packages/flutter/lib/src/cupertino/scrollbar.dart - external/github.com/flutter/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 'package:flutter/gestures.dart';
 import 'package:flutter/services.dart';
 import 'package:flutter/widgets.dart';

 import 'colors.dart';

 // All values eyeballed.
 const double _kScrollbarMinLength = 36.0;
 const double _kScrollbarMinOverscrollLength = 8.0;
 const Duration _kScrollbarTimeToFade = Duration(milliseconds: 1200);
 const Duration _kScrollbarFadeDuration = Duration(milliseconds: 250);
 const Duration _kScrollbarResizeDuration = Duration(milliseconds: 100);

 // Extracted from iOS 13.1 beta using Debug View Hierarchy.
 const Color _kScrollbarColor = CupertinoDynamicColor.withBrightness(
   color: Color(0x59000000),
   darkColor: Color(0x80FFFFFF),
 );
 const double _kScrollbarThickness = 3;
 const double _kScrollbarThicknessDragging = 8.0;
 const Radius _kScrollbarRadius = Radius.circular(1.5);
 const Radius _kScrollbarRadiusDragging = Radius.circular(4.0);

 // This is the amount of space from the top of a vertical scrollbar to the
 // top edge of the scrollable, measured when the vertical scrollbar overscrolls
 // to the top.
 // TODO(LongCatIsLooong): fix https://github.com/flutter/flutter/issues/32175
 const double _kScrollbarMainAxisMargin = 3.0;
 const double _kScrollbarCrossAxisMargin = 3.0;

 /// An iOS style scrollbar.
 ///
 /// A scrollbar indicates which portion of a [Scrollable] widget is actually
 /// visible.
 ///
 /// To add a scrollbar to a [ScrollView], simply wrap the scroll view widget in
 /// a [CupertinoScrollbar] widget.
 ///
 /// By default, the CupertinoScrollbar will be draggable (a feature introduced
 /// in iOS 13), it uses the PrimaryScrollController. For multiple scrollbars, or
 /// other more complicated situations, see the [controller] parameter.
 ///
 /// See also:
 ///
 ///  * [ListView], which display a linear, scrollable list of children.
 ///  * [GridView], which display a 2 dimensional, scrollable array of children.
 ///  * [Scrollbar], a Material Design scrollbar that dynamically adapts to the
 ///    platform showing either an Android style or iOS style scrollbar.
 class CupertinoScrollbar extends StatefulWidget {
   /// Creates an iOS style scrollbar that wraps the given [child].
   ///
   /// The [child] should be a source of [ScrollNotification] notifications,
   /// typically a [Scrollable] widget.
   const CupertinoScrollbar({
     Key key,
     this.controller,
     this.isAlwaysShown = false,
     @required this.child,
   }) : super(key: key);

   /// The subtree to place inside the [CupertinoScrollbar].
   ///
   /// This should include a source of [ScrollNotification] notifications,
   /// typically a [Scrollable] widget.
   final Widget child;

   /// {@template flutter.cupertino.cupertinoScrollbar.controller}
   /// The [ScrollController] used to implement Scrollbar dragging.
   ///
   /// introduced in iOS 13.
   ///
   /// If nothing is passed to controller, the default behavior is to automatically
   /// enable scrollbar dragging on the nearest ScrollController using
   /// [PrimaryScrollController.of].
   ///
   /// If a ScrollController is passed, then scrollbar dragging will be enabled on
   /// the given ScrollController. A stateful ancestor of this CupertinoScrollbar
   /// needs to manage the ScrollController and either pass it to a scrollable
   /// descendant or use a PrimaryScrollController to share it.
   ///
   /// Here is an example of using the `controller` parameter to enable
   /// scrollbar dragging for multiple independent ListViews:
   ///
   /// {@tool snippet}
   ///
   /// ```dart
   /// final ScrollController _controllerOne = ScrollController();
   /// final ScrollController _controllerTwo = ScrollController();
   ///
   /// build(BuildContext context) {
   /// return Column(
   ///   children: <Widget>[
   ///     Container(
   ///        height: 200,
   ///        child: CupertinoScrollbar(
   ///          controller: _controllerOne,
   ///          child: ListView.builder(
   ///            controller: _controllerOne,
   ///            itemCount: 120,
   ///            itemBuilder: (BuildContext context, int index) => Text('item $index'),
   ///          ),
   ///        ),
   ///      ),
   ///      Container(
   ///        height: 200,
   ///        child: CupertinoScrollbar(
   ///          controller: _controllerTwo,
   ///          child: ListView.builder(
   ///            controller: _controllerTwo,
   ///            itemCount: 120,
   ///            itemBuilder: (BuildContext context, int index) => Text('list 2 item $index'),
   ///          ),
   ///        ),
   ///      ),
   ///    ],
   ///   );
   /// }
   /// ```
   /// {@end-tool}
   /// {@endtemplate}
   final ScrollController controller;

   /// {@template flutter.cupertino.cupertinoScrollbar.isAlwaysShown}
   /// Indicates whether the [Scrollbar] should always be visible.
   ///
   /// When false, the scrollbar will be shown during scrolling
   /// and will fade out otherwise.
   ///
   /// When true, the scrollbar will always be visible and never fade out.
   ///
   /// The [controller] property must be set in this case.
   /// It should be passed the relevant [Scrollable]'s [ScrollController].
   ///
   /// Defaults to false.
   ///
   /// {@tool snippet}
   ///
   /// ```dart
   /// final ScrollController _controllerOne = ScrollController();
   /// final ScrollController _controllerTwo = ScrollController();
   ///
   /// build(BuildContext context) {
   /// return Column(
   ///   children: <Widget>[
   ///     Container(
   ///        height: 200,
   ///        child: Scrollbar(
   ///          isAlwaysShown: true,
   ///          controller: _controllerOne,
   ///          child: ListView.builder(
   ///            controller: _controllerOne,
   ///            itemCount: 120,
   ///            itemBuilder: (BuildContext context, int index)
   ///                => Text('item $index'),
   ///          ),
   ///        ),
   ///      ),
   ///      Container(
   ///        height: 200,
   ///        child: CupertinoScrollbar(
   ///          isAlwaysShown: true,
   ///          controller: _controllerTwo,
   ///          child: SingleChildScrollView(
   ///            controller: _controllerTwo,
   ///            child: SizedBox(height: 2000, width: 500,),
   ///          ),
   ///        ),
   ///      ),
   ///    ],
   ///   );
   /// }
   /// ```
   /// {@end-tool}
   /// {@endtemplate}
   final bool isAlwaysShown;

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

 class _CupertinoScrollbarState extends State<CupertinoScrollbar> with TickerProviderStateMixin {
   final GlobalKey _customPaintKey = GlobalKey();
   ScrollbarPainter _painter;

   AnimationController _fadeoutAnimationController;
   Animation<double> _fadeoutOpacityAnimation;
   AnimationController _thicknessAnimationController;
   Timer _fadeoutTimer;
   double _dragScrollbarPositionY;
   Drag _drag;

   double get _thickness {
     return _kScrollbarThickness + _thicknessAnimationController.value * (_kScrollbarThicknessDragging - _kScrollbarThickness);
   }

   Radius get _radius {
     return Radius.lerp(_kScrollbarRadius, _kScrollbarRadiusDragging, _thicknessAnimationController.value);
   }

   ScrollController _currentController;
   ScrollController get _controller =>
       widget.controller ?? PrimaryScrollController.of(context);

   @override
   void initState() {
     super.initState();
     _fadeoutAnimationController = AnimationController(
       vsync: this,
       duration: _kScrollbarFadeDuration,
     );
     _fadeoutOpacityAnimation = CurvedAnimation(
       parent: _fadeoutAnimationController,
       curve: Curves.fastOutSlowIn,
     );
     _thicknessAnimationController = AnimationController(
       vsync: this,
       duration: _kScrollbarResizeDuration,
     );
     _thicknessAnimationController.addListener(() {
       _painter.updateThickness(_thickness, _radius);
     });
   }

   @override
   void didChangeDependencies() {
     super.didChangeDependencies();
     if (_painter == null) {
       _painter = _buildCupertinoScrollbarPainter(context);
     } else {
       _painter
         ..textDirection = Directionality.of(context)
         ..color = CupertinoDynamicColor.resolve(_kScrollbarColor, context)
         ..padding = MediaQuery.of(context).padding;
     }
     _triggerScrollbar();
   }

   @override
   void didUpdateWidget(CupertinoScrollbar oldWidget) {
     super.didUpdateWidget(oldWidget);
     if (widget.isAlwaysShown != oldWidget.isAlwaysShown) {
       if (widget.isAlwaysShown == true) {
         _triggerScrollbar();
         _fadeoutAnimationController.animateTo(1.0);
       } else {
         _fadeoutAnimationController.reverse();
       }
     }
   }

   /// Returns a [ScrollbarPainter] visually styled like the iOS scrollbar.
   ScrollbarPainter _buildCupertinoScrollbarPainter(BuildContext context) {
     return ScrollbarPainter(
       color: CupertinoDynamicColor.resolve(_kScrollbarColor, context),
       textDirection: Directionality.of(context),
       thickness: _thickness,
       fadeoutOpacityAnimation: _fadeoutOpacityAnimation,
       mainAxisMargin: _kScrollbarMainAxisMargin,
       crossAxisMargin: _kScrollbarCrossAxisMargin,
       radius: _radius,
       padding: MediaQuery.of(context).padding,
       minLength: _kScrollbarMinLength,
       minOverscrollLength: _kScrollbarMinOverscrollLength,
     );
   }

   // Wait one frame and cause an empty scroll event.  This allows the thumb to
   // show immediately when isAlwaysShown is true.  A scroll event is required in
   // order to paint the thumb.
   void _triggerScrollbar() {
     WidgetsBinding.instance.addPostFrameCallback((Duration duration) {
       if (widget.isAlwaysShown) {
         assert(widget.controller != null);
         _fadeoutTimer?.cancel();
         widget.controller.position.didUpdateScrollPositionBy(0);
       }
     });
   }

   // Handle a gesture that drags the scrollbar by the given amount.
   void _dragScrollbar(double primaryDelta) {
     assert(_currentController != null);

     // Convert primaryDelta, the amount that the scrollbar moved since the last
     // time _dragScrollbar was called, into the coordinate space of the scroll
     // position, and create/update the drag event with that position.
     final double scrollOffsetLocal = _painter.getTrackToScroll(primaryDelta);
     final double scrollOffsetGlobal = scrollOffsetLocal + _currentController.position.pixels;

     if (_drag == null) {
       _drag = _currentController.position.drag(
         DragStartDetails(
           globalPosition: Offset(0.0, scrollOffsetGlobal),
         ),
         () {},
       );
     } else {
       _drag.update(DragUpdateDetails(
         globalPosition: Offset(0.0, scrollOffsetGlobal),
         delta: Offset(0.0, -scrollOffsetLocal),
         primaryDelta: -scrollOffsetLocal,
       ));
     }
   }

   void _startFadeoutTimer() {
     if (!widget.isAlwaysShown) {
       _fadeoutTimer?.cancel();
       _fadeoutTimer = Timer(_kScrollbarTimeToFade, () {
         _fadeoutAnimationController.reverse();
         _fadeoutTimer = null;
       });
     }
   }

   bool _checkVertical() {
     try {
       return _currentController.position.axis == Axis.vertical;
     } catch (_) {
       // Ignore the gesture if we cannot determine the direction.
       return false;
     }
   }

   double _pressStartY = 0.0;

   // Long press event callbacks handle the gesture where the user long presses
   // on the scrollbar thumb and then drags the scrollbar without releasing.
   void _handleLongPressStart(LongPressStartDetails details) {
     _currentController = _controller;
     if (!_checkVertical()) {
       return;
     }
     _pressStartY = details.localPosition.dy;
     _fadeoutTimer?.cancel();
     _fadeoutAnimationController.forward();
     _dragScrollbar(details.localPosition.dy);
     _dragScrollbarPositionY = details.localPosition.dy;
   }

   void _handleLongPress() {
     if (!_checkVertical()) {
       return;
     }
     _fadeoutTimer?.cancel();
     _thicknessAnimationController.forward().then<void>(
           (_) => HapticFeedback.mediumImpact(),
     );
   }

   void _handleLongPressMoveUpdate(LongPressMoveUpdateDetails details) {
     if (!_checkVertical()) {
       return;
     }
     _dragScrollbar(details.localPosition.dy - _dragScrollbarPositionY);
     _dragScrollbarPositionY = details.localPosition.dy;
   }

   void _handleLongPressEnd(LongPressEndDetails details) {
     if (!_checkVertical()) {
       return;
     }
     _handleDragScrollEnd(details.velocity.pixelsPerSecond.dy);
     if (details.velocity.pixelsPerSecond.dy.abs() < 10 &&
         (details.localPosition.dy - _pressStartY).abs() > 0) {
       HapticFeedback.mediumImpact();
     }
     _currentController = null;
   }

   void _handleDragScrollEnd(double trackVelocityY) {
     _startFadeoutTimer();
     _thicknessAnimationController.reverse();
     _dragScrollbarPositionY = null;
     final double scrollVelocityY = _painter.getTrackToScroll(trackVelocityY);
     _drag?.end(DragEndDetails(
       primaryVelocity: -scrollVelocityY,
       velocity: Velocity(
         pixelsPerSecond: Offset(
           0.0,
           -scrollVelocityY,
         ),
       ),
     ));
     _drag = null;
   }

   bool _handleScrollNotification(ScrollNotification notification) {
     final ScrollMetrics metrics = notification.metrics;
     if (metrics.maxScrollExtent <= metrics.minScrollExtent) {
       return false;
     }

     if (notification is ScrollUpdateNotification ||
         notification is OverscrollNotification) {
       // Any movements always makes the scrollbar start showing up.
       if (_fadeoutAnimationController.status != AnimationStatus.forward) {
         _fadeoutAnimationController.forward();
       }

       _fadeoutTimer?.cancel();
       _painter.update(notification.metrics, notification.metrics.axisDirection);
     } else if (notification is ScrollEndNotification) {
       // On iOS, the scrollbar can only go away once the user lifted the finger.
       if (_dragScrollbarPositionY == null) {
         _startFadeoutTimer();
       }
     }
     return false;
   }

   // Get the GestureRecognizerFactories used to detect gestures on the scrollbar
   // thumb.
   Map<Type, GestureRecognizerFactory> get _gestures {
     final Map<Type, GestureRecognizerFactory> gestures =
         <Type, GestureRecognizerFactory>{};

     gestures[_ThumbPressGestureRecognizer] =
         GestureRecognizerFactoryWithHandlers<_ThumbPressGestureRecognizer>(
       () => _ThumbPressGestureRecognizer(
         debugOwner: this,
         customPaintKey: _customPaintKey,
       ),
       (_ThumbPressGestureRecognizer instance) {
         instance
           ..onLongPressStart = _handleLongPressStart
           ..onLongPress = _handleLongPress
           ..onLongPressMoveUpdate = _handleLongPressMoveUpdate
           ..onLongPressEnd = _handleLongPressEnd;
       },
     );

     return gestures;
   }

   @override
   void dispose() {
     _fadeoutAnimationController.dispose();
     _thicknessAnimationController.dispose();
     _fadeoutTimer?.cancel();
     _painter.dispose();
     super.dispose();
   }

   @override
   Widget build(BuildContext context) {
     return NotificationListener<ScrollNotification>(
       onNotification: _handleScrollNotification,
       child: RepaintBoundary(
         child: RawGestureDetector(
           gestures: _gestures,
           child: CustomPaint(
             key: _customPaintKey,
             foregroundPainter: _painter,
             child: RepaintBoundary(child: widget.child),
           ),
         ),
       ),
     );
   }
 }

 // A longpress gesture detector that only responds to events on the scrollbar's
 // thumb and ignores everything else.
 class _ThumbPressGestureRecognizer extends LongPressGestureRecognizer {
   _ThumbPressGestureRecognizer({
     double postAcceptSlopTolerance,
     PointerDeviceKind kind,
     Object debugOwner,
     GlobalKey customPaintKey,
   }) :  _customPaintKey = customPaintKey,
         super(
           postAcceptSlopTolerance: postAcceptSlopTolerance,
           kind: kind,
           debugOwner: debugOwner,
           duration: const Duration(milliseconds: 100),
         );

   final GlobalKey _customPaintKey;

   @override
   bool isPointerAllowed(PointerDownEvent event) {
     if (!_hitTestInteractive(_customPaintKey, event.position)) {
       return false;
     }
     return super.isPointerAllowed(event);
   }
 }

 // foregroundPainter also hit tests its children by default, but the
 // scrollbar should only respond to a gesture directly on its thumb, so
 // manually check for a hit on the thumb here.
 bool _hitTestInteractive(GlobalKey customPaintKey, Offset offset) {
   if (customPaintKey.currentContext == null) {
     return false;
   }
   final CustomPaint customPaint = customPaintKey.currentContext.widget as CustomPaint;
   final ScrollbarPainter painter = customPaint.foregroundPainter as ScrollbarPainter;
   final RenderBox renderBox = customPaintKey.currentContext.findRenderObject() as RenderBox;
   final Offset localOffset = renderBox.globalToLocal(offset);
   return painter.hitTestInteractive(localOffset);
 }
	// 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 'package:flutter/gestures.dart';
	import 'package:flutter/services.dart';
	import 'package:flutter/widgets.dart';

	import 'colors.dart';

	// All values eyeballed.
	const double _kScrollbarMinLength = 36.0;
	const double _kScrollbarMinOverscrollLength = 8.0;
	const Duration _kScrollbarTimeToFade = Duration(milliseconds: 1200);
	const Duration _kScrollbarFadeDuration = Duration(milliseconds: 250);
	const Duration _kScrollbarResizeDuration = Duration(milliseconds: 100);

	// Extracted from iOS 13.1 beta using Debug View Hierarchy.
	const Color _kScrollbarColor = CupertinoDynamicColor.withBrightness(
	color: Color(0x59000000),
	darkColor: Color(0x80FFFFFF),
	);
	const double _kScrollbarThickness = 3;
	const double _kScrollbarThicknessDragging = 8.0;
	const Radius _kScrollbarRadius = Radius.circular(1.5);
	const Radius _kScrollbarRadiusDragging = Radius.circular(4.0);

	// This is the amount of space from the top of a vertical scrollbar to the
	// top edge of the scrollable, measured when the vertical scrollbar overscrolls
	// to the top.
	// TODO(LongCatIsLooong): fix https://github.com/flutter/flutter/issues/32175
	const double _kScrollbarMainAxisMargin = 3.0;
	const double _kScrollbarCrossAxisMargin = 3.0;

	/// An iOS style scrollbar.
	///
	/// A scrollbar indicates which portion of a [Scrollable] widget is actually
	/// visible.
	///
	/// To add a scrollbar to a [ScrollView], simply wrap the scroll view widget in
	/// a [CupertinoScrollbar] widget.
	///
	/// By default, the CupertinoScrollbar will be draggable (a feature introduced
	/// in iOS 13), it uses the PrimaryScrollController. For multiple scrollbars, or
	/// other more complicated situations, see the [controller] parameter.
	///
	/// See also:
	///
	/// * [ListView], which display a linear, scrollable list of children.
	/// * [GridView], which display a 2 dimensional, scrollable array of children.
	/// * [Scrollbar], a Material Design scrollbar that dynamically adapts to the
	/// platform showing either an Android style or iOS style scrollbar.
	class CupertinoScrollbar extends StatefulWidget {
	/// Creates an iOS style scrollbar that wraps the given [child].
	///
	/// The [child] should be a source of [ScrollNotification] notifications,
	/// typically a [Scrollable] widget.
	const CupertinoScrollbar({
	Key key,
	this.controller,
	this.isAlwaysShown = false,
	@required this.child,
	}) : super(key: key);

	/// The subtree to place inside the [CupertinoScrollbar].
	///
	/// This should include a source of [ScrollNotification] notifications,
	/// typically a [Scrollable] widget.
	final Widget child;

	/// {@template flutter.cupertino.cupertinoScrollbar.controller}
	/// The [ScrollController] used to implement Scrollbar dragging.
	///
	/// introduced in iOS 13.
	///
	/// If nothing is passed to controller, the default behavior is to automatically
	/// enable scrollbar dragging on the nearest ScrollController using
	/// [PrimaryScrollController.of].
	///
	/// If a ScrollController is passed, then scrollbar dragging will be enabled on
	/// the given ScrollController. A stateful ancestor of this CupertinoScrollbar
	/// needs to manage the ScrollController and either pass it to a scrollable
	/// descendant or use a PrimaryScrollController to share it.
	///
	/// Here is an example of using the `controller` parameter to enable
	/// scrollbar dragging for multiple independent ListViews:
	///
	/// {@tool snippet}
	///
	/// ```dart
	/// final ScrollController _controllerOne = ScrollController();
	/// final ScrollController _controllerTwo = ScrollController();
	///
	/// build(BuildContext context) {
	/// return Column(
	/// children: <Widget>[
	/// Container(
	/// height: 200,
	/// child: CupertinoScrollbar(
	/// controller: _controllerOne,
	/// child: ListView.builder(
	/// controller: _controllerOne,
	/// itemCount: 120,
	/// itemBuilder: (BuildContext context, int index) => Text('item $index'),
	/// ),
	/// ),
	/// ),
	/// Container(
	/// height: 200,
	/// child: CupertinoScrollbar(
	/// controller: _controllerTwo,
	/// child: ListView.builder(
	/// controller: _controllerTwo,
	/// itemCount: 120,
	/// itemBuilder: (BuildContext context, int index) => Text('list 2 item $index'),
	/// ),
	/// ),
	/// ),
	/// ],
	/// );
	/// }
	/// ```
	/// {@end-tool}
	/// {@endtemplate}
	final ScrollController controller;

	/// {@template flutter.cupertino.cupertinoScrollbar.isAlwaysShown}
	/// Indicates whether the [Scrollbar] should always be visible.
	///
	/// When false, the scrollbar will be shown during scrolling
	/// and will fade out otherwise.
	///
	/// When true, the scrollbar will always be visible and never fade out.
	///
	/// The [controller] property must be set in this case.
	/// It should be passed the relevant [Scrollable]'s [ScrollController].
	///
	/// Defaults to false.
	///
	/// {@tool snippet}
	///
	/// ```dart
	/// final ScrollController _controllerOne = ScrollController();
	/// final ScrollController _controllerTwo = ScrollController();
	///
	/// build(BuildContext context) {
	/// return Column(
	/// children: <Widget>[
	/// Container(
	/// height: 200,
	/// child: Scrollbar(
	/// isAlwaysShown: true,
	/// controller: _controllerOne,
	/// child: ListView.builder(
	/// controller: _controllerOne,
	/// itemCount: 120,
	/// itemBuilder: (BuildContext context, int index)
	/// => Text('item $index'),
	/// ),
	/// ),
	/// ),
	/// Container(
	/// height: 200,
	/// child: CupertinoScrollbar(
	/// isAlwaysShown: true,
	/// controller: _controllerTwo,
	/// child: SingleChildScrollView(
	/// controller: _controllerTwo,
	/// child: SizedBox(height: 2000, width: 500,),
	/// ),
	/// ),
	/// ),
	/// ],
	/// );
	/// }
	/// ```
	/// {@end-tool}
	/// {@endtemplate}
	final bool isAlwaysShown;

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

	class _CupertinoScrollbarState extends State<CupertinoScrollbar> with TickerProviderStateMixin {
	final GlobalKey _customPaintKey = GlobalKey();
	ScrollbarPainter _painter;

	AnimationController _fadeoutAnimationController;
	Animation<double> _fadeoutOpacityAnimation;
	AnimationController _thicknessAnimationController;
	Timer _fadeoutTimer;
	double _dragScrollbarPositionY;
	Drag _drag;

	double get _thickness {
	return _kScrollbarThickness + _thicknessAnimationController.value * (_kScrollbarThicknessDragging - _kScrollbarThickness);
	}

	Radius get _radius {
	return Radius.lerp(_kScrollbarRadius, _kScrollbarRadiusDragging, _thicknessAnimationController.value);
	}

	ScrollController _currentController;
	ScrollController get _controller =>
	widget.controller ?? PrimaryScrollController.of(context);

	@override
	void initState() {
	super.initState();
	_fadeoutAnimationController = AnimationController(
	vsync: this,
	duration: _kScrollbarFadeDuration,
	);
	_fadeoutOpacityAnimation = CurvedAnimation(
	parent: _fadeoutAnimationController,
	curve: Curves.fastOutSlowIn,
	);
	_thicknessAnimationController = AnimationController(
	vsync: this,
	duration: _kScrollbarResizeDuration,
	);
	_thicknessAnimationController.addListener(() {
	_painter.updateThickness(_thickness, _radius);
	});
	}

	@override
	void didChangeDependencies() {
	super.didChangeDependencies();
	if (_painter == null) {
	_painter = _buildCupertinoScrollbarPainter(context);
	} else {
	_painter
	..textDirection = Directionality.of(context)
	..color = CupertinoDynamicColor.resolve(_kScrollbarColor, context)
	..padding = MediaQuery.of(context).padding;
	}
	_triggerScrollbar();
	}

	@override
	void didUpdateWidget(CupertinoScrollbar oldWidget) {
	super.didUpdateWidget(oldWidget);
	if (widget.isAlwaysShown != oldWidget.isAlwaysShown) {
	if (widget.isAlwaysShown == true) {
	_triggerScrollbar();
	_fadeoutAnimationController.animateTo(1.0);
	} else {
	_fadeoutAnimationController.reverse();
	}
	}
	}

	/// Returns a [ScrollbarPainter] visually styled like the iOS scrollbar.
	ScrollbarPainter _buildCupertinoScrollbarPainter(BuildContext context) {
	return ScrollbarPainter(
	color: CupertinoDynamicColor.resolve(_kScrollbarColor, context),
	textDirection: Directionality.of(context),
	thickness: _thickness,
	fadeoutOpacityAnimation: _fadeoutOpacityAnimation,
	mainAxisMargin: _kScrollbarMainAxisMargin,
	crossAxisMargin: _kScrollbarCrossAxisMargin,
	radius: _radius,
	padding: MediaQuery.of(context).padding,
	minLength: _kScrollbarMinLength,
	minOverscrollLength: _kScrollbarMinOverscrollLength,
	);
	}

	// Wait one frame and cause an empty scroll event. This allows the thumb to
	// show immediately when isAlwaysShown is true. A scroll event is required in
	// order to paint the thumb.
	void _triggerScrollbar() {
	WidgetsBinding.instance.addPostFrameCallback((Duration duration) {
	if (widget.isAlwaysShown) {
	assert(widget.controller != null);
	_fadeoutTimer?.cancel();
	widget.controller.position.didUpdateScrollPositionBy(0);
	}
	});
	}

	// Handle a gesture that drags the scrollbar by the given amount.
	void _dragScrollbar(double primaryDelta) {
	assert(_currentController != null);

	// Convert primaryDelta, the amount that the scrollbar moved since the last
	// time _dragScrollbar was called, into the coordinate space of the scroll
	// position, and create/update the drag event with that position.
	final double scrollOffsetLocal = _painter.getTrackToScroll(primaryDelta);
	final double scrollOffsetGlobal = scrollOffsetLocal + _currentController.position.pixels;

	if (_drag == null) {
	_drag = _currentController.position.drag(
	DragStartDetails(
	globalPosition: Offset(0.0, scrollOffsetGlobal),
	),
	() {},
	);
	} else {
	_drag.update(DragUpdateDetails(
	globalPosition: Offset(0.0, scrollOffsetGlobal),
	delta: Offset(0.0, -scrollOffsetLocal),
	primaryDelta: -scrollOffsetLocal,
	));
	}
	}

	void _startFadeoutTimer() {
	if (!widget.isAlwaysShown) {
	_fadeoutTimer?.cancel();
	_fadeoutTimer = Timer(_kScrollbarTimeToFade, () {
	_fadeoutAnimationController.reverse();
	_fadeoutTimer = null;
	});
	}
	}

	bool _checkVertical() {
	try {
	return _currentController.position.axis == Axis.vertical;
	} catch (_) {
	// Ignore the gesture if we cannot determine the direction.
	return false;
	}
	}

	double _pressStartY = 0.0;

	// Long press event callbacks handle the gesture where the user long presses
	// on the scrollbar thumb and then drags the scrollbar without releasing.
	void _handleLongPressStart(LongPressStartDetails details) {
	_currentController = _controller;
	if (!_checkVertical()) {
	return;
	}
	_pressStartY = details.localPosition.dy;
	_fadeoutTimer?.cancel();
	_fadeoutAnimationController.forward();
	_dragScrollbar(details.localPosition.dy);
	_dragScrollbarPositionY = details.localPosition.dy;
	}

	void _handleLongPress() {
	if (!_checkVertical()) {
	return;
	}
	_fadeoutTimer?.cancel();
	_thicknessAnimationController.forward().then<void>(
	(_) => HapticFeedback.mediumImpact(),
	);
	}

	void _handleLongPressMoveUpdate(LongPressMoveUpdateDetails details) {
	if (!_checkVertical()) {
	return;
	}
	_dragScrollbar(details.localPosition.dy - _dragScrollbarPositionY);
	_dragScrollbarPositionY = details.localPosition.dy;
	}

	void _handleLongPressEnd(LongPressEndDetails details) {
	if (!_checkVertical()) {
	return;
	}
	_handleDragScrollEnd(details.velocity.pixelsPerSecond.dy);
	if (details.velocity.pixelsPerSecond.dy.abs() < 10 &&
	(details.localPosition.dy - _pressStartY).abs() > 0) {
	HapticFeedback.mediumImpact();
	}
	_currentController = null;
	}

	void _handleDragScrollEnd(double trackVelocityY) {
	_startFadeoutTimer();
	_thicknessAnimationController.reverse();
	_dragScrollbarPositionY = null;
	final double scrollVelocityY = _painter.getTrackToScroll(trackVelocityY);
	_drag?.end(DragEndDetails(
	primaryVelocity: -scrollVelocityY,
	velocity: Velocity(
	pixelsPerSecond: Offset(
	0.0,
	-scrollVelocityY,
	),
	),
	));
	_drag = null;
	}

	bool _handleScrollNotification(ScrollNotification notification) {
	final ScrollMetrics metrics = notification.metrics;
	if (metrics.maxScrollExtent <= metrics.minScrollExtent) {
	return false;
	}

	if (notification is ScrollUpdateNotification \|\|
	notification is OverscrollNotification) {
	// Any movements always makes the scrollbar start showing up.
	if (_fadeoutAnimationController.status != AnimationStatus.forward) {
	_fadeoutAnimationController.forward();
	}

	_fadeoutTimer?.cancel();
	_painter.update(notification.metrics, notification.metrics.axisDirection);
	} else if (notification is ScrollEndNotification) {
	// On iOS, the scrollbar can only go away once the user lifted the finger.
	if (_dragScrollbarPositionY == null) {
	_startFadeoutTimer();
	}
	}
	return false;
	}

	// Get the GestureRecognizerFactories used to detect gestures on the scrollbar
	// thumb.
	Map<Type, GestureRecognizerFactory> get _gestures {
	final Map<Type, GestureRecognizerFactory> gestures =
	<Type, GestureRecognizerFactory>{};

	gestures[_ThumbPressGestureRecognizer] =
	GestureRecognizerFactoryWithHandlers<_ThumbPressGestureRecognizer>(
	() => _ThumbPressGestureRecognizer(
	debugOwner: this,
	customPaintKey: _customPaintKey,
	),
	(_ThumbPressGestureRecognizer instance) {
	instance
	..onLongPressStart = _handleLongPressStart
	..onLongPress = _handleLongPress
	..onLongPressMoveUpdate = _handleLongPressMoveUpdate
	..onLongPressEnd = _handleLongPressEnd;
	},
	);

	return gestures;
	}

	@override
	void dispose() {
	_fadeoutAnimationController.dispose();
	_thicknessAnimationController.dispose();
	_fadeoutTimer?.cancel();
	_painter.dispose();
	super.dispose();
	}

	@override
	Widget build(BuildContext context) {
	return NotificationListener<ScrollNotification>(
	onNotification: _handleScrollNotification,
	child: RepaintBoundary(
	child: RawGestureDetector(
	gestures: _gestures,
	child: CustomPaint(
	key: _customPaintKey,
	foregroundPainter: _painter,
	child: RepaintBoundary(child: widget.child),
	),
	),
	),
	);
	}
	}

	// A longpress gesture detector that only responds to events on the scrollbar's
	// thumb and ignores everything else.
	class _ThumbPressGestureRecognizer extends LongPressGestureRecognizer {
	_ThumbPressGestureRecognizer({
	double postAcceptSlopTolerance,
	PointerDeviceKind kind,
	Object debugOwner,
	GlobalKey customPaintKey,
	}) : _customPaintKey = customPaintKey,
	super(
	postAcceptSlopTolerance: postAcceptSlopTolerance,
	kind: kind,
	debugOwner: debugOwner,
	duration: const Duration(milliseconds: 100),
	);

	final GlobalKey _customPaintKey;

	@override
	bool isPointerAllowed(PointerDownEvent event) {
	if (!_hitTestInteractive(_customPaintKey, event.position)) {
	return false;
	}
	return super.isPointerAllowed(event);
	}
	}

	// foregroundPainter also hit tests its children by default, but the
	// scrollbar should only respond to a gesture directly on its thumb, so
	// manually check for a hit on the thumb here.
	bool _hitTestInteractive(GlobalKey customPaintKey, Offset offset) {
	if (customPaintKey.currentContext == null) {
	return false;
	}
	final CustomPaint customPaint = customPaintKey.currentContext.widget as CustomPaint;
	final ScrollbarPainter painter = customPaint.foregroundPainter as ScrollbarPainter;
	final RenderBox renderBox = customPaintKey.currentContext.findRenderObject() as RenderBox;
	final Offset localOffset = renderBox.globalToLocal(offset);
	return painter.hitTestInteractive(localOffset);
	}