packages/flutter/lib/src/widgets/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:math' as math;

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

 import 'scroll_metrics.dart';

 const double _kMinThumbExtent = 18.0;
 const double _kMinInteractiveSize = 48.0;

 /// A [CustomPainter] for painting scrollbars.
 ///
 /// The size of the scrollbar along its scroll direction is typically
 /// proportional to the percentage of content completely visible on screen,
 /// as long as its size isn't less than [minLength] and it isn't overscrolling.
 ///
 /// Unlike [CustomPainter]s that subclasses [CustomPainter] and only repaint
 /// when [shouldRepaint] returns true (which requires this [CustomPainter] to
 /// be rebuilt), this painter has the added optimization of repainting and not
 /// rebuilding when:
 ///
 ///  * the scroll position changes; and
 ///  * when the scrollbar fades away.
 ///
 /// Calling [update] with the new [ScrollMetrics] will repaint the new scrollbar
 /// position.
 ///
 /// Updating the value on the provided [fadeoutOpacityAnimation] will repaint
 /// with the new opacity.
 ///
 /// You must call [dispose] on this [ScrollbarPainter] when it's no longer used.
 ///
 /// See also:
 ///
 ///  * [Scrollbar] for a widget showing a scrollbar around a [Scrollable] in the
 ///    Material Design style.
 ///  * [CupertinoScrollbar] for a widget showing a scrollbar around a
 ///    [Scrollable] in the iOS style.
 class ScrollbarPainter extends ChangeNotifier implements CustomPainter {
   /// Creates a scrollbar with customizations given by construction arguments.
   ScrollbarPainter({
     @required Color color,
     @required TextDirection textDirection,
     @required this.thickness,
     @required this.fadeoutOpacityAnimation,
     EdgeInsets padding = EdgeInsets.zero,
     this.mainAxisMargin = 0.0,
     this.crossAxisMargin = 0.0,
     this.radius,
     this.minLength = _kMinThumbExtent,
     double minOverscrollLength,
   }) : assert(color != null),
        assert(textDirection != null),
        assert(thickness != null),
        assert(fadeoutOpacityAnimation != null),
        assert(mainAxisMargin != null),
        assert(crossAxisMargin != null),
        assert(minLength != null),
        assert(minLength >= 0),
        assert(minOverscrollLength == null || minOverscrollLength <= minLength),
        assert(minOverscrollLength == null || minOverscrollLength >= 0),
        assert(padding != null),
        assert(padding.isNonNegative),
        _color = color,
        _textDirection = textDirection,
        _padding = padding,
        minOverscrollLength = minOverscrollLength ?? minLength {
     fadeoutOpacityAnimation.addListener(notifyListeners);
   }

   /// [Color] of the thumb. Mustn't be null.
   Color get color => _color;
   Color _color;
   set color(Color value) {
     assert(value != null);
     if (color == value)
       return;

     _color = value;
     notifyListeners();
   }

   /// [TextDirection] of the [BuildContext] which dictates the side of the
   /// screen the scrollbar appears in (the trailing side). Mustn't be null.
   TextDirection get textDirection => _textDirection;
   TextDirection _textDirection;
   set textDirection(TextDirection value) {
     assert(value != null);
     if (textDirection == value)
       return;

     _textDirection = value;
     notifyListeners();
   }

   /// Thickness of the scrollbar in its cross-axis in logical pixels. Mustn't be null.
   double thickness;

   /// An opacity [Animation] that dictates the opacity of the thumb.
   /// Changes in value of this [Listenable] will automatically trigger repaints.
   /// Mustn't be null.
   final Animation<double> fadeoutOpacityAnimation;

   /// Distance from the scrollbar's start and end to the edge of the viewport
   /// in logical pixels. It affects the amount of available paint area.
   ///
   /// Mustn't be null and defaults to 0.
   final double mainAxisMargin;

   /// Distance from the scrollbar's side to the nearest edge in logical pixels.
   ///
   /// Must not be null and defaults to 0.
   final double crossAxisMargin;

   /// [Radius] of corners if the scrollbar should have rounded corners.
   ///
   /// Scrollbar will be rectangular if [radius] is null.
   Radius radius;

   /// The amount of space by which to inset the scrollbar's start and end, as
   /// well as its side to the nearest edge, in logical pixels.
   ///
   /// This is typically set to the current [MediaQueryData.padding] to avoid
   /// partial obstructions such as display notches. If you only want additional
   /// margins around the scrollbar, see [mainAxisMargin].
   ///
   /// Defaults to [EdgeInsets.zero]. Must not be null and offsets from all four
   /// directions must be greater than or equal to zero.
   EdgeInsets get padding => _padding;
   EdgeInsets _padding;
   set padding(EdgeInsets value) {
     assert(value != null);
     if (padding == value)
       return;

     _padding = value;
     notifyListeners();
   }


   /// The preferred smallest size the scrollbar can shrink to when the total
   /// scrollable extent is large, the current visible viewport is small, and the
   /// viewport is not overscrolled.
   ///
   /// The size of the scrollbar may shrink to a smaller size than [minLength]
   /// to fit in the available paint area. E.g., when [minLength] is
   /// `double.infinity`, it will not be respected if [viewportDimension] and
   /// [mainAxisMargin] are finite.
   ///
   /// Mustn't be null and the value has to be within the range of 0 to
   /// [minOverscrollLength], inclusive. Defaults to 18.0.
   final double minLength;

   /// The preferred smallest size the scrollbar can shrink to when viewport is
   /// overscrolled.
   ///
   /// When overscrolling, the size of the scrollbar may shrink to a smaller size
   /// than [minOverscrollLength] to fit in the available paint area. E.g., when
   /// [minOverscrollLength] is `double.infinity`, it will not be respected if
   /// the [viewportDimension] and [mainAxisMargin] are finite.
   ///
   /// The value is less than or equal to [minLength] and greater than or equal to 0.
   /// If unspecified or set to null, it will defaults to the value of [minLength].
   final double minOverscrollLength;

   ScrollMetrics _lastMetrics;
   AxisDirection _lastAxisDirection;
   Rect _thumbRect;

   /// Update with new [ScrollMetrics]. The scrollbar will show and redraw itself
   /// based on these new metrics.
   ///
   /// The scrollbar will remain on screen.
   void update(
     ScrollMetrics metrics,
     AxisDirection axisDirection,
   ) {
     _lastMetrics = metrics;
     _lastAxisDirection = axisDirection;
     notifyListeners();
   }

   /// Update and redraw with new scrollbar thickness and radius.
   void updateThickness(double nextThickness, Radius nextRadius) {
     thickness = nextThickness;
     radius = nextRadius;
     notifyListeners();
   }

   Paint get _paint {
     return Paint()
       ..color = color.withOpacity(color.opacity * fadeoutOpacityAnimation.value);
   }

   void _paintThumbCrossAxis(Canvas canvas, Size size, double thumbOffset, double thumbExtent, AxisDirection direction) {
     double x, y;
     Size thumbSize;

     switch (direction) {
       case AxisDirection.down:
         thumbSize = Size(thickness, thumbExtent);
         x = textDirection == TextDirection.rtl
           ? crossAxisMargin + padding.left
           : size.width - thickness - crossAxisMargin - padding.right;
         y = thumbOffset;
         break;
       case AxisDirection.up:
         thumbSize = Size(thickness, thumbExtent);
         x = textDirection == TextDirection.rtl
           ? crossAxisMargin + padding.left
           : size.width - thickness - crossAxisMargin - padding.right;
         y = thumbOffset;
         break;
       case AxisDirection.left:
         thumbSize = Size(thumbExtent, thickness);
         x = thumbOffset;
         y = size.height - thickness - crossAxisMargin - padding.bottom;
         break;
       case AxisDirection.right:
         thumbSize = Size(thumbExtent, thickness);
         x = thumbOffset;
         y = size.height - thickness - crossAxisMargin - padding.bottom;
         break;
     }

     _thumbRect = Offset(x, y) & thumbSize;
     if (radius == null)
       canvas.drawRect(_thumbRect, _paint);
     else
       canvas.drawRRect(RRect.fromRectAndRadius(_thumbRect, radius), _paint);
   }

   double _thumbExtent() {
     // Thumb extent reflects fraction of content visible, as long as this
     // isn't less than the absolute minimum size.
     // _totalContentExtent >= viewportDimension, so (_totalContentExtent - _mainAxisPadding) > 0
     final double fractionVisible = ((_lastMetrics.extentInside - _mainAxisPadding) / (_totalContentExtent - _mainAxisPadding))
       .clamp(0.0, 1.0) as double;

     final double thumbExtent = math.max(
       math.min(_trackExtent, minOverscrollLength),
       _trackExtent * fractionVisible,
     );

     final double fractionOverscrolled = 1.0 - _lastMetrics.extentInside / _lastMetrics.viewportDimension;
     final double safeMinLength = math.min(minLength, _trackExtent);
     final double newMinLength = (_beforeExtent > 0 && _afterExtent > 0)
       // Thumb extent is no smaller than minLength if scrolling normally.
       ? safeMinLength
       // User is overscrolling. Thumb extent can be less than minLength
       // but no smaller than minOverscrollLength. We can't use the
       // fractionVisible to produce intermediate values between minLength and
       // minOverscrollLength when the user is transitioning from regular
       // scrolling to overscrolling, so we instead use the percentage of the
       // content that is still in the viewport to determine the size of the
       // thumb. iOS behavior appears to have the thumb reach its minimum size
       // with ~20% of overscroll. We map the percentage of minLength from
       // [0.8, 1.0] to [0.0, 1.0], so 0% to 20% of overscroll will produce
       // values for the thumb that range between minLength and the smallest
       // possible value, minOverscrollLength.
       : safeMinLength * (1.0 - fractionOverscrolled.clamp(0.0, 0.2) / 0.2);

     // The `thumbExtent` should be no greater than `trackSize`, otherwise
     // the scrollbar may scroll towards the wrong direction.
     return thumbExtent.clamp(newMinLength, _trackExtent) as double;
   }

   @override
   void dispose() {
     fadeoutOpacityAnimation.removeListener(notifyListeners);
     super.dispose();
   }

   bool get _isVertical => _lastAxisDirection == AxisDirection.down || _lastAxisDirection == AxisDirection.up;
   bool get _isReversed => _lastAxisDirection == AxisDirection.up || _lastAxisDirection == AxisDirection.left;
   // The amount of scroll distance before and after the current position.
   double get _beforeExtent => _isReversed ? _lastMetrics.extentAfter : _lastMetrics.extentBefore;
   double get _afterExtent => _isReversed ? _lastMetrics.extentBefore : _lastMetrics.extentAfter;
   // Padding of the thumb track.
   double get _mainAxisPadding => _isVertical ? padding.vertical : padding.horizontal;
   // The size of the thumb track.
   double get _trackExtent => _lastMetrics.viewportDimension - 2 * mainAxisMargin - _mainAxisPadding;

   // The total size of the scrollable content.
   double get _totalContentExtent {
     return _lastMetrics.maxScrollExtent
       - _lastMetrics.minScrollExtent
       + _lastMetrics.viewportDimension;
   }

   /// Convert between a thumb track position and the corresponding scroll
   /// position.
   ///
   /// thumbOffsetLocal is a position in the thumb track. Cannot be null.
   double getTrackToScroll(double thumbOffsetLocal) {
     assert(thumbOffsetLocal != null);
     final double scrollableExtent = _lastMetrics.maxScrollExtent - _lastMetrics.minScrollExtent;
     final double thumbMovableExtent = _trackExtent - _thumbExtent();

     return scrollableExtent * thumbOffsetLocal / thumbMovableExtent;
   }

   // Converts between a scroll position and the corresponding position in the
   // thumb track.
   double _getScrollToTrack(ScrollMetrics metrics, double thumbExtent) {
     final double scrollableExtent = metrics.maxScrollExtent - metrics.minScrollExtent;

     final double fractionPast = (scrollableExtent > 0)
       ? ((metrics.pixels - metrics.minScrollExtent) / scrollableExtent).clamp(0.0, 1.0) as double
       : 0;

     return (_isReversed ? 1 - fractionPast : fractionPast) * (_trackExtent - thumbExtent);
   }

   @override
   void paint(Canvas canvas, Size size) {
     if (_lastAxisDirection == null
         || _lastMetrics == null
         || fadeoutOpacityAnimation.value == 0.0)
       return;

     // Skip painting if there's not enough space.
     if (_lastMetrics.viewportDimension <= _mainAxisPadding || _trackExtent <= 0) {
       return;
     }

     final double beforePadding = _isVertical ? padding.top : padding.left;
     final double thumbExtent = _thumbExtent();
     final double thumbOffsetLocal = _getScrollToTrack(_lastMetrics, thumbExtent);
     final double thumbOffset = thumbOffsetLocal + mainAxisMargin + beforePadding;

     return _paintThumbCrossAxis(canvas, size, thumbOffset, thumbExtent, _lastAxisDirection);
   }

   /// Same as hitTest, but includes some padding to make sure that the region
   /// isn't too small to be interacted with by the user.
   bool hitTestInteractive(Offset position) {
     if (_thumbRect == null) {
       return false;
     }
     // The thumb is not able to be hit when transparent.
     if (fadeoutOpacityAnimation.value == 0.0) {
       return false;
     }
     final Rect interactiveThumbRect = _thumbRect.expandToInclude(
       Rect.fromCircle(center: _thumbRect.center, radius: _kMinInteractiveSize / 2),
     );
     return interactiveThumbRect.contains(position);
   }

   // Scrollbars can be interactive in Cupertino.
   @override
   bool hitTest(Offset position) {
     if (_thumbRect == null) {
       return null;
     }
     // The thumb is not able to be hit when transparent.
     if (fadeoutOpacityAnimation.value == 0.0) {
       return false;
     }
     return _thumbRect.contains(position);
   }

   @override
   bool shouldRepaint(ScrollbarPainter old) {
     // Should repaint if any properties changed.
     return color != old.color
         || textDirection != old.textDirection
         || thickness != old.thickness
         || fadeoutOpacityAnimation != old.fadeoutOpacityAnimation
         || mainAxisMargin != old.mainAxisMargin
         || crossAxisMargin != old.crossAxisMargin
         || radius != old.radius
         || minLength != old.minLength
         || padding != old.padding;
   }

   @override
   bool shouldRebuildSemantics(CustomPainter oldDelegate) => false;

   @override
   SemanticsBuilderCallback get semanticsBuilder => null;
 }
	// 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:math' as math;

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

	import 'scroll_metrics.dart';

	const double _kMinThumbExtent = 18.0;
	const double _kMinInteractiveSize = 48.0;

	/// A [CustomPainter] for painting scrollbars.
	///
	/// The size of the scrollbar along its scroll direction is typically
	/// proportional to the percentage of content completely visible on screen,
	/// as long as its size isn't less than [minLength] and it isn't overscrolling.
	///
	/// Unlike [CustomPainter]s that subclasses [CustomPainter] and only repaint
	/// when [shouldRepaint] returns true (which requires this [CustomPainter] to
	/// be rebuilt), this painter has the added optimization of repainting and not
	/// rebuilding when:
	///
	/// * the scroll position changes; and
	/// * when the scrollbar fades away.
	///
	/// Calling [update] with the new [ScrollMetrics] will repaint the new scrollbar
	/// position.
	///
	/// Updating the value on the provided [fadeoutOpacityAnimation] will repaint
	/// with the new opacity.
	///
	/// You must call [dispose] on this [ScrollbarPainter] when it's no longer used.
	///
	/// See also:
	///
	/// * [Scrollbar] for a widget showing a scrollbar around a [Scrollable] in the
	/// Material Design style.
	/// * [CupertinoScrollbar] for a widget showing a scrollbar around a
	/// [Scrollable] in the iOS style.
	class ScrollbarPainter extends ChangeNotifier implements CustomPainter {
	/// Creates a scrollbar with customizations given by construction arguments.
	ScrollbarPainter({
	@required Color color,
	@required TextDirection textDirection,
	@required this.thickness,
	@required this.fadeoutOpacityAnimation,
	EdgeInsets padding = EdgeInsets.zero,
	this.mainAxisMargin = 0.0,
	this.crossAxisMargin = 0.0,
	this.radius,
	this.minLength = _kMinThumbExtent,
	double minOverscrollLength,
	}) : assert(color != null),
	assert(textDirection != null),
	assert(thickness != null),
	assert(fadeoutOpacityAnimation != null),
	assert(mainAxisMargin != null),
	assert(crossAxisMargin != null),
	assert(minLength != null),
	assert(minLength >= 0),
	assert(minOverscrollLength == null \|\| minOverscrollLength <= minLength),
	assert(minOverscrollLength == null \|\| minOverscrollLength >= 0),
	assert(padding != null),
	assert(padding.isNonNegative),
	_color = color,
	_textDirection = textDirection,
	_padding = padding,
	minOverscrollLength = minOverscrollLength ?? minLength {
	fadeoutOpacityAnimation.addListener(notifyListeners);
	}

	/// [Color] of the thumb. Mustn't be null.
	Color get color => _color;
	Color _color;
	set color(Color value) {
	assert(value != null);
	if (color == value)
	return;

	_color = value;
	notifyListeners();
	}

	/// [TextDirection] of the [BuildContext] which dictates the side of the
	/// screen the scrollbar appears in (the trailing side). Mustn't be null.
	TextDirection get textDirection => _textDirection;
	TextDirection _textDirection;
	set textDirection(TextDirection value) {
	assert(value != null);
	if (textDirection == value)
	return;

	_textDirection = value;
	notifyListeners();
	}

	/// Thickness of the scrollbar in its cross-axis in logical pixels. Mustn't be null.
	double thickness;

	/// An opacity [Animation] that dictates the opacity of the thumb.
	/// Changes in value of this [Listenable] will automatically trigger repaints.
	/// Mustn't be null.
	final Animation<double> fadeoutOpacityAnimation;

	/// Distance from the scrollbar's start and end to the edge of the viewport
	/// in logical pixels. It affects the amount of available paint area.
	///
	/// Mustn't be null and defaults to 0.
	final double mainAxisMargin;

	/// Distance from the scrollbar's side to the nearest edge in logical pixels.
	///
	/// Must not be null and defaults to 0.
	final double crossAxisMargin;

	/// [Radius] of corners if the scrollbar should have rounded corners.
	///
	/// Scrollbar will be rectangular if [radius] is null.
	Radius radius;

	/// The amount of space by which to inset the scrollbar's start and end, as
	/// well as its side to the nearest edge, in logical pixels.
	///
	/// This is typically set to the current [MediaQueryData.padding] to avoid
	/// partial obstructions such as display notches. If you only want additional
	/// margins around the scrollbar, see [mainAxisMargin].
	///
	/// Defaults to [EdgeInsets.zero]. Must not be null and offsets from all four
	/// directions must be greater than or equal to zero.
	EdgeInsets get padding => _padding;
	EdgeInsets _padding;
	set padding(EdgeInsets value) {
	assert(value != null);
	if (padding == value)
	return;

	_padding = value;
	notifyListeners();
	}


	/// The preferred smallest size the scrollbar can shrink to when the total
	/// scrollable extent is large, the current visible viewport is small, and the
	/// viewport is not overscrolled.
	///
	/// The size of the scrollbar may shrink to a smaller size than [minLength]
	/// to fit in the available paint area. E.g., when [minLength] is
	/// `double.infinity`, it will not be respected if [viewportDimension] and
	/// [mainAxisMargin] are finite.
	///
	/// Mustn't be null and the value has to be within the range of 0 to
	/// [minOverscrollLength], inclusive. Defaults to 18.0.
	final double minLength;

	/// The preferred smallest size the scrollbar can shrink to when viewport is
	/// overscrolled.
	///
	/// When overscrolling, the size of the scrollbar may shrink to a smaller size
	/// than [minOverscrollLength] to fit in the available paint area. E.g., when
	/// [minOverscrollLength] is `double.infinity`, it will not be respected if
	/// the [viewportDimension] and [mainAxisMargin] are finite.
	///
	/// The value is less than or equal to [minLength] and greater than or equal to 0.
	/// If unspecified or set to null, it will defaults to the value of [minLength].
	final double minOverscrollLength;

	ScrollMetrics _lastMetrics;
	AxisDirection _lastAxisDirection;
	Rect _thumbRect;

	/// Update with new [ScrollMetrics]. The scrollbar will show and redraw itself
	/// based on these new metrics.
	///
	/// The scrollbar will remain on screen.
	void update(
	ScrollMetrics metrics,
	AxisDirection axisDirection,
	) {
	_lastMetrics = metrics;
	_lastAxisDirection = axisDirection;
	notifyListeners();
	}

	/// Update and redraw with new scrollbar thickness and radius.
	void updateThickness(double nextThickness, Radius nextRadius) {
	thickness = nextThickness;
	radius = nextRadius;
	notifyListeners();
	}

	Paint get _paint {
	return Paint()
	..color = color.withOpacity(color.opacity * fadeoutOpacityAnimation.value);
	}

	void _paintThumbCrossAxis(Canvas canvas, Size size, double thumbOffset, double thumbExtent, AxisDirection direction) {
	double x, y;
	Size thumbSize;

	switch (direction) {
	case AxisDirection.down:
	thumbSize = Size(thickness, thumbExtent);
	x = textDirection == TextDirection.rtl
	? crossAxisMargin + padding.left
	: size.width - thickness - crossAxisMargin - padding.right;
	y = thumbOffset;
	break;
	case AxisDirection.up:
	thumbSize = Size(thickness, thumbExtent);
	x = textDirection == TextDirection.rtl
	? crossAxisMargin + padding.left
	: size.width - thickness - crossAxisMargin - padding.right;
	y = thumbOffset;
	break;
	case AxisDirection.left:
	thumbSize = Size(thumbExtent, thickness);
	x = thumbOffset;
	y = size.height - thickness - crossAxisMargin - padding.bottom;
	break;
	case AxisDirection.right:
	thumbSize = Size(thumbExtent, thickness);
	x = thumbOffset;
	y = size.height - thickness - crossAxisMargin - padding.bottom;
	break;
	}

	_thumbRect = Offset(x, y) & thumbSize;
	if (radius == null)
	canvas.drawRect(_thumbRect, _paint);
	else
	canvas.drawRRect(RRect.fromRectAndRadius(_thumbRect, radius), _paint);
	}

	double _thumbExtent() {
	// Thumb extent reflects fraction of content visible, as long as this
	// isn't less than the absolute minimum size.
	// _totalContentExtent >= viewportDimension, so (_totalContentExtent - _mainAxisPadding) > 0
	final double fractionVisible = ((_lastMetrics.extentInside - _mainAxisPadding) / (_totalContentExtent - _mainAxisPadding))
	.clamp(0.0, 1.0) as double;

	final double thumbExtent = math.max(
	math.min(_trackExtent, minOverscrollLength),
	_trackExtent * fractionVisible,
	);

	final double fractionOverscrolled = 1.0 - _lastMetrics.extentInside / _lastMetrics.viewportDimension;
	final double safeMinLength = math.min(minLength, _trackExtent);
	final double newMinLength = (_beforeExtent > 0 && _afterExtent > 0)
	// Thumb extent is no smaller than minLength if scrolling normally.
	? safeMinLength
	// User is overscrolling. Thumb extent can be less than minLength
	// but no smaller than minOverscrollLength. We can't use the
	// fractionVisible to produce intermediate values between minLength and
	// minOverscrollLength when the user is transitioning from regular
	// scrolling to overscrolling, so we instead use the percentage of the
	// content that is still in the viewport to determine the size of the
	// thumb. iOS behavior appears to have the thumb reach its minimum size
	// with ~20% of overscroll. We map the percentage of minLength from
	// [0.8, 1.0] to [0.0, 1.0], so 0% to 20% of overscroll will produce
	// values for the thumb that range between minLength and the smallest
	// possible value, minOverscrollLength.
	: safeMinLength * (1.0 - fractionOverscrolled.clamp(0.0, 0.2) / 0.2);

	// The `thumbExtent` should be no greater than `trackSize`, otherwise
	// the scrollbar may scroll towards the wrong direction.
	return thumbExtent.clamp(newMinLength, _trackExtent) as double;
	}

	@override
	void dispose() {
	fadeoutOpacityAnimation.removeListener(notifyListeners);
	super.dispose();
	}

	bool get _isVertical => _lastAxisDirection == AxisDirection.down \|\| _lastAxisDirection == AxisDirection.up;
	bool get _isReversed => _lastAxisDirection == AxisDirection.up \|\| _lastAxisDirection == AxisDirection.left;
	// The amount of scroll distance before and after the current position.
	double get _beforeExtent => _isReversed ? _lastMetrics.extentAfter : _lastMetrics.extentBefore;
	double get _afterExtent => _isReversed ? _lastMetrics.extentBefore : _lastMetrics.extentAfter;
	// Padding of the thumb track.
	double get _mainAxisPadding => _isVertical ? padding.vertical : padding.horizontal;
	// The size of the thumb track.
	double get _trackExtent => _lastMetrics.viewportDimension - 2 * mainAxisMargin - _mainAxisPadding;

	// The total size of the scrollable content.
	double get _totalContentExtent {
	return _lastMetrics.maxScrollExtent
	- _lastMetrics.minScrollExtent
	+ _lastMetrics.viewportDimension;
	}

	/// Convert between a thumb track position and the corresponding scroll
	/// position.
	///
	/// thumbOffsetLocal is a position in the thumb track. Cannot be null.
	double getTrackToScroll(double thumbOffsetLocal) {
	assert(thumbOffsetLocal != null);
	final double scrollableExtent = _lastMetrics.maxScrollExtent - _lastMetrics.minScrollExtent;
	final double thumbMovableExtent = _trackExtent - _thumbExtent();

	return scrollableExtent * thumbOffsetLocal / thumbMovableExtent;
	}

	// Converts between a scroll position and the corresponding position in the
	// thumb track.
	double _getScrollToTrack(ScrollMetrics metrics, double thumbExtent) {
	final double scrollableExtent = metrics.maxScrollExtent - metrics.minScrollExtent;

	final double fractionPast = (scrollableExtent > 0)
	? ((metrics.pixels - metrics.minScrollExtent) / scrollableExtent).clamp(0.0, 1.0) as double
	: 0;

	return (_isReversed ? 1 - fractionPast : fractionPast) * (_trackExtent - thumbExtent);
	}

	@override
	void paint(Canvas canvas, Size size) {
	if (_lastAxisDirection == null
	\|\| _lastMetrics == null
	\|\| fadeoutOpacityAnimation.value == 0.0)
	return;

	// Skip painting if there's not enough space.
	if (_lastMetrics.viewportDimension <= _mainAxisPadding \|\| _trackExtent <= 0) {
	return;
	}

	final double beforePadding = _isVertical ? padding.top : padding.left;
	final double thumbExtent = _thumbExtent();
	final double thumbOffsetLocal = _getScrollToTrack(_lastMetrics, thumbExtent);
	final double thumbOffset = thumbOffsetLocal + mainAxisMargin + beforePadding;

	return _paintThumbCrossAxis(canvas, size, thumbOffset, thumbExtent, _lastAxisDirection);
	}

	/// Same as hitTest, but includes some padding to make sure that the region
	/// isn't too small to be interacted with by the user.
	bool hitTestInteractive(Offset position) {
	if (_thumbRect == null) {
	return false;
	}
	// The thumb is not able to be hit when transparent.
	if (fadeoutOpacityAnimation.value == 0.0) {
	return false;
	}
	final Rect interactiveThumbRect = _thumbRect.expandToInclude(
	Rect.fromCircle(center: _thumbRect.center, radius: _kMinInteractiveSize / 2),
	);
	return interactiveThumbRect.contains(position);
	}

	// Scrollbars can be interactive in Cupertino.
	@override
	bool hitTest(Offset position) {
	if (_thumbRect == null) {
	return null;
	}
	// The thumb is not able to be hit when transparent.
	if (fadeoutOpacityAnimation.value == 0.0) {
	return false;
	}
	return _thumbRect.contains(position);
	}

	@override
	bool shouldRepaint(ScrollbarPainter old) {
	// Should repaint if any properties changed.
	return color != old.color
	\|\| textDirection != old.textDirection
	\|\| thickness != old.thickness
	\|\| fadeoutOpacityAnimation != old.fadeoutOpacityAnimation
	\|\| mainAxisMargin != old.mainAxisMargin
	\|\| crossAxisMargin != old.crossAxisMargin
	\|\| radius != old.radius
	\|\| minLength != old.minLength
	\|\| padding != old.padding;
	}

	@override
	bool shouldRebuildSemantics(CustomPainter oldDelegate) => false;

	@override
	SemanticsBuilderCallback get semanticsBuilder => null;
	}