Merge pull request #1074 from abarth/scrollable_list_features

Complete features of ScrollableList2

examples/fitness/lib/feed.dart

@@ -14,11 +14,10 @@ class FitnessItemList extends StatelessComponent {
   final FitnessItemHandler onDismissed;
 
   Widget build(BuildContext context) {
-    return new ScrollableList<FitnessItem>(
+    return new ScrollableList2(
       padding: const EdgeDims.all(4.0),
-      items: items,
       itemExtent: kFitnessItemHeight,
-      itemBuilder: (BuildContext context, FitnessItem item, int index) => item.toRow(onDismissed: onDismissed)
+      children: items.map((FitnessItem item) => item.toRow(onDismissed: onDismissed))
     );
   }
 }

examples/stocks/lib/stock_list.dart

@@ -14,10 +14,9 @@ class StockList extends StatelessComponent {
   final StockRowActionCallback onAction;
 
   Widget build(BuildContext context) {
-    return new ScrollableList<Stock>(
-      items: stocks,
+    return new ScrollableList2(
       itemExtent: StockRow.kHeight,
-      itemBuilder: (BuildContext context, Stock stock, int index) {
+      children: stocks.map((Stock stock) {
         return new StockRow(
           keySalt: keySalt,
           stock: stock,
@@ -25,7 +24,7 @@ class StockList extends StatelessComponent {
           onDoubleTap: onShow,
           onLongPressed: onAction
         );
-      }
+      })
     );
   }
 }

examples/widgets/card_collection.dart

@@ -393,12 +393,11 @@ class CardCollectionState extends State<CardCollection> {
   Widget build(BuildContext context) {
     Widget cardCollection;
     if (_fixedSizeCards) {
-      cardCollection = new ScrollableList<CardModel> (
+      cardCollection = new ScrollableList2 (
         snapOffsetCallback: _snapToCenter ? _toSnapOffset : null,
         snapAlignmentOffset: _cardCollectionSize.height / 2.0,
-        items: _cardModels,
-        itemBuilder: (BuildContext context, CardModel card, int index) => _buildCard(context, card.value),
-        itemExtent: _cardModels[0].height
+        itemExtent: _cardModels[0].height,
+        children: _cardModels.map((CardModel card) => _buildCard(context, card.value))
       );
     } else {
       cardCollection = new ScrollableMixedWidgetList(

examples/widgets/media_query.dart

@@ -74,12 +74,12 @@ class MediaQueryExample extends StatelessComponent {
     if (MediaQuery.of(context).size.width < _gridViewBreakpoint) {
       return new ScrollableList2(
         itemExtent: 50.0,
-        children: items.map((AdaptiveItem item) => item.toListItem()).toList()
+        children: items.map((AdaptiveItem item) => item.toListItem())
       );
     } else {
       return new ScrollableGrid(
         delegate: new MaxTileWidthGridDelegate(maxTileWidth: _maxTileWidth),
-        children: items.map((AdaptiveItem item) => item.toCard()).toList()
+        children: items.map((AdaptiveItem item) => item.toCard())
       );
     }
   }

examples/widgets/scrollbar.dart

@@ -5,6 +5,21 @@
 import 'package:intl/intl.dart';
 import 'package:flutter/material.dart';
 
+final NumberFormat _dd = new NumberFormat("00", "en_US");
+
+class _Item extends StatelessComponent {
+  _Item(this.index);
+
+  int index;
+
+  Widget build(BuildContext context) {
+    return new Text('Item ${_dd.format(index)}',
+      key: new ValueKey<int>(index),
+      style: Theme.of(context).text.title
+    );
+  }
+}
+
 class ScrollbarApp extends StatefulComponent {
   ScrollbarAppState createState() => new ScrollbarAppState();
 }
@@ -15,17 +30,10 @@ class ScrollbarAppState extends State<ScrollbarApp> {
   final ScrollbarPainter _scrollbarPainter = new ScrollbarPainter();
 
   Widget _buildMenu(BuildContext context) {
-    NumberFormat dd = new NumberFormat("00", "en_US");
-    return new ScrollableList<int>(
-      items: new List<int>.generate(_itemCount, (int i) => i),
+    return new ScrollableList2(
       itemExtent: _itemExtent,
-      itemBuilder: (_, __, int index) {
-        return new Text('Item ${dd.format(index)}',
-          key: new ValueKey<int>(index),
-          style: Theme.of(context).text.title
-        );
-      },
-      scrollableListPainter: _scrollbarPainter
+      scrollableListPainter: _scrollbarPainter,
+      children: new List<Widget>.generate(_itemCount, (int i) => new _Item(i))
     );
   }
 

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

@@ -21,44 +21,35 @@ Map<MaterialListType, double> _kItemExtent = const <MaterialListType, double>{
   MaterialListType.threeLine: kThreeLineListItemHeight,
 };
 
-class MaterialList<T> extends StatefulComponent {
+class MaterialList extends StatefulComponent {
   MaterialList({
     Key key,
     this.initialScrollOffset,
     this.onScroll,
-    this.items,
-    this.itemBuilder,
-    this.type: MaterialListType.twoLine
+    this.type: MaterialListType.twoLine,
+    this.children
   }) : super(key: key);
 
   final double initialScrollOffset;
   final ScrollListener onScroll;
-  final List<T> items;
-  final ItemBuilder<T> itemBuilder;
   final MaterialListType type;
+  final Iterable<Widget> children;
 
-  _MaterialListState<T> createState() => new _MaterialListState<T>();
+  _MaterialListState createState() => new _MaterialListState();
 }
 
-class _MaterialListState<T> extends State<MaterialList<T>> {
-
-  void initState() {
-    super.initState();
-    _scrollbarPainter = new ScrollbarPainter();
-  }
-
-  ScrollbarPainter _scrollbarPainter;
+class _MaterialListState extends State<MaterialList> {
+  ScrollbarPainter _scrollbarPainter = new ScrollbarPainter();
 
   Widget build(BuildContext context) {
-    return new ScrollableList<T>(
+    return new ScrollableList2(
       initialScrollOffset: config.initialScrollOffset,
       scrollDirection: ScrollDirection.vertical,
       onScroll: config.onScroll,
-      items: config.items,
-      itemBuilder: config.itemBuilder,
       itemExtent: _kItemExtent[config.type],
       padding: const EdgeDims.symmetric(vertical: 8.0),
-      scrollableListPainter: _scrollbarPainter
+      scrollableListPainter: _scrollbarPainter,
+      children: config.children
     );
   }
 }

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

@@ -30,22 +30,25 @@ class ScrollbarPainter extends ScrollableListPainter {
     Point thumbOrigin;
     Size thumbSize;
 
-    if (isVertical) {
-      double thumbHeight = viewportBounds.height * viewportBounds.height / contentExtent;
-      thumbHeight = thumbHeight.clamp(_kMinScrollbarThumbLength, viewportBounds.height);
-      final double maxThumbTop = viewportBounds.height - thumbHeight;
-      double thumbTop = (scrollOffset / (contentExtent - viewportBounds.height)) * maxThumbTop;
-      thumbTop = viewportBounds.top + thumbTop.clamp(0.0, maxThumbTop);
-      thumbOrigin = new Point(viewportBounds.right - _kScrollbarThumbGirth, thumbTop);
-      thumbSize = new Size(_kScrollbarThumbGirth, thumbHeight);
-    } else {
-      double thumbWidth = viewportBounds.width * viewportBounds.width / contentExtent;
-      thumbWidth = thumbWidth.clamp(_kMinScrollbarThumbLength, viewportBounds.width);
-      final double maxThumbLeft = viewportBounds.width - thumbWidth;
-      double thumbLeft = (scrollOffset / (contentExtent - viewportBounds.width)) * maxThumbLeft;
-      thumbLeft = viewportBounds.left + thumbLeft.clamp(0.0, maxThumbLeft);
-      thumbOrigin = new Point(thumbLeft, viewportBounds.height - _kScrollbarThumbGirth);
-      thumbSize = new Size(thumbWidth, _kScrollbarThumbGirth);
+    switch (scrollDirection) {
+      case ScrollDirection.vertical:
+        double thumbHeight = viewportBounds.height * viewportBounds.height / contentExtent;
+        thumbHeight = thumbHeight.clamp(_kMinScrollbarThumbLength, viewportBounds.height);
+        final double maxThumbTop = viewportBounds.height - thumbHeight;
+        double thumbTop = (scrollOffset / (contentExtent - viewportBounds.height)) * maxThumbTop;
+        thumbTop = viewportBounds.top + thumbTop.clamp(0.0, maxThumbTop);
+        thumbOrigin = new Point(viewportBounds.right - _kScrollbarThumbGirth, thumbTop);
+        thumbSize = new Size(_kScrollbarThumbGirth, thumbHeight);
+        break;
+      case ScrollDirection.horizontal:
+        double thumbWidth = viewportBounds.width * viewportBounds.width / contentExtent;
+        thumbWidth = thumbWidth.clamp(_kMinScrollbarThumbLength, viewportBounds.width);
+        final double maxThumbLeft = viewportBounds.width - thumbWidth;
+        double thumbLeft = (scrollOffset / (contentExtent - viewportBounds.width)) * maxThumbLeft;
+        thumbLeft = viewportBounds.left + thumbLeft.clamp(0.0, maxThumbLeft);
+        thumbOrigin = new Point(thumbLeft, viewportBounds.height - _kScrollbarThumbGirth);
+        thumbSize = new Size(thumbWidth, _kScrollbarThumbGirth);
+        break;
     }
 
     paintThumb(context, thumbOrigin & thumbSize);
@@ -65,7 +68,7 @@ class ScrollbarPainter extends ScrollableListPainter {
       ..variable = new AnimatedValue<double>(0.0, end: 1.0, curve: Curves.ease)
       ..addListener(() {
         _opacity = _fade.value;
-        renderer?.markNeedsPaint();
+        renderObject?.markNeedsPaint();
       });
     return _fade.forward();
   }

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

@@ -8,6 +8,7 @@ import 'package:vector_math/vector_math_64.dart';
 
 import 'box.dart';
 import 'object.dart';
+import 'viewport.dart';
 
 /// Parent data for use with [RenderBlockBase].
 class BlockParentData extends ContainerBoxParentDataMixin<RenderBox> { }
@@ -32,8 +33,10 @@ typedef double _Constrainer(double value);
 /// children. Because blocks expand in the main axis, blocks must be given
 /// unlimited space in the main axis, typically by being contained in a
 /// viewport with a scrolling direction that matches the block's main axis.
-abstract class RenderBlockBase extends RenderBox with ContainerRenderObjectMixin<RenderBox, BlockParentData>,
-                                                      RenderBoxContainerDefaultsMixin<RenderBox, BlockParentData> {
+abstract class RenderBlockBase extends RenderBox
+    with ContainerRenderObjectMixin<RenderBox, BlockParentData>,
+         RenderBoxContainerDefaultsMixin<RenderBox, BlockParentData>
+    implements HasScrollDirection {
 
   RenderBlockBase({
     List<RenderBox> children,
@@ -82,6 +85,9 @@ abstract class RenderBlockBase extends RenderBox with ContainerRenderObjectMixin
   /// Whether the main axis is vertical.
   bool get isVertical => _direction == BlockDirection.vertical;
 
+  // TODO(abarth): Remove BlockDirection in favor of ScrollDirection.
+  ScrollDirection get scrollDirection => isVertical ? ScrollDirection.vertical : ScrollDirection.horizontal;
+
   BoxConstraints _getInnerConstraints(BoxConstraints constraints) {
     if (isVertical)
       return new BoxConstraints.tightFor(width: constraints.constrainWidth(constraints.maxWidth),

packages/flutter/lib/src/rendering/list.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 'box.dart';
 import 'object.dart';
 import 'viewport.dart';
@@ -9,18 +11,23 @@ import 'viewport.dart';
 /// Parent data for use with [RenderList].
 class ListParentData extends ContainerBoxParentDataMixin<RenderBox> { }
 
-class RenderList extends RenderVirtualViewport<ListParentData> {
+class RenderList extends RenderVirtualViewport<ListParentData> implements HasScrollDirection {
   RenderList({
     List<RenderBox> children,
     double itemExtent,
+    EdgeDims padding,
     int virtualChildCount,
     Offset paintOffset: Offset.zero,
+    ScrollDirection scrollDirection: ScrollDirection.vertical,
     LayoutCallback callback
-  }) : _itemExtent = itemExtent, super(
-    virtualChildCount: virtualChildCount,
-    paintOffset: paintOffset,
-    callback: callback
-  ) {
+  }) : _itemExtent = itemExtent,
+       _padding = padding,
+       _scrollDirection = scrollDirection,
+       super(
+         virtualChildCount: virtualChildCount,
+         paintOffset: paintOffset,
+         callback: callback
+       ) {
     assert(itemExtent != null);
     addAll(children);
   }
@@ -35,50 +42,124 @@ class RenderList extends RenderVirtualViewport<ListParentData> {
     markNeedsLayout();
   }
 
+  EdgeDims get padding => _padding;
+  EdgeDims _padding;
+  void set padding (EdgeDims newValue) {
+    if (_padding == newValue)
+      return;
+    _padding = newValue;
+    markNeedsLayout();
+  }
+
+  ScrollDirection get scrollDirection => _scrollDirection;
+  ScrollDirection _scrollDirection;
+  void set scrollDirection (ScrollDirection newValue) {
+    if (_scrollDirection == newValue)
+      return;
+    _scrollDirection = newValue;
+    markNeedsLayout();
+  }
+
   void setupParentData(RenderBox child) {
     if (child.parentData is! ListParentData)
       child.parentData = new ListParentData();
   }
 
-  double get _preferredMainAxisExtent => itemExtent * virtualChildCount;
+  double get _scrollAxisPadding {
+    switch (scrollDirection) {
+      case ScrollDirection.vertical:
+        return padding.vertical;
+      case ScrollDirection.horizontal:
+        return padding.horizontal;
+    }
+  }
 
-  double getMinIntrinsicWidth(BoxConstraints constraints) {
+  double get _preferredExtent {
+    double extent = itemExtent * virtualChildCount;
+    if (padding != null)
+      extent += _scrollAxisPadding;
+    return extent;
+  }
+
+  double _getIntrinsicWidth(BoxConstraints constraints) {
     assert(constraints.isNormalized);
-    return constraints.constrainWidth(0.0);
+    switch (scrollDirection) {
+      case ScrollDirection.vertical:
+        return constraints.constrainWidth(0.0);
+      case ScrollDirection.horizontal:
+        return constraints.constrainWidth(_preferredExtent);
+    }
+  }
+
+  double getMinIntrinsicWidth(BoxConstraints constraints) {
+    return _getIntrinsicWidth(constraints);
   }
 
   double getMaxIntrinsicWidth(BoxConstraints constraints) {
+    return _getIntrinsicWidth(constraints);
+  }
+
+  double _getIntrinsicHeight(BoxConstraints constraints) {
     assert(constraints.isNormalized);
-    return constraints.constrainWidth(0.0);
+    switch (scrollDirection) {
+      case ScrollDirection.vertical:
+        return constraints.constrainHeight(_preferredExtent);
+      case ScrollDirection.horizontal:
+        return constraints.constrainHeight(0.0);
+    }
   }
 
   double getMinIntrinsicHeight(BoxConstraints constraints) {
-    assert(constraints.isNormalized);
-    return constraints.constrainHeight(_preferredMainAxisExtent);
+    return _getIntrinsicHeight(constraints);
   }
 
   double getMaxIntrinsicHeight(BoxConstraints constraints) {
-    assert(constraints.isNormalized);
-    return constraints.constrainHeight(_preferredMainAxisExtent);
+    return _getIntrinsicHeight(constraints);
   }
 
   void performLayout() {
-    double height = _preferredMainAxisExtent;
-    size = new Size(constraints.maxWidth, constraints.constrainHeight(height));
+    size = new Size(constraints.maxWidth,
+                    constraints.constrainHeight(_preferredExtent));
 
     if (callback != null)
       invokeLayoutCallback(callback);
 
+    double itemWidth;
+    double itemHeight;
+
+    double x = 0.0;
+    double dx = 0.0;
+
+    double y = 0.0;
+    double dy = 0.0;
+
+    switch (scrollDirection) {
+      case ScrollDirection.vertical:
+        itemWidth = math.max(0, size.width - (padding == null ? 0.0 : padding.horizontal));
+        itemHeight = itemExtent;
+        y = padding != null ? padding.top : 0.0;
+        dy = itemExtent;
+        break;
+      case ScrollDirection.horizontal:
+        itemWidth = itemExtent;
+        itemHeight = math.max(0, size.height - (padding == null ? 0.0 : padding.vertical));
+        x = padding != null ? padding.left : 0.0;
+        dx = itemExtent;
+        break;
+    }
+
     BoxConstraints innerConstraints =
-        new BoxConstraints.tightFor(width: size.width, height: itemExtent);
+        new BoxConstraints.tightFor(width: itemWidth, height: itemHeight);
 
-    int childIndex = 0;
     RenderBox child = firstChild;
     while (child != null) {
       child.layout(innerConstraints);
+
       final ListParentData childParentData = child.parentData;
-      childParentData.offset = new Offset(0.0, childIndex * itemExtent);
-      childIndex += 1;
+      childParentData.offset = new Offset(x, y);
+      x += dx;
+      y += dy;
+
       assert(child.parentData == childParentData);
       child = childParentData.nextSibling;
     }

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

@@ -18,6 +18,10 @@ enum ScrollDirection {
   vertical,
 }
 
+abstract class HasScrollDirection {
+  ScrollDirection get scrollDirection;
+}
+
 /// A render object that's bigger on the inside.
 ///
 /// The child of a viewport can layout to a larger size than the viewport
@@ -27,7 +31,8 @@ enum ScrollDirection {
 ///
 /// Viewport is the core scrolling primitive in the system, but it can be used
 /// in other situations.
-class RenderViewport extends RenderBox with RenderObjectWithChildMixin<RenderBox> {
+class RenderViewport extends RenderBox with RenderObjectWithChildMixin<RenderBox>
+    implements HasScrollDirection {
 
   RenderViewport({
     RenderBox child,
@@ -177,10 +182,12 @@ abstract class RenderVirtualViewport<T extends ContainerBoxParentDataMixin<Rende
   RenderVirtualViewport({
     int virtualChildCount,
     Offset paintOffset,
-    LayoutCallback callback
+    LayoutCallback callback,
+    Painter overlayPainter
   }) : _virtualChildCount = virtualChildCount,
        _paintOffset = paintOffset,
-       _callback = callback;
+       _callback = callback,
+       _overlayPainter = overlayPainter;
 
   int get virtualChildCount => _virtualChildCount ?? childCount;
   int _virtualChildCount;
@@ -217,8 +224,31 @@ abstract class RenderVirtualViewport<T extends ContainerBoxParentDataMixin<Rende
     markNeedsLayout();
   }
 
+  Painter get overlayPainter => _overlayPainter;
+  Painter _overlayPainter;
+  void set overlayPainter(Painter value) {
+    if (_overlayPainter == value)
+      return;
+    if (attached)
+      _overlayPainter?.detach();
+    _overlayPainter = value;
+    if (attached)
+      _overlayPainter?.attach(this);
+    markNeedsPaint();
+  }
+
+  void attach() {
+    super.attach();
+    _overlayPainter?.attach(this);
+  }
+
+  void detach() {
+    super.detach();
+    _overlayPainter?.detach();
+  }
+
   void applyPaintTransform(RenderBox child, Matrix4 transform) {
-    super.applyPaintTransform(child, transform.translate(paintOffset));
+    super.applyPaintTransform(child, transform.translate(paintOffset.dx, paintOffset.dy));
   }
 
   bool hitTestChildren(HitTestResult result, { Point position }) {
@@ -227,6 +257,7 @@ abstract class RenderVirtualViewport<T extends ContainerBoxParentDataMixin<Rende
 
   void _paintContents(PaintingContext context, Offset offset) {
     defaultPaint(context, offset + paintOffset);
+    _overlayPainter?.paint(context, offset);
   }
 
   void paint(PaintingContext context, Offset offset) {

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

@@ -445,15 +445,19 @@ class Block extends StatelessComponent {
 
 abstract class ScrollableListPainter extends Painter {
   void attach(RenderObject renderObject) {
-    assert(renderObject is RenderBlockViewport);
+    assert(renderObject is RenderBox);
+    assert(renderObject is HasScrollDirection);
     super.attach(renderObject);
   }
 
-  RenderBlockViewport get renderer => renderObject;
+  RenderBox get renderObject => super.renderObject;
 
-  bool get isVertical => renderer.isVertical;
+  ScrollDirection get scrollDirection {
+    HasScrollDirection scrollable = renderObject as dynamic;
+    return scrollable?.scrollDirection;
+  }
 
-  Size get viewportSize => renderer.size;
+  Size get viewportSize => renderObject.size;
 
   double get contentExtent => _contentExtent;
   double _contentExtent = 0.0;
@@ -463,7 +467,7 @@ abstract class ScrollableListPainter extends Painter {
     if (_contentExtent == value)
       return;
     _contentExtent = value;
-    renderer?.markNeedsPaint();
+    renderObject?.markNeedsPaint();
   }
 
   double get scrollOffset => _scrollOffset;
@@ -473,7 +477,7 @@ abstract class ScrollableListPainter extends Painter {
     if (_scrollOffset == value)
       return;
     _scrollOffset = value;
-    renderer?.markNeedsPaint();
+    renderObject?.markNeedsPaint();
   }
 
   /// Called when a scroll starts. Subclasses may override this method to
@@ -675,7 +679,7 @@ class ScrollableList<T> extends ScrollableWidgetList {
     double snapAlignmentOffset: 0.0,
     this.items,
     this.itemBuilder,
-    itemsWrap: false,
+    bool itemsWrap: false,
     double itemExtent,
     EdgeDims padding,
     ScrollableListPainter scrollableListPainter

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

@@ -36,7 +36,7 @@ class ScrollableGrid extends Scrollable {
   );
 
   final GridDelegate delegate;
-  final List<Widget> children;
+  final Iterable<Widget> children;
 
   ScrollableState createState() => new _ScrollableGridState();
 }
@@ -77,7 +77,10 @@ class GridViewport extends VirtualViewport {
   final double startOffset;
   final GridDelegate delegate;
   final ExtentsChangedCallback onExtentsChanged;
-  final List<Widget> children;
+  final Iterable<Widget> children;
+
+  // TODO(abarth): Support horizontal scrolling;
+  ScrollDirection get scrollDirection => ScrollDirection.vertical;
 
   RenderGrid createRenderObject() => new RenderGrid(delegate: delegate);
 
@@ -136,8 +139,8 @@ class _GridViewportElement extends VirtualViewportElement<GridViewport> {
     int materializedRowBase = math.max(0, _lowerBound(_specification.rowOffsets, widget.startOffset) - 1);
     int materializedRowLimit = math.min(_specification.rowCount, _lowerBound(_specification.rowOffsets, widget.startOffset + containerExtent));
 
-    _materializedChildBase = (materializedRowBase * _specification.columnCount).clamp(0, widget.children.length);
-    _materializedChildCount = (materializedRowLimit * _specification.columnCount).clamp(0, widget.children.length) - _materializedChildBase;
+    _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];
 

packages/flutter/lib/src/widgets/scrollable_list.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 'framework.dart';
 import 'scrollable.dart';
 import 'virtual_viewport.dart';
@@ -13,23 +15,31 @@ class ScrollableList2 extends Scrollable {
   ScrollableList2({
     Key key,
     double initialScrollOffset,
+    ScrollDirection scrollDirection: ScrollDirection.vertical,
     ScrollListener onScroll,
     SnapOffsetCallback snapOffsetCallback,
     double snapAlignmentOffset: 0.0,
     this.itemExtent,
+    this.itemsWrap: false,
+    this.padding,
+    this.scrollableListPainter,
     this.children
   }) : super(
     key: key,
     initialScrollOffset: initialScrollOffset,
-    // TODO(abarth): Support horizontal offsets.
-    scrollDirection: ScrollDirection.vertical,
+    scrollDirection: scrollDirection,
     onScroll: onScroll,
     snapOffsetCallback: snapOffsetCallback,
     snapAlignmentOffset: snapAlignmentOffset
-  );
+  ) {
+    assert(itemExtent != null);
+  }
 
   final double itemExtent;
-  final List<Widget> children;
+  final bool itemsWrap;
+  final EdgeDims padding;
+  final ScrollableListPainter scrollableListPainter;
+  final Iterable<Widget> children;
 
   ScrollableState createState() => new _ScrollableList2State();
 }
@@ -39,20 +49,40 @@ class _ScrollableList2State extends ScrollableState<ScrollableList2> {
   ExtentScrollBehavior get scrollBehavior => super.scrollBehavior;
 
   void _handleExtentsChanged(double contentExtent, double containerExtent) {
+    config.scrollableListPainter?.contentExtent = contentExtent;
     setState(() {
       scrollTo(scrollBehavior.updateExtents(
-        contentExtent: contentExtent,
+        contentExtent: config.itemsWrap ? double.INFINITY : contentExtent,
         containerExtent: containerExtent,
         scrollOffset: scrollOffset
       ));
     });
   }
 
+  void dispatchOnScrollStart() {
+    super.dispatchOnScrollStart();
+    config.scrollableListPainter?.scrollStarted();
+  }
+
+  void dispatchOnScroll() {
+    super.dispatchOnScroll();
+    config.scrollableListPainter?.scrollOffset = scrollOffset;
+  }
+
+  void dispatchOnScrollEnd() {
+    super.dispatchOnScrollEnd();
+    config.scrollableListPainter?.scrollEnded();
+  }
+
   Widget buildContent(BuildContext context) {
     return new ListViewport(
+      onExtentsChanged: _handleExtentsChanged,
       startOffset: scrollOffset,
+      scrollDirection: config.scrollDirection,
       itemExtent: config.itemExtent,
-      onExtentsChanged: _handleExtentsChanged,
+      itemsWrap: config.itemsWrap,
+      padding: config.padding,
+      overlayPainter: config.scrollableListPainter,
       children: config.children
     );
   }
@@ -61,16 +91,27 @@ class _ScrollableList2State extends ScrollableState<ScrollableList2> {
 class ListViewport extends VirtualViewport {
   ListViewport({
     Key key,
-    this.startOffset,
-    this.itemExtent,
     this.onExtentsChanged,
+    this.startOffset: 0.0,
+    this.scrollDirection: ScrollDirection.vertical,
+    this.itemExtent,
+    this.itemsWrap: false,
+    this.padding,
+    this.overlayPainter,
     this.children
-  });
+  }) {
+    assert(scrollDirection != null);
+    assert(itemExtent != null);
+  }
 
+  final ExtentsChangedCallback onExtentsChanged;
   final double startOffset;
+  final ScrollDirection scrollDirection;
   final double itemExtent;
-  final ExtentsChangedCallback onExtentsChanged;
-  final List<Widget> children;
+  final bool itemsWrap;
+  final EdgeDims padding;
+  final Painter overlayPainter;
+  final Iterable<Widget> children;
 
   RenderList createRenderObject() => new RenderList(itemExtent: itemExtent);
 
@@ -95,21 +136,39 @@ class _ListViewportElement extends VirtualViewportElement<ListViewport> {
   double _repaintOffsetLimit;
 
   void updateRenderObject() {
+    renderObject.scrollDirection = widget.scrollDirection;
     renderObject.itemExtent = widget.itemExtent;
+    renderObject.padding = widget.padding;
+    renderObject.overlayPainter = widget.overlayPainter;
     super.updateRenderObject();
   }
 
   double _contentExtent;
   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) {
-    double contentExtent = widget.itemExtent * widget.children.length;
-    double containerExtent = renderObject.size.height;
+    int length = renderObject.virtualChildCount;
+    double contentExtent = widget.itemExtent * length;
+    double containerExtent = _getContainerExtentFromRenderObject();
 
-    _materializedChildBase = (widget.startOffset ~/ widget.itemExtent).clamp(0, widget.children.length);
-    int materializedChildLimit = ((widget.startOffset + containerExtent) / widget.itemExtent).ceil().clamp(0, widget.children.length);
-    _materializedChildCount = materializedChildLimit - _materializedChildBase;
+    _materializedChildBase = math.max(0, widget.startOffset ~/ widget.itemExtent);
+    int materializedChildLimit = math.max(0, ((widget.startOffset + containerExtent) / widget.itemExtent).ceil());
 
+    if (!widget.itemsWrap) {
+      _materializedChildBase = math.min(length, _materializedChildBase);
+      materializedChildLimit = math.min(length, materializedChildLimit);
+    }
+
+    _materializedChildCount = materializedChildLimit - _materializedChildBase;
     _repaintOffsetBase = _materializedChildBase * widget.itemExtent;
     _repaintOffsetLimit = materializedChildLimit * widget.itemExtent;
 

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

@@ -11,7 +11,8 @@ typedef void ExtentsChangedCallback(double contentExtent, double containerExtent
 
 abstract class VirtualViewport extends RenderObjectWidget {
   double get startOffset;
-  List<Widget> get children;
+  ScrollDirection get scrollDirection;
+  Iterable<Widget> get children;
 }
 
 abstract class VirtualViewportElement<T extends VirtualViewport> extends RenderObjectElement<T> {
@@ -35,6 +36,8 @@ abstract class VirtualViewportElement<T extends VirtualViewport> extends RenderO
 
   void mount(Element parent, dynamic newSlot) {
     super.mount(parent, newSlot);
+    _iterator = null;
+    _widgets = <Widget>[];
     renderObject.callback = layout;
     updateRenderObject();
   }
@@ -45,6 +48,10 @@ abstract class VirtualViewportElement<T extends VirtualViewport> extends RenderO
   }
 
   void update(T newWidget) {
+    if (widget.children != newWidget.children) {
+      _iterator = null;
+      _widgets = <Widget>[];
+    }
     super.update(newWidget);
     updateRenderObject();
     if (!renderObject.needsLayout)
@@ -52,8 +59,23 @@ abstract class VirtualViewportElement<T extends VirtualViewport> extends RenderO
   }
 
   void _updatePaintOffset() {
-    renderObject.paintOffset =
-    renderObject.paintOffset = new Offset(0.0, -(widget.startOffset - repaintOffsetBase));
+    switch (widget.scrollDirection) {
+      case ScrollDirection.vertical:
+        renderObject.paintOffset = new Offset(0.0, -(widget.startOffset - repaintOffsetBase));
+        break;
+      case ScrollDirection.horizontal:
+        renderObject.paintOffset = new Offset(-(widget.startOffset - repaintOffsetBase), 0.0);
+        break;
+    }
+  }
+
+  double get _containerExtent {
+    switch (widget.scrollDirection) {
+      case ScrollDirection.vertical:
+        return renderObject.size.height;
+      case ScrollDirection.horizontal:
+        return renderObject.size.width;
+    }
   }
 
   void updateRenderObject() {
@@ -67,7 +89,7 @@ abstract class VirtualViewportElement<T extends VirtualViewport> extends RenderO
       if (!renderObject.needsLayout) {
         if (repaintOffsetBase != null && widget.startOffset < repaintOffsetBase)
           renderObject.markNeedsLayout();
-        else if (repaintOffsetLimit != null && widget.startOffset + renderObject.size.height > repaintOffsetLimit)
+        else if (repaintOffsetLimit != null && widget.startOffset + _containerExtent > repaintOffsetLimit)
           renderObject.markNeedsLayout();
       }
     }
@@ -80,15 +102,37 @@ abstract class VirtualViewportElement<T extends VirtualViewport> extends RenderO
     BuildableElement.lockState(_materializeChildren);
   }
 
+  Iterator<Widget> _iterator;
+  List<Widget> _widgets;
+
+  void _populateWidgets(int limit) {
+    if (limit <= _widgets.length)
+      return;
+    if (widget.children is List<Widget>) {
+      _widgets = widget.children;
+      return;
+    }
+    _iterator ??= widget.children.iterator;
+    while (_widgets.length < limit) {
+      bool moved = _iterator.moveNext();
+      assert(moved);
+      Widget current = _iterator.current;
+      assert(current != null);
+      _widgets.add(current);
+    }
+  }
+
   void _materializeChildren() {
     int base = materializedChildBase;
     int count = materializedChildCount;
+    int length = renderObject.virtualChildCount;
     assert(base != null);
     assert(count != null);
+    _populateWidgets(base + count);
     List<Widget> newWidgets = new List<Widget>(count);
     for (int i = 0; i < count; ++i) {
       int childIndex = base + i;
-      Widget child = widget.children[childIndex];
+      Widget child = _widgets[childIndex % length];
       Key key = new ValueKey(child.key ?? childIndex);
       newWidgets[i] = new RepaintBoundary(key: key, child: child);
     }

packages/flutter/test/widget/dismissable_test.dart

@@ -21,7 +21,7 @@ void handleOnDismissed(int item) {
   dismissedItems.add(item);
 }
 
-Widget buildDismissableItem(BuildContext context, int item, int index) {
+Widget buildDismissableItem(int item) {
   return new Dismissable(
     key: new ValueKey<int>(item),
     direction: dismissDirection,
@@ -38,11 +38,12 @@ Widget buildDismissableItem(BuildContext context, int item, int index) {
 Widget widgetBuilder() {
   return new Container(
     padding: const EdgeDims.all(10.0),
-    child: new ScrollableList<int>(
-      items: <int>[0, 1, 2, 3, 4].where((int i) => !dismissedItems.contains(i)).toList(),
-      itemBuilder: buildDismissableItem,
+    child: new ScrollableList2(
       scrollDirection: scrollDirection,
-      itemExtent: itemExtent
+      itemExtent: itemExtent,
+      children: <int>[0, 1, 2, 3, 4].where(
+        (int i) => !dismissedItems.contains(i)
+      ).map(buildDismissableItem)
     )
   );
 }

packages/flutter/test/widget/reparent_state_test.dart

@@ -99,16 +99,15 @@ void main() {
 
       (key.currentState as StateMarkerState).marker = "marked";
 
-      tester.pumpWidget(new ScrollableList<int>(
-        items: <int>[0],
+      tester.pumpWidget(new ScrollableList2(
         itemExtent: 100.0,
-        itemBuilder: (BuildContext context, int item, int index) {
-          return new Container(
+        children: <Widget>[
+          new Container(
             key: new Key('container'),
             height: 100.0,
             child: new StateMarker(key: key)
-          );
-        }
+          )
+        ]
       ));
 
       expect((key.currentState as StateMarkerState).marker, equals("marked"));

packages/flutter/test/widget/scrollable_list_hit_testing_test.dart

@@ -17,20 +17,18 @@ void main() {
       tester.pumpWidget(new Center(
         child: new Container(
           height: 50.0,
-          child: new ScrollableList<int>(
+          child: new ScrollableList2(
             key: new GlobalKey(),
-            items: items,
-            itemBuilder: (BuildContext context, int item, int index) {
+            itemExtent: 290.0,
+            scrollDirection: ScrollDirection.horizontal,
+            children: items.map((int item) {
               return new Container(
-                key: new ValueKey<int>(item),
                 child: new GestureDetector(
                   onTap: () { tapped.add(item); },
                   child: new Text('$item')
                 )
               );
-            },
-            itemExtent: 290.0,
-            scrollDirection: ScrollDirection.horizontal
+            })
           )
         )
       ));
@@ -59,20 +57,18 @@ void main() {
       tester.pumpWidget(new Center(
         child: new Container(
           width: 50.0,
-          child: new ScrollableList<int>(
+          child: new ScrollableList2(
             key: new GlobalKey(),
-            items: items,
-            itemBuilder: (BuildContext context, int item, int index) {
+            itemExtent: 290.0,
+            scrollDirection: ScrollDirection.vertical,
+            children: items.map((int item) {
               return new Container(
-                key: new ValueKey<int>(item),
                 child: new GestureDetector(
                   onTap: () { tapped.add(item); },
                   child: new Text('$item')
                 )
               );
-            },
-            itemExtent: 290.0,
-            scrollDirection: ScrollDirection.vertical
+            })
           )
         )
       ));

packages/flutter/test/widget/scrollable_list_horizontal_test.dart

@@ -13,16 +13,14 @@ Widget buildFrame() {
   return new Center(
     child: new Container(
       height: 50.0,
-      child: new ScrollableList<int>(
-        items: items,
-        itemBuilder: (BuildContext context, int item, int index) {
+      child: new ScrollableList2(
+        itemExtent: 290.0,
+        scrollDirection: ScrollDirection.horizontal,
+        children: items.map((int item) {
           return new Container(
-            key: new ValueKey<int>(item),
             child: new Text('$item')
           );
-        },
-        itemExtent: 290.0,
-        scrollDirection: ScrollDirection.horizontal
+        })
       )
     )
   );

packages/flutter/test/widget/scrollable_list_vertical_test.dart

@@ -9,16 +9,14 @@ import 'package:test/test.dart';
 const List<int> items = const <int>[0, 1, 2, 3, 4, 5];
 
 Widget buildFrame() {
-  return new ScrollableList<int>(
-    items: items,
-    itemBuilder: (BuildContext context, int item, int index) {
+  return new ScrollableList2(
+    itemExtent: 290.0,
+    scrollDirection: ScrollDirection.vertical,
+    children: items.map((int item) {
       return new Container(
-        key: new ValueKey<int>(item),
         child: new Text('$item')
       );
-    },
-    itemExtent: 290.0,
-    scrollDirection: ScrollDirection.vertical
+    })
   );
 }
 

packages/flutter/test/widget/snap_scrolling_test.dart

@@ -12,9 +12,8 @@ const double itemExtent = 200.0;
 ScrollDirection scrollDirection = ScrollDirection.vertical;
 GlobalKey scrollableListKey;
 
-Widget buildItem(BuildContext context, int item, int index) {
+Widget buildItem(int item) {
   return new Container(
-    key: new ValueKey<int>(item),
     width: itemExtent,
     height: itemExtent,
     child: new Text(item.toString())
@@ -30,13 +29,12 @@ Widget buildFrame() {
   return new Center(
     child: new Container(
       height: itemExtent * 2.0,
-      child: new ScrollableList<int>(
+      child: new ScrollableList2(
         key: scrollableListKey,
         snapOffsetCallback: snapOffsetCallback,
         scrollDirection: scrollDirection,
-        items: <int>[0, 1, 2, 3, 4, 5, 7, 8, 9],
-        itemBuilder: buildItem,
-        itemExtent: itemExtent
+        itemExtent: itemExtent,
+        children: <int>[0, 1, 2, 3, 4, 5, 7, 8, 9].map(buildItem)
       )
     )
   );