packages/flutter/lib/src/widgets/overscroll_indicator.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' show Timer;
 import 'dart:math' as math;

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

 import 'basic.dart';
 import 'framework.dart';
 import 'notification_listener.dart';
 import 'scroll_notification.dart';
 import 'ticker_provider.dart';

 /// A visual indication that a scroll view has overscrolled.
 ///
 /// A [GlowingOverscrollIndicator] listens for [ScrollNotification]s in order
 /// to control the overscroll indication. These notifications are typically
 /// generated by a [ScrollView], such as a [ListView] or a [GridView].
 ///
 /// [GlowingOverscrollIndicator] generates [OverscrollIndicatorNotification]
 /// before showing an overscroll indication. To prevent the indicator from
 /// showing the indication, call [OverscrollIndicatorNotification.disallowGlow]
 /// on the notification.
 ///
 /// Created automatically by [ScrollBehavior.buildViewportChrome] on platforms
 /// (e.g., Android) that commonly use this type of overscroll indication.
 ///
 /// In a [MaterialApp], the edge glow color is the [ThemeData.accentColor].
 class GlowingOverscrollIndicator extends StatefulWidget {
   /// Creates a visual indication that a scroll view has overscrolled.
   ///
   /// In order for this widget to display an overscroll indication, the [child]
   /// widget must contain a widget that generates a [ScrollNotification], such
   /// as a [ListView] or a [GridView].
   ///
   /// The [showLeading], [showTrailing], [axisDirection], [color], and
   /// [notificationPredicate] arguments must not be null.
   const GlowingOverscrollIndicator({
     Key key,
     this.showLeading = true,
     this.showTrailing = true,
     @required this.axisDirection,
     @required this.color,
     this.notificationPredicate = defaultScrollNotificationPredicate,
     this.child,
   }) : assert(showLeading != null),
        assert(showTrailing != null),
        assert(axisDirection != null),
        assert(color != null),
        assert(notificationPredicate != null),
        super(key: key);

   /// Whether to show the overscroll glow on the side with negative scroll
   /// offsets.
   ///
   /// For a vertical downwards viewport, this is the top side.
   ///
   /// Defaults to true.
   ///
   /// See [showTrailing] for the corresponding control on the other side of the
   /// viewport.
   final bool showLeading;

   /// Whether to show the overscroll glow on the side with positive scroll
   /// offsets.
   ///
   /// For a vertical downwards viewport, this is the bottom side.
   ///
   /// Defaults to true.
   ///
   /// See [showLeading] for the corresponding control on the other side of the
   /// viewport.
   final bool showTrailing;

   /// The direction of positive scroll offsets in the [Scrollable] whose
   /// overscrolls are to be visualized.
   final AxisDirection axisDirection;

   /// The axis along which scrolling occurs in the [Scrollable] whose
   /// overscrolls are to be visualized.
   Axis get axis => axisDirectionToAxis(axisDirection);

   /// The color of the glow. The alpha channel is ignored.
   final Color color;

   /// A check that specifies whether a [ScrollNotification] should be
   /// handled by this widget.
   ///
   /// By default, checks whether `notification.depth == 0`. Set it to something
   /// else for more complicated layouts.
   final ScrollNotificationPredicate notificationPredicate;

   /// The widget below this widget in the tree.
   ///
   /// The overscroll indicator will paint on top of this child. This child (and its
   /// subtree) should include a source of [ScrollNotification] notifications.
   ///
   /// Typically a [GlowingOverscrollIndicator] is created by a
   /// [ScrollBehavior.buildViewportChrome] method, in which case
   /// the child is usually the one provided as an argument to that method.
   final Widget child;

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

   @override
   void debugFillProperties(DiagnosticPropertiesBuilder properties) {
     super.debugFillProperties(properties);
     properties.add(EnumProperty<AxisDirection>('axisDirection', axisDirection));
     String showDescription;
     if (showLeading && showTrailing) {
       showDescription = 'both sides';
     } else if (showLeading) {
       showDescription = 'leading side only';
     } else if (showTrailing) {
       showDescription = 'trailing side only';
     } else {
       showDescription = 'neither side (!)';
     }
     properties.add(MessageProperty('show', showDescription));
     properties.add(ColorProperty('color', color, showName: false));
   }
 }

 class _GlowingOverscrollIndicatorState extends State<GlowingOverscrollIndicator> with TickerProviderStateMixin {
   _GlowController _leadingController;
   _GlowController _trailingController;
   Listenable _leadingAndTrailingListener;

   @override
   void initState() {
     super.initState();
     _leadingController = _GlowController(vsync: this, color: widget.color, axis: widget.axis);
     _trailingController = _GlowController(vsync: this, color: widget.color, axis: widget.axis);
     _leadingAndTrailingListener = Listenable.merge(<Listenable>[_leadingController, _trailingController]);
   }

   @override
   void didUpdateWidget(GlowingOverscrollIndicator oldWidget) {
     super.didUpdateWidget(oldWidget);
     if (oldWidget.color != widget.color || oldWidget.axis != widget.axis) {
       _leadingController.color = widget.color;
       _leadingController.axis = widget.axis;
       _trailingController.color = widget.color;
       _trailingController.axis = widget.axis;
     }
   }

   Type _lastNotificationType;
   final Map<bool, bool> _accepted = <bool, bool>{false: true, true: true};

   bool _handleScrollNotification(ScrollNotification notification) {
     if (!widget.notificationPredicate(notification))
       return false;
     if (notification is OverscrollNotification) {
       _GlowController controller;
       if (notification.overscroll < 0.0) {
         controller = _leadingController;
       } else if (notification.overscroll > 0.0) {
         controller = _trailingController;
       } else {
         assert(false);
       }
       final bool isLeading = controller == _leadingController;
       if (_lastNotificationType != OverscrollNotification) {
         final OverscrollIndicatorNotification confirmationNotification = OverscrollIndicatorNotification(leading: isLeading);
         confirmationNotification.dispatch(context);
         _accepted[isLeading] = confirmationNotification._accepted;
       }
       assert(controller != null);
       assert(notification.metrics.axis == widget.axis);
       if (_accepted[isLeading]) {
         if (notification.velocity != 0.0) {
           assert(notification.dragDetails == null);
           controller.absorbImpact(notification.velocity.abs());
         } else {
           assert(notification.overscroll != 0.0);
           if (notification.dragDetails != null) {
             assert(notification.dragDetails.globalPosition != null);
             final RenderBox renderer = notification.context.findRenderObject() as RenderBox;
             assert(renderer != null);
             assert(renderer.hasSize);
             final Size size = renderer.size;
             final Offset position = renderer.globalToLocal(notification.dragDetails.globalPosition);
             switch (notification.metrics.axis) {
               case Axis.horizontal:
                 controller.pull(notification.overscroll.abs(), size.width, position.dy.clamp(0.0, size.height) as double, size.height);
                 break;
               case Axis.vertical:
                 controller.pull(notification.overscroll.abs(), size.height, position.dx.clamp(0.0, size.width) as double, size.width);
                 break;
             }
           }
         }
       }
     } else if (notification is ScrollEndNotification || notification is ScrollUpdateNotification) {
       if ((notification as dynamic).dragDetails != null) {
         _leadingController.scrollEnd();
         _trailingController.scrollEnd();
       }
     }
     _lastNotificationType = notification.runtimeType;
     return false;
   }

   @override
   void dispose() {
     _leadingController.dispose();
     _trailingController.dispose();
     super.dispose();
   }

   @override
   Widget build(BuildContext context) {
     return NotificationListener<ScrollNotification>(
       onNotification: _handleScrollNotification,
       child: RepaintBoundary(
         child: CustomPaint(
           foregroundPainter: _GlowingOverscrollIndicatorPainter(
             leadingController: widget.showLeading ? _leadingController : null,
             trailingController: widget.showTrailing ? _trailingController : null,
             axisDirection: widget.axisDirection,
             repaint: _leadingAndTrailingListener,
           ),
           child: RepaintBoundary(
             child: widget.child,
           ),
         ),
       ),
     );
   }
 }

 // The Glow logic is a port of the logic in the following file:
 // https://android.googlesource.com/platform/frameworks/base/+/master/core/java/android/widget/EdgeEffect.java
 // as of December 2016.

 enum _GlowState { idle, absorb, pull, recede }

 class _GlowController extends ChangeNotifier {
   _GlowController({
     @required TickerProvider vsync,
     @required Color color,
     @required Axis axis,
   }) : assert(vsync != null),
        assert(color != null),
        assert(axis != null),
        _color = color,
        _axis = axis {
     _glowController = AnimationController(vsync: vsync)
       ..addStatusListener(_changePhase);
     final Animation<double> decelerator = CurvedAnimation(
       parent: _glowController,
       curve: Curves.decelerate,
     )..addListener(notifyListeners);
     _glowOpacity = decelerator.drive(_glowOpacityTween);
     _glowSize = decelerator.drive(_glowSizeTween);
     _displacementTicker = vsync.createTicker(_tickDisplacement);
   }

   // animation of the main axis direction
   _GlowState _state = _GlowState.idle;
   AnimationController _glowController;
   Timer _pullRecedeTimer;

   // animation values
   final Tween<double> _glowOpacityTween = Tween<double>(begin: 0.0, end: 0.0);
   Animation<double> _glowOpacity;
   final Tween<double> _glowSizeTween = Tween<double>(begin: 0.0, end: 0.0);
   Animation<double> _glowSize;

   // animation of the cross axis position
   Ticker _displacementTicker;
   Duration _displacementTickerLastElapsed;
   double _displacementTarget = 0.5;
   double _displacement = 0.5;

   // tracking the pull distance
   double _pullDistance = 0.0;

   Color get color => _color;
   Color _color;
   set color(Color value) {
     assert(color != null);
     if (color == value)
       return;
     _color = value;
     notifyListeners();
   }

   Axis get axis => _axis;
   Axis _axis;
   set axis(Axis value) {
     assert(axis != null);
     if (axis == value)
       return;
     _axis = value;
     notifyListeners();
   }

   static const Duration _recedeTime = Duration(milliseconds: 600);
   static const Duration _pullTime = Duration(milliseconds: 167);
   static const Duration _pullHoldTime = Duration(milliseconds: 167);
   static const Duration _pullDecayTime = Duration(milliseconds: 2000);
   static final Duration _crossAxisHalfTime = Duration(microseconds: (Duration.microsecondsPerSecond / 60.0).round());

   static const double _maxOpacity = 0.5;
   static const double _pullOpacityGlowFactor = 0.8;
   static const double _velocityGlowFactor = 0.00006;
   static const double _sqrt3 = 1.73205080757; // const math.sqrt(3)
   static const double _widthToHeightFactor = (3.0 / 4.0) * (2.0 - _sqrt3);

   // absorbed velocities are clamped to the range _minVelocity.._maxVelocity
   static const double _minVelocity = 100.0; // logical pixels per second
   static const double _maxVelocity = 10000.0; // logical pixels per second

   @override
   void dispose() {
     _glowController.dispose();
     _displacementTicker.dispose();
     _pullRecedeTimer?.cancel();
     super.dispose();
   }

   /// Handle a scroll slamming into the edge at a particular velocity.
   ///
   /// The velocity must be positive.
   void absorbImpact(double velocity) {
     assert(velocity >= 0.0);
     _pullRecedeTimer?.cancel();
     _pullRecedeTimer = null;
     velocity = velocity.clamp(_minVelocity, _maxVelocity) as double;
     _glowOpacityTween.begin = _state == _GlowState.idle ? 0.3 : _glowOpacity.value;
     _glowOpacityTween.end = (velocity * _velocityGlowFactor).clamp(_glowOpacityTween.begin, _maxOpacity) as double;
     _glowSizeTween.begin = _glowSize.value;
     _glowSizeTween.end = math.min(0.025 + 7.5e-7 * velocity * velocity, 1.0);
     _glowController.duration = Duration(milliseconds: (0.15 + velocity * 0.02).round());
     _glowController.forward(from: 0.0);
     _displacement = 0.5;
     _state = _GlowState.absorb;
   }

   /// Handle a user-driven overscroll.
   ///
   /// The `overscroll` argument should be the scroll distance in logical pixels,
   /// the `extent` argument should be the total dimension of the viewport in the
   /// main axis in logical pixels, the `crossAxisOffset` argument should be the
   /// distance from the leading (left or top) edge of the cross axis of the
   /// viewport, and the `crossExtent` should be the size of the cross axis. For
   /// example, a pull of 50 pixels up the middle of a 200 pixel high and 100
   /// pixel wide vertical viewport should result in a call of `pull(50.0, 200.0,
   /// 50.0, 100.0)`. The `overscroll` value should be positive regardless of the
   /// direction.
   void pull(double overscroll, double extent, double crossAxisOffset, double crossExtent) {
     _pullRecedeTimer?.cancel();
     _pullDistance += overscroll / 200.0; // This factor is magic. Not clear why we need it to match Android.
     _glowOpacityTween.begin = _glowOpacity.value;
     _glowOpacityTween.end = math.min(_glowOpacity.value + overscroll / extent * _pullOpacityGlowFactor, _maxOpacity);
     final double height = math.min(extent, crossExtent * _widthToHeightFactor);
     _glowSizeTween.begin = _glowSize.value;
     _glowSizeTween.end = math.max(1.0 - 1.0 / (0.7 * math.sqrt(_pullDistance * height)), _glowSize.value);
     _displacementTarget = crossAxisOffset / crossExtent;
     if (_displacementTarget != _displacement) {
       if (!_displacementTicker.isTicking) {
         assert(_displacementTickerLastElapsed == null);
         _displacementTicker.start();
       }
     } else {
       _displacementTicker.stop();
       _displacementTickerLastElapsed = null;
     }
     _glowController.duration = _pullTime;
     if (_state != _GlowState.pull) {
       _glowController.forward(from: 0.0);
       _state = _GlowState.pull;
     } else {
       if (!_glowController.isAnimating) {
         assert(_glowController.value == 1.0);
         notifyListeners();
       }
     }
     _pullRecedeTimer = Timer(_pullHoldTime, () => _recede(_pullDecayTime));
   }

   void scrollEnd() {
     if (_state == _GlowState.pull)
       _recede(_recedeTime);
   }

   void _changePhase(AnimationStatus status) {
     if (status != AnimationStatus.completed)
       return;
     switch (_state) {
       case _GlowState.absorb:
         _recede(_recedeTime);
         break;
       case _GlowState.recede:
         _state = _GlowState.idle;
         _pullDistance = 0.0;
         break;
       case _GlowState.pull:
       case _GlowState.idle:
         break;
     }
   }

   void _recede(Duration duration) {
     if (_state == _GlowState.recede || _state == _GlowState.idle)
       return;
     _pullRecedeTimer?.cancel();
     _pullRecedeTimer = null;
     _glowOpacityTween.begin = _glowOpacity.value;
     _glowOpacityTween.end = 0.0;
     _glowSizeTween.begin = _glowSize.value;
     _glowSizeTween.end = 0.0;
     _glowController.duration = duration;
     _glowController.forward(from: 0.0);
     _state = _GlowState.recede;
   }

   void _tickDisplacement(Duration elapsed) {
     if (_displacementTickerLastElapsed != null) {
       final double t = (elapsed.inMicroseconds - _displacementTickerLastElapsed.inMicroseconds).toDouble();
       _displacement = _displacementTarget - (_displacementTarget - _displacement) * math.pow(2.0, -t / _crossAxisHalfTime.inMicroseconds);
       notifyListeners();
     }
     if (nearEqual(_displacementTarget, _displacement, Tolerance.defaultTolerance.distance)) {
       _displacementTicker.stop();
       _displacementTickerLastElapsed = null;
     } else {
       _displacementTickerLastElapsed = elapsed;
     }
   }

   void paint(Canvas canvas, Size size) {
     if (_glowOpacity.value == 0.0)
       return;
     final double baseGlowScale = size.width > size.height ? size.height / size.width : 1.0;
     final double radius = size.width * 3.0 / 2.0;
     final double height = math.min(size.height, size.width * _widthToHeightFactor);
     final double scaleY = _glowSize.value * baseGlowScale;
     final Rect rect = Rect.fromLTWH(0.0, 0.0, size.width, height);
     final Offset center = Offset((size.width / 2.0) * (0.5 + _displacement), height - radius);
     final Paint paint = Paint()..color = color.withOpacity(_glowOpacity.value);
     canvas.save();
     canvas.scale(1.0, scaleY);
     canvas.clipRect(rect);
     canvas.drawCircle(center, radius, paint);
     canvas.restore();
   }
 }

 class _GlowingOverscrollIndicatorPainter extends CustomPainter {
   _GlowingOverscrollIndicatorPainter({
     this.leadingController,
     this.trailingController,
     this.axisDirection,
     Listenable repaint,
   }) : super(
     repaint: repaint,
   );

   /// The controller for the overscroll glow on the side with negative scroll offsets.
   ///
   /// For a vertical downwards viewport, this is the top side.
   final _GlowController leadingController;

   /// The controller for the overscroll glow on the side with positive scroll offsets.
   ///
   /// For a vertical downwards viewport, this is the bottom side.
   final _GlowController trailingController;

   /// The direction of the viewport.
   final AxisDirection axisDirection;

   static const double piOver2 = math.pi / 2.0;

   void _paintSide(Canvas canvas, Size size, _GlowController controller, AxisDirection axisDirection, GrowthDirection growthDirection) {
     if (controller == null)
       return;
     switch (applyGrowthDirectionToAxisDirection(axisDirection, growthDirection)) {
       case AxisDirection.up:
         controller.paint(canvas, size);
         break;
       case AxisDirection.down:
         canvas.save();
         canvas.translate(0.0, size.height);
         canvas.scale(1.0, -1.0);
         controller.paint(canvas, size);
         canvas.restore();
         break;
       case AxisDirection.left:
         canvas.save();
         canvas.rotate(piOver2);
         canvas.scale(1.0, -1.0);
         controller.paint(canvas, Size(size.height, size.width));
         canvas.restore();
         break;
       case AxisDirection.right:
         canvas.save();
         canvas.translate(size.width, 0.0);
         canvas.rotate(piOver2);
         controller.paint(canvas, Size(size.height, size.width));
         canvas.restore();
         break;
     }
   }

   @override
   void paint(Canvas canvas, Size size) {
     _paintSide(canvas, size, leadingController, axisDirection, GrowthDirection.reverse);
     _paintSide(canvas, size, trailingController, axisDirection, GrowthDirection.forward);
   }

   @override
   bool shouldRepaint(_GlowingOverscrollIndicatorPainter oldDelegate) {
     return oldDelegate.leadingController != leadingController
         || oldDelegate.trailingController != trailingController;
   }
 }

 /// A notification that an [GlowingOverscrollIndicator] will start showing an
 /// overscroll indication.
 ///
 /// To prevent the indicator from showing the indication, call [disallowGlow] on
 /// the notification.
 ///
 /// See also:
 ///
 ///  * [GlowingOverscrollIndicator], which generates this type of notification.
 class OverscrollIndicatorNotification extends Notification with ViewportNotificationMixin {
   /// Creates a notification that an [GlowingOverscrollIndicator] will start
   /// showing an overscroll indication.
   ///
   /// The [leading] argument must not be null.
   OverscrollIndicatorNotification({
     @required this.leading,
   });

   /// Whether the indication will be shown on the leading edge of the scroll
   /// view.
   final bool leading;

   bool _accepted = true;

   /// Call this method if the glow should be prevented.
   void disallowGlow() {
     _accepted = false;
   }

   @override
   void debugFillDescription(List<String> description) {
     super.debugFillDescription(description);
     description.add('side: ${leading ? "leading edge" : "trailing edge"}');
   }
 }
	// 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' show Timer;
	import 'dart:math' as math;

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

	import 'basic.dart';
	import 'framework.dart';
	import 'notification_listener.dart';
	import 'scroll_notification.dart';
	import 'ticker_provider.dart';

	/// A visual indication that a scroll view has overscrolled.
	///
	/// A [GlowingOverscrollIndicator] listens for [ScrollNotification]s in order
	/// to control the overscroll indication. These notifications are typically
	/// generated by a [ScrollView], such as a [ListView] or a [GridView].
	///
	/// [GlowingOverscrollIndicator] generates [OverscrollIndicatorNotification]
	/// before showing an overscroll indication. To prevent the indicator from
	/// showing the indication, call [OverscrollIndicatorNotification.disallowGlow]
	/// on the notification.
	///
	/// Created automatically by [ScrollBehavior.buildViewportChrome] on platforms
	/// (e.g., Android) that commonly use this type of overscroll indication.
	///
	/// In a [MaterialApp], the edge glow color is the [ThemeData.accentColor].
	class GlowingOverscrollIndicator extends StatefulWidget {
	/// Creates a visual indication that a scroll view has overscrolled.
	///
	/// In order for this widget to display an overscroll indication, the [child]
	/// widget must contain a widget that generates a [ScrollNotification], such
	/// as a [ListView] or a [GridView].
	///
	/// The [showLeading], [showTrailing], [axisDirection], [color], and
	/// [notificationPredicate] arguments must not be null.
	const GlowingOverscrollIndicator({
	Key key,
	this.showLeading = true,
	this.showTrailing = true,
	@required this.axisDirection,
	@required this.color,
	this.notificationPredicate = defaultScrollNotificationPredicate,
	this.child,
	}) : assert(showLeading != null),
	assert(showTrailing != null),
	assert(axisDirection != null),
	assert(color != null),
	assert(notificationPredicate != null),
	super(key: key);

	/// Whether to show the overscroll glow on the side with negative scroll
	/// offsets.
	///
	/// For a vertical downwards viewport, this is the top side.
	///
	/// Defaults to true.
	///
	/// See [showTrailing] for the corresponding control on the other side of the
	/// viewport.
	final bool showLeading;

	/// Whether to show the overscroll glow on the side with positive scroll
	/// offsets.
	///
	/// For a vertical downwards viewport, this is the bottom side.
	///
	/// Defaults to true.
	///
	/// See [showLeading] for the corresponding control on the other side of the
	/// viewport.
	final bool showTrailing;

	/// The direction of positive scroll offsets in the [Scrollable] whose
	/// overscrolls are to be visualized.
	final AxisDirection axisDirection;

	/// The axis along which scrolling occurs in the [Scrollable] whose
	/// overscrolls are to be visualized.
	Axis get axis => axisDirectionToAxis(axisDirection);

	/// The color of the glow. The alpha channel is ignored.
	final Color color;

	/// A check that specifies whether a [ScrollNotification] should be
	/// handled by this widget.
	///
	/// By default, checks whether `notification.depth == 0`. Set it to something
	/// else for more complicated layouts.
	final ScrollNotificationPredicate notificationPredicate;

	/// The widget below this widget in the tree.
	///
	/// The overscroll indicator will paint on top of this child. This child (and its
	/// subtree) should include a source of [ScrollNotification] notifications.
	///
	/// Typically a [GlowingOverscrollIndicator] is created by a
	/// [ScrollBehavior.buildViewportChrome] method, in which case
	/// the child is usually the one provided as an argument to that method.
	final Widget child;

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

	@override
	void debugFillProperties(DiagnosticPropertiesBuilder properties) {
	super.debugFillProperties(properties);
	properties.add(EnumProperty<AxisDirection>('axisDirection', axisDirection));
	String showDescription;
	if (showLeading && showTrailing) {
	showDescription = 'both sides';
	} else if (showLeading) {
	showDescription = 'leading side only';
	} else if (showTrailing) {
	showDescription = 'trailing side only';
	} else {
	showDescription = 'neither side (!)';
	}
	properties.add(MessageProperty('show', showDescription));
	properties.add(ColorProperty('color', color, showName: false));
	}
	}

	class _GlowingOverscrollIndicatorState extends State<GlowingOverscrollIndicator> with TickerProviderStateMixin {
	_GlowController _leadingController;
	_GlowController _trailingController;
	Listenable _leadingAndTrailingListener;

	@override
	void initState() {
	super.initState();
	_leadingController = _GlowController(vsync: this, color: widget.color, axis: widget.axis);
	_trailingController = _GlowController(vsync: this, color: widget.color, axis: widget.axis);
	_leadingAndTrailingListener = Listenable.merge(<Listenable>[_leadingController, _trailingController]);
	}

	@override
	void didUpdateWidget(GlowingOverscrollIndicator oldWidget) {
	super.didUpdateWidget(oldWidget);
	if (oldWidget.color != widget.color \|\| oldWidget.axis != widget.axis) {
	_leadingController.color = widget.color;
	_leadingController.axis = widget.axis;
	_trailingController.color = widget.color;
	_trailingController.axis = widget.axis;
	}
	}

	Type _lastNotificationType;
	final Map<bool, bool> _accepted = <bool, bool>{false: true, true: true};

	bool _handleScrollNotification(ScrollNotification notification) {
	if (!widget.notificationPredicate(notification))
	return false;
	if (notification is OverscrollNotification) {
	_GlowController controller;
	if (notification.overscroll < 0.0) {
	controller = _leadingController;
	} else if (notification.overscroll > 0.0) {
	controller = _trailingController;
	} else {
	assert(false);
	}
	final bool isLeading = controller == _leadingController;
	if (_lastNotificationType != OverscrollNotification) {
	final OverscrollIndicatorNotification confirmationNotification = OverscrollIndicatorNotification(leading: isLeading);
	confirmationNotification.dispatch(context);
	_accepted[isLeading] = confirmationNotification._accepted;
	}
	assert(controller != null);
	assert(notification.metrics.axis == widget.axis);
	if (_accepted[isLeading]) {
	if (notification.velocity != 0.0) {
	assert(notification.dragDetails == null);
	controller.absorbImpact(notification.velocity.abs());
	} else {
	assert(notification.overscroll != 0.0);
	if (notification.dragDetails != null) {
	assert(notification.dragDetails.globalPosition != null);
	final RenderBox renderer = notification.context.findRenderObject() as RenderBox;
	assert(renderer != null);
	assert(renderer.hasSize);
	final Size size = renderer.size;
	final Offset position = renderer.globalToLocal(notification.dragDetails.globalPosition);
	switch (notification.metrics.axis) {
	case Axis.horizontal:
	controller.pull(notification.overscroll.abs(), size.width, position.dy.clamp(0.0, size.height) as double, size.height);
	break;
	case Axis.vertical:
	controller.pull(notification.overscroll.abs(), size.height, position.dx.clamp(0.0, size.width) as double, size.width);
	break;
	}
	}
	}
	}
	} else if (notification is ScrollEndNotification \|\| notification is ScrollUpdateNotification) {
	if ((notification as dynamic).dragDetails != null) {
	_leadingController.scrollEnd();
	_trailingController.scrollEnd();
	}
	}
	_lastNotificationType = notification.runtimeType;
	return false;
	}

	@override
	void dispose() {
	_leadingController.dispose();
	_trailingController.dispose();
	super.dispose();
	}

	@override
	Widget build(BuildContext context) {
	return NotificationListener<ScrollNotification>(
	onNotification: _handleScrollNotification,
	child: RepaintBoundary(
	child: CustomPaint(
	foregroundPainter: _GlowingOverscrollIndicatorPainter(
	leadingController: widget.showLeading ? _leadingController : null,
	trailingController: widget.showTrailing ? _trailingController : null,
	axisDirection: widget.axisDirection,
	repaint: _leadingAndTrailingListener,
	),
	child: RepaintBoundary(
	child: widget.child,
	),
	),
	),
	);
	}
	}

	// The Glow logic is a port of the logic in the following file:
	// https://android.googlesource.com/platform/frameworks/base/+/master/core/java/android/widget/EdgeEffect.java
	// as of December 2016.

	enum _GlowState { idle, absorb, pull, recede }

	class _GlowController extends ChangeNotifier {
	_GlowController({
	@required TickerProvider vsync,
	@required Color color,
	@required Axis axis,
	}) : assert(vsync != null),
	assert(color != null),
	assert(axis != null),
	_color = color,
	_axis = axis {
	_glowController = AnimationController(vsync: vsync)
	..addStatusListener(_changePhase);
	final Animation<double> decelerator = CurvedAnimation(
	parent: _glowController,
	curve: Curves.decelerate,
	)..addListener(notifyListeners);
	_glowOpacity = decelerator.drive(_glowOpacityTween);
	_glowSize = decelerator.drive(_glowSizeTween);
	_displacementTicker = vsync.createTicker(_tickDisplacement);
	}

	// animation of the main axis direction
	_GlowState _state = _GlowState.idle;
	AnimationController _glowController;
	Timer _pullRecedeTimer;

	// animation values
	final Tween<double> _glowOpacityTween = Tween<double>(begin: 0.0, end: 0.0);
	Animation<double> _glowOpacity;
	final Tween<double> _glowSizeTween = Tween<double>(begin: 0.0, end: 0.0);
	Animation<double> _glowSize;

	// animation of the cross axis position
	Ticker _displacementTicker;
	Duration _displacementTickerLastElapsed;
	double _displacementTarget = 0.5;
	double _displacement = 0.5;

	// tracking the pull distance
	double _pullDistance = 0.0;

	Color get color => _color;
	Color _color;
	set color(Color value) {
	assert(color != null);
	if (color == value)
	return;
	_color = value;
	notifyListeners();
	}

	Axis get axis => _axis;
	Axis _axis;
	set axis(Axis value) {
	assert(axis != null);
	if (axis == value)
	return;
	_axis = value;
	notifyListeners();
	}

	static const Duration _recedeTime = Duration(milliseconds: 600);
	static const Duration _pullTime = Duration(milliseconds: 167);
	static const Duration _pullHoldTime = Duration(milliseconds: 167);
	static const Duration _pullDecayTime = Duration(milliseconds: 2000);
	static final Duration _crossAxisHalfTime = Duration(microseconds: (Duration.microsecondsPerSecond / 60.0).round());

	static const double _maxOpacity = 0.5;
	static const double _pullOpacityGlowFactor = 0.8;
	static const double _velocityGlowFactor = 0.00006;
	static const double _sqrt3 = 1.73205080757; // const math.sqrt(3)
	static const double _widthToHeightFactor = (3.0 / 4.0) * (2.0 - _sqrt3);

	// absorbed velocities are clamped to the range _minVelocity.._maxVelocity
	static const double _minVelocity = 100.0; // logical pixels per second
	static const double _maxVelocity = 10000.0; // logical pixels per second

	@override
	void dispose() {
	_glowController.dispose();
	_displacementTicker.dispose();
	_pullRecedeTimer?.cancel();
	super.dispose();
	}

	/// Handle a scroll slamming into the edge at a particular velocity.
	///
	/// The velocity must be positive.
	void absorbImpact(double velocity) {
	assert(velocity >= 0.0);
	_pullRecedeTimer?.cancel();
	_pullRecedeTimer = null;
	velocity = velocity.clamp(_minVelocity, _maxVelocity) as double;
	_glowOpacityTween.begin = _state == _GlowState.idle ? 0.3 : _glowOpacity.value;
	_glowOpacityTween.end = (velocity * _velocityGlowFactor).clamp(_glowOpacityTween.begin, _maxOpacity) as double;
	_glowSizeTween.begin = _glowSize.value;
	_glowSizeTween.end = math.min(0.025 + 7.5e-7 * velocity * velocity, 1.0);
	_glowController.duration = Duration(milliseconds: (0.15 + velocity * 0.02).round());
	_glowController.forward(from: 0.0);
	_displacement = 0.5;
	_state = _GlowState.absorb;
	}

	/// Handle a user-driven overscroll.
	///
	/// The `overscroll` argument should be the scroll distance in logical pixels,
	/// the `extent` argument should be the total dimension of the viewport in the
	/// main axis in logical pixels, the `crossAxisOffset` argument should be the
	/// distance from the leading (left or top) edge of the cross axis of the
	/// viewport, and the `crossExtent` should be the size of the cross axis. For
	/// example, a pull of 50 pixels up the middle of a 200 pixel high and 100
	/// pixel wide vertical viewport should result in a call of `pull(50.0, 200.0,
	/// 50.0, 100.0)`. The `overscroll` value should be positive regardless of the
	/// direction.
	void pull(double overscroll, double extent, double crossAxisOffset, double crossExtent) {
	_pullRecedeTimer?.cancel();
	_pullDistance += overscroll / 200.0; // This factor is magic. Not clear why we need it to match Android.
	_glowOpacityTween.begin = _glowOpacity.value;
	_glowOpacityTween.end = math.min(_glowOpacity.value + overscroll / extent * _pullOpacityGlowFactor, _maxOpacity);
	final double height = math.min(extent, crossExtent * _widthToHeightFactor);
	_glowSizeTween.begin = _glowSize.value;
	_glowSizeTween.end = math.max(1.0 - 1.0 / (0.7 * math.sqrt(_pullDistance * height)), _glowSize.value);
	_displacementTarget = crossAxisOffset / crossExtent;
	if (_displacementTarget != _displacement) {
	if (!_displacementTicker.isTicking) {
	assert(_displacementTickerLastElapsed == null);
	_displacementTicker.start();
	}
	} else {
	_displacementTicker.stop();
	_displacementTickerLastElapsed = null;
	}
	_glowController.duration = _pullTime;
	if (_state != _GlowState.pull) {
	_glowController.forward(from: 0.0);
	_state = _GlowState.pull;
	} else {
	if (!_glowController.isAnimating) {
	assert(_glowController.value == 1.0);
	notifyListeners();
	}
	}
	_pullRecedeTimer = Timer(_pullHoldTime, () => _recede(_pullDecayTime));
	}

	void scrollEnd() {
	if (_state == _GlowState.pull)
	_recede(_recedeTime);
	}

	void _changePhase(AnimationStatus status) {
	if (status != AnimationStatus.completed)
	return;
	switch (_state) {
	case _GlowState.absorb:
	_recede(_recedeTime);
	break;
	case _GlowState.recede:
	_state = _GlowState.idle;
	_pullDistance = 0.0;
	break;
	case _GlowState.pull:
	case _GlowState.idle:
	break;
	}
	}

	void _recede(Duration duration) {
	if (_state == _GlowState.recede \|\| _state == _GlowState.idle)
	return;
	_pullRecedeTimer?.cancel();
	_pullRecedeTimer = null;
	_glowOpacityTween.begin = _glowOpacity.value;
	_glowOpacityTween.end = 0.0;
	_glowSizeTween.begin = _glowSize.value;
	_glowSizeTween.end = 0.0;
	_glowController.duration = duration;
	_glowController.forward(from: 0.0);
	_state = _GlowState.recede;
	}

	void _tickDisplacement(Duration elapsed) {
	if (_displacementTickerLastElapsed != null) {
	final double t = (elapsed.inMicroseconds - _displacementTickerLastElapsed.inMicroseconds).toDouble();
	_displacement = _displacementTarget - (_displacementTarget - _displacement) * math.pow(2.0, -t / _crossAxisHalfTime.inMicroseconds);
	notifyListeners();
	}
	if (nearEqual(_displacementTarget, _displacement, Tolerance.defaultTolerance.distance)) {
	_displacementTicker.stop();
	_displacementTickerLastElapsed = null;
	} else {
	_displacementTickerLastElapsed = elapsed;
	}
	}

	void paint(Canvas canvas, Size size) {
	if (_glowOpacity.value == 0.0)
	return;
	final double baseGlowScale = size.width > size.height ? size.height / size.width : 1.0;
	final double radius = size.width * 3.0 / 2.0;
	final double height = math.min(size.height, size.width * _widthToHeightFactor);
	final double scaleY = _glowSize.value * baseGlowScale;
	final Rect rect = Rect.fromLTWH(0.0, 0.0, size.width, height);
	final Offset center = Offset((size.width / 2.0) * (0.5 + _displacement), height - radius);
	final Paint paint = Paint()..color = color.withOpacity(_glowOpacity.value);
	canvas.save();
	canvas.scale(1.0, scaleY);
	canvas.clipRect(rect);
	canvas.drawCircle(center, radius, paint);
	canvas.restore();
	}
	}

	class _GlowingOverscrollIndicatorPainter extends CustomPainter {
	_GlowingOverscrollIndicatorPainter({
	this.leadingController,
	this.trailingController,
	this.axisDirection,
	Listenable repaint,
	}) : super(
	repaint: repaint,
	);

	/// The controller for the overscroll glow on the side with negative scroll offsets.
	///
	/// For a vertical downwards viewport, this is the top side.
	final _GlowController leadingController;

	/// The controller for the overscroll glow on the side with positive scroll offsets.
	///
	/// For a vertical downwards viewport, this is the bottom side.
	final _GlowController trailingController;

	/// The direction of the viewport.
	final AxisDirection axisDirection;

	static const double piOver2 = math.pi / 2.0;

	void _paintSide(Canvas canvas, Size size, _GlowController controller, AxisDirection axisDirection, GrowthDirection growthDirection) {
	if (controller == null)
	return;
	switch (applyGrowthDirectionToAxisDirection(axisDirection, growthDirection)) {
	case AxisDirection.up:
	controller.paint(canvas, size);
	break;
	case AxisDirection.down:
	canvas.save();
	canvas.translate(0.0, size.height);
	canvas.scale(1.0, -1.0);
	controller.paint(canvas, size);
	canvas.restore();
	break;
	case AxisDirection.left:
	canvas.save();
	canvas.rotate(piOver2);
	canvas.scale(1.0, -1.0);
	controller.paint(canvas, Size(size.height, size.width));
	canvas.restore();
	break;
	case AxisDirection.right:
	canvas.save();
	canvas.translate(size.width, 0.0);
	canvas.rotate(piOver2);
	controller.paint(canvas, Size(size.height, size.width));
	canvas.restore();
	break;
	}
	}

	@override
	void paint(Canvas canvas, Size size) {
	_paintSide(canvas, size, leadingController, axisDirection, GrowthDirection.reverse);
	_paintSide(canvas, size, trailingController, axisDirection, GrowthDirection.forward);
	}

	@override
	bool shouldRepaint(_GlowingOverscrollIndicatorPainter oldDelegate) {
	return oldDelegate.leadingController != leadingController
	\|\| oldDelegate.trailingController != trailingController;
	}
	}

	/// A notification that an [GlowingOverscrollIndicator] will start showing an
	/// overscroll indication.
	///
	/// To prevent the indicator from showing the indication, call [disallowGlow] on
	/// the notification.
	///
	/// See also:
	///
	/// * [GlowingOverscrollIndicator], which generates this type of notification.
	class OverscrollIndicatorNotification extends Notification with ViewportNotificationMixin {
	/// Creates a notification that an [GlowingOverscrollIndicator] will start
	/// showing an overscroll indication.
	///
	/// The [leading] argument must not be null.
	OverscrollIndicatorNotification({
	@required this.leading,
	});

	/// Whether the indication will be shown on the leading edge of the scroll
	/// view.
	final bool leading;

	bool _accepted = true;

	/// Call this method if the glow should be prevented.
	void disallowGlow() {
	_accepted = false;
	}

	@override
	void debugFillDescription(List<String> description) {
	super.debugFillDescription(description);
	description.add('side: ${leading ? "leading edge" : "trailing edge"}');
	}
	}