packages/flutter/lib/src/widgets/scrollable.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 'dart:async';
import 'dart:math' as math;

import 'package:flutter/foundation.dart';
import 'package:flutter/gestures.dart';
import 'package:flutter/rendering.dart';
import 'package:flutter/scheduler.dart';
import 'package:flutter/services.dart';

import 'basic.dart';
import 'framework.dart';
import 'gesture_detector.dart';
import 'media_query.dart';
import 'notification_listener.dart';
import 'restoration.dart';
import 'restoration_properties.dart';
import 'scroll_activity.dart';
import 'scroll_configuration.dart';
import 'scroll_context.dart';
import 'scroll_controller.dart';
import 'scroll_physics.dart';
import 'scroll_position.dart';
import 'scrollable_helpers.dart';
import 'selectable_region.dart';
import 'selection_container.dart';
import 'ticker_provider.dart';
import 'viewport.dart';

export 'package:flutter/physics.dart' show Tolerance;

// Examples can assume:
// late BuildContext context;

/// Signature used by [Scrollable] to build the viewport through which the
/// scrollable content is displayed.
typedef ViewportBuilder = Widget Function(BuildContext context, ViewportOffset position);

/// A widget that scrolls.
///
/// [Scrollable] implements the interaction model for a scrollable widget,
/// including gesture recognition, but does not have an opinion about how the
/// viewport, which actually displays the children, is constructed.
///
/// It's rare to construct a [Scrollable] directly. Instead, consider [ListView]
/// or [GridView], which combine scrolling, viewporting, and a layout model. To
/// combine layout models (or to use a custom layout mode), consider using
/// [CustomScrollView].
///
/// The static [Scrollable.of] and [Scrollable.ensureVisible] functions are
/// often used to interact with the [Scrollable] widget inside a [ListView] or
/// a [GridView].
///
/// To further customize scrolling behavior with a [Scrollable]:
///
/// 1. You can provide a [viewportBuilder] to customize the child model. For
///    example, [SingleChildScrollView] uses a viewport that displays a single
///    box child whereas [CustomScrollView] uses a [Viewport] or a
///    [ShrinkWrappingViewport], both of which display a list of slivers.
///
/// 2. You can provide a custom [ScrollController] that creates a custom
///    [ScrollPosition] subclass. For example, [PageView] uses a
///    [PageController], which creates a page-oriented scroll position subclass
///    that keeps the same page visible when the [Scrollable] resizes.
///
/// See also:
///
///  * [ListView], which is a commonly used [ScrollView] that displays a
///    scrolling, linear list of child widgets.
///  * [PageView], which is a scrolling list of child widgets that are each the
///    size of the viewport.
///  * [GridView], which is a [ScrollView] that displays a scrolling, 2D array
///    of child widgets.
///  * [CustomScrollView], which is a [ScrollView] that creates custom scroll
///    effects using slivers.
///  * [SingleChildScrollView], which is a scrollable widget that has a single
///    child.
///  * [ScrollNotification] and [NotificationListener], which can be used to watch
///    the scroll position without using a [ScrollController].
class Scrollable extends StatefulWidget {
  /// Creates a widget that scrolls.
  ///
  /// The [axisDirection] and [viewportBuilder] arguments must not be null.
  const Scrollable({
    super.key,
    this.axisDirection = AxisDirection.down,
    this.controller,
    this.physics,
    required this.viewportBuilder,
    this.incrementCalculator,
    this.excludeFromSemantics = false,
    this.semanticChildCount,
    this.dragStartBehavior = DragStartBehavior.start,
    this.restorationId,
    this.scrollBehavior,
    this.clipBehavior = Clip.hardEdge,
  }) : assert(semanticChildCount == null || semanticChildCount >= 0);

  /// {@template flutter.widgets.Scrollable.axisDirection}
  /// The direction in which this widget scrolls.
  ///
  /// For example, if the [Scrollable.axisDirection] is [AxisDirection.down],
  /// increasing the scroll position will cause content below the bottom of the
  /// viewport to become visible through the viewport. Similarly, if the
  /// axisDirection is [AxisDirection.right], increasing the scroll position
  /// will cause content beyond the right edge of the viewport to become visible
  /// through the viewport.
  ///
  /// Defaults to [AxisDirection.down].
  /// {@endtemplate}
  final AxisDirection axisDirection;

  /// {@template flutter.widgets.Scrollable.controller}
  /// An object that can be used to control the position to which this widget is
  /// scrolled.
  ///
  /// A [ScrollController] serves several purposes. It can be used to control
  /// the initial scroll position (see [ScrollController.initialScrollOffset]).
  /// It can be used to control whether the scroll view should automatically
  /// save and restore its scroll position in the [PageStorage] (see
  /// [ScrollController.keepScrollOffset]). It can be used to read the current
  /// scroll position (see [ScrollController.offset]), or change it (see
  /// [ScrollController.animateTo]).
  ///
  /// If null, a [ScrollController] will be created internally by [Scrollable]
  /// in order to create and manage the [ScrollPosition].
  ///
  /// See also:
  ///
  ///  * [Scrollable.ensureVisible], which animates the scroll position to
  ///    reveal a given [BuildContext].
  /// {@endtemplate}
  final ScrollController? controller;

  /// {@template flutter.widgets.Scrollable.physics}
  /// How the widgets should respond to user input.
  ///
  /// For example, determines how the widget continues to animate after the
  /// user stops dragging the scroll view.
  ///
  /// Defaults to matching platform conventions via the physics provided from
  /// the ambient [ScrollConfiguration].
  ///
  /// If an explicit [ScrollBehavior] is provided to
  /// [Scrollable.scrollBehavior], the [ScrollPhysics] provided by that behavior
  /// will take precedence after [Scrollable.physics].
  ///
  /// The physics can be changed dynamically, but new physics will only take
  /// effect if the _class_ of the provided object changes. Merely constructing
  /// a new instance with a different configuration is insufficient to cause the
  /// physics to be reapplied. (This is because the final object used is
  /// generated dynamically, which can be relatively expensive, and it would be
  /// inefficient to speculatively create this object each frame to see if the
  /// physics should be updated.)
  ///
  /// See also:
  ///
  ///  * [AlwaysScrollableScrollPhysics], which can be used to indicate that the
  ///    scrollable should react to scroll requests (and possible overscroll)
  ///    even if the scrollable's contents fit without scrolling being necessary.
  /// {@endtemplate}
  final ScrollPhysics? physics;

  /// Builds the viewport through which the scrollable content is displayed.
  ///
  /// A typical viewport uses the given [ViewportOffset] to determine which part
  /// of its content is actually visible through the viewport.
  ///
  /// See also:
  ///
  ///  * [Viewport], which is a viewport that displays a list of slivers.
  ///  * [ShrinkWrappingViewport], which is a viewport that displays a list of
  ///    slivers and sizes itself based on the size of the slivers.
  final ViewportBuilder viewportBuilder;

  /// An optional function that will be called to calculate the distance to
  /// scroll when the scrollable is asked to scroll via the keyboard using a
  /// [ScrollAction].
  ///
  /// If not supplied, the [Scrollable] will scroll a default amount when a
  /// keyboard navigation key is pressed (e.g. pageUp/pageDown, control-upArrow,
  /// etc.), or otherwise invoked by a [ScrollAction].
  ///
  /// If [incrementCalculator] is null, the default for
  /// [ScrollIncrementType.page] is 80% of the size of the scroll window, and
  /// for [ScrollIncrementType.line], 50 logical pixels.
  final ScrollIncrementCalculator? incrementCalculator;

  /// Whether the scroll actions introduced by this [Scrollable] are exposed
  /// in the semantics tree.
  ///
  /// Text fields with an overflow are usually scrollable to make sure that the
  /// user can get to the beginning/end of the entered text. However, these
  /// scrolling actions are generally not exposed to the semantics layer.
  ///
  /// See also:
  ///
  ///  * [GestureDetector.excludeFromSemantics], which is used to accomplish the
  ///    exclusion.
  final bool excludeFromSemantics;

  /// The number of children that will contribute semantic information.
  ///
  /// The value will be null if the number of children is unknown or unbounded.
  ///
  /// Some subtypes of [ScrollView] can infer this value automatically. For
  /// example [ListView] will use the number of widgets in the child list,
  /// while the [ListView.separated] constructor will use half that amount.
  ///
  /// For [CustomScrollView] and other types which do not receive a builder
  /// or list of widgets, the child count must be explicitly provided.
  ///
  /// See also:
  ///
  ///  * [CustomScrollView], for an explanation of scroll semantics.
  ///  * [SemanticsConfiguration.scrollChildCount], the corresponding semantics property.
  final int? semanticChildCount;

  // TODO(jslavitz): Set the DragStartBehavior default to be start across all widgets.
  /// {@template flutter.widgets.scrollable.dragStartBehavior}
  /// Determines the way that drag start behavior is handled.
  ///
  /// If set to [DragStartBehavior.start], scrolling drag behavior will
  /// begin at the position where the drag gesture won the arena. If set to
  /// [DragStartBehavior.down] it will begin at the position where a down
  /// event is first detected.
  ///
  /// In general, setting this to [DragStartBehavior.start] will make drag
  /// animation smoother and setting it to [DragStartBehavior.down] will make
  /// drag behavior feel slightly more reactive.
  ///
  /// By default, the drag start behavior is [DragStartBehavior.start].
  ///
  /// See also:
  ///
  ///  * [DragGestureRecognizer.dragStartBehavior], which gives an example for
  ///    the different behaviors.
  ///
  /// {@endtemplate}
  final DragStartBehavior dragStartBehavior;

  /// {@template flutter.widgets.scrollable.restorationId}
  /// Restoration ID to save and restore the scroll offset of the scrollable.
  ///
  /// If a restoration id is provided, the scrollable will persist its current
  /// scroll offset and restore it during state restoration.
  ///
  /// The scroll offset is persisted in a [RestorationBucket] claimed from
  /// the surrounding [RestorationScope] using the provided restoration ID.
  ///
  /// See also:
  ///
  ///  * [RestorationManager], which explains how state restoration works in
  ///    Flutter.
  /// {@endtemplate}
  final String? restorationId;

  /// {@macro flutter.widgets.shadow.scrollBehavior}
  ///
  /// [ScrollBehavior]s also provide [ScrollPhysics]. If an explicit
  /// [ScrollPhysics] is provided in [physics], it will take precedence,
  /// followed by [scrollBehavior], and then the inherited ancestor
  /// [ScrollBehavior].
  final ScrollBehavior? scrollBehavior;

  /// {@macro flutter.material.Material.clipBehavior}
  ///
  /// Defaults to [Clip.hardEdge].
  ///
  /// This is passed to decorators in [ScrollableDetails], and does not directly affect
  /// clipping of the [Scrollable]. This reflects the same [Clip] that is provided
  /// to [ScrollView.clipBehavior] and is supplied to the [Viewport].
  final Clip clipBehavior;

  /// The axis along which the scroll view scrolls.
  ///
  /// Determined by the [axisDirection].
  Axis get axis => axisDirectionToAxis(axisDirection);

  @override
  ScrollableState createState() => ScrollableState();

  @override
  void debugFillProperties(DiagnosticPropertiesBuilder properties) {
    super.debugFillProperties(properties);
    properties.add(EnumProperty<AxisDirection>('axisDirection', axisDirection));
    properties.add(DiagnosticsProperty<ScrollPhysics>('physics', physics));
    properties.add(StringProperty('restorationId', restorationId));
  }

  /// The state from the closest instance of this class that encloses the given
  /// context, or null if none is found.
  ///
  /// Typical usage is as follows:
  ///
  /// ```dart
  /// ScrollableState? scrollable = Scrollable.maybeOf(context);
  /// ```
  ///
  /// Calling this method will create a dependency on the closest [Scrollable]
  /// in the [context], if there is one.
  ///
  /// See also:
  ///
  /// * [Scrollable.of], which is similar to this method, but asserts
  ///   if no [Scrollable] ancestor is found.
  static ScrollableState? maybeOf(BuildContext context) {
    final _ScrollableScope? widget = context.dependOnInheritedWidgetOfExactType<_ScrollableScope>();
    return widget?.scrollable;
  }

  /// The state from the closest instance of this class that encloses the given
  /// context.
  ///
  /// Typical usage is as follows:
  ///
  /// ```dart
  /// ScrollableState scrollable = Scrollable.of(context);
  /// ```
  ///
  /// Calling this method will create a dependency on the closest [Scrollable]
  /// in the [context].
  ///
  /// If no [Scrollable] ancestor is found, then this method will assert in
  /// debug mode, and throw an exception in release mode.
  ///
  /// See also:
  ///
  /// * [Scrollable.maybeOf], which is similar to this method, but returns null
  ///   if no [Scrollable] ancestor is found.
  static ScrollableState of(BuildContext context) {
    final ScrollableState? scrollableState = maybeOf(context);
    assert(() {
      if (scrollableState == null) {
        throw FlutterError(
          'Scrollable.of() was called with a context that does not contain a '
          'Scrollable widget.\n'
          'No Scrollable widget ancestor could be found starting from the '
          'context that was passed to Scrollable.of(). This can happen '
          'because you are using a widget that looks for a Scrollable '
          'ancestor, but no such ancestor exists.\n'
          'The context used was:\n'
          '  $context',
        );
      }
      return true;
    }());
    return scrollableState!;
  }

  /// Provides a heuristic to determine if expensive frame-bound tasks should be
  /// deferred for the [context] at a specific point in time.
  ///
  /// Calling this method does _not_ create a dependency on any other widget.
  /// This also means that the value returned is only good for the point in time
  /// when it is called, and callers will not get updated if the value changes.
  ///
  /// The heuristic used is determined by the [physics] of this [Scrollable]
  /// via [ScrollPhysics.recommendDeferredLoading]. That method is called with
  /// the current [ScrollPosition.activity]'s [ScrollActivity.velocity].
  ///
  /// If there is no [Scrollable] in the widget tree above the [context], this
  /// method returns false.
  static bool recommendDeferredLoadingForContext(BuildContext context) {
    final _ScrollableScope? widget = context.getInheritedWidgetOfExactType<_ScrollableScope>();
    if (widget == null) {
      return false;
    }
    return widget.position.recommendDeferredLoading(context);
  }

  /// Scrolls the scrollables that enclose the given context so as to make the
  /// given context visible.
  static Future<void> ensureVisible(
    BuildContext context, {
    double alignment = 0.0,
    Duration duration = Duration.zero,
    Curve curve = Curves.ease,
    ScrollPositionAlignmentPolicy alignmentPolicy = ScrollPositionAlignmentPolicy.explicit,
  }) {
    final List<Future<void>> futures = <Future<void>>[];

    // The `targetRenderObject` is used to record the first target renderObject.
    // If there are multiple scrollable widgets nested, we should let
    // the `targetRenderObject` as visible as possible to improve the user experience.
    // Otherwise, let the outer renderObject as visible as possible maybe cause
    // the `targetRenderObject` invisible.
    // Also see https://github.com/flutter/flutter/issues/65100
    RenderObject? targetRenderObject;
    ScrollableState? scrollable = Scrollable.maybeOf(context);
    while (scrollable != null) {
      futures.add(scrollable.position.ensureVisible(
        context.findRenderObject()!,
        alignment: alignment,
        duration: duration,
        curve: curve,
        alignmentPolicy: alignmentPolicy,
        targetRenderObject: targetRenderObject,
      ));

      targetRenderObject = targetRenderObject ?? context.findRenderObject();
      context = scrollable.context;
      scrollable = Scrollable.maybeOf(context);
    }

    if (futures.isEmpty || duration == Duration.zero) {
      return Future<void>.value();
    }
    if (futures.length == 1) {
      return futures.single;
    }
    return Future.wait<void>(futures).then<void>((List<void> _) => null);
  }
}

// Enable Scrollable.of() to work as if ScrollableState was an inherited widget.
// ScrollableState.build() always rebuilds its _ScrollableScope.
class _ScrollableScope extends InheritedWidget {
  const _ScrollableScope({
    required this.scrollable,
    required this.position,
    required super.child,
  });

  final ScrollableState scrollable;
  final ScrollPosition position;

  @override
  bool updateShouldNotify(_ScrollableScope old) {
    return position != old.position;
  }
}

/// State object for a [Scrollable] widget.
///
/// To manipulate a [Scrollable] widget's scroll position, use the object
/// obtained from the [position] property.
///
/// To be informed of when a [Scrollable] widget is scrolling, use a
/// [NotificationListener] to listen for [ScrollNotification] notifications.
///
/// This class is not intended to be subclassed. To specialize the behavior of a
/// [Scrollable], provide it with a [ScrollPhysics].
class ScrollableState extends State<Scrollable> with TickerProviderStateMixin, RestorationMixin
    implements ScrollContext {

  // GETTERS

  /// The manager for this [Scrollable] widget's viewport position.
  ///
  /// To control what kind of [ScrollPosition] is created for a [Scrollable],
  /// provide it with custom [ScrollController] that creates the appropriate
  /// [ScrollPosition] in its [ScrollController.createScrollPosition] method.
  ScrollPosition get position => _position!;
  ScrollPosition? _position;

  /// The resolved [ScrollPhysics] of the [ScrollableState].
  ScrollPhysics? get resolvedPhysics => _physics;
  ScrollPhysics? _physics;

  /// An [Offset] that represents the absolute distance from the origin, or 0,
  /// of the [ScrollPosition] expressed in the associated [Axis].
  ///
  /// Used by [EdgeDraggingAutoScroller] to progress the position forward when a
  /// drag gesture reaches the edge of the [Viewport].
  Offset get deltaToScrollOrigin {
    switch (axisDirection) {
      case AxisDirection.down:
        return Offset(0, position.pixels);
      case AxisDirection.up:
        return Offset(0, -position.pixels);
      case AxisDirection.left:
        return Offset(-position.pixels, 0);
      case AxisDirection.right:
        return Offset(position.pixels, 0);
    }
  }

  ScrollController get _effectiveScrollController => widget.controller ?? _fallbackScrollController!;

  @override
  AxisDirection get axisDirection => widget.axisDirection;

  @override
  TickerProvider get vsync => this;

  @override
  BuildContext? get notificationContext => _gestureDetectorKey.currentContext;

  @override
  BuildContext get storageContext => context;

  @override
  String? get restorationId => widget.restorationId;
  final _RestorableScrollOffset _persistedScrollOffset = _RestorableScrollOffset();

  late ScrollBehavior _configuration;
  ScrollController? _fallbackScrollController;
  DeviceGestureSettings? _mediaQueryGestureSettings;

  // Only call this from places that will definitely trigger a rebuild.
  void _updatePosition() {
    _configuration = widget.scrollBehavior ?? ScrollConfiguration.of(context);
    _physics = _configuration.getScrollPhysics(context);
    if (widget.physics != null) {
      _physics = widget.physics!.applyTo(_physics);
    } else if (widget.scrollBehavior != null) {
      _physics = widget.scrollBehavior!.getScrollPhysics(context).applyTo(_physics);
    }
    final ScrollPosition? oldPosition = _position;
    if (oldPosition != null) {
      _effectiveScrollController.detach(oldPosition);
      // It's important that we not dispose the old position until after the
      // viewport has had a chance to unregister its listeners from the old
      // position. So, schedule a microtask to do it.
      scheduleMicrotask(oldPosition.dispose);
    }

    _position = _effectiveScrollController.createScrollPosition(_physics!, this, oldPosition);
    assert(_position != null);
    _effectiveScrollController.attach(position);
  }

  @override
  void restoreState(RestorationBucket? oldBucket, bool initialRestore) {
    registerForRestoration(_persistedScrollOffset, 'offset');
    assert(_position != null);
    if (_persistedScrollOffset.value != null) {
      position.restoreOffset(_persistedScrollOffset.value!, initialRestore: initialRestore);
    }
  }

  @override
  void saveOffset(double offset) {
    assert(debugIsSerializableForRestoration(offset));
    _persistedScrollOffset.value = offset;
    // [saveOffset] is called after a scrolling ends and it is usually not
    // followed by a frame. Therefore, manually flush restoration data.
    ServicesBinding.instance.restorationManager.flushData();
  }

  @override
  void initState() {
    if (widget.controller == null) {
      _fallbackScrollController = ScrollController();
    }
    super.initState();
  }

  @override
  void didChangeDependencies() {
    _mediaQueryGestureSettings = MediaQuery.maybeGestureSettingsOf(context);
    _updatePosition();
    super.didChangeDependencies();
  }

  bool _shouldUpdatePosition(Scrollable oldWidget) {
    ScrollPhysics? newPhysics = widget.physics ?? widget.scrollBehavior?.getScrollPhysics(context);
    ScrollPhysics? oldPhysics = oldWidget.physics ?? oldWidget.scrollBehavior?.getScrollPhysics(context);
    do {
      if (newPhysics?.runtimeType != oldPhysics?.runtimeType) {
        return true;
      }
      newPhysics = newPhysics?.parent;
      oldPhysics = oldPhysics?.parent;
    } while (newPhysics != null || oldPhysics != null);

    return widget.controller?.runtimeType != oldWidget.controller?.runtimeType;
  }

  @override
  void didUpdateWidget(Scrollable oldWidget) {
    super.didUpdateWidget(oldWidget);

    if (widget.controller != oldWidget.controller) {
      if (oldWidget.controller == null) {
        // The old controller was null, meaning the fallback cannot be null.
        // Dispose of the fallback.
        assert(_fallbackScrollController !=  null);
        assert(widget.controller != null);
        _fallbackScrollController!.detach(position);
        _fallbackScrollController!.dispose();
        _fallbackScrollController = null;
      } else {
        // The old controller was not null, detach.
        oldWidget.controller?.detach(position);
        if (widget.controller == null) {
          // If the new controller is null, we need to set up the fallback
          // ScrollController.
          _fallbackScrollController = ScrollController();
        }
      }
      // Attach the updated effective scroll controller.
      _effectiveScrollController.attach(position);
    }

    if (_shouldUpdatePosition(oldWidget)) {
      _updatePosition();
    }
  }

  @override
  void dispose() {
    if (widget.controller != null) {
      widget.controller!.detach(position);
    } else {
      _fallbackScrollController?.detach(position);
      _fallbackScrollController?.dispose();
    }

    position.dispose();
    _persistedScrollOffset.dispose();
    super.dispose();
  }

  // SEMANTICS

  final GlobalKey _scrollSemanticsKey = GlobalKey();

  @override
  @protected
  void setSemanticsActions(Set<SemanticsAction> actions) {
    if (_gestureDetectorKey.currentState != null) {
      _gestureDetectorKey.currentState!.replaceSemanticsActions(actions);
    }
  }

  // GESTURE RECOGNITION AND POINTER IGNORING

  final GlobalKey<RawGestureDetectorState> _gestureDetectorKey = GlobalKey<RawGestureDetectorState>();
  final GlobalKey _ignorePointerKey = GlobalKey();

  // This field is set during layout, and then reused until the next time it is set.
  Map<Type, GestureRecognizerFactory> _gestureRecognizers = const <Type, GestureRecognizerFactory>{};
  bool _shouldIgnorePointer = false;

  bool? _lastCanDrag;
  Axis? _lastAxisDirection;

  @override
  @protected
  void setCanDrag(bool value) {
    if (value == _lastCanDrag && (!value || widget.axis == _lastAxisDirection)) {
      return;
    }
    if (!value) {
      _gestureRecognizers = const <Type, GestureRecognizerFactory>{};
      // Cancel the active hold/drag (if any) because the gesture recognizers
      // will soon be disposed by our RawGestureDetector, and we won't be
      // receiving pointer up events to cancel the hold/drag.
      _handleDragCancel();
    } else {
      switch (widget.axis) {
        case Axis.vertical:
          _gestureRecognizers = <Type, GestureRecognizerFactory>{
            VerticalDragGestureRecognizer: GestureRecognizerFactoryWithHandlers<VerticalDragGestureRecognizer>(
              () => VerticalDragGestureRecognizer(supportedDevices: _configuration.dragDevices),
              (VerticalDragGestureRecognizer instance) {
                instance
                  ..onDown = _handleDragDown
                  ..onStart = _handleDragStart
                  ..onUpdate = _handleDragUpdate
                  ..onEnd = _handleDragEnd
                  ..onCancel = _handleDragCancel
                  ..minFlingDistance = _physics?.minFlingDistance
                  ..minFlingVelocity = _physics?.minFlingVelocity
                  ..maxFlingVelocity = _physics?.maxFlingVelocity
                  ..velocityTrackerBuilder = _configuration.velocityTrackerBuilder(context)
                  ..dragStartBehavior = widget.dragStartBehavior
                  ..gestureSettings = _mediaQueryGestureSettings
                  ..supportedDevices = _configuration.dragDevices;
              },
            ),
          };
        case Axis.horizontal:
          _gestureRecognizers = <Type, GestureRecognizerFactory>{
            HorizontalDragGestureRecognizer: GestureRecognizerFactoryWithHandlers<HorizontalDragGestureRecognizer>(
              () => HorizontalDragGestureRecognizer(supportedDevices: _configuration.dragDevices),
              (HorizontalDragGestureRecognizer instance) {
                instance
                  ..onDown = _handleDragDown
                  ..onStart = _handleDragStart
                  ..onUpdate = _handleDragUpdate
                  ..onEnd = _handleDragEnd
                  ..onCancel = _handleDragCancel
                  ..minFlingDistance = _physics?.minFlingDistance
                  ..minFlingVelocity = _physics?.minFlingVelocity
                  ..maxFlingVelocity = _physics?.maxFlingVelocity
                  ..velocityTrackerBuilder = _configuration.velocityTrackerBuilder(context)
                  ..dragStartBehavior = widget.dragStartBehavior
                  ..gestureSettings = _mediaQueryGestureSettings
                  ..supportedDevices = _configuration.dragDevices;
              },
            ),
          };
      }
    }
    _lastCanDrag = value;
    _lastAxisDirection = widget.axis;
    if (_gestureDetectorKey.currentState != null) {
      _gestureDetectorKey.currentState!.replaceGestureRecognizers(_gestureRecognizers);
    }
  }

  @override
  @protected
  void setIgnorePointer(bool value) {
    if (_shouldIgnorePointer == value) {
      return;
    }
    _shouldIgnorePointer = value;
    if (_ignorePointerKey.currentContext != null) {
      final RenderIgnorePointer renderBox = _ignorePointerKey.currentContext!.findRenderObject()! as RenderIgnorePointer;
      renderBox.ignoring = _shouldIgnorePointer;
    }
  }

  // TOUCH HANDLERS

  Drag? _drag;
  ScrollHoldController? _hold;

  void _handleDragDown(DragDownDetails details) {
    assert(_drag == null);
    assert(_hold == null);
    _hold = position.hold(_disposeHold);
  }

  void _handleDragStart(DragStartDetails details) {
    // It's possible for _hold to become null between _handleDragDown and
    // _handleDragStart, for example if some user code calls jumpTo or otherwise
    // triggers a new activity to begin.
    assert(_drag == null);
    _drag = position.drag(details, _disposeDrag);
    assert(_drag != null);
    assert(_hold == null);
  }

  void _handleDragUpdate(DragUpdateDetails details) {
    // _drag might be null if the drag activity ended and called _disposeDrag.
    assert(_hold == null || _drag == null);
    _drag?.update(details);
  }

  void _handleDragEnd(DragEndDetails details) {
    // _drag might be null if the drag activity ended and called _disposeDrag.
    assert(_hold == null || _drag == null);
    _drag?.end(details);
    assert(_drag == null);
  }

  void _handleDragCancel() {
    if (_gestureDetectorKey.currentContext == null) {
      // The cancel was caused by the GestureDetector getting disposed, which
      // means we will get disposed momentarily as well and shouldn't do
      // any work.
      return;
    }
    // _hold might be null if the drag started.
    // _drag might be null if the drag activity ended and called _disposeDrag.
    assert(_hold == null || _drag == null);
    _hold?.cancel();
    _drag?.cancel();
    assert(_hold == null);
    assert(_drag == null);
  }

  void _disposeHold() {
    _hold = null;
  }

  void _disposeDrag() {
    _drag = null;
  }

  // SCROLL WHEEL

  // Returns the offset that should result from applying [event] to the current
  // position, taking min/max scroll extent into account.
  double _targetScrollOffsetForPointerScroll(double delta) {
    return math.min(
      math.max(position.pixels + delta, position.minScrollExtent),
      position.maxScrollExtent,
    );
  }

  // Returns the delta that should result from applying [event] with axis,
  // direction, and any modifiers specified by the ScrollBehavior taken into
  // account.
  double _pointerSignalEventDelta(PointerScrollEvent event) {
    late double delta;
    final Set<LogicalKeyboardKey> pressed = HardwareKeyboard.instance.logicalKeysPressed;
    final bool flipAxes = pressed.any(_configuration.pointerAxisModifiers.contains) &&
      // Axes are only flipped for physical mouse wheel input.
      // On some platforms, like web, trackpad input is handled through pointer
      // signals, but should not be included in this axis modifying behavior.
      // This is because on a trackpad, all directional axes are available to
      // the user, while mouse scroll wheels typically are restricted to one
      // axis.
      event.kind == PointerDeviceKind.mouse;

    switch (widget.axis) {
      case Axis.horizontal:
        delta = flipAxes
          ? event.scrollDelta.dy
          : event.scrollDelta.dx;
      case Axis.vertical:
        delta = flipAxes
          ? event.scrollDelta.dx
          : event.scrollDelta.dy;
    }

    if (axisDirectionIsReversed(widget.axisDirection)) {
      delta *= -1;
    }
    return delta;
  }

  void _receivedPointerSignal(PointerSignalEvent event) {
    if (event is PointerScrollEvent && _position != null) {
      if (_physics != null && !_physics!.shouldAcceptUserOffset(position)) {
        return;
      }
      final double delta = _pointerSignalEventDelta(event);
      final double targetScrollOffset = _targetScrollOffsetForPointerScroll(delta);
      // Only express interest in the event if it would actually result in a scroll.
      if (delta != 0.0 && targetScrollOffset != position.pixels) {
        GestureBinding.instance.pointerSignalResolver.register(event, _handlePointerScroll);
      }
    } else if (event is PointerScrollInertiaCancelEvent) {
      position.pointerScroll(0);
      // Don't use the pointer signal resolver, all hit-tested scrollables should stop.
    }
  }

  void _handlePointerScroll(PointerEvent event) {
    assert(event is PointerScrollEvent);
    final double delta = _pointerSignalEventDelta(event as PointerScrollEvent);
    final double targetScrollOffset = _targetScrollOffsetForPointerScroll(delta);
    if (delta != 0.0 && targetScrollOffset != position.pixels) {
      position.pointerScroll(delta);
    }
  }

  bool _handleScrollMetricsNotification(ScrollMetricsNotification notification) {
    if (notification.depth == 0) {
      final RenderObject? scrollSemanticsRenderObject = _scrollSemanticsKey.currentContext?.findRenderObject();
      if (scrollSemanticsRenderObject != null) {
        scrollSemanticsRenderObject.markNeedsSemanticsUpdate();
      }
    }
    return false;
  }

  // DESCRIPTION

  @override
  Widget build(BuildContext context) {
    assert(_position != null);
    // _ScrollableScope must be placed above the BuildContext returned by notificationContext
    // so that we can get this ScrollableState by doing the following:
    //
    // ScrollNotification notification;
    // Scrollable.of(notification.context)
    //
    // Since notificationContext is pointing to _gestureDetectorKey.context, _ScrollableScope
    // must be placed above the widget using it: RawGestureDetector
    Widget result = _ScrollableScope(
      scrollable: this,
      position: position,
      // TODO(ianh): Having all these global keys is sad.
      child: Listener(
        onPointerSignal: _receivedPointerSignal,
        child: RawGestureDetector(
          key: _gestureDetectorKey,
          gestures: _gestureRecognizers,
          behavior: HitTestBehavior.opaque,
          excludeFromSemantics: widget.excludeFromSemantics,
          child: Semantics(
            explicitChildNodes: !widget.excludeFromSemantics,
            child: IgnorePointer(
              key: _ignorePointerKey,
              ignoring: _shouldIgnorePointer,
              ignoringSemantics: false,
              child: widget.viewportBuilder(context, position),
            ),
          ),
        ),
      ),
    );

    if (!widget.excludeFromSemantics) {
      result = NotificationListener<ScrollMetricsNotification>(
        onNotification: _handleScrollMetricsNotification,
        child: _ScrollSemantics(
          key: _scrollSemanticsKey,
          position: position,
          allowImplicitScrolling: _physics!.allowImplicitScrolling,
          semanticChildCount: widget.semanticChildCount,
          child: result,
        )
      );
    }

    final ScrollableDetails details = ScrollableDetails(
      direction: widget.axisDirection,
      controller: _effectiveScrollController,
      decorationClipBehavior: widget.clipBehavior,
    );

    result = _configuration.buildScrollbar(
      context,
      _configuration.buildOverscrollIndicator(context, result, details),
      details,
    );

    // Selection is only enabled when there is a parent registrar.
    final SelectionRegistrar? registrar = SelectionContainer.maybeOf(context);
    if (registrar != null) {
      result = _ScrollableSelectionHandler(
        state: this,
        position: position,
        registrar: registrar,
        child: result,
      );
    }

    return result;
  }

  @override
  void debugFillProperties(DiagnosticPropertiesBuilder properties) {
    super.debugFillProperties(properties);
    properties.add(DiagnosticsProperty<ScrollPosition>('position', position));
    properties.add(DiagnosticsProperty<ScrollPhysics>('effective physics', _physics));
  }
}

/// A widget to handle selection for a scrollable.
///
/// This widget registers itself to the [registrar] and uses
/// [SelectionContainer] to collect selectables from its subtree.
class _ScrollableSelectionHandler extends StatefulWidget {
  const _ScrollableSelectionHandler({
    required this.state,
    required this.position,
    required this.registrar,
    required this.child,
  });

  final ScrollableState state;
  final ScrollPosition position;
  final Widget child;
  final SelectionRegistrar registrar;

  @override
  _ScrollableSelectionHandlerState createState() => _ScrollableSelectionHandlerState();
}

class _ScrollableSelectionHandlerState extends State<_ScrollableSelectionHandler> {
  late _ScrollableSelectionContainerDelegate _selectionDelegate;

  @override
  void initState() {
    super.initState();
    _selectionDelegate = _ScrollableSelectionContainerDelegate(
      state: widget.state,
      position: widget.position,
    );
  }

  @override
  void didUpdateWidget(_ScrollableSelectionHandler oldWidget) {
    super.didUpdateWidget(oldWidget);
    if (oldWidget.position != widget.position) {
      _selectionDelegate.position = widget.position;
    }
  }

  @override
  void dispose() {
    _selectionDelegate.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return SelectionContainer(
      registrar: widget.registrar,
      delegate: _selectionDelegate,
      child: widget.child,
    );
  }
}

/// This updater handles the case where the selectables change frequently, and
/// it optimizes toward scrolling updates.
///
/// It keeps track of the drag start offset relative to scroll origin for every
/// selectable. The records are used to determine whether the selection is up to
/// date with the scroll position when it sends the drag update event to a
/// selectable.
class _ScrollableSelectionContainerDelegate extends MultiSelectableSelectionContainerDelegate {
  _ScrollableSelectionContainerDelegate({
    required this.state,
    required ScrollPosition position
  }) : _position = position,
       _autoScroller = EdgeDraggingAutoScroller(state, velocityScalar: _kDefaultSelectToScrollVelocityScalar) {
    _position.addListener(_scheduleLayoutChange);
  }

  // Pointer drag is a single point, it should not have a size.
  static const double _kDefaultDragTargetSize = 0;

  // An eye-balled value for a smooth scrolling speed.
  static const double _kDefaultSelectToScrollVelocityScalar = 30;

  final ScrollableState state;
  final EdgeDraggingAutoScroller _autoScroller;
  bool _scheduledLayoutChange = false;
  Offset? _currentDragStartRelatedToOrigin;
  Offset? _currentDragEndRelatedToOrigin;

  // The scrollable only auto scrolls if the selection starts in the scrollable.
  bool _selectionStartsInScrollable = false;

  ScrollPosition get position => _position;
  ScrollPosition _position;
  set position(ScrollPosition other) {
    if (other == _position) {
      return;
    }
    _position.removeListener(_scheduleLayoutChange);
    _position = other;
    _position.addListener(_scheduleLayoutChange);
  }

  // The layout will only be updated a frame later than position changes.
  // Schedule PostFrameCallback to capture the accurate layout.
  void _scheduleLayoutChange() {
    if (_scheduledLayoutChange) {
      return;
    }
    _scheduledLayoutChange = true;
    SchedulerBinding.instance.addPostFrameCallback((Duration timeStamp) {
      if (!_scheduledLayoutChange) {
        return;
      }
      _scheduledLayoutChange = false;
      layoutDidChange();
    });
  }

  /// Stores the scroll offset when a scrollable receives the last
  /// [SelectionEdgeUpdateEvent].
  ///
  /// The stored scroll offset may be null if a scrollable never receives a
  /// [SelectionEdgeUpdateEvent].
  ///
  /// When a new [SelectionEdgeUpdateEvent] is dispatched to a selectable, this
  /// updater checks the current scroll offset against the one stored in these
  /// records. If the scroll offset is different, it synthesizes an opposite
  /// [SelectionEdgeUpdateEvent] and dispatches the event before dispatching the
  /// new event.
  ///
  /// For example, if a selectable receives an end [SelectionEdgeUpdateEvent]
  /// and its scroll offset in the records is different from the current value,
  /// it synthesizes a start [SelectionEdgeUpdateEvent] and dispatches it before
  /// dispatching the original end [SelectionEdgeUpdateEvent].
  final Map<Selectable, double> _selectableStartEdgeUpdateRecords = <Selectable, double>{};
  final Map<Selectable, double> _selectableEndEdgeUpdateRecords = <Selectable, double>{};

  @override
  void didChangeSelectables() {
    final Set<Selectable> selectableSet = selectables.toSet();
    _selectableStartEdgeUpdateRecords.removeWhere((Selectable key, double value) => !selectableSet.contains(key));
    _selectableEndEdgeUpdateRecords.removeWhere((Selectable key, double value) => !selectableSet.contains(key));
    super.didChangeSelectables();
  }

  @override
  SelectionResult handleClearSelection(ClearSelectionEvent event) {
    _selectableStartEdgeUpdateRecords.clear();
    _selectableEndEdgeUpdateRecords.clear();
    _currentDragStartRelatedToOrigin = null;
    _currentDragEndRelatedToOrigin = null;
    _selectionStartsInScrollable = false;
    return super.handleClearSelection(event);
  }

  @override
  SelectionResult handleSelectionEdgeUpdate(SelectionEdgeUpdateEvent event) {
    if (_currentDragEndRelatedToOrigin == null && _currentDragStartRelatedToOrigin == null) {
      assert(!_selectionStartsInScrollable);
      _selectionStartsInScrollable = _globalPositionInScrollable(event.globalPosition);
    }
    final Offset deltaToOrigin = _getDeltaToScrollOrigin(state);
    if (event.type == SelectionEventType.endEdgeUpdate) {
      _currentDragEndRelatedToOrigin = _inferPositionRelatedToOrigin(event.globalPosition);
      final Offset endOffset = _currentDragEndRelatedToOrigin!.translate(-deltaToOrigin.dx, -deltaToOrigin.dy);
      event = SelectionEdgeUpdateEvent.forEnd(globalPosition: endOffset);
    } else {
      _currentDragStartRelatedToOrigin = _inferPositionRelatedToOrigin(event.globalPosition);
      final Offset startOffset = _currentDragStartRelatedToOrigin!.translate(-deltaToOrigin.dx, -deltaToOrigin.dy);
      event = SelectionEdgeUpdateEvent.forStart(globalPosition: startOffset);
    }
    final SelectionResult result = super.handleSelectionEdgeUpdate(event);

    // Result may be pending if one of the selectable child is also a scrollable.
    // In that case, the parent scrollable needs to wait for the child to finish
    // scrolling.
    if (result == SelectionResult.pending) {
      _autoScroller.stopAutoScroll();
      return result;
    }
    if (_selectionStartsInScrollable) {
      _autoScroller.startAutoScrollIfNecessary(_dragTargetFromEvent(event));
      if (_autoScroller.scrolling) {
        return SelectionResult.pending;
      }
    }
    return result;
  }

  Offset _inferPositionRelatedToOrigin(Offset globalPosition) {
    final RenderBox box = state.context.findRenderObject()! as RenderBox;
    final Offset localPosition = box.globalToLocal(globalPosition);
    if (!_selectionStartsInScrollable) {
      // If the selection starts outside of the scrollable, selecting across the
      // scrollable boundary will act as selecting the entire content in the
      // scrollable. This logic move the offset to the 0.0 or infinity to cover
      // the entire content if the input position is outside of the scrollable.
      if (localPosition.dy < 0 || localPosition.dx < 0) {
        return box.localToGlobal(Offset.zero);
      }
      if (localPosition.dy > box.size.height || localPosition.dx > box.size.width) {
        return Offset.infinite;
      }
    }
    final Offset deltaToOrigin = _getDeltaToScrollOrigin(state);
    return box.localToGlobal(localPosition.translate(deltaToOrigin.dx, deltaToOrigin.dy));
  }

  /// Infers the [_currentDragStartRelatedToOrigin] and
  /// [_currentDragEndRelatedToOrigin] from the geometry.
  ///
  /// This method is called after a select word and select all event where the
  /// selection is triggered by none drag events. The
  /// [_currentDragStartRelatedToOrigin] and [_currentDragEndRelatedToOrigin]
  /// are essential to handle future [SelectionEdgeUpdateEvent]s.
  void _updateDragLocationsFromGeometries({bool forceUpdateStart = true, bool forceUpdateEnd = true}) {
    final Offset deltaToOrigin = _getDeltaToScrollOrigin(state);
    final RenderBox box = state.context.findRenderObject()! as RenderBox;
    final Matrix4 transform = box.getTransformTo(null);
    if (currentSelectionStartIndex != -1 && (_currentDragStartRelatedToOrigin == null || forceUpdateStart)) {
      final SelectionGeometry geometry = selectables[currentSelectionStartIndex].value;
      assert(geometry.hasSelection);
      final SelectionPoint start = geometry.startSelectionPoint!;
      final Matrix4 childTransform = selectables[currentSelectionStartIndex].getTransformTo(box);
      final Offset localDragStart = MatrixUtils.transformPoint(
        childTransform,
        start.localPosition + Offset(0, - start.lineHeight / 2),
      );
      _currentDragStartRelatedToOrigin = MatrixUtils.transformPoint(transform, localDragStart + deltaToOrigin);
    }
    if (currentSelectionEndIndex != -1 && (_currentDragEndRelatedToOrigin == null || forceUpdateEnd)) {
      final SelectionGeometry geometry = selectables[currentSelectionEndIndex].value;
      assert(geometry.hasSelection);
      final SelectionPoint end = geometry.endSelectionPoint!;
      final Matrix4 childTransform = selectables[currentSelectionEndIndex].getTransformTo(box);
      final Offset localDragEnd = MatrixUtils.transformPoint(
        childTransform,
        end.localPosition + Offset(0, - end.lineHeight / 2),
      );
      _currentDragEndRelatedToOrigin = MatrixUtils.transformPoint(transform, localDragEnd + deltaToOrigin);
    }
  }

  @override
  SelectionResult handleSelectAll(SelectAllSelectionEvent event) {
    assert(!_selectionStartsInScrollable);
    final SelectionResult result = super.handleSelectAll(event);
    assert((currentSelectionStartIndex == -1) == (currentSelectionEndIndex == -1));
    if (currentSelectionStartIndex != -1) {
      _updateDragLocationsFromGeometries();
    }
    return result;
  }

  @override
  SelectionResult handleSelectWord(SelectWordSelectionEvent event) {
    _selectionStartsInScrollable = _globalPositionInScrollable(event.globalPosition);
    final SelectionResult result = super.handleSelectWord(event);
    _updateDragLocationsFromGeometries();
    return result;
  }

  @override
  SelectionResult handleGranularlyExtendSelection(GranularlyExtendSelectionEvent event) {
    final SelectionResult result = super.handleGranularlyExtendSelection(event);
    // The selection geometry may not have the accurate offset for the edges
    // that are outside of the viewport whose transform may not be valid. Only
    // the edge this event is updating is sure to be accurate.
    _updateDragLocationsFromGeometries(
      forceUpdateStart: !event.isEnd,
      forceUpdateEnd: event.isEnd,
    );
    if (_selectionStartsInScrollable) {
      _jumpToEdge(event.isEnd);
    }
    return result;
  }

  @override
  SelectionResult handleDirectionallyExtendSelection(DirectionallyExtendSelectionEvent event) {
    final SelectionResult result = super.handleDirectionallyExtendSelection(event);
    // The selection geometry may not have the accurate offset for the edges
    // that are outside of the viewport whose transform may not be valid. Only
    // the edge this event is updating is sure to be accurate.
    _updateDragLocationsFromGeometries(
      forceUpdateStart: !event.isEnd,
      forceUpdateEnd: event.isEnd,
    );
    if (_selectionStartsInScrollable) {
      _jumpToEdge(event.isEnd);
    }
    return result;
  }

  void _jumpToEdge(bool isExtent) {
    final Selectable selectable;
    final double? lineHeight;
    final SelectionPoint? edge;
    if (isExtent) {
      selectable = selectables[currentSelectionEndIndex];
      edge = selectable.value.endSelectionPoint;
      lineHeight = selectable.value.endSelectionPoint!.lineHeight;
    } else {
      selectable = selectables[currentSelectionStartIndex];
      edge = selectable.value.startSelectionPoint;
      lineHeight = selectable.value.startSelectionPoint?.lineHeight;
    }
    if (lineHeight == null || edge == null) {
      return;
    }
    final RenderBox scrollableBox = state.context.findRenderObject()! as RenderBox;
    final Matrix4 transform = selectable.getTransformTo(scrollableBox);
    final Offset edgeOffsetInScrollableCoordinates = MatrixUtils.transformPoint(transform, edge.localPosition);
    final Rect scrollableRect = Rect.fromLTRB(0, 0, scrollableBox.size.width, scrollableBox.size.height);
    switch (state.axisDirection) {
      case AxisDirection.up:
        final double edgeBottom = edgeOffsetInScrollableCoordinates.dy;
        final double edgeTop = edgeOffsetInScrollableCoordinates.dy - lineHeight;
        if (edgeBottom >= scrollableRect.bottom && edgeTop <= scrollableRect.top) {
          return;
        }
        if (edgeBottom > scrollableRect.bottom) {
          position.jumpTo(position.pixels + scrollableRect.bottom - edgeBottom);
          return;
        }
        if (edgeTop < scrollableRect.top) {
          position.jumpTo(position.pixels + scrollableRect.top - edgeTop);
        }
        return;
      case AxisDirection.right:
        final double edge = edgeOffsetInScrollableCoordinates.dx;
        if (edge >= scrollableRect.right && edge <= scrollableRect.left) {
          return;
        }
        if (edge > scrollableRect.right) {
          position.jumpTo(position.pixels + edge - scrollableRect.right);
          return;
        }
        if (edge < scrollableRect.left) {
          position.jumpTo(position.pixels + edge - scrollableRect.left);
        }
        return;
      case AxisDirection.down:
        final double edgeBottom = edgeOffsetInScrollableCoordinates.dy;
        final double edgeTop = edgeOffsetInScrollableCoordinates.dy - lineHeight;
        if (edgeBottom >= scrollableRect.bottom && edgeTop <= scrollableRect.top) {
          return;
        }
        if (edgeBottom > scrollableRect.bottom) {
          position.jumpTo(position.pixels + edgeBottom - scrollableRect.bottom);
          return;
        }
        if (edgeTop < scrollableRect.top) {
          position.jumpTo(position.pixels + edgeTop - scrollableRect.top);
        }
        return;
      case AxisDirection.left:
        final double edge = edgeOffsetInScrollableCoordinates.dx;
        if (edge >= scrollableRect.right && edge <= scrollableRect.left) {
          return;
        }
        if (edge > scrollableRect.right) {
          position.jumpTo(position.pixels + scrollableRect.right - edge);
          return;
        }
        if (edge < scrollableRect.left) {
          position.jumpTo(position.pixels + scrollableRect.left - edge);
        }
        return;
    }
  }

  bool _globalPositionInScrollable(Offset globalPosition) {
    final RenderBox box = state.context.findRenderObject()! as RenderBox;
    final Offset localPosition = box.globalToLocal(globalPosition);
    final Rect rect = Rect.fromLTWH(0, 0, box.size.width, box.size.height);
    return rect.contains(localPosition);
  }

  Rect _dragTargetFromEvent(SelectionEdgeUpdateEvent event) {
    return Rect.fromCenter(center: event.globalPosition, width: _kDefaultDragTargetSize, height: _kDefaultDragTargetSize);
  }

  @override
  SelectionResult dispatchSelectionEventToChild(Selectable selectable, SelectionEvent event) {
    switch (event.type) {
      case SelectionEventType.startEdgeUpdate:
        _selectableStartEdgeUpdateRecords[selectable] = state.position.pixels;
        ensureChildUpdated(selectable);
      case SelectionEventType.endEdgeUpdate:
        _selectableEndEdgeUpdateRecords[selectable] = state.position.pixels;
        ensureChildUpdated(selectable);
      case SelectionEventType.granularlyExtendSelection:
      case SelectionEventType.directionallyExtendSelection:
        ensureChildUpdated(selectable);
        _selectableStartEdgeUpdateRecords[selectable] = state.position.pixels;
        _selectableEndEdgeUpdateRecords[selectable] = state.position.pixels;
      case SelectionEventType.clear:
        _selectableEndEdgeUpdateRecords.remove(selectable);
        _selectableStartEdgeUpdateRecords.remove(selectable);
      case SelectionEventType.selectAll:
      case SelectionEventType.selectWord:
        _selectableEndEdgeUpdateRecords[selectable] = state.position.pixels;
        _selectableStartEdgeUpdateRecords[selectable] = state.position.pixels;
    }
    return super.dispatchSelectionEventToChild(selectable, event);
  }

  @override
  void ensureChildUpdated(Selectable selectable) {
    final double newRecord = state.position.pixels;
    final double? previousStartRecord = _selectableStartEdgeUpdateRecords[selectable];
    if (_currentDragStartRelatedToOrigin != null &&
        (previousStartRecord == null || (newRecord - previousStartRecord).abs() > precisionErrorTolerance)) {
      // Make sure the selectable has up to date events.
      final Offset deltaToOrigin = _getDeltaToScrollOrigin(state);
      final Offset startOffset = _currentDragStartRelatedToOrigin!.translate(-deltaToOrigin.dx, -deltaToOrigin.dy);
      selectable.dispatchSelectionEvent(SelectionEdgeUpdateEvent.forStart(globalPosition: startOffset));
    }
    final double? previousEndRecord = _selectableEndEdgeUpdateRecords[selectable];
    if (_currentDragEndRelatedToOrigin != null &&
        (previousEndRecord == null || (newRecord - previousEndRecord).abs() > precisionErrorTolerance)) {
      // Make sure the selectable has up to date events.
      final Offset deltaToOrigin = _getDeltaToScrollOrigin(state);
      final Offset endOffset = _currentDragEndRelatedToOrigin!.translate(-deltaToOrigin.dx, -deltaToOrigin.dy);
      selectable.dispatchSelectionEvent(SelectionEdgeUpdateEvent.forEnd(globalPosition: endOffset));
    }
  }

  @override
  void dispose() {
    _selectableStartEdgeUpdateRecords.clear();
    _selectableEndEdgeUpdateRecords.clear();
    _scheduledLayoutChange = false;
    _autoScroller.stopAutoScroll();
    super.dispose();
  }
}

Offset _getDeltaToScrollOrigin(ScrollableState scrollableState) {
  switch (scrollableState.axisDirection) {
    case AxisDirection.down:
      return Offset(0, scrollableState.position.pixels);
    case AxisDirection.up:
      return Offset(0, -scrollableState.position.pixels);
    case AxisDirection.left:
      return Offset(-scrollableState.position.pixels, 0);
    case AxisDirection.right:
      return Offset(scrollableState.position.pixels, 0);
  }
}

/// With [_ScrollSemantics] certain child [SemanticsNode]s can be
/// excluded from the scrollable area for semantics purposes.
///
/// Nodes, that are to be excluded, have to be tagged with
/// [RenderViewport.excludeFromScrolling] and the [RenderAbstractViewport] in
/// use has to add the [RenderViewport.useTwoPaneSemantics] tag to its
/// [SemanticsConfiguration] by overriding
/// [RenderObject.describeSemanticsConfiguration].
///
/// If the tag [RenderViewport.useTwoPaneSemantics] is present on the viewport,
/// two semantics nodes will be used to represent the [Scrollable]: The outer
/// node will contain all children, that are excluded from scrolling. The inner
/// node, which is annotated with the scrolling actions, will house the
/// scrollable children.
class _ScrollSemantics extends SingleChildRenderObjectWidget {
  const _ScrollSemantics({
    super.key,
    required this.position,
    required this.allowImplicitScrolling,
    required this.semanticChildCount,
    super.child,
  }) : assert(semanticChildCount == null || semanticChildCount >= 0);

  final ScrollPosition position;
  final bool allowImplicitScrolling;
  final int? semanticChildCount;

  @override
  _RenderScrollSemantics createRenderObject(BuildContext context) {
    return _RenderScrollSemantics(
      position: position,
      allowImplicitScrolling: allowImplicitScrolling,
      semanticChildCount: semanticChildCount,
    );
  }

  @override
  void updateRenderObject(BuildContext context, _RenderScrollSemantics renderObject) {
    renderObject
      ..allowImplicitScrolling = allowImplicitScrolling
      ..position = position
      ..semanticChildCount = semanticChildCount;
  }
}

class _RenderScrollSemantics extends RenderProxyBox {
  _RenderScrollSemantics({
    required ScrollPosition position,
    required bool allowImplicitScrolling,
    required int? semanticChildCount,
    RenderBox? child,
  }) : _position = position,
       _allowImplicitScrolling = allowImplicitScrolling,
       _semanticChildCount = semanticChildCount,
       super(child) {
    position.addListener(markNeedsSemanticsUpdate);
  }

  /// Whether this render object is excluded from the semantic tree.
  ScrollPosition get position => _position;
  ScrollPosition _position;
  set position(ScrollPosition value) {
    if (value == _position) {
      return;
    }
    _position.removeListener(markNeedsSemanticsUpdate);
    _position = value;
    _position.addListener(markNeedsSemanticsUpdate);
    markNeedsSemanticsUpdate();
  }

  /// Whether this node can be scrolled implicitly.
  bool get allowImplicitScrolling => _allowImplicitScrolling;
  bool _allowImplicitScrolling;
  set allowImplicitScrolling(bool value) {
    if (value == _allowImplicitScrolling) {
      return;
    }
    _allowImplicitScrolling = value;
    markNeedsSemanticsUpdate();
  }

  int? get semanticChildCount => _semanticChildCount;
  int? _semanticChildCount;
  set semanticChildCount(int? value) {
    if (value == semanticChildCount) {
      return;
    }
    _semanticChildCount = value;
    markNeedsSemanticsUpdate();
  }

  @override
  void describeSemanticsConfiguration(SemanticsConfiguration config) {
    super.describeSemanticsConfiguration(config);
    config.isSemanticBoundary = true;
    if (position.haveDimensions) {
      config
          ..hasImplicitScrolling = allowImplicitScrolling
          ..scrollPosition = _position.pixels
          ..scrollExtentMax = _position.maxScrollExtent
          ..scrollExtentMin = _position.minScrollExtent
          ..scrollChildCount = semanticChildCount;
    }
  }

  SemanticsNode? _innerNode;

  @override
  void assembleSemanticsNode(SemanticsNode node, SemanticsConfiguration config, Iterable<SemanticsNode> children) {
    if (children.isEmpty || !children.first.isTagged(RenderViewport.useTwoPaneSemantics)) {
      _innerNode = null;
      super.assembleSemanticsNode(node, config, children);
      return;
    }

    _innerNode ??= SemanticsNode(showOnScreen: showOnScreen);
    _innerNode!
      ..isMergedIntoParent = node.isPartOfNodeMerging
      ..rect = node.rect;

    int? firstVisibleIndex;
    final List<SemanticsNode> excluded = <SemanticsNode>[_innerNode!];
    final List<SemanticsNode> included = <SemanticsNode>[];
    for (final SemanticsNode child in children) {
      assert(child.isTagged(RenderViewport.useTwoPaneSemantics));
      if (child.isTagged(RenderViewport.excludeFromScrolling)) {
        excluded.add(child);
      } else {
        if (!child.hasFlag(SemanticsFlag.isHidden)) {
          firstVisibleIndex ??= child.indexInParent;
        }
        included.add(child);
      }
    }
    config.scrollIndex = firstVisibleIndex;
    node.updateWith(config: null, childrenInInversePaintOrder: excluded);
    _innerNode!.updateWith(config: config, childrenInInversePaintOrder: included);
  }

  @override
  void clearSemantics() {
    super.clearSemantics();
    _innerNode = null;
  }
}

// Not using a RestorableDouble because we want to allow null values and override
// [enabled].
class _RestorableScrollOffset extends RestorableValue<double?> {
  @override
  double? createDefaultValue() => null;

  @override
  void didUpdateValue(double? oldValue) {
    notifyListeners();
  }

  @override
  double fromPrimitives(Object? data) {
    return data! as double;
  }

  @override
  Object? toPrimitives() {
    return value;
  }

  @override
  bool get enabled => value != null;
}