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

// 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/foundation.dart';
import 'package:flutter/rendering.dart';

import 'automatic_keep_alive.dart';
import 'basic.dart';
import 'framework.dart';
import 'selection_container.dart';

export 'package:flutter/rendering.dart' show
  SliverGridDelegate,
  SliverGridDelegateWithFixedCrossAxisCount,
  SliverGridDelegateWithMaxCrossAxisExtent;

// Examples can assume:
// late SliverGridDelegateWithMaxCrossAxisExtent _gridDelegate;
// abstract class SomeWidget extends StatefulWidget { const SomeWidget({super.key}); }
// typedef ChildWidget = Placeholder;

/// A callback which produces a semantic index given a widget and the local index.
///
/// Return a null value to prevent a widget from receiving an index.
///
/// A semantic index is used to tag child semantic nodes for accessibility
/// announcements in scroll view.
///
/// See also:
///
///  * [CustomScrollView], for an explanation of scroll semantics.
///  * [SliverChildBuilderDelegate], for an explanation of how this is used to
///    generate indexes.
typedef SemanticIndexCallback = int? Function(Widget widget, int localIndex);

int _kDefaultSemanticIndexCallback(Widget _, int localIndex) => localIndex;

/// A delegate that supplies children for slivers.
///
/// Many slivers lazily construct their box children to avoid creating more
/// children than are visible through the [Viewport]. Rather than receiving
/// their children as an explicit [List], they receive their children using a
/// [SliverChildDelegate].
///
/// It's uncommon to subclass [SliverChildDelegate]. Instead, consider using one
/// of the existing subclasses that provide adaptors to builder callbacks or
/// explicit child lists.
///
/// {@template flutter.widgets.SliverChildDelegate.lifecycle}
/// ## Child elements' lifecycle
///
/// ### Creation
///
/// While laying out the list, visible children's elements, states and render
/// objects will be created lazily based on existing widgets (such as in the
/// case of [SliverChildListDelegate]) or lazily provided ones (such as in the
/// case of [SliverChildBuilderDelegate]).
///
/// ### Destruction
///
/// When a child is scrolled out of view, the associated element subtree, states
/// and render objects are destroyed. A new child at the same position in the
/// sliver will be lazily recreated along with new elements, states and render
/// objects when it is scrolled back.
///
/// ### Destruction mitigation
///
/// In order to preserve state as child elements are scrolled in and out of
/// view, the following options are possible:
///
///  * Moving the ownership of non-trivial UI-state-driving business logic
///    out of the sliver child subtree. For instance, if a list contains posts
///    with their number of upvotes coming from a cached network response, store
///    the list of posts and upvote number in a data model outside the list. Let
///    the sliver child UI subtree be easily recreate-able from the
///    source-of-truth model object. Use [StatefulWidget]s in the child widget
///    subtree to store instantaneous UI state only.
///
///  * Letting [KeepAlive] be the root widget of the sliver child widget subtree
///    that needs to be preserved. The [KeepAlive] widget marks the child
///    subtree's top render object child for keepalive. When the associated top
///    render object is scrolled out of view, the sliver keeps the child's
///    render object (and by extension, its associated elements and states) in a
///    cache list instead of destroying them. When scrolled back into view, the
///    render object is repainted as-is (if it wasn't marked dirty in the
///    interim).
///
///    This only works if the [SliverChildDelegate] subclasses don't wrap the
///    child widget subtree with other widgets such as [AutomaticKeepAlive] and
///    [RepaintBoundary] via `addAutomaticKeepAlives` and
///    `addRepaintBoundaries`.
///
///  * Using [AutomaticKeepAlive] widgets (inserted by default in
///    [SliverChildListDelegate] or [SliverChildListDelegate]).
///    [AutomaticKeepAlive] allows descendant widgets to control whether the
///    subtree is actually kept alive or not. This behavior is in contrast with
///    [KeepAlive], which will unconditionally keep the subtree alive.
///
///    As an example, the [EditableText] widget signals its sliver child element
///    subtree to stay alive while its text field has input focus. If it doesn't
///    have focus and no other descendants signaled for keepalive via a
///    [KeepAliveNotification], the sliver child element subtree will be
///    destroyed when scrolled away.
///
///    [AutomaticKeepAlive] descendants typically signal it to be kept alive by
///    using the [AutomaticKeepAliveClientMixin], then implementing the
///    [AutomaticKeepAliveClientMixin.wantKeepAlive] getter and calling
///    [AutomaticKeepAliveClientMixin.updateKeepAlive].
///
/// ## Using more than one delegate in a [Viewport]
///
/// If multiple delegates are used in a single scroll view, the first child of
/// each delegate will always be laid out, even if it extends beyond the
/// currently viewable area. This is because at least one child is required in
/// order to [estimateMaxScrollOffset] for the whole scroll view, as it uses the
/// currently built children to estimate the remaining children's extent.
/// {@endtemplate}
///
/// See also:
///
///  * [SliverChildBuilderDelegate], which is a delegate that uses a builder
///    callback to construct the children.
///  * [SliverChildListDelegate], which is a delegate that has an explicit list
///    of children.
abstract class SliverChildDelegate {
  /// Abstract const constructor. This constructor enables subclasses to provide
  /// const constructors so that they can be used in const expressions.
  const SliverChildDelegate();

  /// Returns the child with the given index.
  ///
  /// Should return null if asked to build a widget with a greater
  /// index than exists. If this returns null, [estimatedChildCount]
  /// must subsequently return a precise non-null value (which is then
  /// used to implement [RenderSliverBoxChildManager.childCount]).
  ///
  /// Subclasses typically override this function and wrap their children in
  /// [AutomaticKeepAlive], [IndexedSemantics], and [RepaintBoundary] widgets.
  ///
  /// The values returned by this method are cached. To indicate that the
  /// widgets have changed, a new delegate must be provided, and the new
  /// delegate's [shouldRebuild] method must return true.
  Widget? build(BuildContext context, int index);

  /// Returns an estimate of the number of children this delegate will build.
  ///
  /// Used to estimate the maximum scroll offset if [estimateMaxScrollOffset]
  /// returns null.
  ///
  /// Return null if there are an unbounded number of children or if it would
  /// be too difficult to estimate the number of children.
  ///
  /// This must return a precise number once [build] has returned null, as it
  /// used to implement [RenderSliverBoxChildManager.childCount].
  int? get estimatedChildCount => null;

  /// Returns an estimate of the max scroll extent for all the children.
  ///
  /// Subclasses should override this function if they have additional
  /// information about their max scroll extent.
  ///
  /// The default implementation returns null, which causes the caller to
  /// extrapolate the max scroll offset from the given parameters.
  double? estimateMaxScrollOffset(
    int firstIndex,
    int lastIndex,
    double leadingScrollOffset,
    double trailingScrollOffset,
  ) => null;

  /// Called at the end of layout to indicate that layout is now complete.
  ///
  /// The `firstIndex` argument is the index of the first child that was
  /// included in the current layout. The `lastIndex` argument is the index of
  /// the last child that was included in the current layout.
  ///
  /// Useful for subclasses that which to track which children are included in
  /// the underlying render tree.
  void didFinishLayout(int firstIndex, int lastIndex) { }

  /// Called whenever a new instance of the child delegate class is
  /// provided to the sliver.
  ///
  /// If the new instance represents different information than the old
  /// instance, then the method should return true, otherwise it should return
  /// false.
  ///
  /// If the method returns false, then the [build] call might be optimized
  /// away.
  bool shouldRebuild(covariant SliverChildDelegate oldDelegate);

  /// Find index of child element with associated key.
  ///
  /// This will be called during `performRebuild` in [SliverMultiBoxAdaptorElement]
  /// to check if a child has moved to a different position. It should return the
  /// index of the child element with associated key, null if not found.
  ///
  /// If not provided, a child widget may not map to its existing [RenderObject]
  /// when the order of children returned from the children builder changes.
  /// This may result in state-loss.
  int? findIndexByKey(Key key) => null;

  @override
  String toString() {
    final List<String> description = <String>[];
    debugFillDescription(description);
    return '${describeIdentity(this)}(${description.join(", ")})';
  }

  /// Add additional information to the given description for use by [toString].
  @protected
  @mustCallSuper
  void debugFillDescription(List<String> description) {
    try {
      final int? children = estimatedChildCount;
      if (children != null) {
        description.add('estimated child count: $children');
      }
    } catch (e) {
      // The exception is forwarded to widget inspector.
      description.add('estimated child count: EXCEPTION (${e.runtimeType})');
    }
  }
}

class _SaltedValueKey extends ValueKey<Key> {
  const _SaltedValueKey(super.key);
}

/// Called to find the new index of a child based on its `key` in case of
/// reordering.
///
/// If the child with the `key` is no longer present, null is returned.
///
/// Used by [SliverChildBuilderDelegate.findChildIndexCallback].
typedef ChildIndexGetter = int? Function(Key key);

/// A delegate that supplies children for slivers using a builder callback.
///
/// Many slivers lazily construct their box children to avoid creating more
/// children than are visible through the [Viewport]. This delegate provides
/// children using a [NullableIndexedWidgetBuilder] callback, so that the children do
/// not even have to be built until they are displayed.
///
/// The widgets returned from the builder callback are automatically wrapped in
/// [AutomaticKeepAlive] widgets if [addAutomaticKeepAlives] is true (the
/// default) and in [RepaintBoundary] widgets if [addRepaintBoundaries] is true
/// (also the default).
///
/// ## Accessibility
///
/// The [CustomScrollView] requires that its semantic children are annotated
/// using [IndexedSemantics]. This is done by default in the delegate with
/// the `addSemanticIndexes` parameter set to true.
///
/// If multiple delegates are used in a single scroll view, then the indexes
/// will not be correct by default. The `semanticIndexOffset` can be used to
/// offset the semantic indexes of each delegate so that the indexes are
/// monotonically increasing. For example, if a scroll view contains two
/// delegates where the first has 10 children contributing semantics, then the
/// second delegate should offset its children by 10.
///
/// {@tool snippet}
///
/// This sample code shows how to use `semanticIndexOffset` to handle multiple
/// delegates in a single scroll view.
///
/// ```dart
/// CustomScrollView(
///   semanticChildCount: 4,
///   slivers: <Widget>[
///     SliverGrid(
///       gridDelegate: _gridDelegate,
///       delegate: SliverChildBuilderDelegate(
///         (BuildContext context, int index) {
///            return const Text('...');
///          },
///          childCount: 2,
///        ),
///      ),
///     SliverGrid(
///       gridDelegate: _gridDelegate,
///       delegate: SliverChildBuilderDelegate(
///         (BuildContext context, int index) {
///            return const Text('...');
///          },
///          childCount: 2,
///          semanticIndexOffset: 2,
///        ),
///      ),
///   ],
/// )
/// ```
/// {@end-tool}
///
/// In certain cases, only a subset of child widgets should be annotated
/// with a semantic index. For example, in [ListView.separated()] the
/// separators do not have an index associated with them. This is done by
/// providing a `semanticIndexCallback` which returns null for separators
/// indexes and rounds the non-separator indexes down by half.
///
/// {@tool snippet}
///
/// This sample code shows how to use `semanticIndexCallback` to handle
/// annotating a subset of child nodes with a semantic index. There is
/// a [Spacer] widget at odd indexes which should not have a semantic
/// index.
///
/// ```dart
/// CustomScrollView(
///   semanticChildCount: 5,
///   slivers: <Widget>[
///     SliverGrid(
///       gridDelegate: _gridDelegate,
///       delegate: SliverChildBuilderDelegate(
///         (BuildContext context, int index) {
///            if (index.isEven) {
///              return const Text('...');
///            }
///            return const Spacer();
///          },
///          semanticIndexCallback: (Widget widget, int localIndex) {
///            if (localIndex.isEven) {
///              return localIndex ~/ 2;
///            }
///            return null;
///          },
///          childCount: 10,
///        ),
///      ),
///   ],
/// )
/// ```
/// {@end-tool}
///
/// See also:
///
///  * [SliverChildListDelegate], which is a delegate that has an explicit list
///    of children.
///  * [IndexedSemantics], for an example of manually annotating child nodes
///    with semantic indexes.
class SliverChildBuilderDelegate extends SliverChildDelegate {
  /// Creates a delegate that supplies children for slivers using the given
  /// builder callback.
  ///
  /// The [builder], [addAutomaticKeepAlives], [addRepaintBoundaries],
  /// [addSemanticIndexes], and [semanticIndexCallback] arguments must not be
  /// null.
  ///
  /// If the order in which [builder] returns children ever changes, consider
  /// providing a [findChildIndexCallback]. This allows the delegate to find the
  /// new index for a child that was previously located at a different index to
  /// attach the existing state to the [Widget] at its new location.
  const SliverChildBuilderDelegate(
    this.builder, {
    this.findChildIndexCallback,
    this.childCount,
    this.addAutomaticKeepAlives = true,
    this.addRepaintBoundaries = true,
    this.addSemanticIndexes = true,
    this.semanticIndexCallback = _kDefaultSemanticIndexCallback,
    this.semanticIndexOffset = 0,
  });

  /// Called to build children for the sliver.
  ///
  /// Will be called only for indices greater than or equal to zero and less
  /// than [childCount] (if [childCount] is non-null).
  ///
  /// Should return null if asked to build a widget with a greater index than
  /// exists.
  ///
  /// May result in an infinite loop or run out of memory if [childCount] is null
  /// and the [builder] always provides a zero-size widget (such as `Container()`
  /// or `SizedBox.shrink()`). If possible, provide children with non-zero size,
  /// return null from [builder], or set a [childCount].
  ///
  /// The delegate wraps the children returned by this builder in
  /// [RepaintBoundary] widgets.
  final NullableIndexedWidgetBuilder builder;

  /// The total number of children this delegate can provide.
  ///
  /// If null, the number of children is determined by the least index for which
  /// [builder] returns null.
  ///
  /// May result in an infinite loop or run out of memory if [childCount] is null
  /// and the [builder] always provides a zero-size widget (such as `Container()`
  /// or `SizedBox.shrink()`). If possible, provide children with non-zero size,
  /// return null from [builder], or set a [childCount].
  final int? childCount;

  /// {@template flutter.widgets.SliverChildBuilderDelegate.addAutomaticKeepAlives}
  /// Whether to wrap each child in an [AutomaticKeepAlive].
  ///
  /// Typically, children in lazy list are wrapped in [AutomaticKeepAlive]
  /// widgets so that children can use [KeepAliveNotification]s to preserve
  /// their state when they would otherwise be garbage collected off-screen.
  ///
  /// This feature (and [addRepaintBoundaries]) must be disabled if the children
  /// are going to manually maintain their [KeepAlive] state. It may also be
  /// more efficient to disable this feature if it is known ahead of time that
  /// none of the children will ever try to keep themselves alive.
  ///
  /// Defaults to true.
  /// {@endtemplate}
  final bool addAutomaticKeepAlives;

  /// {@template flutter.widgets.SliverChildBuilderDelegate.addRepaintBoundaries}
  /// Whether to wrap each child in a [RepaintBoundary].
  ///
  /// Typically, children in a scrolling container are wrapped in repaint
  /// boundaries so that they do not need to be repainted as the list scrolls.
  /// If the children are easy to repaint (e.g., solid color blocks or a short
  /// snippet of text), it might be more efficient to not add a repaint boundary
  /// and instead always repaint the children during scrolling.
  ///
  /// Defaults to true.
  /// {@endtemplate}
  final bool addRepaintBoundaries;

  /// {@template flutter.widgets.SliverChildBuilderDelegate.addSemanticIndexes}
  /// Whether to wrap each child in an [IndexedSemantics].
  ///
  /// Typically, children in a scrolling container must be annotated with a
  /// semantic index in order to generate the correct accessibility
  /// announcements. This should only be set to false if the indexes have
  /// already been provided by an [IndexedSemantics] widget.
  ///
  /// Defaults to true.
  ///
  /// See also:
  ///
  ///  * [IndexedSemantics], for an explanation of how to manually
  ///    provide semantic indexes.
  /// {@endtemplate}
  final bool addSemanticIndexes;

  /// {@template flutter.widgets.SliverChildBuilderDelegate.semanticIndexOffset}
  /// An initial offset to add to the semantic indexes generated by this widget.
  ///
  /// Defaults to zero.
  /// {@endtemplate}
  final int semanticIndexOffset;

  /// {@template flutter.widgets.SliverChildBuilderDelegate.semanticIndexCallback}
  /// A [SemanticIndexCallback] which is used when [addSemanticIndexes] is true.
  ///
  /// Defaults to providing an index for each widget.
  /// {@endtemplate}
  final SemanticIndexCallback semanticIndexCallback;

  /// {@template flutter.widgets.SliverChildBuilderDelegate.findChildIndexCallback}
  /// Called to find the new index of a child based on its key in case of reordering.
  ///
  /// If not provided, a child widget may not map to its existing [RenderObject]
  /// when the order of children returned from the children builder changes.
  /// This may result in state-loss.
  ///
  /// This callback should take an input [Key], and it should return the
  /// index of the child element with that associated key, or null if not found.
  /// {@endtemplate}
  final ChildIndexGetter? findChildIndexCallback;

  @override
  int? findIndexByKey(Key key) {
    if (findChildIndexCallback == null) {
      return null;
    }
    final Key childKey;
    if (key is _SaltedValueKey) {
      final _SaltedValueKey saltedValueKey = key;
      childKey = saltedValueKey.value;
    } else {
      childKey = key;
    }
    return findChildIndexCallback!(childKey);
  }

  @override
  @pragma('vm:notify-debugger-on-exception')
  Widget? build(BuildContext context, int index) {
    if (index < 0 || (childCount != null && index >= childCount!)) {
      return null;
    }
    Widget? child;
    try {
      child = builder(context, index);
    } catch (exception, stackTrace) {
      child = _createErrorWidget(exception, stackTrace);
    }
    if (child == null) {
      return null;
    }
    final Key? key = child.key != null ? _SaltedValueKey(child.key!) : null;
    if (addRepaintBoundaries) {
      child = RepaintBoundary(child: child);
    }
    if (addSemanticIndexes) {
      final int? semanticIndex = semanticIndexCallback(child, index);
      if (semanticIndex != null) {
        child = IndexedSemantics(index: semanticIndex + semanticIndexOffset, child: child);
      }
    }
    if (addAutomaticKeepAlives) {
      child = AutomaticKeepAlive(child: _SelectionKeepAlive(child: child));
    }
    return KeyedSubtree(key: key, child: child);
  }

  @override
  int? get estimatedChildCount => childCount;

  @override
  bool shouldRebuild(covariant SliverChildBuilderDelegate oldDelegate) => true;
}

/// A delegate that supplies children for slivers using an explicit list.
///
/// Many slivers lazily construct their box children to avoid creating more
/// children than are visible through the [Viewport]. This delegate provides
/// children using an explicit list, which is convenient but reduces the benefit
/// of building children lazily.
///
/// In general building all the widgets in advance is not efficient. It is
/// better to create a delegate that builds them on demand using
/// [SliverChildBuilderDelegate] or by subclassing [SliverChildDelegate]
/// directly.
///
/// This class is provided for the cases where either the list of children is
/// known well in advance (ideally the children are themselves compile-time
/// constants, for example), and therefore will not be built each time the
/// delegate itself is created, or the list is small, such that it's likely
/// always visible (and thus there is nothing to be gained by building it on
/// demand). For example, the body of a dialog box might fit both of these
/// conditions.
///
/// The widgets in the given [children] list are automatically wrapped in
/// [AutomaticKeepAlive] widgets if [addAutomaticKeepAlives] is true (the
/// default) and in [RepaintBoundary] widgets if [addRepaintBoundaries] is true
/// (also the default).
///
/// ## Accessibility
///
/// The [CustomScrollView] requires that its semantic children are annotated
/// using [IndexedSemantics]. This is done by default in the delegate with
/// the `addSemanticIndexes` parameter set to true.
///
/// If multiple delegates are used in a single scroll view, then the indexes
/// will not be correct by default. The `semanticIndexOffset` can be used to
/// offset the semantic indexes of each delegate so that the indexes are
/// monotonically increasing. For example, if a scroll view contains two
/// delegates where the first has 10 children contributing semantics, then the
/// second delegate should offset its children by 10.
///
/// In certain cases, only a subset of child widgets should be annotated
/// with a semantic index. For example, in [ListView.separated()] the
/// separators do not have an index associated with them. This is done by
/// providing a `semanticIndexCallback` which returns null for separators
/// indexes and rounds the non-separator indexes down by half.
///
/// See [SliverChildBuilderDelegate] for sample code using
/// `semanticIndexOffset` and `semanticIndexCallback`.
///
/// See also:
///
///  * [SliverChildBuilderDelegate], which is a delegate that uses a builder
///    callback to construct the children.
class SliverChildListDelegate extends SliverChildDelegate {
  /// Creates a delegate that supplies children for slivers using the given
  /// list.
  ///
  /// The [children], [addAutomaticKeepAlives], [addRepaintBoundaries],
  /// [addSemanticIndexes], and [semanticIndexCallback] arguments must not be
  /// null.
  ///
  /// If the order of children never changes, consider using the constant
  /// [SliverChildListDelegate.fixed] constructor.
  SliverChildListDelegate(
    this.children, {
    this.addAutomaticKeepAlives = true,
    this.addRepaintBoundaries = true,
    this.addSemanticIndexes = true,
    this.semanticIndexCallback = _kDefaultSemanticIndexCallback,
    this.semanticIndexOffset = 0,
  }) : _keyToIndex = <Key?, int>{null: 0};

  /// Creates a constant version of the delegate that supplies children for
  /// slivers using the given list.
  ///
  /// If the order of the children will change, consider using the regular
  /// [SliverChildListDelegate] constructor.
  ///
  /// The [children], [addAutomaticKeepAlives], [addRepaintBoundaries],
  /// [addSemanticIndexes], and [semanticIndexCallback] arguments must not be
  /// null.
  const SliverChildListDelegate.fixed(
    this.children, {
    this.addAutomaticKeepAlives = true,
    this.addRepaintBoundaries = true,
    this.addSemanticIndexes = true,
    this.semanticIndexCallback = _kDefaultSemanticIndexCallback,
    this.semanticIndexOffset = 0,
  }) : _keyToIndex = null;

  /// {@macro flutter.widgets.SliverChildBuilderDelegate.addAutomaticKeepAlives}
  final bool addAutomaticKeepAlives;

  /// {@macro flutter.widgets.SliverChildBuilderDelegate.addRepaintBoundaries}
  final bool addRepaintBoundaries;

  /// {@macro flutter.widgets.SliverChildBuilderDelegate.addSemanticIndexes}
  final bool addSemanticIndexes;

  /// {@macro flutter.widgets.SliverChildBuilderDelegate.semanticIndexOffset}
  final int semanticIndexOffset;

  /// {@macro flutter.widgets.SliverChildBuilderDelegate.semanticIndexCallback}
  final SemanticIndexCallback semanticIndexCallback;

  /// The widgets to display.
  ///
  /// If this list is going to be mutated, it is usually wise to put a [Key] on
  /// each of the child widgets, so that the framework can match old
  /// configurations to new configurations and maintain the underlying render
  /// objects.
  ///
  /// Also, a [Widget] in Flutter is immutable, so directly modifying the
  /// [children] such as `someWidget.children.add(...)` or
  /// passing a reference of the original list value to the [children] parameter
  /// will result in incorrect behaviors. Whenever the
  /// children list is modified, a new list object should be provided.
  ///
  /// The following code corrects the problem mentioned above.
  ///
  /// ```dart
  /// class SomeWidgetState extends State<SomeWidget> {
  ///   final List<Widget> _children = <Widget>[];
  ///
  ///   void someHandler() {
  ///     setState(() {
  ///       // The key here allows Flutter to reuse the underlying render
  ///       // objects even if the children list is recreated.
  ///       _children.add(ChildWidget(key: UniqueKey()));
  ///     });
  ///   }
  ///
  ///   @override
  ///   Widget build(BuildContext context) {
  ///     // Always create a new list of children as a Widget is immutable.
  ///     return PageView(children: List<Widget>.of(_children));
  ///   }
  /// }
  /// ```
  final List<Widget> children;

  /// A map to cache key to index lookup for children.
  ///
  /// _keyToIndex[null] is used as current index during the lazy loading process
  /// in [_findChildIndex]. _keyToIndex should never be used for looking up null key.
  final Map<Key?, int>? _keyToIndex;

  bool get _isConstantInstance => _keyToIndex == null;

  int? _findChildIndex(Key key) {
    if (_isConstantInstance) {
      return null;
    }
    // Lazily fill the [_keyToIndex].
    if (!_keyToIndex!.containsKey(key)) {
      int index = _keyToIndex![null]!;
      while (index < children.length) {
        final Widget child = children[index];
        if (child.key != null) {
          _keyToIndex![child.key] = index;
        }
        if (child.key == key) {
          // Record current index for next function call.
          _keyToIndex![null] = index + 1;
          return index;
        }
        index += 1;
      }
      _keyToIndex![null] = index;
    } else {
      return _keyToIndex![key];
    }
    return null;
  }

  @override
  int? findIndexByKey(Key key) {
    final Key childKey;
    if (key is _SaltedValueKey) {
      final _SaltedValueKey saltedValueKey = key;
      childKey = saltedValueKey.value;
    } else {
      childKey = key;
    }
    return _findChildIndex(childKey);
  }

  @override
  Widget? build(BuildContext context, int index) {
    if (index < 0 || index >= children.length) {
      return null;
    }
    Widget child = children[index];
    final Key? key = child.key != null? _SaltedValueKey(child.key!) : null;
    if (addRepaintBoundaries) {
      child = RepaintBoundary(child: child);
    }
    if (addSemanticIndexes) {
      final int? semanticIndex = semanticIndexCallback(child, index);
      if (semanticIndex != null) {
        child = IndexedSemantics(index: semanticIndex + semanticIndexOffset, child: child);
      }
    }
    if (addAutomaticKeepAlives) {
      child = AutomaticKeepAlive(child: _SelectionKeepAlive(child: child));
    }

    return KeyedSubtree(key: key, child: child);
  }

  @override
  int? get estimatedChildCount => children.length;

  @override
  bool shouldRebuild(covariant SliverChildListDelegate oldDelegate) {
    return children != oldDelegate.children;
  }
}

class _SelectionKeepAlive extends StatefulWidget {
  /// Creates a widget that listens to [KeepAliveNotification]s and maintains a
  /// [KeepAlive] widget appropriately.
  const _SelectionKeepAlive({
    required this.child,
  });

  /// The widget below this widget in the tree.
  ///
  /// {@macro flutter.widgets.ProxyWidget.child}
  final Widget child;

  @override
  State<_SelectionKeepAlive> createState() => _SelectionKeepAliveState();
}

class _SelectionKeepAliveState extends State<_SelectionKeepAlive> with AutomaticKeepAliveClientMixin implements SelectionRegistrar {
  Set<Selectable>? _selectablesWithSelections;
  Map<Selectable, VoidCallback>? _selectableAttachments;
  SelectionRegistrar? _registrar;

  @override
  bool get wantKeepAlive => _wantKeepAlive;
  bool _wantKeepAlive = false;
  set wantKeepAlive(bool value) {
    if (_wantKeepAlive != value) {
      _wantKeepAlive = value;
      updateKeepAlive();
    }
  }

  VoidCallback listensTo(Selectable selectable) {
    return () {
      if (selectable.value.hasSelection) {
        _updateSelectablesWithSelections(selectable, add: true);
      } else {
        _updateSelectablesWithSelections(selectable, add: false);
      }
    };
  }

  void _updateSelectablesWithSelections(Selectable selectable, {required bool add}) {
    if (add) {
      assert(selectable.value.hasSelection);
      _selectablesWithSelections ??= <Selectable>{};
      _selectablesWithSelections!.add(selectable);
    } else {
      _selectablesWithSelections?.remove(selectable);
    }
    wantKeepAlive = _selectablesWithSelections?.isNotEmpty ?? false;
  }

  @override
  void didChangeDependencies() {
    super.didChangeDependencies();
    final SelectionRegistrar? newRegistrar = SelectionContainer.maybeOf(context);
    if (_registrar != newRegistrar) {
      if (_registrar != null) {
        _selectableAttachments?.keys.forEach(_registrar!.remove);
      }
      _registrar = newRegistrar;
      if (_registrar != null) {
        _selectableAttachments?.keys.forEach(_registrar!.add);
      }
    }
  }

  @override
  void add(Selectable selectable) {
    final VoidCallback attachment = listensTo(selectable);
    selectable.addListener(attachment);
    _selectableAttachments ??= <Selectable, VoidCallback>{};
    _selectableAttachments![selectable] = attachment;
    _registrar!.add(selectable);
    if (selectable.value.hasSelection) {
      _updateSelectablesWithSelections(selectable, add: true);
    }
  }

  @override
  void remove(Selectable selectable) {
    if (_selectableAttachments == null) {
      return;
    }
    assert(_selectableAttachments!.containsKey(selectable));
    final VoidCallback attachment = _selectableAttachments!.remove(selectable)!;
    selectable.removeListener(attachment);
    _registrar!.remove(selectable);
    _updateSelectablesWithSelections(selectable, add: false);
  }

  @override
  void dispose() {
    if (_selectableAttachments != null) {
      for (final Selectable selectable in _selectableAttachments!.keys) {
        _registrar!.remove(selectable);
        selectable.removeListener(_selectableAttachments![selectable]!);
      }
      _selectableAttachments = null;
    }
    _selectablesWithSelections = null;
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    super.build(context);
    if (_registrar == null) {
      return widget.child;
    }
    return SelectionRegistrarScope(
      registrar: this,
      child: widget.child,
    );
  }
}

// Return a Widget for the given Exception
Widget _createErrorWidget(Object exception, StackTrace stackTrace) {
  final FlutterErrorDetails details = FlutterErrorDetails(
    exception: exception,
    stack: stackTrace,
    library: 'widgets library',
    context: ErrorDescription('building'),
  );
  FlutterError.reportError(details);
  return ErrorWidget.builder(details);
}