.itemCount != null ? widget.itemCount * widget.itemExtent : double.INFINITY;
   }
 }
-
-class HomogeneousPageViewport extends _ViewportBase {
-  HomogeneousPageViewport({
-    Key key,
-    ListBuilder builder,
-    bool itemsWrap: false,
-    int itemCount, // optional, but you cannot shrink-wrap this class or otherwise use its intrinsic dimensions if you don't specify it
-    ScrollDirection direction: ScrollDirection.vertical,
-    double startOffset: 0.0,
-    Painter overlayPainter
-  }) : super(
-    key: key,
-    builder: builder,
-    itemsWrap: itemsWrap,
-    itemCount: itemCount,
-    direction: direction,
-    startOffset: startOffset,
-    overlayPainter: overlayPainter
-   );
-
-  _HomogeneousPageViewportElement createElement() => new _HomogeneousPageViewportElement(this);
-}
-
-class _HomogeneousPageViewportElement extends _ViewportBaseElement<HomogeneousPageViewport> {
-  _HomogeneousPageViewportElement(HomogeneousPageViewport widget) : super(widget);
-
-  void layout(BoxConstraints constraints) {
-    // We enter a build scope (meaning that markNeedsBuild() is forbidden)
-    // because we are in the middle of layout and if we allowed people to set
-    // state, they'd expect to have that state reflected immediately, which, if
-    // we were to try to honour it, would potentially result in assertions
-    // because you can't normally mutate the render object tree during layout.
-    // (If there were a way to limit these writes to descendants of this, it'd
-    // be ok because we are exempt from that assert since we are still actively
-    // doing our own layout.)
-    BuildableElement.lockState(() {
-      double itemExtent = widget.direction == ScrollDirection.vertical ? constraints.maxHeight : constraints.maxWidth;
-      double offset;
-      if (widget.startOffset <= 0.0 && !widget.itemsWrap) {
-        _layoutFirstIndex = 0;
-        offset = -widget.startOffset * itemExtent;
-      } else {
-        _layoutFirstIndex = widget.startOffset.floor();
-        offset = -((widget.startOffset * itemExtent) % itemExtent);
-      }
-      if (itemExtent < double.INFINITY && widget.itemCount != null) {
-        final double contentExtent = itemExtent * widget.itemCount;
-        _layoutItemCount = contentExtent == 0.0 ? 0 : ((contentExtent - offset) / contentExtent).ceil();
-        if (!widget.itemsWrap)
-          _layoutItemCount = math.min(_layoutItemCount, widget.itemCount - _layoutFirstIndex);
-      } else {
-        assert(() {
-          'This HomogeneousPageViewport has no specified number of items (meaning it has infinite items), ' +
-          'and has been placed in an unconstrained environment where all items can be rendered. ' +
-          'It is most likely that you have placed your HomogeneousPageViewport (which is an internal ' +
-          'component of several scrollable widgets) inside either another scrolling box, a flexible ' +
-          'box (Row, Column), or a Stack, without giving it a specific size.';
-          return widget.itemCount != null;
-        });
-        _layoutItemCount = widget.itemCount - _layoutFirstIndex;
-      }
-      _layoutItemCount = math.max(0, _layoutItemCount);
-      _updateChildren();
-      // Update the renderObject configuration
-      renderObject.direction = widget.direction == ScrollDirection.vertical ? BlockDirection.vertical : BlockDirection.horizontal;
-      renderObject.itemExtent = itemExtent;
-      renderObject.minExtent = itemExtent;
-      renderObject.startOffset = offset;
-      renderObject.overlayPainter = widget.overlayPainter;
-    }, building: true);
-  }
-
-  double getTotalExtent(BoxConstraints constraints) {
-    double itemExtent = widget.direction == ScrollDirection.vertical ? constraints.maxHeight : constraints.maxWidth;
-    return widget.itemCount != null ? widget.itemCount * itemExtent : double.INFINITY;
-  }
-}

packages/flutter/lib/src/widgets/pageable_list.dart

@@ -3,7 +3,6 @@
 // found in the LICENSE file.
 
 import 'dart:async';
-import 'dart:math' as math;
 
 import 'package:flutter/animation.dart';
 import 'package:flutter/gestures.dart';
@@ -11,8 +10,8 @@ import 'package:flutter/rendering.dart';
 
 import 'basic.dart';
 import 'framework.dart';
-import 'homogeneous_viewport.dart';
 import 'scrollable.dart';
+import 'virtual_viewport.dart';
 
 /// Controls what alignment items use when settling.
 enum ItemsSnapAlignment {
@@ -22,7 +21,7 @@ enum ItemsSnapAlignment {
 
 typedef void PageChangedCallback(int newPage);
 
-class PageableList<T> extends Scrollable {
+class PageableList extends Scrollable {
   PageableList({
     Key key,
     initialScrollOffset,
@@ -32,14 +31,13 @@ class PageableList<T> extends Scrollable {
     ScrollListener onScrollEnd,
     SnapOffsetCallback snapOffsetCallback,
     double snapAlignmentOffset: 0.0,
-    this.items,
-    this.itemBuilder,
     this.itemsWrap: false,
     this.itemsSnapAlignment: ItemsSnapAlignment.adjacentItem,
     this.onPageChanged,
     this.scrollableListPainter,
     this.duration: const Duration(milliseconds: 200),
-    this.curve: Curves.ease
+    this.curve: Curves.ease,
+    this.children
   }) : super(
     key: key,
     initialScrollOffset: initialScrollOffset,
@@ -51,21 +49,19 @@ class PageableList<T> extends Scrollable {
     snapAlignmentOffset: snapAlignmentOffset
   );
 
-  final List<T> items;
-  final ItemBuilder<T> itemBuilder;
-  final ItemsSnapAlignment itemsSnapAlignment;
   final bool itemsWrap;
+  final ItemsSnapAlignment itemsSnapAlignment;
   final PageChangedCallback onPageChanged;
   final ScrollableListPainter scrollableListPainter;
   final Duration duration;
   final Curve curve;
+  final Iterable<Widget> children;
 
-  PageableListState<T, PageableList<T>> createState() => new PageableListState<T, PageableList<T>>();
+  PageableListState createState() => new PageableListState();
 }
 
-class PageableListState<T, Config extends PageableList<T>> extends ScrollableState<Config> {
-
-  int get itemCount => config.items?.length ?? 0;
+class PageableListState<T extends PageableList> extends ScrollableState<T> {
+  int get itemCount => config.children?.length ?? 0;
   int _previousItemCount;
 
   double pixelToScrollOffset(double value) {
@@ -76,7 +72,12 @@ class PageableListState<T, Config extends PageableList<T>> extends ScrollableSta
     return pixelScrollExtent == 0.0 ? 0.0 : value / pixelScrollExtent;
   }
 
-  void didUpdateConfig(Config oldConfig) {
+  void initState() {
+    super.initState();
+    _updateScrollBehavior();
+  }
+
+  void didUpdateConfig(PageableList oldConfig) {
     super.didUpdateConfig(oldConfig);
 
     bool scrollBehaviorUpdateNeeded = config.scrollDirection != oldConfig.scrollDirection;
@@ -94,9 +95,7 @@ class PageableListState<T, Config extends PageableList<T>> extends ScrollableSta
   }
 
   void _updateScrollBehavior() {
-    // if you don't call this from build(), you must call it from setState().
-    if (config.scrollableListPainter != null)
-      config.scrollableListPainter.contentExtent = itemCount.toDouble();
+    config.scrollableListPainter?.contentExtent = itemCount.toDouble();
     scrollTo(scrollBehavior.updateExtents(
       contentExtent: itemCount.toDouble(),
       containerExtent: 1.0,
@@ -111,8 +110,7 @@ class PageableListState<T, Config extends PageableList<T>> extends ScrollableSta
 
   void dispatchOnScroll() {
     super.dispatchOnScroll();
-    if (config.scrollableListPainter != null)
-      config.scrollableListPainter.scrollOffset = scrollOffset;
+    config.scrollableListPainter?.scrollOffset = scrollOffset;
   }
 
   void dispatchOnScrollEnd() {
@@ -121,17 +119,12 @@ class PageableListState<T, Config extends PageableList<T>> extends ScrollableSta
   }
 
   Widget buildContent(BuildContext context) {
-    if (itemCount != _previousItemCount) {
-      _previousItemCount = itemCount;
-      _updateScrollBehavior();
-    }
-    return new HomogeneousPageViewport(
-      builder: buildItems,
+    return new PageViewport(
       itemsWrap: config.itemsWrap,
-      itemCount: itemCount,
-      direction: config.scrollDirection,
+      scrollDirection: config.scrollDirection,
       startOffset: scrollOffset,
-      overlayPainter: config.scrollableListPainter
+      overlayPainter: config.scrollableListPainter,
+      children: config.children
     );
   }
 
@@ -180,18 +173,94 @@ class PageableListState<T, Config extends PageableList<T>> extends ScrollableSta
       .then(_notifyPageChanged);
   }
 
-  List<Widget> buildItems(BuildContext context, int start, int count) {
-    final List<Widget> result = new List<Widget>();
-    final int begin = config.itemsWrap ? start : math.max(0, start);
-    final int end = config.itemsWrap ? begin + count : math.min(begin + count, itemCount);
-    for (int i = begin; i < end; ++i)
-      result.add(config.itemBuilder(context, config.items[i % itemCount], i));
-    assert(result.every((Widget item) => item.key != null));
-    return result;
-  }
-
   void _notifyPageChanged(_) {
     if (config.onPageChanged != null)
       config.onPageChanged(itemCount == 0 ? 0 : scrollOffset.floor() % itemCount);
   }
 }
+
+class PageViewport extends VirtualViewport {
+  PageViewport({
+    Key key,
+    this.startOffset: 0.0,
+    this.scrollDirection: ScrollDirection.vertical,
+    this.itemsWrap: false,
+    this.overlayPainter,
+    this.children
+  }) {
+    assert(scrollDirection != null);
+  }
+
+  final double startOffset;
+  final ScrollDirection scrollDirection;
+  final bool itemsWrap;
+  final Painter overlayPainter;
+  final Iterable<Widget> children;
+
+  RenderList createRenderObject() => new RenderList();
+
+  _PageViewportElement createElement() => new _PageViewportElement(this);
+}
+
+class _PageViewportElement extends VirtualViewportElement<PageViewport> {
+  _PageViewportElement(PageViewport widget) : super(widget);
+
+  RenderList get renderObject => super.renderObject;
+
+  int get materializedChildBase => _materializedChildBase;
+  int _materializedChildBase;
+
+  int get materializedChildCount => _materializedChildCount;
+  int _materializedChildCount;
+
+  double get startOffsetBase => _repaintOffsetBase;
+  double _repaintOffsetBase;
+
+  double get startOffsetLimit =>_repaintOffsetLimit;
+  double _repaintOffsetLimit;
+
+  double get paintOffset {
+    if (_containerExtent == null)
+      return 0.0;
+    return -(widget.startOffset - startOffsetBase) * _containerExtent;
+  }
+
+  void updateRenderObject() {
+    renderObject.scrollDirection = widget.scrollDirection;
+    renderObject.overlayPainter = widget.overlayPainter;
+    super.updateRenderObject();
+  }
+
+  double _containerExtent;
+
+  double _getContainerExtentFromRenderObject() {
+    switch (widget.scrollDirection) {
+      case ScrollDirection.vertical:
+        return renderObject.size.height;
+      case ScrollDirection.horizontal:
+        return renderObject.size.width;
+    }
+  }
+
+  void layout(BoxConstraints constraints) {
+    int length = renderObject.virtualChildCount;
+    _containerExtent = _getContainerExtentFromRenderObject();
+
+    _materializedChildBase = widget.startOffset.floor();
+    int materializedChildLimit = (widget.startOffset + 1.0).ceil();
+
+    if (!widget.itemsWrap) {
+      _materializedChildBase = _materializedChildBase.clamp(0, length);
+      materializedChildLimit = materializedChildLimit.clamp(0, length);
+    } else if (length == 0) {
+      materializedChildLimit = _materializedChildBase;
+    }
+
+    _materializedChildCount = materializedChildLimit - _materializedChildBase;
+
+    _repaintOffsetBase = _materializedChildBase.toDouble();
+    _repaintOffsetLimit = (materializedChildLimit - 1).toDouble();
+
+    super.layout(constraints);
+  }
+}

packages/flutter/lib/src/widgets/scrollable.dart

@@ -664,8 +664,6 @@ abstract class ScrollableWidgetListState<T extends ScrollableWidgetList> extends
 
 }
 
-typedef Widget ItemBuilder<T>(BuildContext context, T item, int index);
-
 /// A general scrollable list for a large number of children that might not all
 /// have the same height. Prefer [ScrollableWidgetList] when all the children
 /// have the same height because it can use that property to be more efficient.

packages/flutter/lib/src/widgets/scrollable_grid.dart

@@ -116,11 +116,11 @@ class _GridViewportElement extends VirtualViewportElement<GridViewport> {
   int get materializedChildCount => _materializedChildCount;
   int _materializedChildCount;
 
-  double get repaintOffsetBase => _repaintOffsetBase;
-  double _repaintOffsetBase;
+  double get startOffsetBase => _startOffsetBase;
+  double _startOffsetBase;
 
-  double get repaintOffsetLimit =>_repaintOffsetLimit;
-  double _repaintOffsetLimit;
+  double get startOffsetLimit =>_startOffsetLimit;
+  double _startOffsetLimit;
 
   void updateRenderObject() {
     renderObject.delegate = widget.delegate;
@@ -141,8 +141,8 @@ class _GridViewportElement extends VirtualViewportElement<GridViewport> {
 
     _materializedChildBase = (materializedRowBase * _specification.columnCount).clamp(0, renderObject.virtualChildCount);
     _materializedChildCount = (materializedRowLimit * _specification.columnCount).clamp(0, renderObject.virtualChildCount) - _materializedChildBase;
-    _repaintOffsetBase = _specification.rowOffsets[materializedRowBase];
-    _repaintOffsetLimit = _specification.rowOffsets[materializedRowLimit] - containerExtent;
+    _startOffsetBase = _specification.rowOffsets[materializedRowBase];
+    _startOffsetLimit = _specification.rowOffsets[materializedRowLimit] - containerExtent;
 
     super.layout(constraints);
 

packages/flutter/lib/src/widgets/scrollable_list.dart

@@ -129,11 +129,11 @@ class _ListViewportElement extends VirtualViewportElement<ListViewport> {
   int get materializedChildCount => _materializedChildCount;
   int _materializedChildCount;
 
-  double get repaintOffsetBase => _repaintOffsetBase;
-  double _repaintOffsetBase;
+  double get startOffsetBase => _startOffsetBase;
+  double _startOffsetBase;
 
-  double get repaintOffsetLimit =>_repaintOffsetLimit;
-  double _repaintOffsetLimit;
+  double get startOffsetLimit =>_startOffsetLimit;
+  double _startOffsetLimit;
 
   void updateRenderObject() {
     renderObject.scrollDirection = widget.scrollDirection;
@@ -156,21 +156,25 @@ class _ListViewportElement extends VirtualViewportElement<ListViewport> {
   }
 
   void layout(BoxConstraints constraints) {
-    int length = renderObject.virtualChildCount;
+    final int length = renderObject.virtualChildCount;
+    final double itemExtent = widget.itemExtent;
+
     double contentExtent = widget.itemExtent * length;
     double containerExtent = _getContainerExtentFromRenderObject();
 
-    _materializedChildBase = math.max(0, widget.startOffset ~/ widget.itemExtent);
-    int materializedChildLimit = math.max(0, ((widget.startOffset + containerExtent) / widget.itemExtent).ceil());
+    _materializedChildBase = math.max(0, widget.startOffset ~/ itemExtent);
+    int materializedChildLimit = math.max(0, ((widget.startOffset + containerExtent) / itemExtent).ceil());
 
     if (!widget.itemsWrap) {
       _materializedChildBase = math.min(length, _materializedChildBase);
       materializedChildLimit = math.min(length, materializedChildLimit);
+    } else if (length == 0) {
+      materializedChildLimit = _materializedChildBase;
     }
 
     _materializedChildCount = materializedChildLimit - _materializedChildBase;
-    _repaintOffsetBase = _materializedChildBase * widget.itemExtent;
-    _repaintOffsetLimit = materializedChildLimit * widget.itemExtent - containerExtent;
+    _startOffsetBase = _materializedChildBase * itemExtent;
+    _startOffsetLimit = materializedChildLimit * itemExtent - containerExtent;
 
     super.layout(constraints);
 

packages/flutter/lib/src/widgets/virtual_viewport.dart

@@ -2,6 +2,8 @@
 // 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 'basic.dart';
 import 'framework.dart';
 
@@ -20,8 +22,10 @@ abstract class VirtualViewportElement<T extends VirtualViewport> extends RenderO
 
   int get materializedChildBase;
   int get materializedChildCount;
-  double get repaintOffsetBase;
-  double get repaintOffsetLimit;
+  double get startOffsetBase;
+  double get startOffsetLimit;
+
+  double get paintOffset => -(widget.startOffset - startOffsetBase);
 
   List<Element> _materializedChildren = const <Element>[];
 
@@ -61,10 +65,10 @@ abstract class VirtualViewportElement<T extends VirtualViewport> extends RenderO
   void _updatePaintOffset() {
     switch (widget.scrollDirection) {
       case ScrollDirection.vertical:
-        renderObject.paintOffset = new Offset(0.0, -(widget.startOffset - repaintOffsetBase));
+        renderObject.paintOffset = new Offset(0.0, paintOffset);
         break;
       case ScrollDirection.horizontal:
-        renderObject.paintOffset = new Offset(-(widget.startOffset - repaintOffsetBase), 0.0);
+        renderObject.paintOffset = new Offset(paintOffset, 0.0);
         break;
     }
   }
@@ -72,24 +76,25 @@ abstract class VirtualViewportElement<T extends VirtualViewport> extends RenderO
   void updateRenderObject() {
     renderObject.virtualChildCount = widget.children.length;
 
-    if (repaintOffsetBase != null) {
+    if (startOffsetBase != null) {
       _updatePaintOffset();
 
       // If we don't already need layout, we need to request a layout if the
       // viewport has shifted to expose new children.
       if (!renderObject.needsLayout) {
-        if (repaintOffsetBase != null && widget.startOffset < repaintOffsetBase)
+        if (startOffsetBase != null && widget.startOffset < startOffsetBase)
           renderObject.markNeedsLayout();
-        else if (repaintOffsetLimit != null && widget.startOffset > repaintOffsetLimit)
+        else if (startOffsetLimit != null && widget.startOffset > startOffsetLimit)
           renderObject.markNeedsLayout();
       }
     }
   }
 
   void layout(BoxConstraints constraints) {
-    assert(repaintOffsetBase != null);
-    assert(repaintOffsetLimit != null);
+    assert(startOffsetBase != null);
+    assert(startOffsetLimit != null);
     _updatePaintOffset();
+    // TODO(abarth): Set building: true here.
     BuildableElement.lockState(_materializeChildren);
   }
 
@@ -119,11 +124,11 @@ abstract class VirtualViewportElement<T extends VirtualViewport> extends RenderO
     int length = renderObject.virtualChildCount;
     assert(base != null);
     assert(count != null);
-    _populateWidgets(base + count);
+    _populateWidgets(base < 0 ? length : math.min(length, base + count));
     List<Widget> newWidgets = new List<Widget>(count);
     for (int i = 0; i < count; ++i) {
       int childIndex = base + i;
-      Widget child = _widgets[childIndex % length];
+      Widget child = _widgets[(childIndex % length).abs()];
       Key key = new ValueKey(child.key ?? childIndex);
       newWidgets[i] = new RepaintBoundary(key: key, child: child);
     }

packages/flutter/test/widget/pageable_list_test.dart

@@ -13,7 +13,7 @@ final List<GlobalKey> globalKeys = defaultPages.map((_) => new GlobalKey()).toLi
 int currentPage = null;
 bool itemsWrap = false;
 
-Widget buildPage(BuildContext context, int page, int index) {
+Widget buildPage(int page) {
   return new Container(
     key: globalKeys[page],
     width: pageSize.width,
@@ -23,9 +23,8 @@ Widget buildPage(BuildContext context, int page, int index) {
 }
 
 Widget buildFrame({ List<int> pages: defaultPages }) {
-  final list = new PageableList<int>(
-    items: pages,
-    itemBuilder: buildPage,
+  final list = new PageableList(
+    children: pages.map(buildPage),
     itemsWrap: itemsWrap,
     scrollDirection: ScrollDirection.horizontal,
     onPageChanged: (int page) { currentPage = page; }
@@ -136,7 +135,7 @@ void main() {
     testWidgets((WidgetTester tester) {
       currentPage = null;
       itemsWrap = true;
-      tester.pumpWidget(buildFrame(pages: null));
+      tester.pumpWidget(buildFrame(pages: <int>[]));
       expect(currentPage, isNull);
     });
   });