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

 // Copyright 2014 The Flutter Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.

 import 'dart:async';
 import 'dart:math' as math;

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

 import 'actions.dart';
 import 'basic.dart';
 import 'framework.dart';
 import 'primary_scroll_controller.dart';
 import 'scroll_configuration.dart';
 import 'scroll_controller.dart';
 import 'scroll_metrics.dart';
 import 'scroll_physics.dart';
 import 'scrollable.dart';

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

 /// Describes the aspects of a Scrollable widget to inform inherited widgets
 /// like [ScrollBehavior] for decorating or enumerate the properties of combined
 /// Scrollables, such as [TwoDimensionalScrollable].
 ///
 /// Decorations like [GlowingOverscrollIndicator]s and [Scrollbar]s require
 /// information about the Scrollable in order to be initialized.
 @immutable
 class ScrollableDetails {
   /// Creates a set of details describing the [Scrollable]. The [direction]
   /// cannot be null.
   const ScrollableDetails({
     required this.direction,
     this.controller,
     this.physics,
     @Deprecated(
       'Migrate to decorationClipBehavior. '
       'This property was deprecated so that its application is clearer. This clip '
       'applies to decorators, and does not directly clip a scroll view. '
       'This feature was deprecated after v3.9.0-1.0.pre.'
     )
     Clip? clipBehavior,
     Clip? decorationClipBehavior,
   }) : decorationClipBehavior = clipBehavior ?? decorationClipBehavior;

   /// A constructor specific to a [Scrollable] with an [Axis.vertical].
   const ScrollableDetails.vertical({
     bool reverse = false,
     this.controller,
     this.physics,
     this.decorationClipBehavior,
   }) : direction = reverse ? AxisDirection.up : AxisDirection.down;

   /// A constructor specific to a [Scrollable] with an [Axis.horizontal].
   const ScrollableDetails.horizontal({
     bool reverse = false,
     this.controller,
     this.physics,
     this.decorationClipBehavior,
   }) : direction = reverse ? AxisDirection.left : AxisDirection.right;

   /// {@macro flutter.widgets.Scrollable.axisDirection}
   final AxisDirection direction;

   /// {@macro flutter.widgets.Scrollable.controller}
   final ScrollController? controller;

   /// {@macro flutter.widgets.Scrollable.physics}
   final ScrollPhysics? physics;

   /// {@macro flutter.material.Material.clipBehavior}
   ///
   /// This can be used by [MaterialScrollBehavior] to clip a
   /// [StretchingOverscrollIndicator].
   ///
   /// This [Clip] does not affect the [Viewport.clipBehavior], but is rather
   /// passed from the same value by [Scrollable] so that decorators like
   /// [StretchingOverscrollIndicator] honor the same clip.
   ///
   /// Defaults to null.
   final Clip? decorationClipBehavior;

   /// Deprecated getter for [decorationClipBehavior].
   @Deprecated(
     'Migrate to decorationClipBehavior. '
     'This property was deprecated so that its application is clearer. This clip '
     'applies to decorators, and does not directly clip a scroll view. '
     'This feature was deprecated after v3.9.0-1.0.pre.'
   )
   Clip? get clipBehavior => decorationClipBehavior;

   /// Copy the current [ScrollableDetails] with the given values replacing the
   /// current values.
   ScrollableDetails copyWith({
     AxisDirection? direction,
     ScrollController? controller,
     ScrollPhysics? physics,
     Clip? decorationClipBehavior,
   }) {
     return ScrollableDetails(
       direction: direction ?? this.direction,
       controller: controller ?? this.controller,
       physics: physics ?? this.physics,
       decorationClipBehavior: decorationClipBehavior ?? this.decorationClipBehavior,
     );
   }

   @override
   String toString() {
     final List<String> description = <String>[];
     description.add('axisDirection: $direction');

     void addIfNonNull(String prefix, Object? value) {
       if (value != null) {
         description.add(prefix + value.toString());
       }
     }
     addIfNonNull('scroll controller: ', controller);
     addIfNonNull('scroll physics: ', physics);
     addIfNonNull('decorationClipBehavior: ', decorationClipBehavior);
     return '${describeIdentity(this)}(${description.join(", ")})';
   }

   @override
   int get hashCode => Object.hash(
     direction,
     controller,
     physics,
     decorationClipBehavior,
   );

   @override
   bool operator ==(Object other) {
     if (identical(this, other)) {
       return true;
     }
     if (other.runtimeType != runtimeType) {
       return false;
     }
     return other is ScrollableDetails
       && other.direction == direction
       && other.controller == controller
       && other.physics == physics
       && other.decorationClipBehavior == decorationClipBehavior;
   }
 }

 /// An auto scroller that scrolls the [scrollable] if a drag gesture drags close
 /// to its edge.
 ///
 /// The scroll velocity is controlled by the [velocityScalar]:
 ///
 /// velocity = <distance of overscroll> * [velocityScalar].
 class EdgeDraggingAutoScroller {
   /// Creates a auto scroller that scrolls the [scrollable].
   EdgeDraggingAutoScroller(
     this.scrollable, {
     this.onScrollViewScrolled,
     required this.velocityScalar,
   });

   /// The [Scrollable] this auto scroller is scrolling.
   final ScrollableState scrollable;

   /// Called when a scroll view is scrolled.
   ///
   /// The scroll view may be scrolled multiple times in a row until the drag
   /// target no longer triggers the auto scroll. This callback will be called
   /// in between each scroll.
   final VoidCallback? onScrollViewScrolled;

   /// {@template flutter.widgets.EdgeDraggingAutoScroller.velocityScalar}
   /// The velocity scalar per pixel over scroll.
   ///
   /// It represents how the velocity scale with the over scroll distance. The
   /// auto-scroll velocity = <distance of overscroll> * velocityScalar.
   /// {@endtemplate}
   final double velocityScalar;

   late Rect _dragTargetRelatedToScrollOrigin;

   /// Whether the auto scroll is in progress.
   bool get scrolling => _scrolling;
   bool _scrolling = false;

   double _offsetExtent(Offset offset, Axis scrollDirection) {
     switch (scrollDirection) {
       case Axis.horizontal:
         return offset.dx;
       case Axis.vertical:
         return offset.dy;
     }
   }

   double _sizeExtent(Size size, Axis scrollDirection) {
     switch (scrollDirection) {
       case Axis.horizontal:
         return size.width;
       case Axis.vertical:
         return size.height;
     }
   }

   AxisDirection get _axisDirection => scrollable.axisDirection;
   Axis get _scrollDirection => axisDirectionToAxis(_axisDirection);

   /// Starts the auto scroll if the [dragTarget] is close to the edge.
   ///
   /// The scroll starts to scroll the [scrollable] if the target rect is close
   /// to the edge of the [scrollable]; otherwise, it remains stationary.
   ///
   /// If the scrollable is already scrolling, calling this method updates the
   /// previous dragTarget to the new value and continues scrolling if necessary.
   void startAutoScrollIfNecessary(Rect dragTarget) {
     final Offset deltaToOrigin = scrollable.deltaToScrollOrigin;
     _dragTargetRelatedToScrollOrigin = dragTarget.translate(deltaToOrigin.dx, deltaToOrigin.dy);
     if (_scrolling) {
       // The change will be picked up in the next scroll.
       return;
     }
     assert(!_scrolling);
     _scroll();
   }

   /// Stop any ongoing auto scrolling.
   void stopAutoScroll() {
     _scrolling = false;
   }

   Future<void> _scroll() async {
     final RenderBox scrollRenderBox = scrollable.context.findRenderObject()! as RenderBox;
     final Rect globalRect = MatrixUtils.transformRect(
       scrollRenderBox.getTransformTo(null),
       Rect.fromLTWH(0, 0, scrollRenderBox.size.width, scrollRenderBox.size.height),
     );
     assert(
       globalRect.size.width >= _dragTargetRelatedToScrollOrigin.size.width &&
         globalRect.size.height >= _dragTargetRelatedToScrollOrigin.size.height,
       'Drag target size is larger than scrollable size, which may cause bouncing',
     );
     _scrolling = true;
     double? newOffset;
     const double overDragMax = 20.0;

     final Offset deltaToOrigin = scrollable.deltaToScrollOrigin;
     final Offset viewportOrigin = globalRect.topLeft.translate(deltaToOrigin.dx, deltaToOrigin.dy);
     final double viewportStart = _offsetExtent(viewportOrigin, _scrollDirection);
     final double viewportEnd = viewportStart + _sizeExtent(globalRect.size, _scrollDirection);

     final double proxyStart = _offsetExtent(_dragTargetRelatedToScrollOrigin.topLeft, _scrollDirection);
     final double proxyEnd = _offsetExtent(_dragTargetRelatedToScrollOrigin.bottomRight, _scrollDirection);
     switch (_axisDirection) {
       case AxisDirection.up:
       case AxisDirection.left:
         if (proxyEnd > viewportEnd && scrollable.position.pixels > scrollable.position.minScrollExtent) {
           final double overDrag = math.min(proxyEnd - viewportEnd, overDragMax);
           newOffset = math.max(scrollable.position.minScrollExtent, scrollable.position.pixels - overDrag);
         } else if (proxyStart < viewportStart && scrollable.position.pixels < scrollable.position.maxScrollExtent) {
           final double overDrag = math.min(viewportStart - proxyStart, overDragMax);
           newOffset = math.min(scrollable.position.maxScrollExtent, scrollable.position.pixels + overDrag);
         }
       case AxisDirection.right:
       case AxisDirection.down:
         if (proxyStart < viewportStart && scrollable.position.pixels > scrollable.position.minScrollExtent) {
           final double overDrag = math.min(viewportStart - proxyStart, overDragMax);
           newOffset = math.max(scrollable.position.minScrollExtent, scrollable.position.pixels -  overDrag);
         } else if (proxyEnd > viewportEnd && scrollable.position.pixels < scrollable.position.maxScrollExtent) {
           final double overDrag = math.min(proxyEnd - viewportEnd, overDragMax);
           newOffset = math.min(scrollable.position.maxScrollExtent, scrollable.position.pixels + overDrag);
         }
     }

     if (newOffset == null || (newOffset - scrollable.position.pixels).abs() < 1.0) {
       // Drag should not trigger scroll.
       _scrolling = false;
       return;
     }
     final Duration duration = Duration(milliseconds: (1000 / velocityScalar).round());
     await scrollable.position.animateTo(
       newOffset,
       duration: duration,
       curve: Curves.linear,
     );
     if (onScrollViewScrolled != null) {
       onScrollViewScrolled!();
     }
     if (_scrolling) {
       await _scroll();
     }
   }
 }

 /// A typedef for a function that can calculate the offset for a type of scroll
 /// increment given a [ScrollIncrementDetails].
 ///
 /// This function is used as the type for [Scrollable.incrementCalculator],
 /// which is called from a [ScrollAction].
 typedef ScrollIncrementCalculator = double Function(ScrollIncrementDetails details);

 /// Describes the type of scroll increment that will be performed by a
 /// [ScrollAction] on a [Scrollable].
 ///
 /// This is used to configure a [ScrollIncrementDetails] object to pass to a
 /// [ScrollIncrementCalculator] function on a [Scrollable].
 ///
 /// {@template flutter.widgets.ScrollIncrementType.intent}
 /// This indicates the *intent* of the scroll, not necessarily the size. Not all
 /// scrollable areas will have the concept of a "line" or "page", but they can
 /// respond to the different standard key bindings that cause scrolling, which
 /// are bound to keys that people use to indicate a "line" scroll (e.g.
 /// control-arrowDown keys) or a "page" scroll (e.g. pageDown key). It is
 /// recommended that at least the relative magnitudes of the scrolls match
 /// expectations.
 /// {@endtemplate}
 enum ScrollIncrementType {
   /// Indicates that the [ScrollIncrementCalculator] should return the scroll
   /// distance it should move when the user requests to scroll by a "line".
   ///
   /// The distance a "line" scrolls refers to what should happen when the key
   /// binding for "scroll down/up by a line" is triggered. It's up to the
   /// [ScrollIncrementCalculator] function to decide what that means for a
   /// particular scrollable.
   line,

   /// Indicates that the [ScrollIncrementCalculator] should return the scroll
   /// distance it should move when the user requests to scroll by a "page".
   ///
   /// The distance a "page" scrolls refers to what should happen when the key
   /// binding for "scroll down/up by a page" is triggered. It's up to the
   /// [ScrollIncrementCalculator] function to decide what that means for a
   /// particular scrollable.
   page,
 }

 /// A details object that describes the type of scroll increment being requested
 /// of a [ScrollIncrementCalculator] function, as well as the current metrics
 /// for the scrollable.
 class ScrollIncrementDetails {
   /// A const constructor for a [ScrollIncrementDetails].
   ///
   /// All of the arguments must not be null, and are required.
   const ScrollIncrementDetails({
     required this.type,
     required this.metrics,
   });

   /// The type of scroll this is (e.g. line, page, etc.).
   ///
   /// {@macro flutter.widgets.ScrollIncrementType.intent}
   final ScrollIncrementType type;

   /// The current metrics of the scrollable that is being scrolled.
   final ScrollMetrics metrics;
 }

 /// An [Intent] that represents scrolling the nearest scrollable by an amount
 /// appropriate for the [type] specified.
 ///
 /// The actual amount of the scroll is determined by the
 /// [Scrollable.incrementCalculator], or by its defaults if that is not
 /// specified.
 class ScrollIntent extends Intent {
   /// Creates a const [ScrollIntent] that requests scrolling in the given
   /// [direction], with the given [type].
   const ScrollIntent({
     required this.direction,
     this.type = ScrollIncrementType.line,
   });

   /// The direction in which to scroll the scrollable containing the focused
   /// widget.
   final AxisDirection direction;

   /// The type of scrolling that is intended.
   final ScrollIncrementType type;
 }

 /// An [Action] that scrolls the relevant [Scrollable] by the amount configured
 /// in the [ScrollIntent] given to it.
 ///
 /// If a Scrollable cannot be found above the given [BuildContext], the
 /// [PrimaryScrollController] will be considered for default handling of
 /// [ScrollAction]s.
 ///
 /// If [Scrollable.incrementCalculator] is null for the scrollable, the default
 /// for a [ScrollIntent.type] set to [ScrollIncrementType.page] is 80% of the
 /// size of the scroll window, and for [ScrollIncrementType.line], 50 logical
 /// pixels.
 class ScrollAction extends ContextAction<ScrollIntent> {
   @override
   bool isEnabled(ScrollIntent intent, [BuildContext? context]) {
     if (context == null) {
       return false;
     }
     if (Scrollable.maybeOf(context) != null) {
       return true;
     }
     final ScrollController? primaryScrollController = PrimaryScrollController.maybeOf(context);
     return (primaryScrollController != null) && (primaryScrollController.hasClients);
   }

   /// Returns the scroll increment for a single scroll request, for use when
   /// scrolling using a hardware keyboard.
   ///
   /// Must not be called when the position is null, or when any of the position
   /// metrics (pixels, viewportDimension, maxScrollExtent, minScrollExtent) are
   /// null. The type and state arguments must not be null, and the widget must
   /// have already been laid out so that the position fields are valid.
   static double _calculateScrollIncrement(ScrollableState state, { ScrollIncrementType type = ScrollIncrementType.line }) {
     assert(state.position.hasPixels);
     assert(state.resolvedPhysics == null || state.resolvedPhysics!.shouldAcceptUserOffset(state.position));
     if (state.widget.incrementCalculator != null) {
       return state.widget.incrementCalculator!(
         ScrollIncrementDetails(
           type: type,
           metrics: state.position,
         ),
       );
     }
     switch (type) {
       case ScrollIncrementType.line:
         return 50.0;
       case ScrollIncrementType.page:
         return 0.8 * state.position.viewportDimension;
     }
   }

   /// Find out how much of an increment to move by, taking the different
   /// directions into account.
   static double getDirectionalIncrement(ScrollableState state, ScrollIntent intent) {
     final double increment = _calculateScrollIncrement(state, type: intent.type);
     switch (intent.direction) {
       case AxisDirection.down:
         switch (state.axisDirection) {
           case AxisDirection.up:
             return -increment;
           case AxisDirection.down:
             return increment;
           case AxisDirection.right:
           case AxisDirection.left:
             return 0.0;
         }
       case AxisDirection.up:
         switch (state.axisDirection) {
           case AxisDirection.up:
             return increment;
           case AxisDirection.down:
             return -increment;
           case AxisDirection.right:
           case AxisDirection.left:
             return 0.0;
         }
       case AxisDirection.left:
         switch (state.axisDirection) {
           case AxisDirection.right:
             return -increment;
           case AxisDirection.left:
             return increment;
           case AxisDirection.up:
           case AxisDirection.down:
             return 0.0;
         }
       case AxisDirection.right:
         switch (state.axisDirection) {
           case AxisDirection.right:
             return increment;
           case AxisDirection.left:
             return -increment;
           case AxisDirection.up:
           case AxisDirection.down:
             return 0.0;
         }
     }
   }

   @override
   void invoke(ScrollIntent intent, [BuildContext? context]) {
     assert(context != null, 'Cannot scroll without a context.');
     ScrollableState? state = Scrollable.maybeOf(context!);
     if (state == null) {
       final ScrollController primaryScrollController = PrimaryScrollController.of(context);
       assert (() {
         if (primaryScrollController.positions.length != 1) {
           throw FlutterError.fromParts(<DiagnosticsNode>[
             ErrorSummary(
               'A ScrollAction was invoked with the PrimaryScrollController, but '
               'more than one ScrollPosition is attached.',
             ),
             ErrorDescription(
               'Only one ScrollPosition can be manipulated by a ScrollAction at '
               'a time.',
             ),
             ErrorHint(
               'The PrimaryScrollController can be inherited automatically by '
               'descendant ScrollViews based on the TargetPlatform and scroll '
               'direction. By default, the PrimaryScrollController is '
               'automatically inherited on mobile platforms for vertical '
               'ScrollViews. ScrollView.primary can also override this behavior.',
             ),
           ]);
         }
         return true;
       }());

       if (primaryScrollController.position.context.notificationContext == null
           && Scrollable.maybeOf(primaryScrollController.position.context.notificationContext!) == null) {
         return;
       }
       state = Scrollable.maybeOf(primaryScrollController.position.context.notificationContext!);
     }
     assert(state != null, '$ScrollAction was invoked on a context that has no scrollable parent');
     assert(state!.position.hasPixels, 'Scrollable must be laid out before it can be scrolled via a ScrollAction');

     // Don't do anything if the user isn't allowed to scroll.
     if (state!.resolvedPhysics != null && !state.resolvedPhysics!.shouldAcceptUserOffset(state.position)) {
       return;
     }
     final double increment = getDirectionalIncrement(state, intent);
     if (increment == 0.0) {
       return;
     }
     state.position.moveTo(
       state.position.pixels + increment,
       duration: const Duration(milliseconds: 100),
       curve: Curves.easeInOut,
     );
   }
 }
	// 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/rendering.dart';

	import 'actions.dart';
	import 'basic.dart';
	import 'framework.dart';
	import 'primary_scroll_controller.dart';
	import 'scroll_configuration.dart';
	import 'scroll_controller.dart';
	import 'scroll_metrics.dart';
	import 'scroll_physics.dart';
	import 'scrollable.dart';

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

	/// Describes the aspects of a Scrollable widget to inform inherited widgets
	/// like [ScrollBehavior] for decorating or enumerate the properties of combined
	/// Scrollables, such as [TwoDimensionalScrollable].
	///
	/// Decorations like [GlowingOverscrollIndicator]s and [Scrollbar]s require
	/// information about the Scrollable in order to be initialized.
	@immutable
	class ScrollableDetails {
	/// Creates a set of details describing the [Scrollable]. The [direction]
	/// cannot be null.
	const ScrollableDetails({
	required this.direction,
	this.controller,
	this.physics,
	@Deprecated(
	'Migrate to decorationClipBehavior. '
	'This property was deprecated so that its application is clearer. This clip '
	'applies to decorators, and does not directly clip a scroll view. '
	'This feature was deprecated after v3.9.0-1.0.pre.'
	)
	Clip? clipBehavior,
	Clip? decorationClipBehavior,
	}) : decorationClipBehavior = clipBehavior ?? decorationClipBehavior;

	/// A constructor specific to a [Scrollable] with an [Axis.vertical].
	const ScrollableDetails.vertical({
	bool reverse = false,
	this.controller,
	this.physics,
	this.decorationClipBehavior,
	}) : direction = reverse ? AxisDirection.up : AxisDirection.down;

	/// A constructor specific to a [Scrollable] with an [Axis.horizontal].
	const ScrollableDetails.horizontal({
	bool reverse = false,
	this.controller,
	this.physics,
	this.decorationClipBehavior,
	}) : direction = reverse ? AxisDirection.left : AxisDirection.right;

	/// {@macro flutter.widgets.Scrollable.axisDirection}
	final AxisDirection direction;

	/// {@macro flutter.widgets.Scrollable.controller}
	final ScrollController? controller;

	/// {@macro flutter.widgets.Scrollable.physics}
	final ScrollPhysics? physics;

	/// {@macro flutter.material.Material.clipBehavior}
	///
	/// This can be used by [MaterialScrollBehavior] to clip a
	/// [StretchingOverscrollIndicator].
	///
	/// This [Clip] does not affect the [Viewport.clipBehavior], but is rather
	/// passed from the same value by [Scrollable] so that decorators like
	/// [StretchingOverscrollIndicator] honor the same clip.
	///
	/// Defaults to null.
	final Clip? decorationClipBehavior;

	/// Deprecated getter for [decorationClipBehavior].
	@Deprecated(
	'Migrate to decorationClipBehavior. '
	'This property was deprecated so that its application is clearer. This clip '
	'applies to decorators, and does not directly clip a scroll view. '
	'This feature was deprecated after v3.9.0-1.0.pre.'
	)
	Clip? get clipBehavior => decorationClipBehavior;

	/// Copy the current [ScrollableDetails] with the given values replacing the
	/// current values.
	ScrollableDetails copyWith({
	AxisDirection? direction,
	ScrollController? controller,
	ScrollPhysics? physics,
	Clip? decorationClipBehavior,
	}) {
	return ScrollableDetails(
	direction: direction ?? this.direction,
	controller: controller ?? this.controller,
	physics: physics ?? this.physics,
	decorationClipBehavior: decorationClipBehavior ?? this.decorationClipBehavior,
	);
	}

	@override
	String toString() {
	final List<String> description = <String>[];
	description.add('axisDirection: $direction');

	void addIfNonNull(String prefix, Object? value) {
	if (value != null) {
	description.add(prefix + value.toString());
	}
	}
	addIfNonNull('scroll controller: ', controller);
	addIfNonNull('scroll physics: ', physics);
	addIfNonNull('decorationClipBehavior: ', decorationClipBehavior);
	return '${describeIdentity(this)}(${description.join(", ")})';
	}

	@override
	int get hashCode => Object.hash(
	direction,
	controller,
	physics,
	decorationClipBehavior,
	);

	@override
	bool operator ==(Object other) {
	if (identical(this, other)) {
	return true;
	}
	if (other.runtimeType != runtimeType) {
	return false;
	}
	return other is ScrollableDetails
	&& other.direction == direction
	&& other.controller == controller
	&& other.physics == physics
	&& other.decorationClipBehavior == decorationClipBehavior;
	}
	}

	/// An auto scroller that scrolls the [scrollable] if a drag gesture drags close
	/// to its edge.
	///
	/// The scroll velocity is controlled by the [velocityScalar]:
	///
	/// velocity = <distance of overscroll> * [velocityScalar].
	class EdgeDraggingAutoScroller {
	/// Creates a auto scroller that scrolls the [scrollable].
	EdgeDraggingAutoScroller(
	this.scrollable, {
	this.onScrollViewScrolled,
	required this.velocityScalar,
	});

	/// The [Scrollable] this auto scroller is scrolling.
	final ScrollableState scrollable;

	/// Called when a scroll view is scrolled.
	///
	/// The scroll view may be scrolled multiple times in a row until the drag
	/// target no longer triggers the auto scroll. This callback will be called
	/// in between each scroll.
	final VoidCallback? onScrollViewScrolled;

	/// {@template flutter.widgets.EdgeDraggingAutoScroller.velocityScalar}
	/// The velocity scalar per pixel over scroll.
	///
	/// It represents how the velocity scale with the over scroll distance. The
	/// auto-scroll velocity = <distance of overscroll> * velocityScalar.
	/// {@endtemplate}
	final double velocityScalar;

	late Rect _dragTargetRelatedToScrollOrigin;

	/// Whether the auto scroll is in progress.
	bool get scrolling => _scrolling;
	bool _scrolling = false;

	double _offsetExtent(Offset offset, Axis scrollDirection) {
	switch (scrollDirection) {
	case Axis.horizontal:
	return offset.dx;
	case Axis.vertical:
	return offset.dy;
	}
	}

	double _sizeExtent(Size size, Axis scrollDirection) {
	switch (scrollDirection) {
	case Axis.horizontal:
	return size.width;
	case Axis.vertical:
	return size.height;
	}
	}

	AxisDirection get _axisDirection => scrollable.axisDirection;
	Axis get _scrollDirection => axisDirectionToAxis(_axisDirection);

	/// Starts the auto scroll if the [dragTarget] is close to the edge.
	///
	/// The scroll starts to scroll the [scrollable] if the target rect is close
	/// to the edge of the [scrollable]; otherwise, it remains stationary.
	///
	/// If the scrollable is already scrolling, calling this method updates the
	/// previous dragTarget to the new value and continues scrolling if necessary.
	void startAutoScrollIfNecessary(Rect dragTarget) {
	final Offset deltaToOrigin = scrollable.deltaToScrollOrigin;
	_dragTargetRelatedToScrollOrigin = dragTarget.translate(deltaToOrigin.dx, deltaToOrigin.dy);
	if (_scrolling) {
	// The change will be picked up in the next scroll.
	return;
	}
	assert(!_scrolling);
	_scroll();
	}

	/// Stop any ongoing auto scrolling.
	void stopAutoScroll() {
	_scrolling = false;
	}

	Future<void> _scroll() async {
	final RenderBox scrollRenderBox = scrollable.context.findRenderObject()! as RenderBox;
	final Rect globalRect = MatrixUtils.transformRect(
	scrollRenderBox.getTransformTo(null),
	Rect.fromLTWH(0, 0, scrollRenderBox.size.width, scrollRenderBox.size.height),
	);
	assert(
	globalRect.size.width >= _dragTargetRelatedToScrollOrigin.size.width &&
	globalRect.size.height >= _dragTargetRelatedToScrollOrigin.size.height,
	'Drag target size is larger than scrollable size, which may cause bouncing',
	);
	_scrolling = true;
	double? newOffset;
	const double overDragMax = 20.0;

	final Offset deltaToOrigin = scrollable.deltaToScrollOrigin;
	final Offset viewportOrigin = globalRect.topLeft.translate(deltaToOrigin.dx, deltaToOrigin.dy);
	final double viewportStart = _offsetExtent(viewportOrigin, _scrollDirection);
	final double viewportEnd = viewportStart + _sizeExtent(globalRect.size, _scrollDirection);

	final double proxyStart = _offsetExtent(_dragTargetRelatedToScrollOrigin.topLeft, _scrollDirection);
	final double proxyEnd = _offsetExtent(_dragTargetRelatedToScrollOrigin.bottomRight, _scrollDirection);
	switch (_axisDirection) {
	case AxisDirection.up:
	case AxisDirection.left:
	if (proxyEnd > viewportEnd && scrollable.position.pixels > scrollable.position.minScrollExtent) {
	final double overDrag = math.min(proxyEnd - viewportEnd, overDragMax);
	newOffset = math.max(scrollable.position.minScrollExtent, scrollable.position.pixels - overDrag);
	} else if (proxyStart < viewportStart && scrollable.position.pixels < scrollable.position.maxScrollExtent) {
	final double overDrag = math.min(viewportStart - proxyStart, overDragMax);
	newOffset = math.min(scrollable.position.maxScrollExtent, scrollable.position.pixels + overDrag);
	}
	case AxisDirection.right:
	case AxisDirection.down:
	if (proxyStart < viewportStart && scrollable.position.pixels > scrollable.position.minScrollExtent) {
	final double overDrag = math.min(viewportStart - proxyStart, overDragMax);
	newOffset = math.max(scrollable.position.minScrollExtent, scrollable.position.pixels - overDrag);
	} else if (proxyEnd > viewportEnd && scrollable.position.pixels < scrollable.position.maxScrollExtent) {
	final double overDrag = math.min(proxyEnd - viewportEnd, overDragMax);
	newOffset = math.min(scrollable.position.maxScrollExtent, scrollable.position.pixels + overDrag);
	}
	}

	if (newOffset == null \|\| (newOffset - scrollable.position.pixels).abs() < 1.0) {
	// Drag should not trigger scroll.
	_scrolling = false;
	return;
	}
	final Duration duration = Duration(milliseconds: (1000 / velocityScalar).round());
	await scrollable.position.animateTo(
	newOffset,
	duration: duration,
	curve: Curves.linear,
	);
	if (onScrollViewScrolled != null) {
	onScrollViewScrolled!();
	}
	if (_scrolling) {
	await _scroll();
	}
	}
	}

	/// A typedef for a function that can calculate the offset for a type of scroll
	/// increment given a [ScrollIncrementDetails].
	///
	/// This function is used as the type for [Scrollable.incrementCalculator],
	/// which is called from a [ScrollAction].
	typedef ScrollIncrementCalculator = double Function(ScrollIncrementDetails details);

	/// Describes the type of scroll increment that will be performed by a
	/// [ScrollAction] on a [Scrollable].
	///
	/// This is used to configure a [ScrollIncrementDetails] object to pass to a
	/// [ScrollIncrementCalculator] function on a [Scrollable].
	///
	/// {@template flutter.widgets.ScrollIncrementType.intent}
	/// This indicates the intent of the scroll, not necessarily the size. Not all
	/// scrollable areas will have the concept of a "line" or "page", but they can
	/// respond to the different standard key bindings that cause scrolling, which
	/// are bound to keys that people use to indicate a "line" scroll (e.g.
	/// control-arrowDown keys) or a "page" scroll (e.g. pageDown key). It is
	/// recommended that at least the relative magnitudes of the scrolls match
	/// expectations.
	/// {@endtemplate}
	enum ScrollIncrementType {
	/// Indicates that the [ScrollIncrementCalculator] should return the scroll
	/// distance it should move when the user requests to scroll by a "line".
	///
	/// The distance a "line" scrolls refers to what should happen when the key
	/// binding for "scroll down/up by a line" is triggered. It's up to the
	/// [ScrollIncrementCalculator] function to decide what that means for a
	/// particular scrollable.
	line,

	/// Indicates that the [ScrollIncrementCalculator] should return the scroll
	/// distance it should move when the user requests to scroll by a "page".
	///
	/// The distance a "page" scrolls refers to what should happen when the key
	/// binding for "scroll down/up by a page" is triggered. It's up to the
	/// [ScrollIncrementCalculator] function to decide what that means for a
	/// particular scrollable.
	page,
	}

	/// A details object that describes the type of scroll increment being requested
	/// of a [ScrollIncrementCalculator] function, as well as the current metrics
	/// for the scrollable.
	class ScrollIncrementDetails {
	/// A const constructor for a [ScrollIncrementDetails].
	///
	/// All of the arguments must not be null, and are required.
	const ScrollIncrementDetails({
	required this.type,
	required this.metrics,
	});

	/// The type of scroll this is (e.g. line, page, etc.).
	///
	/// {@macro flutter.widgets.ScrollIncrementType.intent}
	final ScrollIncrementType type;

	/// The current metrics of the scrollable that is being scrolled.
	final ScrollMetrics metrics;
	}

	/// An [Intent] that represents scrolling the nearest scrollable by an amount
	/// appropriate for the [type] specified.
	///
	/// The actual amount of the scroll is determined by the
	/// [Scrollable.incrementCalculator], or by its defaults if that is not
	/// specified.
	class ScrollIntent extends Intent {
	/// Creates a const [ScrollIntent] that requests scrolling in the given
	/// [direction], with the given [type].
	const ScrollIntent({
	required this.direction,
	this.type = ScrollIncrementType.line,
	});

	/// The direction in which to scroll the scrollable containing the focused
	/// widget.
	final AxisDirection direction;

	/// The type of scrolling that is intended.
	final ScrollIncrementType type;
	}

	/// An [Action] that scrolls the relevant [Scrollable] by the amount configured
	/// in the [ScrollIntent] given to it.
	///
	/// If a Scrollable cannot be found above the given [BuildContext], the
	/// [PrimaryScrollController] will be considered for default handling of
	/// [ScrollAction]s.
	///
	/// If [Scrollable.incrementCalculator] is null for the scrollable, the default
	/// for a [ScrollIntent.type] set to [ScrollIncrementType.page] is 80% of the
	/// size of the scroll window, and for [ScrollIncrementType.line], 50 logical
	/// pixels.
	class ScrollAction extends ContextAction<ScrollIntent> {
	@override
	bool isEnabled(ScrollIntent intent, [BuildContext? context]) {
	if (context == null) {
	return false;
	}
	if (Scrollable.maybeOf(context) != null) {
	return true;
	}
	final ScrollController? primaryScrollController = PrimaryScrollController.maybeOf(context);
	return (primaryScrollController != null) && (primaryScrollController.hasClients);
	}

	/// Returns the scroll increment for a single scroll request, for use when
	/// scrolling using a hardware keyboard.
	///
	/// Must not be called when the position is null, or when any of the position
	/// metrics (pixels, viewportDimension, maxScrollExtent, minScrollExtent) are
	/// null. The type and state arguments must not be null, and the widget must
	/// have already been laid out so that the position fields are valid.
	static double _calculateScrollIncrement(ScrollableState state, { ScrollIncrementType type = ScrollIncrementType.line }) {
	assert(state.position.hasPixels);
	assert(state.resolvedPhysics == null \|\| state.resolvedPhysics!.shouldAcceptUserOffset(state.position));
	if (state.widget.incrementCalculator != null) {
	return state.widget.incrementCalculator!(
	ScrollIncrementDetails(
	type: type,
	metrics: state.position,
	),
	);
	}
	switch (type) {
	case ScrollIncrementType.line:
	return 50.0;
	case ScrollIncrementType.page:
	return 0.8 * state.position.viewportDimension;
	}
	}

	/// Find out how much of an increment to move by, taking the different
	/// directions into account.
	static double getDirectionalIncrement(ScrollableState state, ScrollIntent intent) {
	final double increment = _calculateScrollIncrement(state, type: intent.type);
	switch (intent.direction) {
	case AxisDirection.down:
	switch (state.axisDirection) {
	case AxisDirection.up:
	return -increment;
	case AxisDirection.down:
	return increment;
	case AxisDirection.right:
	case AxisDirection.left:
	return 0.0;
	}
	case AxisDirection.up:
	switch (state.axisDirection) {
	case AxisDirection.up:
	return increment;
	case AxisDirection.down:
	return -increment;
	case AxisDirection.right:
	case AxisDirection.left:
	return 0.0;
	}
	case AxisDirection.left:
	switch (state.axisDirection) {
	case AxisDirection.right:
	return -increment;
	case AxisDirection.left:
	return increment;
	case AxisDirection.up:
	case AxisDirection.down:
	return 0.0;
	}
	case AxisDirection.right:
	switch (state.axisDirection) {
	case AxisDirection.right:
	return increment;
	case AxisDirection.left:
	return -increment;
	case AxisDirection.up:
	case AxisDirection.down:
	return 0.0;
	}
	}
	}

	@override
	void invoke(ScrollIntent intent, [BuildContext? context]) {
	assert(context != null, 'Cannot scroll without a context.');
	ScrollableState? state = Scrollable.maybeOf(context!);
	if (state == null) {
	final ScrollController primaryScrollController = PrimaryScrollController.of(context);
	assert (() {
	if (primaryScrollController.positions.length != 1) {
	throw FlutterError.fromParts(<DiagnosticsNode>[
	ErrorSummary(
	'A ScrollAction was invoked with the PrimaryScrollController, but '
	'more than one ScrollPosition is attached.',
	),
	ErrorDescription(
	'Only one ScrollPosition can be manipulated by a ScrollAction at '
	'a time.',
	),
	ErrorHint(
	'The PrimaryScrollController can be inherited automatically by '
	'descendant ScrollViews based on the TargetPlatform and scroll '
	'direction. By default, the PrimaryScrollController is '
	'automatically inherited on mobile platforms for vertical '
	'ScrollViews. ScrollView.primary can also override this behavior.',
	),
	]);
	}
	return true;
	}());

	if (primaryScrollController.position.context.notificationContext == null
	&& Scrollable.maybeOf(primaryScrollController.position.context.notificationContext!) == null) {
	return;
	}
	state = Scrollable.maybeOf(primaryScrollController.position.context.notificationContext!);
	}
	assert(state != null, '$ScrollAction was invoked on a context that has no scrollable parent');
	assert(state!.position.hasPixels, 'Scrollable must be laid out before it can be scrolled via a ScrollAction');

	// Don't do anything if the user isn't allowed to scroll.
	if (state!.resolvedPhysics != null && !state.resolvedPhysics!.shouldAcceptUserOffset(state.position)) {
	return;
	}
	final double increment = getDirectionalIncrement(state, intent);
	if (increment == 0.0) {
	return;
	}
	state.position.moveTo(
	state.position.pixels + increment,
	duration: const Duration(milliseconds: 100),
	curve: Curves.easeInOut,
	);
	}
	}