packages/flutter/lib/src/widgets/viewport.dart - mirrors/flutter - Git at Google

 // Copyright 2014 The Flutter 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:flutter/rendering.dart';

 import 'basic.dart';
 import 'debug.dart';
 import 'framework.dart';
 import 'scroll_notification.dart';

 export 'package:flutter/rendering.dart' show
   AxisDirection,
   GrowthDirection;

 /// A widget that is bigger on the inside.
 ///
 /// [Viewport] is the visual workhorse of the scrolling machinery. It displays a
 /// subset of its children according to its own dimensions and the given
 /// [offset]. As the offset varies, different children are visible through
 /// the viewport.
 ///
 /// [Viewport] hosts a bidirectional list of slivers, anchored on a [center]
 /// sliver, which is placed at the zero scroll offset. The center widget is
 /// displayed in the viewport according to the [anchor] property.
 ///
 /// Slivers that are earlier in the child list than [center] are displayed in
 /// reverse order in the reverse [axisDirection] starting from the [center]. For
 /// example, if the [axisDirection] is [AxisDirection.down], the first sliver
 /// before [center] is placed above the [center]. The slivers that are later in
 /// the child list than [center] are placed in order in the [axisDirection]. For
 /// example, in the preceding scenario, the first sliver after [center] is
 /// placed below the [center].
 ///
 /// [Viewport] cannot contain box children directly. Instead, use a
 /// [SliverList], [SliverFixedExtentList], [SliverGrid], or a
 /// [SliverToBoxAdapter], for example.
 ///
 /// See also:
 ///
 ///  * [ListView], [PageView], [GridView], and [CustomScrollView], which combine
 ///    [Scrollable] and [Viewport] into widgets that are easier to use.
 ///  * [SliverToBoxAdapter], which allows a box widget to be placed inside a
 ///    sliver context (the opposite of this widget).
 ///  * [ShrinkWrappingViewport], a variant of [Viewport] that shrink-wraps its
 ///    contents along the main axis.
 ///  * [ViewportElementMixin], which should be mixed in to the [Element] type used
 ///    by viewport-like widgets to correctly handle scroll notifications.
 class Viewport extends MultiChildRenderObjectWidget {
   /// Creates a widget that is bigger on the inside.
   ///
   /// The viewport listens to the [offset], which means you do not need to
   /// rebuild this widget when the [offset] changes.
   ///
   /// The [offset] argument must not be null.
   ///
   /// The [cacheExtent] must be specified if the [cacheExtentStyle] is
   /// not [CacheExtentStyle.pixel].
   Viewport({
     super.key,
     this.axisDirection = AxisDirection.down,
     this.crossAxisDirection,
     this.anchor = 0.0,
     required this.offset,
     this.center,
     this.cacheExtent,
     this.cacheExtentStyle = CacheExtentStyle.pixel,
     this.clipBehavior = Clip.hardEdge,
     List<Widget> slivers = const <Widget>[],
   }) : assert(center == null || slivers.where((Widget child) => child.key == center).length == 1),
        assert(cacheExtentStyle != CacheExtentStyle.viewport || cacheExtent != null),
        super(children: slivers);

   /// The direction in which the [offset]'s [ViewportOffset.pixels] increases.
   ///
   /// For example, if the [axisDirection] is [AxisDirection.down], a scroll
   /// offset of zero is at the top of the viewport and increases towards the
   /// bottom of the viewport.
   final AxisDirection axisDirection;

   /// The direction in which child should be laid out in the cross axis.
   ///
   /// If the [axisDirection] is [AxisDirection.down] or [AxisDirection.up], this
   /// property defaults to [AxisDirection.left] if the ambient [Directionality]
   /// is [TextDirection.rtl] and [AxisDirection.right] if the ambient
   /// [Directionality] is [TextDirection.ltr].
   ///
   /// If the [axisDirection] is [AxisDirection.left] or [AxisDirection.right],
   /// this property defaults to [AxisDirection.down].
   final AxisDirection? crossAxisDirection;

   /// The relative position of the zero scroll offset.
   ///
   /// For example, if [anchor] is 0.5 and the [axisDirection] is
   /// [AxisDirection.down] or [AxisDirection.up], then the zero scroll offset is
   /// vertically centered within the viewport. If the [anchor] is 1.0, and the
   /// [axisDirection] is [AxisDirection.right], then the zero scroll offset is
   /// on the left edge of the viewport.
   ///
   /// {@macro flutter.rendering.GrowthDirection.sample}
   final double anchor;

   /// Which part of the content inside the viewport should be visible.
   ///
   /// The [ViewportOffset.pixels] value determines the scroll offset that the
   /// viewport uses to select which part of its content to display. As the user
   /// scrolls the viewport, this value changes, which changes the content that
   /// is displayed.
   ///
   /// Typically a [ScrollPosition].
   final ViewportOffset offset;

   /// The first child in the [GrowthDirection.forward] growth direction.
   ///
   /// Children after [center] will be placed in the [axisDirection] relative to
   /// the [center]. Children before [center] will be placed in the opposite of
   /// the [axisDirection] relative to the [center].
   ///
   /// The [center] must be the key of a child of the viewport.
   ///
   /// {@macro flutter.rendering.GrowthDirection.sample}
   final Key? center;

   /// {@macro flutter.rendering.RenderViewportBase.cacheExtent}
   ///
   /// See also:
   ///
   ///  * [cacheExtentStyle], which controls the units of the [cacheExtent].
   final double? cacheExtent;

   /// {@macro flutter.rendering.RenderViewportBase.cacheExtentStyle}
   final CacheExtentStyle cacheExtentStyle;

   /// {@macro flutter.material.Material.clipBehavior}
   ///
   /// Defaults to [Clip.hardEdge].
   final Clip clipBehavior;

   /// Given a [BuildContext] and an [AxisDirection], determine the correct cross
   /// axis direction.
   ///
   /// This depends on the [Directionality] if the `axisDirection` is vertical;
   /// otherwise, the default cross axis direction is downwards.
   static AxisDirection getDefaultCrossAxisDirection(BuildContext context, AxisDirection axisDirection) {
     switch (axisDirection) {
       case AxisDirection.up:
         assert(debugCheckHasDirectionality(
           context,
           why: "to determine the cross-axis direction when the viewport has an 'up' axisDirection",
           alternative: "Alternatively, consider specifying the 'crossAxisDirection' argument on the Viewport.",
         ));
         return textDirectionToAxisDirection(Directionality.of(context));
       case AxisDirection.right:
         return AxisDirection.down;
       case AxisDirection.down:
         assert(debugCheckHasDirectionality(
           context,
           why: "to determine the cross-axis direction when the viewport has a 'down' axisDirection",
           alternative: "Alternatively, consider specifying the 'crossAxisDirection' argument on the Viewport.",
         ));
         return textDirectionToAxisDirection(Directionality.of(context));
       case AxisDirection.left:
         return AxisDirection.down;
     }
   }

   @override
   RenderViewport createRenderObject(BuildContext context) {
     return RenderViewport(
       axisDirection: axisDirection,
       crossAxisDirection: crossAxisDirection ?? Viewport.getDefaultCrossAxisDirection(context, axisDirection),
       anchor: anchor,
       offset: offset,
       cacheExtent: cacheExtent,
       cacheExtentStyle: cacheExtentStyle,
       clipBehavior: clipBehavior,
     );
   }

   @override
   void updateRenderObject(BuildContext context, RenderViewport renderObject) {
     renderObject
       ..axisDirection = axisDirection
       ..crossAxisDirection = crossAxisDirection ?? Viewport.getDefaultCrossAxisDirection(context, axisDirection)
       ..anchor = anchor
       ..offset = offset
       ..cacheExtent = cacheExtent
       ..cacheExtentStyle = cacheExtentStyle
       ..clipBehavior = clipBehavior;
   }

   @override
   MultiChildRenderObjectElement createElement() => _ViewportElement(this);

   @override
   void debugFillProperties(DiagnosticPropertiesBuilder properties) {
     super.debugFillProperties(properties);
     properties.add(EnumProperty<AxisDirection>('axisDirection', axisDirection));
     properties.add(EnumProperty<AxisDirection>('crossAxisDirection', crossAxisDirection, defaultValue: null));
     properties.add(DoubleProperty('anchor', anchor));
     properties.add(DiagnosticsProperty<ViewportOffset>('offset', offset));
     if (center != null) {
       properties.add(DiagnosticsProperty<Key>('center', center));
     } else if (children.isNotEmpty && children.first.key != null) {
       properties.add(DiagnosticsProperty<Key>('center', children.first.key, tooltip: 'implicit'));
     }
     properties.add(DiagnosticsProperty<double>('cacheExtent', cacheExtent));
     properties.add(DiagnosticsProperty<CacheExtentStyle>('cacheExtentStyle', cacheExtentStyle));
   }
 }

 class _ViewportElement extends MultiChildRenderObjectElement with NotifiableElementMixin, ViewportElementMixin {
   /// Creates an element that uses the given widget as its configuration.
   _ViewportElement(Viewport super.widget);

   bool _doingMountOrUpdate = false;
   int? _centerSlotIndex;

   @override
   RenderViewport get renderObject => super.renderObject as RenderViewport;

   @override
   void mount(Element? parent, Object? newSlot) {
     assert(!_doingMountOrUpdate);
     _doingMountOrUpdate = true;
     super.mount(parent, newSlot);
     _updateCenter();
     assert(_doingMountOrUpdate);
     _doingMountOrUpdate = false;
   }

   @override
   void update(MultiChildRenderObjectWidget newWidget) {
     assert(!_doingMountOrUpdate);
     _doingMountOrUpdate = true;
     super.update(newWidget);
     _updateCenter();
     assert(_doingMountOrUpdate);
     _doingMountOrUpdate = false;
   }

   void _updateCenter() {
     // TODO(ianh): cache the keys to make this faster
     final Viewport viewport = widget as Viewport;
     if (viewport.center != null) {
       int elementIndex = 0;
       for (final Element e in children) {
         if (e.widget.key == viewport.center) {
           renderObject.center = e.renderObject as RenderSliver?;
           break;
         }
         elementIndex++;
       }
       assert(elementIndex < children.length);
       _centerSlotIndex = elementIndex;
     } else if (children.isNotEmpty) {
       renderObject.center = children.first.renderObject as RenderSliver?;
       _centerSlotIndex = 0;
     } else {
       renderObject.center = null;
       _centerSlotIndex = null;
     }
   }

   @override
   void insertRenderObjectChild(RenderObject child, IndexedSlot<Element?> slot) {
     super.insertRenderObjectChild(child, slot);
     // Once [mount]/[update] are done, the `renderObject.center` will be updated
     // in [_updateCenter].
     if (!_doingMountOrUpdate && slot.index == _centerSlotIndex) {
       renderObject.center = child as RenderSliver?;
     }
   }

   @override
   void moveRenderObjectChild(RenderObject child, IndexedSlot<Element?> oldSlot, IndexedSlot<Element?> newSlot) {
     super.moveRenderObjectChild(child, oldSlot, newSlot);
     assert(_doingMountOrUpdate);
   }

   @override
   void removeRenderObjectChild(RenderObject child, Object? slot) {
     super.removeRenderObjectChild(child, slot);
     if (!_doingMountOrUpdate && renderObject.center == child) {
       renderObject.center = null;
     }
   }

   @override
   void debugVisitOnstageChildren(ElementVisitor visitor) {
     children.where((Element e) {
       final RenderSliver renderSliver = e.renderObject! as RenderSliver;
       return renderSliver.geometry!.visible;
     }).forEach(visitor);
   }
 }

 /// A widget that is bigger on the inside and shrink wraps its children in the
 /// main axis.
 ///
 /// [ShrinkWrappingViewport] displays a subset of its children according to its
 /// own dimensions and the given [offset]. As the offset varies, different
 /// children are visible through the viewport.
 ///
 /// [ShrinkWrappingViewport] differs from [Viewport] in that [Viewport] expands
 /// to fill the main axis whereas [ShrinkWrappingViewport] sizes itself to match
 /// its children in the main axis. This shrink wrapping behavior is expensive
 /// because the children, and hence the viewport, could potentially change size
 /// whenever the [offset] changes (e.g., because of a collapsing header).
 ///
 /// [ShrinkWrappingViewport] cannot contain box children directly. Instead, use
 /// a [SliverList], [SliverFixedExtentList], [SliverGrid], or a
 /// [SliverToBoxAdapter], for example.
 ///
 /// See also:
 ///
 ///  * [ListView], [PageView], [GridView], and [CustomScrollView], which combine
 ///    [Scrollable] and [ShrinkWrappingViewport] into widgets that are easier to
 ///    use.
 ///  * [SliverToBoxAdapter], which allows a box widget to be placed inside a
 ///    sliver context (the opposite of this widget).
 ///  * [Viewport], a viewport that does not shrink-wrap its contents.
 class ShrinkWrappingViewport extends MultiChildRenderObjectWidget {
   /// Creates a widget that is bigger on the inside and shrink wraps its
   /// children in the main axis.
   ///
   /// The viewport listens to the [offset], which means you do not need to
   /// rebuild this widget when the [offset] changes.
   ///
   /// The [offset] argument must not be null.
   const ShrinkWrappingViewport({
     super.key,
     this.axisDirection = AxisDirection.down,
     this.crossAxisDirection,
     required this.offset,
     this.clipBehavior = Clip.hardEdge,
     List<Widget> slivers = const <Widget>[],
   }) : super(children: slivers);

   /// The direction in which the [offset]'s [ViewportOffset.pixels] increases.
   ///
   /// For example, if the [axisDirection] is [AxisDirection.down], a scroll
   /// offset of zero is at the top of the viewport and increases towards the
   /// bottom of the viewport.
   final AxisDirection axisDirection;

   /// The direction in which child should be laid out in the cross axis.
   ///
   /// If the [axisDirection] is [AxisDirection.down] or [AxisDirection.up], this
   /// property defaults to [AxisDirection.left] if the ambient [Directionality]
   /// is [TextDirection.rtl] and [AxisDirection.right] if the ambient
   /// [Directionality] is [TextDirection.ltr].
   ///
   /// If the [axisDirection] is [AxisDirection.left] or [AxisDirection.right],
   /// this property defaults to [AxisDirection.down].
   final AxisDirection? crossAxisDirection;

   /// Which part of the content inside the viewport should be visible.
   ///
   /// The [ViewportOffset.pixels] value determines the scroll offset that the
   /// viewport uses to select which part of its content to display. As the user
   /// scrolls the viewport, this value changes, which changes the content that
   /// is displayed.
   ///
   /// Typically a [ScrollPosition].
   final ViewportOffset offset;

   /// {@macro flutter.material.Material.clipBehavior}
   ///
   /// Defaults to [Clip.hardEdge].
   final Clip clipBehavior;

   @override
   RenderShrinkWrappingViewport createRenderObject(BuildContext context) {
     return RenderShrinkWrappingViewport(
       axisDirection: axisDirection,
       crossAxisDirection: crossAxisDirection ?? Viewport.getDefaultCrossAxisDirection(context, axisDirection),
       offset: offset,
       clipBehavior: clipBehavior,
     );
   }

   @override
   void updateRenderObject(BuildContext context, RenderShrinkWrappingViewport renderObject) {
     renderObject
       ..axisDirection = axisDirection
       ..crossAxisDirection = crossAxisDirection ?? Viewport.getDefaultCrossAxisDirection(context, axisDirection)
       ..offset = offset
       ..clipBehavior = clipBehavior;
   }

   @override
   void debugFillProperties(DiagnosticPropertiesBuilder properties) {
     super.debugFillProperties(properties);
     properties.add(EnumProperty<AxisDirection>('axisDirection', axisDirection));
     properties.add(EnumProperty<AxisDirection>('crossAxisDirection', crossAxisDirection, defaultValue: null));
     properties.add(DiagnosticsProperty<ViewportOffset>('offset', offset));
   }
 }
	// Copyright 2014 The Flutter 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:flutter/rendering.dart';

	import 'basic.dart';
	import 'debug.dart';
	import 'framework.dart';
	import 'scroll_notification.dart';

	export 'package:flutter/rendering.dart' show
	AxisDirection,
	GrowthDirection;

	/// A widget that is bigger on the inside.
	///
	/// [Viewport] is the visual workhorse of the scrolling machinery. It displays a
	/// subset of its children according to its own dimensions and the given
	/// [offset]. As the offset varies, different children are visible through
	/// the viewport.
	///
	/// [Viewport] hosts a bidirectional list of slivers, anchored on a [center]
	/// sliver, which is placed at the zero scroll offset. The center widget is
	/// displayed in the viewport according to the [anchor] property.
	///
	/// Slivers that are earlier in the child list than [center] are displayed in
	/// reverse order in the reverse [axisDirection] starting from the [center]. For
	/// example, if the [axisDirection] is [AxisDirection.down], the first sliver
	/// before [center] is placed above the [center]. The slivers that are later in
	/// the child list than [center] are placed in order in the [axisDirection]. For
	/// example, in the preceding scenario, the first sliver after [center] is
	/// placed below the [center].
	///
	/// [Viewport] cannot contain box children directly. Instead, use a
	/// [SliverList], [SliverFixedExtentList], [SliverGrid], or a
	/// [SliverToBoxAdapter], for example.
	///
	/// See also:
	///
	/// * [ListView], [PageView], [GridView], and [CustomScrollView], which combine
	/// [Scrollable] and [Viewport] into widgets that are easier to use.
	/// * [SliverToBoxAdapter], which allows a box widget to be placed inside a
	/// sliver context (the opposite of this widget).
	/// * [ShrinkWrappingViewport], a variant of [Viewport] that shrink-wraps its
	/// contents along the main axis.
	/// * [ViewportElementMixin], which should be mixed in to the [Element] type used
	/// by viewport-like widgets to correctly handle scroll notifications.
	class Viewport extends MultiChildRenderObjectWidget {
	/// Creates a widget that is bigger on the inside.
	///
	/// The viewport listens to the [offset], which means you do not need to
	/// rebuild this widget when the [offset] changes.
	///
	/// The [offset] argument must not be null.
	///
	/// The [cacheExtent] must be specified if the [cacheExtentStyle] is
	/// not [CacheExtentStyle.pixel].
	Viewport({
	super.key,
	this.axisDirection = AxisDirection.down,
	this.crossAxisDirection,
	this.anchor = 0.0,
	required this.offset,
	this.center,
	this.cacheExtent,
	this.cacheExtentStyle = CacheExtentStyle.pixel,
	this.clipBehavior = Clip.hardEdge,
	List<Widget> slivers = const <Widget>[],
	}) : assert(center == null \|\| slivers.where((Widget child) => child.key == center).length == 1),
	assert(cacheExtentStyle != CacheExtentStyle.viewport \|\| cacheExtent != null),
	super(children: slivers);

	/// The direction in which the [offset]'s [ViewportOffset.pixels] increases.
	///
	/// For example, if the [axisDirection] is [AxisDirection.down], a scroll
	/// offset of zero is at the top of the viewport and increases towards the
	/// bottom of the viewport.
	final AxisDirection axisDirection;

	/// The direction in which child should be laid out in the cross axis.
	///
	/// If the [axisDirection] is [AxisDirection.down] or [AxisDirection.up], this
	/// property defaults to [AxisDirection.left] if the ambient [Directionality]
	/// is [TextDirection.rtl] and [AxisDirection.right] if the ambient
	/// [Directionality] is [TextDirection.ltr].
	///
	/// If the [axisDirection] is [AxisDirection.left] or [AxisDirection.right],
	/// this property defaults to [AxisDirection.down].
	final AxisDirection? crossAxisDirection;

	/// The relative position of the zero scroll offset.
	///
	/// For example, if [anchor] is 0.5 and the [axisDirection] is
	/// [AxisDirection.down] or [AxisDirection.up], then the zero scroll offset is
	/// vertically centered within the viewport. If the [anchor] is 1.0, and the
	/// [axisDirection] is [AxisDirection.right], then the zero scroll offset is
	/// on the left edge of the viewport.
	///
	/// {@macro flutter.rendering.GrowthDirection.sample}
	final double anchor;

	/// Which part of the content inside the viewport should be visible.
	///
	/// The [ViewportOffset.pixels] value determines the scroll offset that the
	/// viewport uses to select which part of its content to display. As the user
	/// scrolls the viewport, this value changes, which changes the content that
	/// is displayed.
	///
	/// Typically a [ScrollPosition].
	final ViewportOffset offset;

	/// The first child in the [GrowthDirection.forward] growth direction.
	///
	/// Children after [center] will be placed in the [axisDirection] relative to
	/// the [center]. Children before [center] will be placed in the opposite of
	/// the [axisDirection] relative to the [center].
	///
	/// The [center] must be the key of a child of the viewport.
	///
	/// {@macro flutter.rendering.GrowthDirection.sample}
	final Key? center;

	/// {@macro flutter.rendering.RenderViewportBase.cacheExtent}
	///
	/// See also:
	///
	/// * [cacheExtentStyle], which controls the units of the [cacheExtent].
	final double? cacheExtent;

	/// {@macro flutter.rendering.RenderViewportBase.cacheExtentStyle}
	final CacheExtentStyle cacheExtentStyle;

	/// {@macro flutter.material.Material.clipBehavior}
	///
	/// Defaults to [Clip.hardEdge].
	final Clip clipBehavior;

	/// Given a [BuildContext] and an [AxisDirection], determine the correct cross
	/// axis direction.
	///
	/// This depends on the [Directionality] if the `axisDirection` is vertical;
	/// otherwise, the default cross axis direction is downwards.
	static AxisDirection getDefaultCrossAxisDirection(BuildContext context, AxisDirection axisDirection) {
	switch (axisDirection) {
	case AxisDirection.up:
	assert(debugCheckHasDirectionality(
	context,
	why: "to determine the cross-axis direction when the viewport has an 'up' axisDirection",
	alternative: "Alternatively, consider specifying the 'crossAxisDirection' argument on the Viewport.",
	));
	return textDirectionToAxisDirection(Directionality.of(context));
	case AxisDirection.right:
	return AxisDirection.down;
	case AxisDirection.down:
	assert(debugCheckHasDirectionality(
	context,
	why: "to determine the cross-axis direction when the viewport has a 'down' axisDirection",
	alternative: "Alternatively, consider specifying the 'crossAxisDirection' argument on the Viewport.",
	));
	return textDirectionToAxisDirection(Directionality.of(context));
	case AxisDirection.left:
	return AxisDirection.down;
	}
	}

	@override
	RenderViewport createRenderObject(BuildContext context) {
	return RenderViewport(
	axisDirection: axisDirection,
	crossAxisDirection: crossAxisDirection ?? Viewport.getDefaultCrossAxisDirection(context, axisDirection),
	anchor: anchor,
	offset: offset,
	cacheExtent: cacheExtent,
	cacheExtentStyle: cacheExtentStyle,
	clipBehavior: clipBehavior,
	);
	}

	@override
	void updateRenderObject(BuildContext context, RenderViewport renderObject) {
	renderObject
	..axisDirection = axisDirection
	..crossAxisDirection = crossAxisDirection ?? Viewport.getDefaultCrossAxisDirection(context, axisDirection)
	..anchor = anchor
	..offset = offset
	..cacheExtent = cacheExtent
	..cacheExtentStyle = cacheExtentStyle
	..clipBehavior = clipBehavior;
	}

	@override
	MultiChildRenderObjectElement createElement() => _ViewportElement(this);

	@override
	void debugFillProperties(DiagnosticPropertiesBuilder properties) {
	super.debugFillProperties(properties);
	properties.add(EnumProperty<AxisDirection>('axisDirection', axisDirection));
	properties.add(EnumProperty<AxisDirection>('crossAxisDirection', crossAxisDirection, defaultValue: null));
	properties.add(DoubleProperty('anchor', anchor));
	properties.add(DiagnosticsProperty<ViewportOffset>('offset', offset));
	if (center != null) {
	properties.add(DiagnosticsProperty<Key>('center', center));
	} else if (children.isNotEmpty && children.first.key != null) {
	properties.add(DiagnosticsProperty<Key>('center', children.first.key, tooltip: 'implicit'));
	}
	properties.add(DiagnosticsProperty<double>('cacheExtent', cacheExtent));
	properties.add(DiagnosticsProperty<CacheExtentStyle>('cacheExtentStyle', cacheExtentStyle));
	}
	}

	class _ViewportElement extends MultiChildRenderObjectElement with NotifiableElementMixin, ViewportElementMixin {
	/// Creates an element that uses the given widget as its configuration.
	_ViewportElement(Viewport super.widget);

	bool _doingMountOrUpdate = false;
	int? _centerSlotIndex;

	@override
	RenderViewport get renderObject => super.renderObject as RenderViewport;

	@override
	void mount(Element? parent, Object? newSlot) {
	assert(!_doingMountOrUpdate);
	_doingMountOrUpdate = true;
	super.mount(parent, newSlot);
	_updateCenter();
	assert(_doingMountOrUpdate);
	_doingMountOrUpdate = false;
	}

	@override
	void update(MultiChildRenderObjectWidget newWidget) {
	assert(!_doingMountOrUpdate);
	_doingMountOrUpdate = true;
	super.update(newWidget);
	_updateCenter();
	assert(_doingMountOrUpdate);
	_doingMountOrUpdate = false;
	}

	void _updateCenter() {
	// TODO(ianh): cache the keys to make this faster
	final Viewport viewport = widget as Viewport;
	if (viewport.center != null) {
	int elementIndex = 0;
	for (final Element e in children) {
	if (e.widget.key == viewport.center) {
	renderObject.center = e.renderObject as RenderSliver?;
	break;
	}
	elementIndex++;
	}
	assert(elementIndex < children.length);
	_centerSlotIndex = elementIndex;
	} else if (children.isNotEmpty) {
	renderObject.center = children.first.renderObject as RenderSliver?;
	_centerSlotIndex = 0;
	} else {
	renderObject.center = null;
	_centerSlotIndex = null;
	}
	}

	@override
	void insertRenderObjectChild(RenderObject child, IndexedSlot<Element?> slot) {
	super.insertRenderObjectChild(child, slot);
	// Once [mount]/[update] are done, the `renderObject.center` will be updated
	// in [_updateCenter].
	if (!_doingMountOrUpdate && slot.index == _centerSlotIndex) {
	renderObject.center = child as RenderSliver?;
	}
	}

	@override
	void moveRenderObjectChild(RenderObject child, IndexedSlot<Element?> oldSlot, IndexedSlot<Element?> newSlot) {
	super.moveRenderObjectChild(child, oldSlot, newSlot);
	assert(_doingMountOrUpdate);
	}

	@override
	void removeRenderObjectChild(RenderObject child, Object? slot) {
	super.removeRenderObjectChild(child, slot);
	if (!_doingMountOrUpdate && renderObject.center == child) {
	renderObject.center = null;
	}
	}

	@override
	void debugVisitOnstageChildren(ElementVisitor visitor) {
	children.where((Element e) {
	final RenderSliver renderSliver = e.renderObject! as RenderSliver;
	return renderSliver.geometry!.visible;
	}).forEach(visitor);
	}
	}

	/// A widget that is bigger on the inside and shrink wraps its children in the
	/// main axis.
	///
	/// [ShrinkWrappingViewport] displays a subset of its children according to its
	/// own dimensions and the given [offset]. As the offset varies, different
	/// children are visible through the viewport.
	///
	/// [ShrinkWrappingViewport] differs from [Viewport] in that [Viewport] expands
	/// to fill the main axis whereas [ShrinkWrappingViewport] sizes itself to match
	/// its children in the main axis. This shrink wrapping behavior is expensive
	/// because the children, and hence the viewport, could potentially change size
	/// whenever the [offset] changes (e.g., because of a collapsing header).
	///
	/// [ShrinkWrappingViewport] cannot contain box children directly. Instead, use
	/// a [SliverList], [SliverFixedExtentList], [SliverGrid], or a
	/// [SliverToBoxAdapter], for example.
	///
	/// See also:
	///
	/// * [ListView], [PageView], [GridView], and [CustomScrollView], which combine
	/// [Scrollable] and [ShrinkWrappingViewport] into widgets that are easier to
	/// use.
	/// * [SliverToBoxAdapter], which allows a box widget to be placed inside a
	/// sliver context (the opposite of this widget).
	/// * [Viewport], a viewport that does not shrink-wrap its contents.
	class ShrinkWrappingViewport extends MultiChildRenderObjectWidget {
	/// Creates a widget that is bigger on the inside and shrink wraps its
	/// children in the main axis.
	///
	/// The viewport listens to the [offset], which means you do not need to
	/// rebuild this widget when the [offset] changes.
	///
	/// The [offset] argument must not be null.
	const ShrinkWrappingViewport({
	super.key,
	this.axisDirection = AxisDirection.down,
	this.crossAxisDirection,
	required this.offset,
	this.clipBehavior = Clip.hardEdge,
	List<Widget> slivers = const <Widget>[],
	}) : super(children: slivers);

	/// The direction in which the [offset]'s [ViewportOffset.pixels] increases.
	///
	/// For example, if the [axisDirection] is [AxisDirection.down], a scroll
	/// offset of zero is at the top of the viewport and increases towards the
	/// bottom of the viewport.
	final AxisDirection axisDirection;

	/// The direction in which child should be laid out in the cross axis.
	///
	/// If the [axisDirection] is [AxisDirection.down] or [AxisDirection.up], this
	/// property defaults to [AxisDirection.left] if the ambient [Directionality]
	/// is [TextDirection.rtl] and [AxisDirection.right] if the ambient
	/// [Directionality] is [TextDirection.ltr].
	///
	/// If the [axisDirection] is [AxisDirection.left] or [AxisDirection.right],
	/// this property defaults to [AxisDirection.down].
	final AxisDirection? crossAxisDirection;

	/// Which part of the content inside the viewport should be visible.
	///
	/// The [ViewportOffset.pixels] value determines the scroll offset that the
	/// viewport uses to select which part of its content to display. As the user
	/// scrolls the viewport, this value changes, which changes the content that
	/// is displayed.
	///
	/// Typically a [ScrollPosition].
	final ViewportOffset offset;

	/// {@macro flutter.material.Material.clipBehavior}
	///
	/// Defaults to [Clip.hardEdge].
	final Clip clipBehavior;

	@override
	RenderShrinkWrappingViewport createRenderObject(BuildContext context) {
	return RenderShrinkWrappingViewport(
	axisDirection: axisDirection,
	crossAxisDirection: crossAxisDirection ?? Viewport.getDefaultCrossAxisDirection(context, axisDirection),
	offset: offset,
	clipBehavior: clipBehavior,
	);
	}

	@override
	void updateRenderObject(BuildContext context, RenderShrinkWrappingViewport renderObject) {
	renderObject
	..axisDirection = axisDirection
	..crossAxisDirection = crossAxisDirection ?? Viewport.getDefaultCrossAxisDirection(context, axisDirection)
	..offset = offset
	..clipBehavior = clipBehavior;
	}

	@override
	void debugFillProperties(DiagnosticPropertiesBuilder properties) {
	super.debugFillProperties(properties);
	properties.add(EnumProperty<AxisDirection>('axisDirection', axisDirection));
	properties.add(EnumProperty<AxisDirection>('crossAxisDirection', crossAxisDirection, defaultValue: null));
	properties.add(DiagnosticsProperty<ViewportOffset>('offset', offset));
	}
	}