Combine all the scrollable classes into scrollable.dart

We ended up with a bunch of different scrollable widgets and it was confusing.
This CL moves them all into one library to make them easier to find.

packages/flutter/example/address_book/lib/main.dart

@@ -17,7 +17,7 @@ import 'package:sky/widgets/icon_button.dart';
 import 'package:sky/widgets/material.dart';
 import 'package:sky/widgets/navigator.dart';
 import 'package:sky/widgets/scaffold.dart';
-import 'package:sky/widgets/scrollable_viewport.dart';
+import 'package:sky/widgets/scrollable.dart';
 import 'package:sky/widgets/task_description.dart';
 import 'package:sky/widgets/theme.dart';
 import 'package:sky/widgets/tool_bar.dart';

packages/flutter/example/demo_launcher/lib/main.dart

@@ -16,7 +16,7 @@ import 'package:sky/widgets/scaffold.dart';
 import 'package:sky/widgets/task_description.dart';
 import 'package:sky/widgets/theme.dart';
 import 'package:sky/widgets/tool_bar.dart';
-import 'package:sky/widgets/scrollable_list.dart';
+import 'package:sky/widgets/scrollable.dart';
 import 'package:sky/widgets/ink_well.dart';
 
 AssetBundle _initBundle() {

packages/flutter/example/fitness/lib/main.dart

@@ -25,8 +25,7 @@ import 'package:sky/widgets/ink_well.dart';
 import 'package:sky/widgets/material.dart';
 import 'package:sky/widgets/radio.dart';
 import 'package:sky/widgets/scaffold.dart';
-import 'package:sky/widgets/scrollable_list.dart';
-import 'package:sky/widgets/scrollable_viewport.dart';
+import 'package:sky/widgets/scrollable.dart';
 import 'package:sky/widgets/snack_bar.dart';
 import 'package:sky/widgets/tool_bar.dart';
 import 'package:sky/widgets/card.dart';

packages/flutter/example/stocks/lib/main.dart

@@ -7,40 +7,39 @@ library stocks;
 import 'dart:math' as math;
 import 'dart:sky' as sky;
 
+import 'package:sky/editing/input.dart';
+import 'package:sky/painting/text_style.dart';
+import 'package:sky/rendering/box.dart';
 import 'package:sky/theme/colors.dart' as colors;
 import 'package:sky/theme/typography.dart' as typography;
 import 'package:sky/widgets/basic.dart';
+import 'package:sky/widgets/checkbox.dart';
 import 'package:sky/widgets/default_text_style.dart';
-import 'package:sky/widgets/navigator.dart';
-import 'package:sky/widgets/theme.dart';
-import 'package:sky/widgets/widget.dart';
-import 'package:sky/widgets/task_description.dart';
-import 'package:sky/rendering/box.dart';
-import 'package:sky/editing/input.dart';
-import 'package:sky/widgets/drawer.dart';
+import 'package:sky/widgets/dialog.dart';
 import 'package:sky/widgets/drawer_divider.dart';
 import 'package:sky/widgets/drawer_header.dart';
 import 'package:sky/widgets/drawer_item.dart';
+import 'package:sky/widgets/drawer.dart';
+import 'package:sky/widgets/flat_button.dart';
 import 'package:sky/widgets/floating_action_button.dart';
-import 'package:sky/widgets/icon.dart';
 import 'package:sky/widgets/icon_button.dart';
+import 'package:sky/widgets/icon.dart';
+import 'package:sky/widgets/ink_well.dart';
+import 'package:sky/widgets/material.dart';
 import 'package:sky/widgets/modal_overlay.dart';
+import 'package:sky/widgets/navigator.dart';
+import 'package:sky/widgets/popup_menu_item.dart';
 import 'package:sky/widgets/popup_menu.dart';
 import 'package:sky/widgets/radio.dart';
-import 'package:sky/widgets/snack_bar.dart';
 import 'package:sky/widgets/scaffold.dart';
+import 'package:sky/widgets/scrollable.dart';
+import 'package:sky/widgets/snack_bar.dart';
+import 'package:sky/widgets/switch.dart';
 import 'package:sky/widgets/tabs.dart';
+import 'package:sky/widgets/task_description.dart';
+import 'package:sky/widgets/theme.dart';
 import 'package:sky/widgets/tool_bar.dart';
-import 'package:sky/widgets/scrollable_list.dart';
-import 'package:sky/widgets/material.dart';
-import 'package:sky/widgets/checkbox.dart';
-import 'package:sky/widgets/popup_menu_item.dart';
-import 'package:sky/painting/text_style.dart';
-import 'package:sky/widgets/ink_well.dart';
-import 'package:sky/widgets/dialog.dart';
-import 'package:sky/widgets/flat_button.dart';
-import 'package:sky/widgets/scrollable_viewport.dart';
-import 'package:sky/widgets/switch.dart';
+import 'package:sky/widgets/widget.dart';
 
 //TODO(jimsimon): Is this needed?
 export 'package:sky/widgets/popup_menu.dart' show PopupMenuStatus;

packages/flutter/example/widgets/card_collection.dart

@@ -10,7 +10,7 @@ import 'package:sky/widgets/block_viewport.dart';
 import 'package:sky/widgets/card.dart';
 import 'package:sky/widgets/dismissable.dart';
 import 'package:sky/widgets/icon.dart';
-import 'package:sky/widgets/variable_height_scrollable.dart';
+import 'package:sky/widgets/scrollable.dart';
 import 'package:sky/widgets/scaffold.dart';
 import 'package:sky/widgets/theme.dart';
 import 'package:sky/widgets/tool_bar.dart';

packages/flutter/lib/widgets/dialog.dart

@@ -10,7 +10,7 @@ import 'package:sky/widgets/default_text_style.dart';
 import 'package:sky/widgets/focus.dart';
 import 'package:sky/widgets/material.dart';
 import 'package:sky/widgets/navigator.dart';
-import 'package:sky/widgets/scrollable_viewport.dart';
+import 'package:sky/widgets/scrollable.dart';
 import 'package:sky/widgets/theme.dart';
 
 typedef Widget DialogBuilder(Navigator navigator);

packages/flutter/lib/widgets/drawer.dart

@@ -13,7 +13,7 @@ import 'package:sky/widgets/animated_container.dart';
 import 'package:sky/widgets/animation_intentions.dart';
 import 'package:sky/widgets/basic.dart';
 import 'package:sky/widgets/navigator.dart';
-import 'package:sky/widgets/scrollable_viewport.dart';
+import 'package:sky/widgets/scrollable.dart';
 import 'package:sky/widgets/theme.dart';
 
 export 'package:sky/animation/animation_performance.dart' show AnimationStatus;

packages/flutter/lib/widgets/fixed_height_scrollable.dart

-// Copyright 2015 The Chromium Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
-
-import 'dart:math' as math;
-
-import 'package:sky/animation/scroll_behavior.dart';
-import 'package:sky/widgets/basic.dart';
-import 'package:sky/widgets/scrollable.dart';
-
-abstract class FixedHeightScrollable extends Scrollable {
-
-  FixedHeightScrollable({ Key key, this.itemHeight, this.padding })
-      : super(key: key) {
-    assert(itemHeight != null);
-  }
-
-  EdgeDims padding;
-  double itemHeight;
-
-  /// Subclasses must implement `get itemCount` to tell FixedHeightScrollable
-  /// how many items there are in the list.
-  int get itemCount;
-  int _previousItemCount;
-
-  void syncFields(FixedHeightScrollable source) {
-    padding = source.padding;
-    itemHeight = source.itemHeight;
-    super.syncFields(source);
-  }
-
-  ScrollBehavior createScrollBehavior() => new OverscrollBehavior();
-  OverscrollBehavior get scrollBehavior => super.scrollBehavior;
-
-  double _height;
-  void _handleSizeChanged(Size newSize) {
-    setState(() {
-      _height = newSize.height;
-      scrollBehavior.containerSize = _height;
-    });
-  }
-
-  void _updateContentsHeight() {
-    double contentsHeight = itemHeight * itemCount;
-    if (padding != null)
-      contentsHeight += padding.top + padding.bottom;
-    scrollBehavior.contentsSize = contentsHeight;
-  }
-
-  void _updateScrollOffset() {
-    if (scrollOffset > scrollBehavior.maxScrollOffset)
-      settleScrollOffset();
-  }
-
-  Widget buildContent() {
-    if (itemCount != _previousItemCount) {
-      _previousItemCount = itemCount;
-      _updateContentsHeight();
-      _updateScrollOffset();
-    }
-
-    int itemShowIndex = 0;
-    int itemShowCount = 0;
-    double offsetY = 0.0;
-    if (_height != null && _height > 0.0) {
-      if (scrollOffset < 0.0) {
-        double visibleHeight = _height + scrollOffset;
-        itemShowCount = (visibleHeight / itemHeight).round() + 1;
-        offsetY = scrollOffset;
-      } else {
-        itemShowCount = (_height / itemHeight).ceil();
-        double alignmentDelta = -scrollOffset % itemHeight;
-        double drawStart;
-        if (alignmentDelta != 0.0) {
-          alignmentDelta -= itemHeight;
-          itemShowCount += 1;
-          drawStart = scrollOffset + alignmentDelta;
-          offsetY = -alignmentDelta;
-        } else {
-          drawStart = scrollOffset;
-        }
-        itemShowIndex = math.max(0, (drawStart / itemHeight).floor());
-      }
-    }
-
-    List<Widget> items = buildItems(itemShowIndex, itemShowCount);
-    assert(items.every((item) => item.key != null));
-
-    // TODO(ianh): Refactor this so that it does the building in the
-    // same frame as the size observing, similar to BlockViewport, but
-    // keeping the fixed-height optimisations.
-    return new SizeObserver(
-      callback: _handleSizeChanged,
-      child: new Viewport(
-        offset: offsetY,
-        child: new Container(
-          padding: padding,
-          child: new Block(items)
-        )
-      )
-    );
-  }
-
-  List<Widget> buildItems(int start, int count);
-
-}

packages/flutter/lib/widgets/popup_menu.dart

@@ -14,7 +14,7 @@ import 'package:sky/widgets/animated_component.dart';
 import 'package:sky/widgets/basic.dart';
 import 'package:sky/widgets/navigator.dart';
 import 'package:sky/widgets/popup_menu_item.dart';
-import 'package:sky/widgets/scrollable_viewport.dart';
+import 'package:sky/widgets/scrollable.dart';
 
 export 'package:sky/animation/animation_performance.dart' show AnimationStatus;
 

packages/flutter/lib/widgets/scrollable.dart

@@ -2,16 +2,22 @@
 // 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 'dart:sky' as sky;
 
 import 'package:newton/newton.dart';
-import 'package:sky/animation/animation_performance.dart';
 import 'package:sky/animation/animated_simulation.dart';
 import 'package:sky/animation/animated_value.dart';
+import 'package:sky/animation/animation_performance.dart';
 import 'package:sky/animation/curves.dart';
 import 'package:sky/animation/scroll_behavior.dart';
 import 'package:sky/theme/view_configuration.dart' as config;
 import 'package:sky/widgets/basic.dart';
+import 'package:sky/widgets/block_viewport.dart';
+import 'package:sky/widgets/scrollable.dart';
+import 'package:sky/widgets/widget.dart';
+
+export 'package:sky/widgets/block_viewport.dart' show BlockViewportLayoutState;
 
 const double _kMillisecondsPerSecond = 1000.0;
 
@@ -27,6 +33,8 @@ abstract class ScrollClient {
 
 enum ScrollDirection { vertical, horizontal }
 
+/// A base class for scrollable widgets that reacts to user input and generates
+/// a scrollOffset.
 abstract class Scrollable extends StatefulComponent {
 
   Scrollable({
@@ -203,3 +211,282 @@ abstract class Scrollable extends StatefulComponent {
     scrollBy(-event.offsetY);
   }
 }
+
+/// A simple scrollable widget that has a single child. Use this component if
+/// you are not worried about offscreen widgets consuming resources.
+class ScrollableViewport extends Scrollable {
+  ScrollableViewport({ Key key, this.child }) : super(key: key);
+
+  Widget child;
+
+  void syncFields(ScrollableViewport source) {
+    child = source.child;
+    super.syncFields(source);
+  }
+
+  ScrollBehavior createScrollBehavior() => new FlingBehavior();
+  FlingBehavior get scrollBehavior => super.scrollBehavior;
+
+  double _viewportHeight = 0.0;
+  double _childHeight = 0.0;
+  void _handleViewportSizeChanged(Size newSize) {
+    _viewportHeight = newSize.height;
+    _updateScrollBehaviour();
+  }
+  void _handleChildSizeChanged(Size newSize) {
+    _childHeight = newSize.height;
+    _updateScrollBehaviour();
+  }
+  void _updateScrollBehaviour() {
+    scrollBehavior.contentsSize = _childHeight;
+    scrollBehavior.containerSize = _viewportHeight;
+    if (scrollOffset > scrollBehavior.maxScrollOffset)
+      settleScrollOffset();
+  }
+
+  Widget buildContent() {
+    return new SizeObserver(
+      callback: _handleViewportSizeChanged,
+      child: new Viewport(
+        offset: scrollOffset,
+        child: new SizeObserver(
+          callback: _handleChildSizeChanged,
+          child: child
+        )
+      )
+    );
+  }
+}
+
+/// A mashup of [ScrollableViewport] and [Block]. Useful when you have a small,
+/// fixed number of children that you wish to arrange in a block layout and that
+/// might exceed the height of its container (and therefore need to scroll).
+class ScrollableBlock extends Component {
+  ScrollableBlock(this.children, { Key key }) : super(key: key);
+
+  final List<Widget> children;
+
+  Widget build() {
+    return new ScrollableViewport(
+      child: new Block(children)
+    );
+  }
+}
+
+/// An optimized scrollable widget for a large number of children that are all
+/// of the same height. Use this widget when you have a large number of children
+/// or when you are concerned about offscreen widgets consuming resources.
+abstract class FixedHeightScrollable extends Scrollable {
+
+  FixedHeightScrollable({ Key key, this.itemHeight, this.padding })
+      : super(key: key) {
+    assert(itemHeight != null);
+  }
+
+  EdgeDims padding;
+  double itemHeight;
+
+  /// Subclasses must implement `get itemCount` to tell FixedHeightScrollable
+  /// how many items there are in the list.
+  int get itemCount;
+  int _previousItemCount;
+
+  void syncFields(FixedHeightScrollable source) {
+    padding = source.padding;
+    itemHeight = source.itemHeight;
+    super.syncFields(source);
+  }
+
+  ScrollBehavior createScrollBehavior() => new OverscrollBehavior();
+  OverscrollBehavior get scrollBehavior => super.scrollBehavior;
+
+  double _height;
+  void _handleSizeChanged(Size newSize) {
+    setState(() {
+      _height = newSize.height;
+      scrollBehavior.containerSize = _height;
+    });
+  }
+
+  void _updateContentsHeight() {
+    double contentsHeight = itemHeight * itemCount;
+    if (padding != null)
+      contentsHeight += padding.top + padding.bottom;
+    scrollBehavior.contentsSize = contentsHeight;
+  }
+
+  void _updateScrollOffset() {
+    if (scrollOffset > scrollBehavior.maxScrollOffset)
+      settleScrollOffset();
+  }
+
+  Widget buildContent() {
+    if (itemCount != _previousItemCount) {
+      _previousItemCount = itemCount;
+      _updateContentsHeight();
+      _updateScrollOffset();
+    }
+
+    int itemShowIndex = 0;
+    int itemShowCount = 0;
+    double offsetY = 0.0;
+    if (_height != null && _height > 0.0) {
+      if (scrollOffset < 0.0) {
+        double visibleHeight = _height + scrollOffset;
+        itemShowCount = (visibleHeight / itemHeight).round() + 1;
+        offsetY = scrollOffset;
+      } else {
+        itemShowCount = (_height / itemHeight).ceil();
+        double alignmentDelta = -scrollOffset % itemHeight;
+        double drawStart;
+        if (alignmentDelta != 0.0) {
+          alignmentDelta -= itemHeight;
+          itemShowCount += 1;
+          drawStart = scrollOffset + alignmentDelta;
+          offsetY = -alignmentDelta;
+        } else {
+          drawStart = scrollOffset;
+        }
+        itemShowIndex = math.max(0, (drawStart / itemHeight).floor());
+      }
+    }
+
+    List<Widget> items = buildItems(itemShowIndex, itemShowCount);
+    assert(items.every((item) => item.key != null));
+
+    // TODO(ianh): Refactor this so that it does the building in the
+    // same frame as the size observing, similar to BlockViewport, but
+    // keeping the fixed-height optimisations.
+    return new SizeObserver(
+      callback: _handleSizeChanged,
+      child: new Viewport(
+        offset: offsetY,
+        child: new Container(
+          padding: padding,
+          child: new Block(items)
+        )
+      )
+    );
+  }
+
+  List<Widget> buildItems(int start, int count);
+
+}
+
+typedef Widget ItemBuilder<T>(T item);
+
+/// A wrapper around [FixedHeightScrollable] that helps you translate a list of
+/// model objects into a scrollable list of widgets. Assumes all the widgets
+/// have the same height.
+class ScrollableList<T> extends FixedHeightScrollable {
+  ScrollableList({
+    Key key,
+    this.items,
+    this.itemBuilder,
+    double itemHeight,
+    EdgeDims padding
+  }) : super(key: key, itemHeight: itemHeight, padding: padding);
+
+  List<T> items;
+  ItemBuilder<T> itemBuilder;
+
+  void syncFields(ScrollableList<T> source) {
+    items = source.items;
+    itemBuilder = source.itemBuilder;
+    super.syncFields(source);
+  }
+
+  int get itemCount => items.length;
+
+  List<Widget> buildItems(int start, int count) {
+    List<Widget> result = new List<Widget>();
+    int end = math.min(start + count, items.length);
+    for (int i = start; i < end; ++i)
+      result.add(itemBuilder(items[i]));
+    return result;
+  }
+}
+
+/// A general scrollable list for a large number of children that might not all
+/// have the same height. Prefer [FixedHeightScrollable] when all the children
+/// have the same height because it can use that property to be more efficient.
+/// Prefer [ScrollableViewport] with a single child.
+class VariableHeightScrollable extends Scrollable {
+  VariableHeightScrollable({
+    Key key,
+    this.builder,
+    this.token,
+    this.layoutState
+  }) : super(key: key);
+
+  IndexedBuilder builder;
+  Object token;
+  BlockViewportLayoutState layoutState;
+
+  // When the token changes the scrollable's contents may have
+  // changed. Remember as much so that after the new contents
+  // have been laid out we can adjust the scrollOffset so that
+  // the last page of content is still visible.
+  bool _contentsChanged = true;
+
+  void initState() {
+    assert(layoutState != null);
+    super.initState();
+  }
+
+  void didMount() {
+    layoutState.addListener(_handleLayoutChanged);
+    super.didMount();
+  }
+
+  void didUnmount() {
+    layoutState.removeListener(_handleLayoutChanged);
+    super.didUnmount();
+  }
+
+  void syncFields(VariableHeightScrollable source) {
+    builder = source.builder;
+    if (token != source.token)
+      _contentsChanged = true;
+    token = source.token;
+    if (layoutState != source.layoutState) {
+      // Warning: this is unlikely to be what you intended.
+      assert(source.layoutState != null);
+      layoutState.removeListener(_handleLayoutChanged);
+      layoutState = source.layoutState;
+      layoutState.addListener(_handleLayoutChanged);
+    }
+    super.syncFields(source);
+  }
+
+  ScrollBehavior createScrollBehavior() => new OverscrollBehavior();
+  OverscrollBehavior get scrollBehavior => super.scrollBehavior;
+
+  void _handleSizeChanged(Size newSize) {
+    scrollBehavior.containerSize = newSize.height;
+  }
+
+  void _handleLayoutChanged() {
+    if (layoutState.didReachLastChild) {
+      scrollBehavior.contentsSize = layoutState.contentsSize;
+      if (_contentsChanged && scrollOffset > scrollBehavior.maxScrollOffset) {
+        _contentsChanged = false;
+        settleScrollOffset();
+      }
+    } else {
+      scrollBehavior.contentsSize = double.INFINITY;
+    }
+  }
+
+  Widget buildContent() {
+    return new SizeObserver(
+      callback: _handleSizeChanged,
+      child: new BlockViewport(
+        builder: builder,
+        layoutState: layoutState,
+        startOffset: scrollOffset,
+        token: token
+      )
+    );
+  }
+}

packages/flutter/lib/widgets/scrollable_list.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:sky/widgets/fixed_height_scrollable.dart';
-import 'package:sky/widgets/basic.dart';
-
-typedef Widget ItemBuilder<T>(T item);
-
-class ScrollableList<T> extends FixedHeightScrollable {
-  ScrollableList({
-    Key key,
-    this.items,
-    this.itemBuilder,
-    double itemHeight,
-    EdgeDims padding
-  }) : super(key: key, itemHeight: itemHeight, padding: padding);
-
-  List<T> items;
-  ItemBuilder<T> itemBuilder;
-
-  void syncFields(ScrollableList<T> source) {
-    items = source.items;
-    itemBuilder = source.itemBuilder;
-    super.syncFields(source);
-  }
-
-  int get itemCount => items.length;
-
-  List<Widget> buildItems(int start, int count) {
-    List<Widget> result = new List<Widget>();
-    int end = math.min(start + count, items.length);
-    for (int i = start; i < end; ++i)
-      result.add(itemBuilder(items[i]));
-    return result;
-  }
-}

packages/flutter/lib/widgets/scrollable_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 'package:sky/animation/scroll_behavior.dart';
-import 'package:sky/widgets/basic.dart';
-import 'package:sky/widgets/scrollable.dart';
-
-class ScrollableViewport extends Scrollable {
-
-  ScrollableViewport({ Key key, this.child }) : super(key: key);
-
-  Widget child;
-
-  void syncFields(ScrollableViewport source) {
-    child = source.child;
-    super.syncFields(source);
-  }
-
-  ScrollBehavior createScrollBehavior() => new FlingBehavior();
-  FlingBehavior get scrollBehavior => super.scrollBehavior;
-
-  double _viewportHeight = 0.0;
-  double _childHeight = 0.0;
-  void _handleViewportSizeChanged(Size newSize) {
-    _viewportHeight = newSize.height;
-    _updateScrollBehaviour();
-  }
-  void _handleChildSizeChanged(Size newSize) {
-    _childHeight = newSize.height;
-    _updateScrollBehaviour();
-  }
-  void _updateScrollBehaviour() {
-    scrollBehavior.contentsSize = _childHeight;
-    scrollBehavior.containerSize = _viewportHeight;
-    if (scrollOffset > scrollBehavior.maxScrollOffset)
-      settleScrollOffset();
-  }
-
-  Widget buildContent() {
-    return new SizeObserver(
-      callback: _handleViewportSizeChanged,
-      child: new Viewport(
-        offset: scrollOffset,
-        child: new SizeObserver(
-          callback: _handleChildSizeChanged,
-          child: child
-        )
-      )
-    );
-  }
-
-}
-
-class ScrollableBlock extends Component {
-
-  ScrollableBlock(this.children, { Key key }) : super(key: key);
-
-  final List<Widget> children;
-
-  Widget build() {
-    return new ScrollableViewport(
-      child: new Block(children)
-    );
-  }
-
-}

packages/flutter/lib/widgets/variable_height_scrollable.dart

-// Copyright 2015 The Chromium Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
-
-import 'package:sky/animation/scroll_behavior.dart';
-import 'package:sky/widgets/basic.dart';
-import 'package:sky/widgets/block_viewport.dart';
-import 'package:sky/widgets/scrollable.dart';
-import 'package:sky/widgets/widget.dart';
-
-export 'package:sky/widgets/block_viewport.dart' show BlockViewportLayoutState;
-
-class VariableHeightScrollable extends Scrollable {
-  VariableHeightScrollable({
-    Key key,
-    this.builder,
-    this.token,
-    this.layoutState
-  }) : super(key: key);
-
-  IndexedBuilder builder;
-  Object token;
-  BlockViewportLayoutState layoutState;
-
-  // When the token changes the scrollable's contents may have
-  // changed. Remember as much so that after the new contents
-  // have been laid out we can adjust the scrollOffset so that
-  // the last page of content is still visible.
-  bool _contentsChanged = true;
-
-  void initState() {
-    assert(layoutState != null);
-    super.initState();
-  }
-
-  void didMount() {
-    layoutState.addListener(_handleLayoutChanged);
-    super.didMount();
-  }
-
-  void didUnmount() {
-    layoutState.removeListener(_handleLayoutChanged);
-    super.didUnmount();
-  }
-
-  void syncFields(VariableHeightScrollable source) {
-    builder = source.builder;
-    if (token != source.token)
-      _contentsChanged = true;
-    token = source.token;
-    if (layoutState != source.layoutState) {
-      // Warning: this is unlikely to be what you intended.
-      assert(source.layoutState != null);
-      layoutState.removeListener(_handleLayoutChanged);
-      layoutState = source.layoutState;
-      layoutState.addListener(_handleLayoutChanged);
-    }
-    super.syncFields(source);
-  }
-
-  ScrollBehavior createScrollBehavior() => new OverscrollBehavior();
-  OverscrollBehavior get scrollBehavior => super.scrollBehavior;
-
-  void _handleSizeChanged(Size newSize) {
-    scrollBehavior.containerSize = newSize.height;
-  }
-
-  void _handleLayoutChanged() {
-    if (layoutState.didReachLastChild) {
-      scrollBehavior.contentsSize = layoutState.contentsSize;
-      if (_contentsChanged && scrollOffset > scrollBehavior.maxScrollOffset) {
-        _contentsChanged = false;
-        settleScrollOffset();
-      }
-    } else {
-      scrollBehavior.contentsSize = double.INFINITY;
-    }
-  }
-
-  Widget buildContent() {
-    return new SizeObserver(
-      callback: _handleSizeChanged,
-      child: new BlockViewport(
-        builder: builder,
-        layoutState: layoutState,
-        startOffset: scrollOffset,
-        token: token
-      )
-    );
-  }
-}