Remove HomogeneousViewport

The virtual viewport machinery now handles all of these use cases.
Previous clients of ScrollableWidgetList can use ScrollableLazyList
instead.

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

@@ -268,15 +268,15 @@ class DayPicker extends StatelessComponent {
   }
 }
 
-// Scrollable list of DayPickers to allow choosing a month
-class MonthPicker extends ScrollableWidgetList {
+class MonthPicker extends StatefulComponent {
   MonthPicker({
+    Key key,
     this.selectedDate,
     this.onChanged,
     this.firstDate,
     this.lastDate,
-    double itemExtent
-  }) : super(itemExtent: itemExtent) {
+    this.itemExtent
+  }) : super(key: key) {
     assert(selectedDate != null);
     assert(onChanged != null);
     assert(lastDate.isAfter(firstDate));
@@ -286,11 +286,12 @@ class MonthPicker extends ScrollableWidgetList {
   final ValueChanged<DateTime> onChanged;
   final DateTime firstDate;
   final DateTime lastDate;
+  final double itemExtent;
 
   _MonthPickerState createState() => new _MonthPickerState();
 }
 
-class _MonthPickerState extends ScrollableWidgetListState<MonthPicker> {
+class _MonthPickerState extends State<MonthPicker> {
   void initState() {
     super.initState();
     _updateCurrentDate();
@@ -313,8 +314,6 @@ class _MonthPickerState extends ScrollableWidgetListState<MonthPicker> {
     });
   }
 
-  int get itemCount => (config.lastDate.year - config.firstDate.year) * 12 + config.lastDate.month - config.firstDate.month + 1;
-
   List<Widget> buildItems(BuildContext context, int start, int count) {
     List<Widget> result = new List<Widget>();
     DateTime startDate = new DateTime(config.firstDate.year + start ~/ 12, config.firstDate.month + start % 12);
@@ -335,6 +334,14 @@ class _MonthPickerState extends ScrollableWidgetListState<MonthPicker> {
     return result;
   }
 
+  Widget build(BuildContext context) {
+    return new ScrollableLazyList(
+      itemExtent: config.itemExtent,
+      itemCount: (config.lastDate.year - config.firstDate.year) * 12 + config.lastDate.month - config.firstDate.month + 1,
+      itemBuilder: buildItems
+    );
+  }
+
   void dispose() {
     if (_timer != null)
       _timer.cancel();
@@ -343,13 +350,14 @@ class _MonthPickerState extends ScrollableWidgetListState<MonthPicker> {
 }
 
 // Scrollable list of years to allow picking a year
-class YearPicker extends ScrollableWidgetList {
+class YearPicker extends StatefulComponent {
   YearPicker({
+    Key key,
     this.selectedDate,
     this.onChanged,
     this.firstDate,
     this.lastDate
-  }) : super(itemExtent: 50.0) {
+  }) : super(key: key) {
     assert(selectedDate != null);
     assert(onChanged != null);
     assert(lastDate.isAfter(firstDate));
@@ -363,8 +371,8 @@ class YearPicker extends ScrollableWidgetList {
   _YearPickerState createState() => new _YearPickerState();
 }
 
-class _YearPickerState extends ScrollableWidgetListState<YearPicker> {
-  int get itemCount => config.lastDate.year - config.firstDate.year + 1;
+class _YearPickerState extends State<YearPicker> {
+  static const double _itemExtent = 50.0;
 
   List<Widget> buildItems(BuildContext context, int start, int count) {
     TextStyle style = Theme.of(context).text.body1.copyWith(color: Colors.black54);
@@ -379,7 +387,7 @@ class _YearPickerState extends ScrollableWidgetListState<YearPicker> {
           config.onChanged(result);
         },
         child: new Container(
-          height: config.itemExtent,
+          height: _itemExtent,
           decoration: year == config.selectedDate.year ? new BoxDecoration(
             backgroundColor: Theme.of(context).primarySwatch[100],
             shape: BoxShape.circle
@@ -393,4 +401,12 @@ class _YearPickerState extends ScrollableWidgetListState<YearPicker> {
     }
     return items;
   }
+
+  Widget build(BuildContext context) {
+    return new ScrollableLazyList(
+      itemExtent: _itemExtent,
+      itemCount: config.lastDate.year - config.firstDate.year + 1,
+      itemBuilder: buildItems
+    );
+  }
 }

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

@@ -344,6 +344,8 @@ class RenderGrid extends RenderVirtualViewport<GridParentData> {
     _delegate = newDelegate;
   }
 
+  int get virtualChildCount => super.virtualChildCount ?? childCount;
+
   /// The virtual index of the first child.
   ///
   /// When asking the delegate for the position of each child, the grid will add

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

@@ -76,7 +76,10 @@ class RenderList extends RenderVirtualViewport<ListParentData> implements HasScr
   double get _preferredExtent {
     if (itemExtent == null)
       return double.INFINITY;
-    double extent = itemExtent * virtualChildCount;
+    int count = virtualChildCount;
+    if (count == null)
+      return double.INFINITY;
+    double extent = itemExtent * count;
     if (padding != null)
       extent += _scrollAxisPadding;
     return extent;

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

@@ -192,7 +192,7 @@ abstract class RenderVirtualViewport<T extends ContainerBoxParentDataMixin<Rende
        _callback = callback,
        _overlayPainter = overlayPainter;
 
-  int get virtualChildCount => _virtualChildCount ?? childCount;
+  int get virtualChildCount => _virtualChildCount;
   int _virtualChildCount;
   void set virtualChildCount(int value) {
     if (_virtualChildCount == value)

packages/flutter/lib/src/widgets/homogeneous_viewport.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:flutter/rendering.dart';
-
-import 'framework.dart';
-import 'basic.dart';
-
-typedef List<Widget> ListBuilder(BuildContext context, int startIndex, int count);
-
-abstract class _ViewportBase extends RenderObjectWidget {
-  _ViewportBase({
-    Key key,
-    this.builder,
-    this.itemsWrap: false,
-    this.itemCount,
-    this.direction: Axis.vertical,
-    this.startOffset: 0.0,
-    this.overlayPainter
-  }) : super(key: key);
-
-  final ListBuilder builder;
-  final bool itemsWrap;
-  final int itemCount;
-  final Axis direction;
-  final double startOffset;
-  final Painter overlayPainter;
-
-  // we don't pass constructor arguments to the RenderBlockViewport() because until
-  // we know our children, the constructor arguments we could give have no effect
-  RenderBlockViewport createRenderObject() => new RenderBlockViewport();
-
-  bool isLayoutDifferentThan(_ViewportBase oldWidget) {
-    // changing the builder doesn't imply the layout changed
-    return itemsWrap != oldWidget.itemsWrap ||
-           itemCount != oldWidget.itemCount ||
-           direction != oldWidget.direction ||
-           startOffset != oldWidget.startOffset;
-  }
-}
-
-abstract class _ViewportBaseElement<T extends _ViewportBase> extends RenderObjectElement<T> {
-  _ViewportBaseElement(T widget) : super(widget);
-
-  List<Element> _children = const <Element>[];
-  int _layoutFirstIndex;
-  int _layoutItemCount;
-
-  RenderBlockViewport get renderObject => super.renderObject;
-
-  void visitChildren(ElementVisitor visitor) {
-    if (_children == null)
-      return;
-    for (Element child in _children)
-      visitor(child);
-  }
-
-  void mount(Element parent, dynamic newSlot) {
-    super.mount(parent, newSlot);
-    renderObject.callback = layout;
-    renderObject.totalExtentCallback = getTotalExtent;
-    renderObject.minCrossAxisExtentCallback = getMinCrossAxisExtent;
-    renderObject.maxCrossAxisExtentCallback = getMaxCrossAxisExtent;
-    renderObject.overlayPainter = widget.overlayPainter;
-  }
-
-  void unmount() {
-    renderObject.callback = null;
-    renderObject.totalExtentCallback = null;
-    renderObject.minCrossAxisExtentCallback = null;
-    renderObject.maxCrossAxisExtentCallback = null;
-    renderObject.overlayPainter = null;
-    super.unmount();
-  }
-
-  void update(T newWidget) {
-    bool needLayout = newWidget.isLayoutDifferentThan(widget);
-    super.update(newWidget);
-    // TODO(abarth): Don't we need to update overlayPainter here?
-    if (needLayout)
-      renderObject.markNeedsLayout();
-    else
-      _updateChildren();
-  }
-
-  void reinvokeBuilders() {
-    _updateChildren();
-  }
-
-  void layout(BoxConstraints constraints);
-
-  void _updateChildren() {
-    assert(_layoutFirstIndex != null);
-    assert(_layoutItemCount != null);
-    List<Widget> newWidgets;
-    if (_layoutItemCount > 0)
-      newWidgets = widget.builder(this, _layoutFirstIndex, _layoutItemCount).map((Widget widget) {
-        return new RepaintBoundary(key: new ValueKey<Key>(widget.key), child: widget);
-      }).toList();
-    else
-      newWidgets = <Widget>[];
-    _children = updateChildren(_children, newWidgets);
-  }
-
-  double getTotalExtent(BoxConstraints constraints);
-
-  double getMinCrossAxisExtent(BoxConstraints constraints) {
-    return 0.0;
-  }
-
-  double getMaxCrossAxisExtent(BoxConstraints constraints) {
-    if (widget.direction == Axis.vertical)
-      return constraints.maxWidth;
-    return constraints.maxHeight;
-  }
-
-  void insertChildRenderObject(RenderObject child, Element slot) {
-    renderObject.insert(child, after: slot?.renderObject);
-  }
-
-  void moveChildRenderObject(RenderObject child, Element slot) {
-    assert(child.parent == renderObject);
-    renderObject.move(child, after: slot?.renderObject);
-  }
-
-  void removeChildRenderObject(RenderObject child) {
-    assert(child.parent == renderObject);
-    renderObject.remove(child);
-  }
-
-}
-
-class HomogeneousViewport extends _ViewportBase {
-  HomogeneousViewport({
-    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
-    Axis direction: Axis.vertical,
-    double startOffset: 0.0,
-    Painter overlayPainter,
-    this.itemExtent // required, must be non-zero
-  }) : super(
-    key: key,
-    builder: builder,
-    itemsWrap: itemsWrap,
-    itemCount: itemCount,
-    direction: direction,
-    startOffset: startOffset,
-    overlayPainter: overlayPainter
-  ) {
-    assert(itemExtent != null);
-    assert(itemExtent > 0);
-  }
-
-  final double itemExtent;
-
-  _HomogeneousViewportElement createElement() => new _HomogeneousViewportElement(this);
-
-  bool isLayoutDifferentThan(HomogeneousViewport oldWidget) {
-    return itemExtent != oldWidget.itemExtent || super.isLayoutDifferentThan(oldWidget);
-  }
-}
-
-class _HomogeneousViewportElement extends _ViewportBaseElement<HomogeneousViewport> {
-  _HomogeneousViewportElement(HomogeneousViewport 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 mainAxisExtent = widget.direction == Axis.vertical ? constraints.maxHeight : constraints.maxWidth;
-      double offset;
-      if (widget.startOffset <= 0.0 && !widget.itemsWrap) {
-        _layoutFirstIndex = 0;
-        offset = -widget.startOffset;
-      } else {
-        _layoutFirstIndex = (widget.startOffset / widget.itemExtent).floor();
-        offset = -(widget.startOffset % widget.itemExtent);
-      }
-      if (mainAxisExtent < double.INFINITY) {
-        _layoutItemCount = ((mainAxisExtent - offset) / widget.itemExtent).ceil();
-        if (widget.itemCount != null && !widget.itemsWrap)
-          _layoutItemCount = math.min(_layoutItemCount, widget.itemCount - _layoutFirstIndex);
-      } else {
-        assert(() {
-          'This HomogeneousViewport 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 HomogeneousViewport (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;
-      renderObject.itemExtent = widget.itemExtent;
-      renderObject.minExtent = getTotalExtent(null);
-      renderObject.startOffset = offset;
-      renderObject.overlayPainter = widget.overlayPainter;
-    }, building: true);
-  }
-
-  double getTotalExtent(BoxConstraints constraints) {
-    // constraints is null when called by layout() above
-    return widget.itemCount != null ? widget.itemCount * widget.itemExtent : double.INFINITY;
-  }
-}

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

@@ -180,9 +180,8 @@ class PageableListState<T extends PageableList> extends ScrollableState<T> {
   }
 }
 
-class PageViewport extends VirtualViewport {
+class PageViewport extends VirtualViewport with VirtualViewportIterableMixin {
   PageViewport({
-    Key key,
     this.startOffset: 0.0,
     this.scrollDirection: Axis.vertical,
     this.itemsWrap: false,

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

@@ -14,7 +14,6 @@ import 'package:flutter/rendering.dart';
 import 'basic.dart';
 import 'framework.dart';
 import 'gesture_detector.dart';
-import 'homogeneous_viewport.dart';
 import 'mixed_viewport.dart';
 import 'notification_listener.dart';
 import 'page_storage.dart';
@@ -523,178 +522,6 @@ abstract class ScrollableListPainter extends Painter {
   Future scrollEnded() => new Future.value();
 }
 
-/// An optimized scrollable widget for a large number of children that are all
-/// the same size (extent) in the scrollDirection. For example for
-/// ScrollDirection.vertical itemExtent is the height of each item. Use this
-/// widget when you have a large number of children or when you are concerned
-// about offscreen widgets consuming resources.
-abstract class ScrollableWidgetList extends Scrollable {
-  ScrollableWidgetList({
-    Key key,
-    double initialScrollOffset,
-    Axis scrollDirection: Axis.vertical,
-    ScrollListener onScroll,
-    SnapOffsetCallback snapOffsetCallback,
-    double snapAlignmentOffset: 0.0,
-    this.itemsWrap: false,
-    this.itemExtent,
-    this.padding,
-    this.scrollableListPainter
-  }) : super(
-    key: key,
-    initialScrollOffset: initialScrollOffset,
-    scrollDirection: scrollDirection,
-    onScroll: onScroll,
-    snapOffsetCallback: snapOffsetCallback,
-    snapAlignmentOffset: snapAlignmentOffset
-  ) {
-    assert(itemExtent != null);
-  }
-
-  final bool itemsWrap;
-  final double itemExtent;
-  final EdgeDims padding;
-  final ScrollableListPainter scrollableListPainter;
-}
-
-abstract class ScrollableWidgetListState<T extends ScrollableWidgetList> extends ScrollableState<T> {
-  /// Subclasses must implement `get itemCount` to tell ScrollableWidgetList
-  /// how many items there are in the list.
-  int get itemCount;
-  int _previousItemCount;
-
-  Size _containerSize = Size.zero;
-
-  void didUpdateConfig(T oldConfig) {
-    super.didUpdateConfig(oldConfig);
-
-    bool scrollBehaviorUpdateNeeded =
-      config.padding != oldConfig.padding ||
-      config.itemExtent != oldConfig.itemExtent ||
-      config.scrollDirection != oldConfig.scrollDirection;
-
-    if (config.itemsWrap != oldConfig.itemsWrap) {
-      _scrollBehavior = null;
-      scrollBehaviorUpdateNeeded = true;
-    }
-
-    if (itemCount != _previousItemCount) {
-      _previousItemCount = itemCount;
-      scrollBehaviorUpdateNeeded = true;
-    }
-
-    if (scrollBehaviorUpdateNeeded)
-      _updateScrollBehavior();
-  }
-
-  ScrollBehavior createScrollBehavior() => new OverscrollBehavior();
-  ExtentScrollBehavior get scrollBehavior => super.scrollBehavior;
-
-  double get _containerExtent {
-    return config.scrollDirection == Axis.vertical
-      ? _containerSize.height
-      : _containerSize.width;
-  }
-
-  void _handleSizeChanged(Size newSize) {
-    setState(() {
-      _containerSize = newSize;
-      _updateScrollBehavior();
-    });
-  }
-
-  double get _leadingPadding {
-    EdgeDims padding = config.padding;
-    if (config.scrollDirection == Axis.vertical)
-      return padding != null ? padding.top : 0.0;
-    return padding != null ? padding.left : -.0;
-  }
-
-  double get _trailingPadding {
-    EdgeDims padding = config.padding;
-    if (config.scrollDirection == Axis.vertical)
-      return padding != null ? padding.bottom : 0.0;
-    return padding != null ? padding.right : 0.0;
-  }
-
-  EdgeDims get _crossAxisPadding {
-    EdgeDims padding = config.padding;
-    if (padding == null)
-      return null;
-    if (config.scrollDirection == Axis.vertical)
-      return new EdgeDims.only(left: padding.left, right: padding.right);
-    return new EdgeDims.only(top: padding.top, bottom: padding.bottom);
-  }
-
-  double get _contentExtent {
-    if (itemCount == null)
-      return null;
-    double contentExtent = config.itemExtent * itemCount;
-    if (config.padding != null)
-      contentExtent += _leadingPadding + _trailingPadding;
-    return contentExtent;
-  }
-
-  void _updateScrollBehavior() {
-    // if you don't call this from build(), you must call it from setState().
-    if (config.scrollableListPainter != null)
-      config.scrollableListPainter.contentExtent = _contentExtent;
-    scrollTo(scrollBehavior.updateExtents(
-      contentExtent: _contentExtent,
-      containerExtent: _containerExtent,
-      scrollOffset: scrollOffset
-    ));
-  }
-
-  void dispatchOnScrollStart() {
-    super.dispatchOnScrollStart();
-    config.scrollableListPainter?.scrollStarted();
-  }
-
-  void dispatchOnScroll() {
-    super.dispatchOnScroll();
-    if (config.scrollableListPainter != null)
-      config.scrollableListPainter.scrollOffset = scrollOffset;
-  }
-
-  void dispatchOnScrollEnd() {
-    super.dispatchOnScrollEnd();
-    config.scrollableListPainter?.scrollEnded();
-  }
-
-  Widget buildContent(BuildContext context) {
-    if (itemCount != _previousItemCount) {
-      _previousItemCount = itemCount;
-      _updateScrollBehavior();
-    }
-
-    return new SizeObserver(
-      onSizeChanged: _handleSizeChanged,
-      child: new Container(
-        padding: _crossAxisPadding,
-        child: new HomogeneousViewport(
-          builder: _buildItems,
-          itemsWrap: config.itemsWrap,
-          itemExtent: config.itemExtent,
-          itemCount: itemCount,
-          direction: config.scrollDirection,
-          startOffset: scrollOffset - _leadingPadding,
-          overlayPainter: config.scrollableListPainter
-        )
-      )
-    );
-  }
-
-  List<Widget> _buildItems(BuildContext context, int start, int count) {
-    List<Widget> result = buildItems(context, start, count);
-    assert(result.every((Widget item) => item.key != null));
-    return result;
-  }
-
-  List<Widget> buildItems(BuildContext context, int start, int count);
-
-}
-
 /// 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

@@ -65,9 +65,8 @@ class _ScrollableGridState extends ScrollableState<ScrollableGrid> {
   }
 }
 
-class GridViewport extends VirtualViewport {
+class GridViewport extends VirtualViewport with VirtualViewportIterableMixin {
   GridViewport({
-    Key key,
     this.startOffset,
     this.delegate,
     this.onExtentsChanged,

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

@@ -88,18 +88,16 @@ class _ScrollableListState extends ScrollableState<ScrollableList> {
   }
 }
 
-class ListViewport extends VirtualViewport {
-  ListViewport({
-    Key key,
+class _VirtualListViewport extends VirtualViewport {
+  _VirtualListViewport(
     this.onExtentsChanged,
-    this.startOffset: 0.0,
-    this.scrollDirection: Axis.vertical,
+    this.startOffset,
+    this.scrollDirection,
     this.itemExtent,
-    this.itemsWrap: false,
+    this.itemsWrap,
     this.padding,
-    this.overlayPainter,
-    this.children
-  }) {
+    this.overlayPainter
+  ) {
     assert(scrollDirection != null);
     assert(itemExtent != null);
   }
@@ -111,15 +109,14 @@ class ListViewport extends VirtualViewport {
   final bool itemsWrap;
   final EdgeDims padding;
   final Painter overlayPainter;
-  final Iterable<Widget> children;
 
   RenderList createRenderObject() => new RenderList(itemExtent: itemExtent);
 
-  _ListViewportElement createElement() => new _ListViewportElement(this);
+  _VirtualListViewportElement createElement() => new _VirtualListViewportElement(this);
 }
 
-class _ListViewportElement extends VirtualViewportElement<ListViewport> {
-  _ListViewportElement(ListViewport widget) : super(widget);
+class _VirtualListViewportElement extends VirtualViewportElement<_VirtualListViewport> {
+  _VirtualListViewportElement(VirtualViewport widget) : super(widget);
 
   RenderList get renderObject => super.renderObject;
 
@@ -135,11 +132,12 @@ class _ListViewportElement extends VirtualViewportElement<ListViewport> {
   double get startOffsetLimit =>_startOffsetLimit;
   double _startOffsetLimit;
 
-  void updateRenderObject(ListViewport oldWidget) {
-    renderObject.scrollDirection = widget.scrollDirection;
-    renderObject.itemExtent = widget.itemExtent;
-    renderObject.padding = widget.padding;
-    renderObject.overlayPainter = widget.overlayPainter;
+  void updateRenderObject(_VirtualListViewport oldWidget) {
+    renderObject
+      ..scrollDirection = widget.scrollDirection
+      ..itemExtent = widget.itemExtent
+      ..padding = widget.padding
+      ..overlayPainter = widget.overlayPainter;
     super.updateRenderObject(oldWidget);
   }
 
@@ -160,13 +158,13 @@ class _ListViewportElement extends VirtualViewportElement<ListViewport> {
     final double itemExtent = widget.itemExtent;
     final EdgeDims padding = widget.padding ?? EdgeDims.zero;
 
-    double contentExtent = widget.itemExtent * length + padding.top + padding.bottom;
+    double contentExtent = length == null ? double.INFINITY : widget.itemExtent * length + padding.top + padding.bottom;
     double containerExtent = _getContainerExtentFromRenderObject();
 
     _materializedChildBase = math.max(0, (widget.startOffset - padding.top) ~/ itemExtent);
     int materializedChildLimit = math.max(0, ((widget.startOffset + containerExtent) / itemExtent).ceil());
 
-    if (!widget.itemsWrap) {
+    if (!widget.itemsWrap && length != null) {
       _materializedChildBase = math.min(length, _materializedChildBase);
       materializedChildLimit = math.min(length, materializedChildLimit);
     } else if (length == 0) {
@@ -186,3 +184,133 @@ class _ListViewportElement extends VirtualViewportElement<ListViewport> {
     }
   }
 }
+
+class ListViewport extends _VirtualListViewport with VirtualViewportIterableMixin {
+  ListViewport({
+    ExtentsChangedCallback onExtentsChanged,
+    double startOffset: 0.0,
+    Axis scrollDirection: Axis.vertical,
+    double itemExtent,
+    bool itemsWrap: false,
+    EdgeDims padding,
+    Painter overlayPainter,
+    this.children
+  }) : super(
+    onExtentsChanged,
+    startOffset,
+    scrollDirection,
+    itemExtent,
+    itemsWrap,
+    padding,
+    overlayPainter
+  );
+
+  final Iterable<Widget> children;
+}
+
+/// An optimized scrollable widget for a large number of children that are all
+/// the same size (extent) in the scrollDirection. For example for
+/// ScrollDirection.vertical itemExtent is the height of each item. Use this
+/// widget when you have a large number of children or when you are concerned
+// about offscreen widgets consuming resources.
+class ScrollableLazyList extends Scrollable {
+  ScrollableLazyList({
+    Key key,
+    double initialScrollOffset,
+    Axis scrollDirection: Axis.vertical,
+    ScrollListener onScroll,
+    SnapOffsetCallback snapOffsetCallback,
+    double snapAlignmentOffset: 0.0,
+    this.itemExtent,
+    this.itemCount,
+    this.itemBuilder,
+    this.padding,
+    this.scrollableListPainter
+  }) : super(
+    key: key,
+    initialScrollOffset: initialScrollOffset,
+    scrollDirection: scrollDirection,
+    onScroll: onScroll,
+    snapOffsetCallback: snapOffsetCallback,
+    snapAlignmentOffset: snapAlignmentOffset
+  ) {
+    assert(itemExtent != null);
+    assert(itemBuilder != null);
+  }
+
+  final double itemExtent;
+  final int itemCount;
+  final ItemListBuilder itemBuilder;
+  final EdgeDims padding;
+  final ScrollableListPainter scrollableListPainter;
+
+  ScrollableState createState() => new _ScrollableLazyListState();
+}
+
+class _ScrollableLazyListState extends ScrollableState<ScrollableLazyList> {
+  ScrollBehavior createScrollBehavior() => new OverscrollBehavior();
+  ExtentScrollBehavior get scrollBehavior => super.scrollBehavior;
+
+  void _handleExtentsChanged(double contentExtent, double containerExtent) {
+    config.scrollableListPainter?.contentExtent = contentExtent;
+    setState(() {
+      scrollTo(scrollBehavior.updateExtents(
+        contentExtent: 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 LazyListViewport(
+      onExtentsChanged: _handleExtentsChanged,
+      startOffset: scrollOffset,
+      scrollDirection: config.scrollDirection,
+      itemExtent: config.itemExtent,
+      itemCount: config.itemCount,
+      itemBuilder: config.itemBuilder,
+      padding: config.padding,
+      overlayPainter: config.scrollableListPainter
+    );
+  }
+}
+
+class LazyListViewport extends _VirtualListViewport with VirtualViewportLazyMixin {
+  LazyListViewport({
+    ExtentsChangedCallback onExtentsChanged,
+    double startOffset: 0.0,
+    Axis scrollDirection: Axis.vertical,
+    double itemExtent,
+    EdgeDims padding,
+    Painter overlayPainter,
+    this.itemCount,
+    this.itemBuilder
+  }) : super(
+    onExtentsChanged,
+    startOffset,
+    scrollDirection,
+    itemExtent,
+    false, // Don't support wrapping yet.
+    padding,
+    overlayPainter
+  );
+
+  final int itemCount;
+  final ItemListBuilder itemBuilder;
+}

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

@@ -14,7 +14,15 @@ typedef void ExtentsChangedCallback(double contentExtent, double containerExtent
 abstract class VirtualViewport extends RenderObjectWidget {
   double get startOffset;
   Axis get scrollDirection;
-  Iterable<Widget> get children;
+
+  _WidgetProvider _createWidgetProvider();
+}
+
+abstract class _WidgetProvider {
+  void didUpdateWidget(VirtualViewport oldWidget, VirtualViewport newWidget);
+  int get virtualChildCount;
+  void prepareChildren(VirtualViewportElement context, int base, int count);
+  Widget getChild(int i);
 }
 
 abstract class VirtualViewportElement<T extends VirtualViewport> extends RenderObjectElement<T> {
@@ -38,10 +46,12 @@ abstract class VirtualViewportElement<T extends VirtualViewport> extends RenderO
       visitor(child);
   }
 
+  _WidgetProvider _widgetProvider;
+
   void mount(Element parent, dynamic newSlot) {
+    _widgetProvider = widget._createWidgetProvider();
+    _widgetProvider.didUpdateWidget(null, widget);
     super.mount(parent, newSlot);
-    _iterator = null;
-    _widgets = <Widget>[];
     renderObject.callback = layout;
     updateRenderObject(null);
   }
@@ -52,11 +62,8 @@ abstract class VirtualViewportElement<T extends VirtualViewport> extends RenderO
   }
 
   void update(T newWidget) {
-    if (widget.children != newWidget.children) {
-      _iterator = null;
-      _widgets = <Widget>[];
-    }
     T oldWidget = widget;
+    _widgetProvider.didUpdateWidget(oldWidget, newWidget);
     super.update(newWidget);
     updateRenderObject(oldWidget);
     if (!renderObject.needsLayout)
@@ -75,7 +82,7 @@ abstract class VirtualViewportElement<T extends VirtualViewport> extends RenderO
   }
 
   void updateRenderObject(T oldWidget) {
-    renderObject.virtualChildCount = widget.children.length;
+    renderObject.virtualChildCount = _widgetProvider.virtualChildCount;
 
     if (startOffsetBase != null) {
       _updatePaintOffset();
@@ -111,37 +118,16 @@ abstract class VirtualViewportElement<T extends VirtualViewport> extends RenderO
     BuildableElement.lockState(_materializeChildren, building: true);
   }
 
-  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 < 0 ? length : math.min(length, base + count));
+    _widgetProvider.prepareChildren(this, 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).abs()];
+      Widget child = _widgetProvider.getChild(childIndex);
       Key key = new ValueKey(child.key ?? childIndex);
       newWidgets[i] = new RepaintBoundary(key: key, child: child);
     }
@@ -162,3 +148,84 @@ abstract class VirtualViewportElement<T extends VirtualViewport> extends RenderO
     renderObject.remove(child);
   }
 }
+
+abstract class VirtualViewportIterableMixin extends VirtualViewport {
+  Iterable<Widget> get children;
+
+  _IterableWidgetProvider _createWidgetProvider() => new _IterableWidgetProvider();
+}
+
+class _IterableWidgetProvider extends _WidgetProvider {
+  int _length;
+  Iterator<Widget> _iterator;
+  List<Widget> _widgets;
+
+  void didUpdateWidget(VirtualViewportIterableMixin oldWidget, VirtualViewportIterableMixin newWidget) {
+    if (oldWidget == null || newWidget.children != oldWidget.children) {
+      _iterator = null;
+      _widgets = <Widget>[];
+      _length = newWidget.children.length;
+    }
+  }
+
+  int get virtualChildCount => _length;
+
+  void prepareChildren(VirtualViewportElement context, int base, int count) {
+    int limit = base < 0 ? _length : math.min(_length, base + count);
+    if (limit <= _widgets.length)
+      return;
+    VirtualViewportIterableMixin widget = context.widget;
+    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);
+    }
+  }
+
+  Widget getChild(int i) => _widgets[(i % _length).abs()];
+}
+
+typedef List<Widget> ItemListBuilder(BuildContext context, int start, int count);
+
+abstract class VirtualViewportLazyMixin extends VirtualViewport {
+  int get itemCount;
+  ItemListBuilder get itemBuilder;
+
+  _LazyWidgetProvider _createWidgetProvider() => new _LazyWidgetProvider();
+}
+
+class _LazyWidgetProvider extends _WidgetProvider {
+  int _length;
+  int _base;
+  List<Widget> _widgets;
+
+  void didUpdateWidget(VirtualViewportLazyMixin oldWidget, VirtualViewportLazyMixin newWidget) {
+    if (_length != newWidget.itemCount || oldWidget?.itemBuilder != newWidget.itemBuilder) {
+      _length = newWidget.itemCount;
+      _base = null;
+      _widgets = null;
+    }
+  }
+
+  int get virtualChildCount => _length;
+
+  void prepareChildren(VirtualViewportElement context, int base, int count) {
+    if (_widgets != null && _widgets.length == count && _base == base)
+      return;
+    VirtualViewportLazyMixin widget = context.widget;
+    _base = base;
+    _widgets = widget.itemBuilder(context, base, count);
+  }
+
+  Widget getChild(int i) {
+    int n = _length ?? _widgets.length;
+    return _widgets[(i % n).abs()];
+  }
+}

packages/flutter/lib/widgets.dart

@@ -18,7 +18,6 @@ export 'src/widgets/framework.dart';
 export 'src/widgets/gesture_detector.dart';
 export 'src/widgets/gridpaper.dart';
 export 'src/widgets/heroes.dart';
-export 'src/widgets/homogeneous_viewport.dart';
 export 'src/widgets/implicit_animations.dart';
 export 'src/widgets/locale_query.dart';
 export 'src/widgets/media_query.dart';

packages/flutter/test/widget/remember_scroll_position_test.dart

@@ -6,26 +6,20 @@ import 'package:flutter_test/flutter_test.dart';
 import 'package:flutter/material.dart';
 import 'package:test/test.dart';
 
-class ThePositiveNumbers extends ScrollableWidgetList {
-  ThePositiveNumbers() : super(itemExtent: 100.0);
-  ThePositiveNumbersState createState() => new ThePositiveNumbersState();
-}
-
-class ThePositiveNumbersState extends ScrollableWidgetListState<ThePositiveNumbers> {
-
-  ScrollBehavior createScrollBehavior() => new UnboundedBehavior();
-
-  int get itemCount => null;
-
-  List<Widget> buildItems(BuildContext context, int start, int count) {
-    List<Widget> result = new List<Widget>();
-    for (int index = start; index < start + count; index += 1)
-      result.add(new Text('$index', key: new ValueKey<int>(index)));
-    return result;
+class ThePositiveNumbers extends StatelessComponent {
+  Widget build(BuildContext context) {
+    return new ScrollableLazyList(
+      itemExtent: 100.0,
+      itemBuilder: (BuildContext context, int start, int count) {
+        List<Widget> result = new List<Widget>();
+        for (int index = start; index < start + count; index += 1)
+          result.add(new Text('$index', key: new ValueKey<int>(index)));
+        return result;
+      }
+    );
   }
 }
 
-
 void main() {
   test('whether we remember our scroll position', () {
     testWidgets((WidgetTester tester) {
@@ -53,8 +47,8 @@ void main() {
       expect(tester.findText('10'), isNull);
       expect(tester.findText('100'), isNull);
 
-      StatefulComponentElement<ThePositiveNumbers, ThePositiveNumbersState> target =
-        tester.findElement((Element element) => element.widget is ThePositiveNumbers);
+      StatefulComponentElement<ScrollableLazyList, ScrollableState<ScrollableLazyList>> target =
+        tester.findElement((Element element) => element.widget is ScrollableLazyList);
       target.state.scrollTo(1000.0);
       tester.pump(new Duration(seconds: 1));
 

packages/flutter/test/widget/homogeneous_viewport_test.dart → packages/flutter/test/widget/scrollable_lazy_list_test.dart

@@ -18,8 +18,8 @@ void main() {
 
       Widget builder() {
         return new FlipComponent(
-          left: new HomogeneousViewport(
-            builder: (BuildContext context, int start, int count) {
+          left: new ScrollableLazyList(
+            itemBuilder: (BuildContext context, int start, int count) {
               List<Widget> result = <Widget>[];
               for (int index = start; index < start + count; index += 1) {
                 callbackTracker.add(index);
@@ -31,7 +31,6 @@ void main() {
               }
               return result;
             },
-            startOffset: 0.0,
             itemExtent: 100.0
           ),
           right: new Text('Not Today')
@@ -67,9 +66,7 @@ void main() {
       // so if our widget is 200 pixels tall, it should fit exactly 3 times.
       // but if we are offset by 300 pixels, there will be 4, numbered 1-4.
 
-      double offset = 300.0;
-
-      ListBuilder itemBuilder = (BuildContext context, int start, int count) {
+      ItemListBuilder itemBuilder = (BuildContext context, int start, int count) {
         List<Widget> result = <Widget>[];
         for (int index = start; index < start + count; index += 1) {
           callbackTracker.add(index);
@@ -83,28 +80,27 @@ void main() {
         return result;
       };
 
-      FlipComponent testComponent;
-      Widget builder() {
-        testComponent = new FlipComponent(
-          left: new HomogeneousViewport(
-            builder: itemBuilder,
-            startOffset: offset,
-            itemExtent: 200.0
-          ),
-          right: new Text('Not Today')
-        );
-        return testComponent;
-      }
+      GlobalKey<ScrollableState<ScrollableLazyList>> scrollableKey = new GlobalKey<ScrollableState<ScrollableLazyList>>();
+      FlipComponent testComponent = new FlipComponent(
+        left: new ScrollableLazyList(
+          key: scrollableKey,
+          itemBuilder: itemBuilder,
+          itemExtent: 200.0,
+          initialScrollOffset: 300.0
+        ),
+        right: new Text('Not Today')
+      );
 
-      tester.pumpWidget(builder());
+      tester.pumpWidget(testComponent);
 
       expect(callbackTracker, equals([1, 2, 3, 4]));
 
       callbackTracker.clear();
 
-      offset = 400.0; // now only 3 should fit, numbered 2-4.
+      scrollableKey.currentState.scrollTo(400.0);
+      // now only 3 should fit, numbered 2-4.
 
-      tester.pumpWidget(builder());
+      tester.pumpWidget(testComponent);
 
       expect(callbackTracker, equals([2, 3, 4]));
 
@@ -120,9 +116,7 @@ void main() {
       // so if our widget is 200 pixels wide, it should fit exactly 4 times.
       // but if we are offset by 300 pixels, there will be 5, numbered 1-5.
 
-      double offset = 300.0;
-
-      ListBuilder itemBuilder = (BuildContext context, int start, int count) {
+      ItemListBuilder itemBuilder = (BuildContext context, int start, int count) {
         List<Widget> result = <Widget>[];
         for (int index = start; index < start + count; index += 1) {
           callbackTracker.add(index);
@@ -136,29 +130,28 @@ void main() {
         return result;
       };
 
-      FlipComponent testComponent;
-      Widget builder() {
-        testComponent = new FlipComponent(
-          left: new HomogeneousViewport(
-            builder: itemBuilder,
-            startOffset: offset,
-            itemExtent: 200.0,
-            direction: Axis.horizontal
-          ),
-          right: new Text('Not Today')
-        );
-        return testComponent;
-      }
+      GlobalKey<ScrollableState<ScrollableLazyList>> scrollableKey = new GlobalKey<ScrollableState<ScrollableLazyList>>();
+      FlipComponent testComponent = new FlipComponent(
+        left: new ScrollableLazyList(
+          key: scrollableKey,
+          itemBuilder: itemBuilder,
+          itemExtent: 200.0,
+          initialScrollOffset: 300.0,
+          scrollDirection: Axis.horizontal
+        ),
+        right: new Text('Not Today')
+      );
 
-      tester.pumpWidget(builder());
+      tester.pumpWidget(testComponent);
 
       expect(callbackTracker, equals([1, 2, 3, 4, 5]));
 
       callbackTracker.clear();
 
-      offset = 400.0; // now only 4 should fit, numbered 2-5.
+      scrollableKey.currentState.scrollTo(400.0);
+      // now only 4 should fit, numbered 2-5.
 
-      tester.pumpWidget(builder());
+      tester.pumpWidget(testComponent);
 
       expect(callbackTracker, equals([2, 3, 4, 5]));