packages/flutter/lib/src/gestures/monodrag.dart - mirrors/flutter - Git at Google

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

 import 'package:flutter/foundation.dart';
 import 'package:vector_math/vector_math_64.dart';

 import 'arena.dart';
 import 'constants.dart';
 import 'drag_details.dart';
 import 'events.dart';
 import 'recognizer.dart';
 import 'velocity_tracker.dart';

 enum _DragState {
   ready,
   possible,
   accepted,
 }

 /// Signature for when a pointer that was previously in contact with the screen
 /// and moving is no longer in contact with the screen.
 ///
 /// The velocity at which the pointer was moving when it stopped contacting
 /// the screen is available in the `details`.
 ///
 /// See [DragGestureRecognizer.onEnd].
 typedef GestureDragEndCallback = void Function(DragEndDetails details);

 /// Signature for when the pointer that previously triggered a
 /// [GestureDragDownCallback] did not complete.
 ///
 /// See [DragGestureRecognizer.onCancel].
 typedef GestureDragCancelCallback = void Function();

 /// Recognizes movement.
 ///
 /// In contrast to [MultiDragGestureRecognizer], [DragGestureRecognizer]
 /// recognizes a single gesture sequence for all the pointers it watches, which
 /// means that the recognizer has at most one drag sequence active at any given
 /// time regardless of how many pointers are in contact with the screen.
 ///
 /// [DragGestureRecognizer] is not intended to be used directly. Instead,
 /// consider using one of its subclasses to recognize specific types for drag
 /// gestures.
 ///
 /// [DragGestureRecognizer] competes on pointer events of [kPrimaryButton]
 /// only when it has at least one non-null callback. If it has no callbacks, it
 /// is a no-op.
 ///
 /// See also:
 ///
 ///  * [HorizontalDragGestureRecognizer], for left and right drags.
 ///  * [VerticalDragGestureRecognizer], for up and down drags.
 ///  * [PanGestureRecognizer], for drags that are not locked to a single axis.
 abstract class DragGestureRecognizer extends OneSequenceGestureRecognizer {
   /// Initialize the object.
   ///
   /// [dragStartBehavior] must not be null.
   ///
   /// {@macro flutter.gestures.gestureRecognizer.kind}
   DragGestureRecognizer({
     Object debugOwner,
     PointerDeviceKind kind,
     this.dragStartBehavior = DragStartBehavior.start,
   }) : assert(dragStartBehavior != null),
        super(debugOwner: debugOwner, kind: kind);

   /// Configure the behavior of offsets sent to [onStart].
   ///
   /// If set to [DragStartBehavior.start], the [onStart] callback will be called
   /// at the time and position when this gesture recognizer wins the arena. If
   /// [DragStartBehavior.down], [onStart] will be called at the time and
   /// position when a down event was first detected.
   ///
   /// For more information about the gesture arena:
   /// https://flutter.dev/docs/development/ui/advanced/gestures#gesture-disambiguation
   ///
   /// By default, the drag start behavior is [DragStartBehavior.start].
   ///
   /// ## Example:
   ///
   /// A finger presses down on the screen with offset (500.0, 500.0), and then
   /// moves to position (510.0, 500.0) before winning the arena. With
   /// [dragStartBehavior] set to [DragStartBehavior.down], the [onStart]
   /// callback will be called at the time corresponding to the touch's position
   /// at (500.0, 500.0). If it is instead set to [DragStartBehavior.start],
   /// [onStart] will be called at the time corresponding to the touch's position
   /// at (510.0, 500.0).
   DragStartBehavior dragStartBehavior;

   /// A pointer has contacted the screen with a primary button and might begin
   /// to move.
   ///
   /// The position of the pointer is provided in the callback's `details`
   /// argument, which is a [DragDownDetails] object.
   ///
   /// See also:
   ///
   ///  * [kPrimaryButton], the button this callback responds to.
   ///  * [DragDownDetails], which is passed as an argument to this callback.
   GestureDragDownCallback onDown;

   /// A pointer has contacted the screen with a primary button and has begun to
   /// move.
   ///
   /// The position of the pointer is provided in the callback's `details`
   /// argument, which is a [DragStartDetails] object.
   ///
   /// Depending on the value of [dragStartBehavior], this function will be
   /// called on the initial touch down, if set to [DragStartBehavior.down] or
   /// when the drag gesture is first detected, if set to
   /// [DragStartBehavior.start].
   ///
   /// See also:
   ///
   ///  * [kPrimaryButton], the button this callback responds to.
   ///  * [DragStartDetails], which is passed as an argument to this callback.
   GestureDragStartCallback onStart;

   /// A pointer that is in contact with the screen with a primary button and
   /// moving has moved again.
   ///
   /// The distance traveled by the pointer since the last update is provided in
   /// the callback's `details` argument, which is a [DragUpdateDetails] object.
   ///
   /// See also:
   ///
   ///  * [kPrimaryButton], the button this callback responds to.
   ///  * [DragUpdateDetails], which is passed as an argument to this callback.
   GestureDragUpdateCallback onUpdate;

   /// A pointer that was previously in contact with the screen with a primary
   /// button and moving is no longer in contact with the screen and was moving
   /// at a specific velocity when it stopped contacting the screen.
   ///
   /// The velocity is provided in the callback's `details` argument, which is a
   /// [DragEndDetails] object.
   ///
   /// See also:
   ///
   ///  * [kPrimaryButton], the button this callback responds to.
   ///  * [DragEndDetails], which is passed as an argument to this callback.
   GestureDragEndCallback onEnd;

   /// The pointer that previously triggered [onDown] did not complete.
   ///
   /// See also:
   ///
   ///  * [kPrimaryButton], the button this callback responds to.
   GestureDragCancelCallback onCancel;

   /// The minimum distance an input pointer drag must have moved to
   /// to be considered a fling gesture.
   ///
   /// This value is typically compared with the distance traveled along the
   /// scrolling axis. If null then [kTouchSlop] is used.
   double minFlingDistance;

   /// The minimum velocity for an input pointer drag to be considered fling.
   ///
   /// This value is typically compared with the magnitude of fling gesture's
   /// velocity along the scrolling axis. If null then [kMinFlingVelocity]
   /// is used.
   double minFlingVelocity;

   /// Fling velocity magnitudes will be clamped to this value.
   ///
   /// If null then [kMaxFlingVelocity] is used.
   double maxFlingVelocity;

   _DragState _state = _DragState.ready;
   OffsetPair _initialPosition;
   OffsetPair _pendingDragOffset;
   Duration _lastPendingEventTimestamp;
   // The buttons sent by `PointerDownEvent`. If a `PointerMoveEvent` comes with a
   // different set of buttons, the gesture is canceled.
   int _initialButtons;
   Matrix4 _lastTransform;

   /// Distance moved in the global coordinate space of the screen in drag direction.
   ///
   /// If drag is only allowed along a defined axis, this value may be negative to
   /// differentiate the direction of the drag.
   double _globalDistanceMoved;

   /// Determines if a gesture is a fling or not based on velocity.
   ///
   /// A fling calls its gesture end callback with a velocity, allowing the
   /// provider of the callback to respond by carrying the gesture forward with
   /// inertia, for example.
   bool isFlingGesture(VelocityEstimate estimate);

   Offset _getDeltaForDetails(Offset delta);
   double _getPrimaryValueFromOffset(Offset value);
   bool get _hasSufficientGlobalDistanceToAccept;

   final Map<int, VelocityTracker> _velocityTrackers = <int, VelocityTracker>{};

   @override
   bool isPointerAllowed(PointerEvent event) {
     if (_initialButtons == null) {
       switch (event.buttons) {
         case kPrimaryButton:
           if (onDown == null &&
               onStart == null &&
               onUpdate == null &&
               onEnd == null &&
               onCancel == null)
             return false;
           break;
         default:
           return false;
       }
     } else {
       // There can be multiple drags simultaneously. Their effects are combined.
       if (event.buttons != _initialButtons) {
         return false;
       }
     }
     return super.isPointerAllowed(event as PointerDownEvent);
   }

   @override
   void addAllowedPointer(PointerEvent event) {
     startTrackingPointer(event.pointer, event.transform);
     _velocityTrackers[event.pointer] = VelocityTracker();
     if (_state == _DragState.ready) {
       _state = _DragState.possible;
       _initialPosition = OffsetPair(global: event.position, local: event.localPosition);
       _initialButtons = event.buttons;
       _pendingDragOffset = OffsetPair.zero;
       _globalDistanceMoved = 0.0;
       _lastPendingEventTimestamp = event.timeStamp;
       _lastTransform = event.transform;
       _checkDown();
     } else if (_state == _DragState.accepted) {
       resolve(GestureDisposition.accepted);
     }
   }

   @override
   void handleEvent(PointerEvent event) {
     assert(_state != _DragState.ready);
     if (!event.synthesized
         && (event is PointerDownEvent || event is PointerMoveEvent)) {
       final VelocityTracker tracker = _velocityTrackers[event.pointer];
       assert(tracker != null);
       tracker.addPosition(event.timeStamp, event.localPosition);
     }

     if (event is PointerMoveEvent) {
       if (event.buttons != _initialButtons) {
         _giveUpPointer(event.pointer);
         return;
       }
       if (_state == _DragState.accepted) {
         _checkUpdate(
           sourceTimeStamp: event.timeStamp,
           delta: _getDeltaForDetails(event.localDelta),
           primaryDelta: _getPrimaryValueFromOffset(event.localDelta),
           globalPosition: event.position,
           localPosition: event.localPosition,
         );
       } else {
         _pendingDragOffset += OffsetPair(local: event.localDelta, global: event.delta);
         _lastPendingEventTimestamp = event.timeStamp;
         _lastTransform = event.transform;
         final Offset movedLocally = _getDeltaForDetails(event.localDelta);
         final Matrix4 localToGlobalTransform = event.transform == null ? null : Matrix4.tryInvert(event.transform);
         _globalDistanceMoved += PointerEvent.transformDeltaViaPositions(
           transform: localToGlobalTransform,
           untransformedDelta: movedLocally,
           untransformedEndPosition: event.localPosition,
         ).distance * (_getPrimaryValueFromOffset(movedLocally) ?? 1).sign;
         if (_hasSufficientGlobalDistanceToAccept)
           resolve(GestureDisposition.accepted);
       }
     }
     if (event is PointerUpEvent || event is PointerCancelEvent) {
       _giveUpPointer(
         event.pointer,
         reject: event is PointerCancelEvent || _state ==_DragState.possible,
       );
     }
   }

   @override
   void acceptGesture(int pointer) {
     if (_state != _DragState.accepted) {
       _state = _DragState.accepted;
       final OffsetPair delta = _pendingDragOffset;
       final Duration timestamp = _lastPendingEventTimestamp;
       final Matrix4 transform = _lastTransform;
       Offset localUpdateDelta;
       switch (dragStartBehavior) {
         case DragStartBehavior.start:
           _initialPosition = _initialPosition + delta;
           localUpdateDelta = Offset.zero;
           break;
         case DragStartBehavior.down:
           localUpdateDelta = _getDeltaForDetails(delta.local);
           break;
       }
       _pendingDragOffset = OffsetPair.zero;
       _lastPendingEventTimestamp = null;
       _lastTransform = null;
       _checkStart(timestamp);
       if (localUpdateDelta != Offset.zero && onUpdate != null) {
         final Matrix4 localToGlobal = transform != null ? Matrix4.tryInvert(transform) : null;
         final Offset correctedLocalPosition = _initialPosition.local + localUpdateDelta;
         final Offset globalUpdateDelta = PointerEvent.transformDeltaViaPositions(
           untransformedEndPosition: correctedLocalPosition,
           untransformedDelta: localUpdateDelta,
           transform: localToGlobal,
         );
         final OffsetPair updateDelta = OffsetPair(local: localUpdateDelta, global: globalUpdateDelta);
         final OffsetPair correctedPosition = _initialPosition + updateDelta; // Only adds delta for down behaviour
         _checkUpdate(
           sourceTimeStamp: timestamp,
           delta: localUpdateDelta,
           primaryDelta: _getPrimaryValueFromOffset(localUpdateDelta),
           globalPosition: correctedPosition.global,
           localPosition: correctedPosition.local,
         );
       }
     }
   }

   @override
   void rejectGesture(int pointer) {
     _giveUpPointer(pointer);
   }

   @override
   void didStopTrackingLastPointer(int pointer) {
     assert(_state != _DragState.ready);
     switch(_state) {
       case _DragState.ready:
         break;

       case _DragState.possible:
         resolve(GestureDisposition.rejected);
         _checkCancel();
         break;

       case _DragState.accepted:
         _checkEnd(pointer);
         break;
     }
     _velocityTrackers.clear();
     _initialButtons = null;
     _state = _DragState.ready;
   }

   void _giveUpPointer(int pointer, {bool reject = true}) {
     stopTrackingPointer(pointer);
     if (reject) {
       if (_velocityTrackers.containsKey(pointer)) {
         _velocityTrackers.remove(pointer);
         resolvePointer(pointer, GestureDisposition.rejected);
       }
     }
   }

   void _checkDown() {
     assert(_initialButtons == kPrimaryButton);
     final DragDownDetails details = DragDownDetails(
       globalPosition: _initialPosition.global,
       localPosition: _initialPosition.local,
     );
     if (onDown != null)
       invokeCallback<void>('onDown', () => onDown(details));
   }

   void _checkStart(Duration timestamp) {
     assert(_initialButtons == kPrimaryButton);
     final DragStartDetails details = DragStartDetails(
       sourceTimeStamp: timestamp,
       globalPosition: _initialPosition.global,
       localPosition: _initialPosition.local,
     );
     if (onStart != null)
       invokeCallback<void>('onStart', () => onStart(details));
   }

   void _checkUpdate({
     Duration sourceTimeStamp,
     Offset delta,
     double primaryDelta,
     Offset globalPosition,
     Offset localPosition,
   }) {
     assert(_initialButtons == kPrimaryButton);
     final DragUpdateDetails details = DragUpdateDetails(
       sourceTimeStamp: sourceTimeStamp,
       delta: delta,
       primaryDelta: primaryDelta,
       globalPosition: globalPosition,
       localPosition: localPosition,
     );
     if (onUpdate != null)
       invokeCallback<void>('onUpdate', () => onUpdate(details));
   }

   void _checkEnd(int pointer) {
     assert(_initialButtons == kPrimaryButton);
     if (onEnd == null)
       return;

     final VelocityTracker tracker = _velocityTrackers[pointer];
     assert(tracker != null);

     DragEndDetails details;
     String Function() debugReport;

     final VelocityEstimate estimate = tracker.getVelocityEstimate();
     if (estimate != null && isFlingGesture(estimate)) {
       final Velocity velocity = Velocity(pixelsPerSecond: estimate.pixelsPerSecond)
         .clampMagnitude(minFlingVelocity ?? kMinFlingVelocity, maxFlingVelocity ?? kMaxFlingVelocity);
       details = DragEndDetails(
         velocity: velocity,
         primaryVelocity: _getPrimaryValueFromOffset(velocity.pixelsPerSecond),
       );
       debugReport = () {
         return '$estimate; fling at $velocity.';
       };
     } else {
       details = DragEndDetails(
         velocity: Velocity.zero,
         primaryVelocity: 0.0,
       );
       debugReport = () {
         if (estimate == null)
           return 'Could not estimate velocity.';
         return '$estimate; judged to not be a fling.';
       };
     }
     invokeCallback<void>('onEnd', () => onEnd(details), debugReport: debugReport);
   }

   void _checkCancel() {
     assert(_initialButtons == kPrimaryButton);
     if (onCancel != null)
       invokeCallback<void>('onCancel', onCancel);
   }

   @override
   void dispose() {
     _velocityTrackers.clear();
     super.dispose();
   }
   @override
   void debugFillProperties(DiagnosticPropertiesBuilder properties) {
     super.debugFillProperties(properties);
     properties.add(EnumProperty<DragStartBehavior>('start behavior', dragStartBehavior));
   }
 }

 /// Recognizes movement in the vertical direction.
 ///
 /// Used for vertical scrolling.
 ///
 /// See also:
 ///
 ///  * [HorizontalDragGestureRecognizer], for a similar recognizer but for
 ///    horizontal movement.
 ///  * [MultiDragGestureRecognizer], for a family of gesture recognizers that
 ///    track each touch point independently.
 class VerticalDragGestureRecognizer extends DragGestureRecognizer {
   /// Create a gesture recognizer for interactions in the vertical axis.
   ///
   /// {@macro flutter.gestures.gestureRecognizer.kind}
   VerticalDragGestureRecognizer({
     Object debugOwner,
     PointerDeviceKind kind,
   }) : super(debugOwner: debugOwner, kind: kind);

   @override
   bool isFlingGesture(VelocityEstimate estimate) {
     final double minVelocity = minFlingVelocity ?? kMinFlingVelocity;
     final double minDistance = minFlingDistance ?? kTouchSlop;
     return estimate.pixelsPerSecond.dy.abs() > minVelocity && estimate.offset.dy.abs() > minDistance;
   }

   @override
   bool get _hasSufficientGlobalDistanceToAccept => _globalDistanceMoved.abs() > kTouchSlop;

   @override
   Offset _getDeltaForDetails(Offset delta) => Offset(0.0, delta.dy);

   @override
   double _getPrimaryValueFromOffset(Offset value) => value.dy;

   @override
   String get debugDescription => 'vertical drag';
 }

 /// Recognizes movement in the horizontal direction.
 ///
 /// Used for horizontal scrolling.
 ///
 /// See also:
 ///
 ///  * [VerticalDragGestureRecognizer], for a similar recognizer but for
 ///    vertical movement.
 ///  * [MultiDragGestureRecognizer], for a family of gesture recognizers that
 ///    track each touch point independently.
 class HorizontalDragGestureRecognizer extends DragGestureRecognizer {
   /// Create a gesture recognizer for interactions in the horizontal axis.
   ///
   /// {@macro flutter.gestures.gestureRecognizer.kind}
   HorizontalDragGestureRecognizer({
     Object debugOwner,
     PointerDeviceKind kind,
   }) : super(debugOwner: debugOwner, kind: kind);

   @override
   bool isFlingGesture(VelocityEstimate estimate) {
     final double minVelocity = minFlingVelocity ?? kMinFlingVelocity;
     final double minDistance = minFlingDistance ?? kTouchSlop;
     return estimate.pixelsPerSecond.dx.abs() > minVelocity && estimate.offset.dx.abs() > minDistance;
   }

   @override
   bool get _hasSufficientGlobalDistanceToAccept => _globalDistanceMoved.abs() > kTouchSlop;

   @override
   Offset _getDeltaForDetails(Offset delta) => Offset(delta.dx, 0.0);

   @override
   double _getPrimaryValueFromOffset(Offset value) => value.dx;

   @override
   String get debugDescription => 'horizontal drag';
 }

 /// Recognizes movement both horizontally and vertically.
 ///
 /// See also:
 ///
 ///  * [ImmediateMultiDragGestureRecognizer], for a similar recognizer that
 ///    tracks each touch point independently.
 ///  * [DelayedMultiDragGestureRecognizer], for a similar recognizer that
 ///    tracks each touch point independently, but that doesn't start until
 ///    some time has passed.
 class PanGestureRecognizer extends DragGestureRecognizer {
   /// Create a gesture recognizer for tracking movement on a plane.
   PanGestureRecognizer({ Object debugOwner }) : super(debugOwner: debugOwner);

   @override
   bool isFlingGesture(VelocityEstimate estimate) {
     final double minVelocity = minFlingVelocity ?? kMinFlingVelocity;
     final double minDistance = minFlingDistance ?? kTouchSlop;
     return estimate.pixelsPerSecond.distanceSquared > minVelocity * minVelocity
         && estimate.offset.distanceSquared > minDistance * minDistance;
   }

   @override
   bool get _hasSufficientGlobalDistanceToAccept {
     return _globalDistanceMoved.abs() > kPanSlop;
   }

   @override
   Offset _getDeltaForDetails(Offset delta) => delta;

   @override
   double _getPrimaryValueFromOffset(Offset value) => null;

   @override
   String get debugDescription => 'pan';
 }
	// 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 'package:flutter/foundation.dart';
	import 'package:vector_math/vector_math_64.dart';

	import 'arena.dart';
	import 'constants.dart';
	import 'drag_details.dart';
	import 'events.dart';
	import 'recognizer.dart';
	import 'velocity_tracker.dart';

	enum _DragState {
	ready,
	possible,
	accepted,
	}

	/// Signature for when a pointer that was previously in contact with the screen
	/// and moving is no longer in contact with the screen.
	///
	/// The velocity at which the pointer was moving when it stopped contacting
	/// the screen is available in the `details`.
	///
	/// See [DragGestureRecognizer.onEnd].
	typedef GestureDragEndCallback = void Function(DragEndDetails details);

	/// Signature for when the pointer that previously triggered a
	/// [GestureDragDownCallback] did not complete.
	///
	/// See [DragGestureRecognizer.onCancel].
	typedef GestureDragCancelCallback = void Function();

	/// Recognizes movement.
	///
	/// In contrast to [MultiDragGestureRecognizer], [DragGestureRecognizer]
	/// recognizes a single gesture sequence for all the pointers it watches, which
	/// means that the recognizer has at most one drag sequence active at any given
	/// time regardless of how many pointers are in contact with the screen.
	///
	/// [DragGestureRecognizer] is not intended to be used directly. Instead,
	/// consider using one of its subclasses to recognize specific types for drag
	/// gestures.
	///
	/// [DragGestureRecognizer] competes on pointer events of [kPrimaryButton]
	/// only when it has at least one non-null callback. If it has no callbacks, it
	/// is a no-op.
	///
	/// See also:
	///
	/// * [HorizontalDragGestureRecognizer], for left and right drags.
	/// * [VerticalDragGestureRecognizer], for up and down drags.
	/// * [PanGestureRecognizer], for drags that are not locked to a single axis.
	abstract class DragGestureRecognizer extends OneSequenceGestureRecognizer {
	/// Initialize the object.
	///
	/// [dragStartBehavior] must not be null.
	///
	/// {@macro flutter.gestures.gestureRecognizer.kind}
	DragGestureRecognizer({
	Object debugOwner,
	PointerDeviceKind kind,
	this.dragStartBehavior = DragStartBehavior.start,
	}) : assert(dragStartBehavior != null),
	super(debugOwner: debugOwner, kind: kind);

	/// Configure the behavior of offsets sent to [onStart].
	///
	/// If set to [DragStartBehavior.start], the [onStart] callback will be called
	/// at the time and position when this gesture recognizer wins the arena. If
	/// [DragStartBehavior.down], [onStart] will be called at the time and
	/// position when a down event was first detected.
	///
	/// For more information about the gesture arena:
	/// https://flutter.dev/docs/development/ui/advanced/gestures#gesture-disambiguation
	///
	/// By default, the drag start behavior is [DragStartBehavior.start].
	///
	/// ## Example:
	///
	/// A finger presses down on the screen with offset (500.0, 500.0), and then
	/// moves to position (510.0, 500.0) before winning the arena. With
	/// [dragStartBehavior] set to [DragStartBehavior.down], the [onStart]
	/// callback will be called at the time corresponding to the touch's position
	/// at (500.0, 500.0). If it is instead set to [DragStartBehavior.start],
	/// [onStart] will be called at the time corresponding to the touch's position
	/// at (510.0, 500.0).
	DragStartBehavior dragStartBehavior;

	/// A pointer has contacted the screen with a primary button and might begin
	/// to move.
	///
	/// The position of the pointer is provided in the callback's `details`
	/// argument, which is a [DragDownDetails] object.
	///
	/// See also:
	///
	/// * [kPrimaryButton], the button this callback responds to.
	/// * [DragDownDetails], which is passed as an argument to this callback.
	GestureDragDownCallback onDown;

	/// A pointer has contacted the screen with a primary button and has begun to
	/// move.
	///
	/// The position of the pointer is provided in the callback's `details`
	/// argument, which is a [DragStartDetails] object.
	///
	/// Depending on the value of [dragStartBehavior], this function will be
	/// called on the initial touch down, if set to [DragStartBehavior.down] or
	/// when the drag gesture is first detected, if set to
	/// [DragStartBehavior.start].
	///
	/// See also:
	///
	/// * [kPrimaryButton], the button this callback responds to.
	/// * [DragStartDetails], which is passed as an argument to this callback.
	GestureDragStartCallback onStart;

	/// A pointer that is in contact with the screen with a primary button and
	/// moving has moved again.
	///
	/// The distance traveled by the pointer since the last update is provided in
	/// the callback's `details` argument, which is a [DragUpdateDetails] object.
	///
	/// See also:
	///
	/// * [kPrimaryButton], the button this callback responds to.
	/// * [DragUpdateDetails], which is passed as an argument to this callback.
	GestureDragUpdateCallback onUpdate;

	/// A pointer that was previously in contact with the screen with a primary
	/// button and moving is no longer in contact with the screen and was moving
	/// at a specific velocity when it stopped contacting the screen.
	///
	/// The velocity is provided in the callback's `details` argument, which is a
	/// [DragEndDetails] object.
	///
	/// See also:
	///
	/// * [kPrimaryButton], the button this callback responds to.
	/// * [DragEndDetails], which is passed as an argument to this callback.
	GestureDragEndCallback onEnd;

	/// The pointer that previously triggered [onDown] did not complete.
	///
	/// See also:
	///
	/// * [kPrimaryButton], the button this callback responds to.
	GestureDragCancelCallback onCancel;

	/// The minimum distance an input pointer drag must have moved to
	/// to be considered a fling gesture.
	///
	/// This value is typically compared with the distance traveled along the
	/// scrolling axis. If null then [kTouchSlop] is used.
	double minFlingDistance;

	/// The minimum velocity for an input pointer drag to be considered fling.
	///
	/// This value is typically compared with the magnitude of fling gesture's
	/// velocity along the scrolling axis. If null then [kMinFlingVelocity]
	/// is used.
	double minFlingVelocity;

	/// Fling velocity magnitudes will be clamped to this value.
	///
	/// If null then [kMaxFlingVelocity] is used.
	double maxFlingVelocity;

	_DragState _state = _DragState.ready;
	OffsetPair _initialPosition;
	OffsetPair _pendingDragOffset;
	Duration _lastPendingEventTimestamp;
	// The buttons sent by `PointerDownEvent`. If a `PointerMoveEvent` comes with a
	// different set of buttons, the gesture is canceled.
	int _initialButtons;
	Matrix4 _lastTransform;

	/// Distance moved in the global coordinate space of the screen in drag direction.
	///
	/// If drag is only allowed along a defined axis, this value may be negative to
	/// differentiate the direction of the drag.
	double _globalDistanceMoved;

	/// Determines if a gesture is a fling or not based on velocity.
	///
	/// A fling calls its gesture end callback with a velocity, allowing the
	/// provider of the callback to respond by carrying the gesture forward with
	/// inertia, for example.
	bool isFlingGesture(VelocityEstimate estimate);

	Offset _getDeltaForDetails(Offset delta);
	double _getPrimaryValueFromOffset(Offset value);
	bool get _hasSufficientGlobalDistanceToAccept;

	final Map<int, VelocityTracker> _velocityTrackers = <int, VelocityTracker>{};

	@override
	bool isPointerAllowed(PointerEvent event) {
	if (_initialButtons == null) {
	switch (event.buttons) {
	case kPrimaryButton:
	if (onDown == null &&
	onStart == null &&
	onUpdate == null &&
	onEnd == null &&
	onCancel == null)
	return false;
	break;
	default:
	return false;
	}
	} else {
	// There can be multiple drags simultaneously. Their effects are combined.
	if (event.buttons != _initialButtons) {
	return false;
	}
	}
	return super.isPointerAllowed(event as PointerDownEvent);
	}

	@override
	void addAllowedPointer(PointerEvent event) {
	startTrackingPointer(event.pointer, event.transform);
	_velocityTrackers[event.pointer] = VelocityTracker();
	if (_state == _DragState.ready) {
	_state = _DragState.possible;
	_initialPosition = OffsetPair(global: event.position, local: event.localPosition);
	_initialButtons = event.buttons;
	_pendingDragOffset = OffsetPair.zero;
	_globalDistanceMoved = 0.0;
	_lastPendingEventTimestamp = event.timeStamp;
	_lastTransform = event.transform;
	_checkDown();
	} else if (_state == _DragState.accepted) {
	resolve(GestureDisposition.accepted);
	}
	}

	@override
	void handleEvent(PointerEvent event) {
	assert(_state != _DragState.ready);
	if (!event.synthesized
	&& (event is PointerDownEvent \|\| event is PointerMoveEvent)) {
	final VelocityTracker tracker = _velocityTrackers[event.pointer];
	assert(tracker != null);
	tracker.addPosition(event.timeStamp, event.localPosition);
	}

	if (event is PointerMoveEvent) {
	if (event.buttons != _initialButtons) {
	_giveUpPointer(event.pointer);
	return;
	}
	if (_state == _DragState.accepted) {
	_checkUpdate(
	sourceTimeStamp: event.timeStamp,
	delta: _getDeltaForDetails(event.localDelta),
	primaryDelta: _getPrimaryValueFromOffset(event.localDelta),
	globalPosition: event.position,
	localPosition: event.localPosition,
	);
	} else {
	_pendingDragOffset += OffsetPair(local: event.localDelta, global: event.delta);
	_lastPendingEventTimestamp = event.timeStamp;
	_lastTransform = event.transform;
	final Offset movedLocally = _getDeltaForDetails(event.localDelta);
	final Matrix4 localToGlobalTransform = event.transform == null ? null : Matrix4.tryInvert(event.transform);
	_globalDistanceMoved += PointerEvent.transformDeltaViaPositions(
	transform: localToGlobalTransform,
	untransformedDelta: movedLocally,
	untransformedEndPosition: event.localPosition,
	).distance * (_getPrimaryValueFromOffset(movedLocally) ?? 1).sign;
	if (_hasSufficientGlobalDistanceToAccept)
	resolve(GestureDisposition.accepted);
	}
	}
	if (event is PointerUpEvent \|\| event is PointerCancelEvent) {
	_giveUpPointer(
	event.pointer,
	reject: event is PointerCancelEvent \|\| _state ==_DragState.possible,
	);
	}
	}

	@override
	void acceptGesture(int pointer) {
	if (_state != _DragState.accepted) {
	_state = _DragState.accepted;
	final OffsetPair delta = _pendingDragOffset;
	final Duration timestamp = _lastPendingEventTimestamp;
	final Matrix4 transform = _lastTransform;
	Offset localUpdateDelta;
	switch (dragStartBehavior) {
	case DragStartBehavior.start:
	_initialPosition = _initialPosition + delta;
	localUpdateDelta = Offset.zero;
	break;
	case DragStartBehavior.down:
	localUpdateDelta = _getDeltaForDetails(delta.local);
	break;
	}
	_pendingDragOffset = OffsetPair.zero;
	_lastPendingEventTimestamp = null;
	_lastTransform = null;
	_checkStart(timestamp);
	if (localUpdateDelta != Offset.zero && onUpdate != null) {
	final Matrix4 localToGlobal = transform != null ? Matrix4.tryInvert(transform) : null;
	final Offset correctedLocalPosition = _initialPosition.local + localUpdateDelta;
	final Offset globalUpdateDelta = PointerEvent.transformDeltaViaPositions(
	untransformedEndPosition: correctedLocalPosition,
	untransformedDelta: localUpdateDelta,
	transform: localToGlobal,
	);
	final OffsetPair updateDelta = OffsetPair(local: localUpdateDelta, global: globalUpdateDelta);
	final OffsetPair correctedPosition = _initialPosition + updateDelta; // Only adds delta for down behaviour
	_checkUpdate(
	sourceTimeStamp: timestamp,
	delta: localUpdateDelta,
	primaryDelta: _getPrimaryValueFromOffset(localUpdateDelta),
	globalPosition: correctedPosition.global,
	localPosition: correctedPosition.local,
	);
	}
	}
	}

	@override
	void rejectGesture(int pointer) {
	_giveUpPointer(pointer);
	}

	@override
	void didStopTrackingLastPointer(int pointer) {
	assert(_state != _DragState.ready);
	switch(_state) {
	case _DragState.ready:
	break;

	case _DragState.possible:
	resolve(GestureDisposition.rejected);
	_checkCancel();
	break;

	case _DragState.accepted:
	_checkEnd(pointer);
	break;
	}
	_velocityTrackers.clear();
	_initialButtons = null;
	_state = _DragState.ready;
	}

	void _giveUpPointer(int pointer, {bool reject = true}) {
	stopTrackingPointer(pointer);
	if (reject) {
	if (_velocityTrackers.containsKey(pointer)) {
	_velocityTrackers.remove(pointer);
	resolvePointer(pointer, GestureDisposition.rejected);
	}
	}
	}

	void _checkDown() {
	assert(_initialButtons == kPrimaryButton);
	final DragDownDetails details = DragDownDetails(
	globalPosition: _initialPosition.global,
	localPosition: _initialPosition.local,
	);
	if (onDown != null)
	invokeCallback<void>('onDown', () => onDown(details));
	}

	void _checkStart(Duration timestamp) {
	assert(_initialButtons == kPrimaryButton);
	final DragStartDetails details = DragStartDetails(
	sourceTimeStamp: timestamp,
	globalPosition: _initialPosition.global,
	localPosition: _initialPosition.local,
	);
	if (onStart != null)
	invokeCallback<void>('onStart', () => onStart(details));
	}

	void _checkUpdate({
	Duration sourceTimeStamp,
	Offset delta,
	double primaryDelta,
	Offset globalPosition,
	Offset localPosition,
	}) {
	assert(_initialButtons == kPrimaryButton);
	final DragUpdateDetails details = DragUpdateDetails(
	sourceTimeStamp: sourceTimeStamp,
	delta: delta,
	primaryDelta: primaryDelta,
	globalPosition: globalPosition,
	localPosition: localPosition,
	);
	if (onUpdate != null)
	invokeCallback<void>('onUpdate', () => onUpdate(details));
	}

	void _checkEnd(int pointer) {
	assert(_initialButtons == kPrimaryButton);
	if (onEnd == null)
	return;

	final VelocityTracker tracker = _velocityTrackers[pointer];
	assert(tracker != null);

	DragEndDetails details;
	String Function() debugReport;

	final VelocityEstimate estimate = tracker.getVelocityEstimate();
	if (estimate != null && isFlingGesture(estimate)) {
	final Velocity velocity = Velocity(pixelsPerSecond: estimate.pixelsPerSecond)
	.clampMagnitude(minFlingVelocity ?? kMinFlingVelocity, maxFlingVelocity ?? kMaxFlingVelocity);
	details = DragEndDetails(
	velocity: velocity,
	primaryVelocity: _getPrimaryValueFromOffset(velocity.pixelsPerSecond),
	);
	debugReport = () {
	return '$estimate; fling at $velocity.';
	};
	} else {
	details = DragEndDetails(
	velocity: Velocity.zero,
	primaryVelocity: 0.0,
	);
	debugReport = () {
	if (estimate == null)
	return 'Could not estimate velocity.';
	return '$estimate; judged to not be a fling.';
	};
	}
	invokeCallback<void>('onEnd', () => onEnd(details), debugReport: debugReport);
	}

	void _checkCancel() {
	assert(_initialButtons == kPrimaryButton);
	if (onCancel != null)
	invokeCallback<void>('onCancel', onCancel);
	}

	@override
	void dispose() {
	_velocityTrackers.clear();
	super.dispose();
	}
	@override
	void debugFillProperties(DiagnosticPropertiesBuilder properties) {
	super.debugFillProperties(properties);
	properties.add(EnumProperty<DragStartBehavior>('start behavior', dragStartBehavior));
	}
	}

	/// Recognizes movement in the vertical direction.
	///
	/// Used for vertical scrolling.
	///
	/// See also:
	///
	/// * [HorizontalDragGestureRecognizer], for a similar recognizer but for
	/// horizontal movement.
	/// * [MultiDragGestureRecognizer], for a family of gesture recognizers that
	/// track each touch point independently.
	class VerticalDragGestureRecognizer extends DragGestureRecognizer {
	/// Create a gesture recognizer for interactions in the vertical axis.
	///
	/// {@macro flutter.gestures.gestureRecognizer.kind}
	VerticalDragGestureRecognizer({
	Object debugOwner,
	PointerDeviceKind kind,
	}) : super(debugOwner: debugOwner, kind: kind);

	@override
	bool isFlingGesture(VelocityEstimate estimate) {
	final double minVelocity = minFlingVelocity ?? kMinFlingVelocity;
	final double minDistance = minFlingDistance ?? kTouchSlop;
	return estimate.pixelsPerSecond.dy.abs() > minVelocity && estimate.offset.dy.abs() > minDistance;
	}

	@override
	bool get _hasSufficientGlobalDistanceToAccept => _globalDistanceMoved.abs() > kTouchSlop;

	@override
	Offset _getDeltaForDetails(Offset delta) => Offset(0.0, delta.dy);

	@override
	double _getPrimaryValueFromOffset(Offset value) => value.dy;

	@override
	String get debugDescription => 'vertical drag';
	}

	/// Recognizes movement in the horizontal direction.
	///
	/// Used for horizontal scrolling.
	///
	/// See also:
	///
	/// * [VerticalDragGestureRecognizer], for a similar recognizer but for
	/// vertical movement.
	/// * [MultiDragGestureRecognizer], for a family of gesture recognizers that
	/// track each touch point independently.
	class HorizontalDragGestureRecognizer extends DragGestureRecognizer {
	/// Create a gesture recognizer for interactions in the horizontal axis.
	///
	/// {@macro flutter.gestures.gestureRecognizer.kind}
	HorizontalDragGestureRecognizer({
	Object debugOwner,
	PointerDeviceKind kind,
	}) : super(debugOwner: debugOwner, kind: kind);

	@override
	bool isFlingGesture(VelocityEstimate estimate) {
	final double minVelocity = minFlingVelocity ?? kMinFlingVelocity;
	final double minDistance = minFlingDistance ?? kTouchSlop;
	return estimate.pixelsPerSecond.dx.abs() > minVelocity && estimate.offset.dx.abs() > minDistance;
	}

	@override
	bool get _hasSufficientGlobalDistanceToAccept => _globalDistanceMoved.abs() > kTouchSlop;

	@override
	Offset _getDeltaForDetails(Offset delta) => Offset(delta.dx, 0.0);

	@override
	double _getPrimaryValueFromOffset(Offset value) => value.dx;

	@override
	String get debugDescription => 'horizontal drag';
	}

	/// Recognizes movement both horizontally and vertically.
	///
	/// See also:
	///
	/// * [ImmediateMultiDragGestureRecognizer], for a similar recognizer that
	/// tracks each touch point independently.
	/// * [DelayedMultiDragGestureRecognizer], for a similar recognizer that
	/// tracks each touch point independently, but that doesn't start until
	/// some time has passed.
	class PanGestureRecognizer extends DragGestureRecognizer {
	/// Create a gesture recognizer for tracking movement on a plane.
	PanGestureRecognizer({ Object debugOwner }) : super(debugOwner: debugOwner);

	@override
	bool isFlingGesture(VelocityEstimate estimate) {
	final double minVelocity = minFlingVelocity ?? kMinFlingVelocity;
	final double minDistance = minFlingDistance ?? kTouchSlop;
	return estimate.pixelsPerSecond.distanceSquared > minVelocity * minVelocity
	&& estimate.offset.distanceSquared > minDistance * minDistance;
	}

	@override
	bool get _hasSufficientGlobalDistanceToAccept {
	return _globalDistanceMoved.abs() > kPanSlop;
	}

	@override
	Offset _getDeltaForDetails(Offset delta) => delta;

	@override
	double _getPrimaryValueFromOffset(Offset value) => null;

	@override
	String get debugDescription => 'pan';
	}