// 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.

/// @docImport 'package:flutter/widgets.dart';
///
/// @docImport 'drag_details.dart';
/// @docImport 'monodrag.dart';
/// @docImport 'multitap.dart';
/// @docImport 'tap.dart';
library;

import 'dart:async';
import 'dart:collection';

import 'package:flutter/foundation.dart';

import 'arena.dart';
import 'binding.dart';
import 'constants.dart';
import 'debug.dart';
import 'events.dart';
import 'pointer_router.dart';
import 'team.dart';

export 'dart:ui' show Offset, PointerDeviceKind;

export 'package:flutter/foundation.dart' show DiagnosticPropertiesBuilder;
export 'package:vector_math/vector_math_64.dart' show Matrix4;

export 'arena.dart' show GestureDisposition;
export 'events.dart' show PointerDownEvent, PointerEvent, PointerPanZoomStartEvent;
export 'gesture_settings.dart' show DeviceGestureSettings;
export 'team.dart' show GestureArenaTeam;

/// Generic signature for callbacks passed to
/// [GestureRecognizer.invokeCallback]. This allows the
/// [GestureRecognizer.invokeCallback] mechanism to be generically used with
/// anonymous functions that return objects of particular types.
typedef RecognizerCallback<T> = T Function();

/// Configuration of offset passed to [DragStartDetails].
///
/// See also:
///
///  * [DragGestureRecognizer.dragStartBehavior], which gives an example for the
///  different behaviors.
enum DragStartBehavior {
  /// Set the initial offset at the position where the first down event was
  /// detected.
  down,

  /// Set the initial position at the position where this gesture recognizer
  /// won the arena.
  start,
}

/// Configuration of multi-finger drag strategy on multi-touch devices.
///
/// When dragging with only one finger, there's no difference in behavior
/// between all the settings.
///
/// Used by [DragGestureRecognizer.multitouchDragStrategy].
enum MultitouchDragStrategy {
  /// Only the latest active pointer is tracked by the recognizer.
  ///
  /// If the tracked pointer is released, the first accepted of the remaining active
  /// pointers will continue to be tracked.
  ///
  /// This is the behavior typically seen on Android.
  latestPointer,

  /// All active pointers will be tracked, and the result is computed from
  /// the boundary pointers.
  ///
  /// The scrolling offset is determined by the maximum deltas of both directions.
  ///
  /// If the user is dragging with 3 pointers at the same time, each having
  /// \[+10, +20, +33\] pixels of offset, the recognizer will report a delta of 33 pixels.
  ///
  /// If the user is dragging with 5 pointers at the same time, each having
  /// \[+10, +20, +33, -1, -12\] pixels of offset, the recognizer will report a
  /// delta of (+33) + (-12) = 21 pixels.
  ///
  /// The panning [PanGestureRecognizer] offset is the average of all pointers.
  ///
  /// If the user is dragging with 3 pointers at the same time, each having
  /// \[+10, +50, -30\] pixels of offset in one direction (horizontal or vertical),
  /// the recognizer will report a delta of (10 + 50 -30) / 3 = 10 pixels in this direction.
  ///
  /// This is the behavior typically seen on iOS.
  averageBoundaryPointers,

  /// All active pointers will be tracked together. The scrolling offset
  /// is the sum of the offsets of all active pointers.
  ///
  /// When a [Scrollable] drives scrolling by this drag strategy, the scrolling
  /// speed will double or triple, depending on how many fingers are dragging
  /// at the same time.
  ///
  /// If the user is dragging with 3 pointers at the same time, each having
  /// \[+10, +20, +33\] pixels of offset, the recognizer will report a delta
  /// of 10 + 20 + 33 = 63 pixels.
  ///
  /// If the user is dragging with 5 pointers at the same time, each having
  /// \[+10, +20, +33, -1, -12\] pixels of offset, the recognizer will report
  /// a delta of 10 + 20 + 33 - 1 - 12 = 50 pixels.
  sumAllPointers,
}

/// Signature for [GestureRecognizer.allowedButtonsFilter].
///
/// Used to filter the input buttons of incoming pointer events.
/// The parameter `buttons` comes from [PointerEvent.buttons].
typedef AllowedButtonsFilter = bool Function(int buttons);

/// The base class that all gesture recognizers inherit from.
///
/// Provides a basic API that can be used by classes that work with
/// gesture recognizers but don't care about the specific details of
/// the gestures recognizers themselves.
///
/// See also:
///
///  * [GestureDetector], the widget that is used to detect built-in gestures.
///  * [RawGestureDetector], the widget that is used to detect custom gestures.
///  * [debugPrintRecognizerCallbacksTrace], a flag that can be set to help
///    debug issues with gesture recognizers.
abstract class GestureRecognizer extends GestureArenaMember with DiagnosticableTreeMixin {
  /// Initializes the gesture recognizer.
  ///
  /// The argument is optional and is only used for debug purposes (e.g. in the
  /// [toString] serialization).
  ///
  /// {@template flutter.gestures.GestureRecognizer.supportedDevices}
  /// It's possible to limit this recognizer to a specific set of [PointerDeviceKind]s
  /// by providing the optional [supportedDevices] argument. If [supportedDevices] is null,
  /// the recognizer will accept pointer events from all device kinds.
  /// {@endtemplate}
  GestureRecognizer({
    this.debugOwner,
    this.supportedDevices,
    this.allowedButtonsFilter = _defaultButtonAcceptBehavior,
  }) {
    // TODO(polina-c): stop duplicating code across disposables
    // https://github.com/flutter/flutter/issues/137435
    if (kFlutterMemoryAllocationsEnabled) {
      FlutterMemoryAllocations.instance.dispatchObjectCreated(
        library: 'package:flutter/gestures.dart',
        className: '$GestureRecognizer',
        object: this,
      );
    }
  }

  /// The recognizer's owner.
  ///
  /// This is used in the [toString] serialization to report the object for which
  /// this gesture recognizer was created, to aid in debugging.
  final Object? debugOwner;

  /// Optional device specific configuration for device gestures that will
  /// take precedence over framework defaults.
  DeviceGestureSettings? gestureSettings;

  /// The kind of devices that are allowed to be recognized as provided by
  /// `supportedDevices` in the constructor, or the currently deprecated `kind`.
  /// These cannot both be set. If both are null, events from all device kinds will be
  /// tracked and recognized.
  Set<PointerDeviceKind>? supportedDevices;

  /// {@template flutter.gestures.multidrag._allowedButtonsFilter}
  /// Called when interaction starts. This limits the dragging behavior
  /// for custom clicks (such as scroll click). Its parameter comes
  /// from [PointerEvent.buttons].
  ///
  /// Due to how [kPrimaryButton], [kSecondaryButton], etc., use integers,
  /// bitwise operations can help filter how buttons are pressed.
  /// For example, if someone simultaneously presses the primary and secondary
  /// buttons, the default behavior will return false. The following code
  /// accepts any button press with primary:
  /// `(int buttons) => buttons & kPrimaryButton != 0`.
  ///
  /// When value is `(int buttons) => false`, allow no interactions.
  /// When value is `(int buttons) => true`, allow all interactions.
  ///
  /// Defaults to all buttons.
  /// {@endtemplate}
  final AllowedButtonsFilter allowedButtonsFilter;

  // The default value for [allowedButtonsFilter].
  // Accept any input.
  static bool _defaultButtonAcceptBehavior(int buttons) => true;

  /// Holds a mapping between pointer IDs and the kind of devices they are
  /// coming from.
  final Map<int, PointerDeviceKind> _pointerToKind = <int, PointerDeviceKind>{};

  /// Registers a new pointer pan/zoom that might be relevant to this gesture
  /// detector.
  ///
  /// A pointer pan/zoom is a stream of events that conveys data covering
  /// pan, zoom, and rotate data from a multi-finger trackpad gesture.
  ///
  /// The owner of this gesture recognizer calls addPointerPanZoom() with the
  /// PointerPanZoomStartEvent of each pointer that should be considered for
  /// this gesture.
  ///
  /// It's the GestureRecognizer's responsibility to then add itself
  /// to the global pointer router (see [PointerRouter]) to receive
  /// subsequent events for this pointer, and to add the pointer to
  /// the global gesture arena manager (see [GestureArenaManager]) to track
  /// that pointer.
  ///
  /// This method is called for each and all pointers being added. In
  /// most cases, you want to override [addAllowedPointerPanZoom] instead.
  void addPointerPanZoom(PointerPanZoomStartEvent event) {
    _pointerToKind[event.pointer] = event.kind;
    if (isPointerPanZoomAllowed(event)) {
      addAllowedPointerPanZoom(event);
    } else {
      handleNonAllowedPointerPanZoom(event);
    }
  }

  /// Registers a new pointer pan/zoom that's been checked to be allowed by this
  /// gesture recognizer.
  ///
  /// Subclasses of [GestureRecognizer] are supposed to override this method
  /// instead of [addPointerPanZoom] because [addPointerPanZoom] will be called for each
  /// pointer being added while [addAllowedPointerPanZoom] is only called for pointers
  /// that are allowed by this recognizer.
  @protected
  void addAllowedPointerPanZoom(PointerPanZoomStartEvent event) {}

  /// Registers a new pointer that might be relevant to this gesture
  /// detector.
  ///
  /// The owner of this gesture recognizer calls addPointer() with the
  /// PointerDownEvent of each pointer that should be considered for
  /// this gesture.
  ///
  /// It's the GestureRecognizer's responsibility to then add itself
  /// to the global pointer router (see [PointerRouter]) to receive
  /// subsequent events for this pointer, and to add the pointer to
  /// the global gesture arena manager (see [GestureArenaManager]) to track
  /// that pointer.
  ///
  /// This method is called for each and all pointers being added. In
  /// most cases, you want to override [addAllowedPointer] instead.
  void addPointer(PointerDownEvent event) {
    _pointerToKind[event.pointer] = event.kind;
    if (isPointerAllowed(event)) {
      addAllowedPointer(event);
    } else {
      handleNonAllowedPointer(event);
    }
  }

  /// Registers a new pointer that's been checked to be allowed by this gesture
  /// recognizer.
  ///
  /// Subclasses of [GestureRecognizer] are supposed to override this method
  /// instead of [addPointer] because [addPointer] will be called for each
  /// pointer being added while [addAllowedPointer] is only called for pointers
  /// that are allowed by this recognizer.
  @protected
  void addAllowedPointer(PointerDownEvent event) {}

  /// Handles a pointer being added that's not allowed by this recognizer.
  ///
  /// Subclasses can override this method and reject the gesture.
  ///
  /// See:
  /// - [OneSequenceGestureRecognizer.handleNonAllowedPointer].
  @protected
  void handleNonAllowedPointer(PointerDownEvent event) {}

  /// Checks whether or not a pointer is allowed to be tracked by this recognizer.
  @protected
  bool isPointerAllowed(PointerDownEvent event) {
    return (supportedDevices == null || supportedDevices!.contains(event.kind)) &&
        allowedButtonsFilter(event.buttons);
  }

  /// Handles a pointer pan/zoom being added that's not allowed by this recognizer.
  ///
  /// Subclasses can override this method and reject the gesture.
  @protected
  void handleNonAllowedPointerPanZoom(PointerPanZoomStartEvent event) {}

  /// Checks whether or not a pointer pan/zoom is allowed to be tracked by this recognizer.
  @protected
  bool isPointerPanZoomAllowed(PointerPanZoomStartEvent event) {
    return supportedDevices == null || supportedDevices!.contains(event.kind);
  }

  /// For a given pointer ID, returns the device kind associated with it.
  ///
  /// The pointer ID is expected to be a valid one i.e. an event was received
  /// with that pointer ID.
  @protected
  PointerDeviceKind getKindForPointer(int pointer) {
    assert(_pointerToKind.containsKey(pointer));
    return _pointerToKind[pointer]!;
  }

  /// Releases any resources used by the object.
  ///
  /// This method is called by the owner of this gesture recognizer
  /// when the object is no longer needed (e.g. when a gesture
  /// recognizer is being unregistered from a [GestureDetector], the
  /// GestureDetector widget calls this method).
  @mustCallSuper
  void dispose() {
    // TODO(polina-c): stop duplicating code across disposables
    // https://github.com/flutter/flutter/issues/137435
    if (kFlutterMemoryAllocationsEnabled) {
      FlutterMemoryAllocations.instance.dispatchObjectDisposed(object: this);
    }
  }

  /// Returns a very short pretty description of the gesture that the
  /// recognizer looks for, like 'tap' or 'horizontal drag'.
  String get debugDescription;

  /// Invoke a callback provided by the application, catching and logging any
  /// exceptions.
  ///
  /// The `name` argument is ignored except when reporting exceptions.
  ///
  /// The `debugReport` argument is optional and is used when
  /// [debugPrintRecognizerCallbacksTrace] is true. If specified, it must be a
  /// callback that returns a string describing useful debugging information,
  /// e.g. the arguments passed to the callback.
  @protected
  @pragma('vm:notify-debugger-on-exception')
  T? invokeCallback<T>(
    String name,
    RecognizerCallback<T> callback, {
    String Function()? debugReport,
  }) {
    T? result;
    try {
      assert(() {
        if (debugPrintRecognizerCallbacksTrace) {
          final String? report = debugReport != null ? debugReport() : null;
          // The 19 in the line below is the width of the prefix used by
          // _debugLogDiagnostic in arena.dart.
          final String prefix = debugPrintGestureArenaDiagnostics ? '${' ' * 19}❙ ' : '';
          debugPrint(
            '$prefix$this calling $name callback.${(report?.isNotEmpty ?? false) ? " $report" : ""}',
          );
        }
        return true;
      }());
      result = callback();
    } catch (exception, stack) {
      InformationCollector? collector;
      assert(() {
        collector =
            () => <DiagnosticsNode>[
              StringProperty('Handler', name),
              DiagnosticsProperty<GestureRecognizer>(
                'Recognizer',
                this,
                style: DiagnosticsTreeStyle.errorProperty,
              ),
            ];
        return true;
      }());
      FlutterError.reportError(
        FlutterErrorDetails(
          exception: exception,
          stack: stack,
          library: 'gesture',
          context: ErrorDescription('while handling a gesture'),
          informationCollector: collector,
        ),
      );
    }
    return result;
  }

  @override
  void debugFillProperties(DiagnosticPropertiesBuilder properties) {
    super.debugFillProperties(properties);
    properties.add(DiagnosticsProperty<Object>('debugOwner', debugOwner, defaultValue: null));
  }
}

/// Base class for gesture recognizers that can only recognize one
/// gesture at a time. For example, a single [TapGestureRecognizer]
/// can never recognize two taps happening simultaneously, even if
/// multiple pointers are placed on the same widget.
///
/// This is in contrast to, for instance, [MultiTapGestureRecognizer],
/// which manages each pointer independently and can consider multiple
/// simultaneous touches to each result in a separate tap.
abstract class OneSequenceGestureRecognizer extends GestureRecognizer {
  /// Initialize the object.
  ///
  /// {@macro flutter.gestures.GestureRecognizer.supportedDevices}
  OneSequenceGestureRecognizer({
    super.debugOwner,
    super.supportedDevices,
    super.allowedButtonsFilter,
  });

  final Map<int, GestureArenaEntry> _entries = <int, GestureArenaEntry>{};
  final Set<int> _trackedPointers = HashSet<int>();

  @override
  @protected
  void addAllowedPointer(PointerDownEvent event) {
    startTrackingPointer(event.pointer, event.transform);
  }

  @override
  @protected
  void handleNonAllowedPointer(PointerDownEvent event) {
    resolve(GestureDisposition.rejected);
  }

  /// Called when a pointer event is routed to this recognizer.
  ///
  /// This will be called for every pointer event while the pointer is being
  /// tracked. Typically, this recognizer will start tracking the pointer in
  /// [addAllowedPointer], which means that [handleEvent] will be called
  /// starting with the [PointerDownEvent] that was passed to [addAllowedPointer].
  ///
  /// See also:
  ///
  ///  * [startTrackingPointer], which causes pointer events to be routed to
  ///    this recognizer.
  ///  * [stopTrackingPointer], which stops events from being routed to this
  ///    recognizer.
  ///  * [stopTrackingIfPointerNoLongerDown], which conditionally stops events
  ///    from being routed to this recognizer.
  @protected
  void handleEvent(PointerEvent event);

  @override
  void acceptGesture(int pointer) {}

  @override
  void rejectGesture(int pointer) {}

  /// Called when the number of pointers this recognizer is tracking changes from one to zero.
  ///
  /// The given pointer ID is the ID of the last pointer this recognizer was
  /// tracking.
  @protected
  void didStopTrackingLastPointer(int pointer);

  /// Resolves this recognizer's participation in each gesture arena with the
  /// given disposition.
  @protected
  @mustCallSuper
  void resolve(GestureDisposition disposition) {
    final List<GestureArenaEntry> localEntries = List<GestureArenaEntry>.of(_entries.values);
    _entries.clear();
    for (final GestureArenaEntry entry in localEntries) {
      entry.resolve(disposition);
    }
  }

  /// Resolves this recognizer's participation in the given gesture arena with
  /// the given disposition.
  @protected
  @mustCallSuper
  void resolvePointer(int pointer, GestureDisposition disposition) {
    final GestureArenaEntry? entry = _entries[pointer];
    if (entry != null) {
      _entries.remove(pointer);
      entry.resolve(disposition);
    }
  }

  @override
  void dispose() {
    resolve(GestureDisposition.rejected);
    for (final int pointer in _trackedPointers) {
      GestureBinding.instance.pointerRouter.removeRoute(pointer, handleEvent);
    }
    _trackedPointers.clear();
    assert(_entries.isEmpty);
    super.dispose();
  }

  /// The team that this recognizer belongs to, if any.
  ///
  /// If [team] is null, this recognizer competes directly in the
  /// [GestureArenaManager] to recognize a sequence of pointer events as a
  /// gesture. If [team] is non-null, this recognizer competes in the arena in
  /// a group with other recognizers on the same team.
  ///
  /// A recognizer can be assigned to a team only when it is not participating
  /// in the arena. For example, a common time to assign a recognizer to a team
  /// is shortly after creating the recognizer.
  GestureArenaTeam? get team => _team;
  GestureArenaTeam? _team;

  /// The [team] can only be set once.
  set team(GestureArenaTeam? value) {
    assert(value != null);
    assert(_entries.isEmpty);
    assert(_trackedPointers.isEmpty);
    assert(_team == null);
    _team = value;
  }

  GestureArenaEntry _addPointerToArena(int pointer) {
    return _team?.add(pointer, this) ?? GestureBinding.instance.gestureArena.add(pointer, this);
  }

  /// Causes events related to the given pointer ID to be routed to this recognizer.
  ///
  /// The pointer events are transformed according to `transform` and then delivered
  /// to [handleEvent]. The value for the `transform` argument is usually obtained
  /// from [PointerDownEvent.transform] to transform the events from the global
  /// coordinate space into the coordinate space of the event receiver. It may be
  /// null if no transformation is necessary.
  ///
  /// Use [stopTrackingPointer] to remove the route added by this function.
  ///
  /// This method also adds this recognizer (or its [team] if it's non-null) to
  /// the gesture arena for the specified pointer.
  ///
  /// This is called by [OneSequenceGestureRecognizer.addAllowedPointer].
  @protected
  void startTrackingPointer(int pointer, [Matrix4? transform]) {
    GestureBinding.instance.pointerRouter.addRoute(pointer, handleEvent, transform);
    _trackedPointers.add(pointer);
    // TODO(goderbauer): Enable assert after recognizers properly clean up their defunct `_entries`, see https://github.com/flutter/flutter/issues/117356.
    // assert(!_entries.containsKey(pointer));
    _entries[pointer] = _addPointerToArena(pointer);
  }

  /// Stops events related to the given pointer ID from being routed to this recognizer.
  ///
  /// If this function reduces the number of tracked pointers to zero, it will
  /// call [didStopTrackingLastPointer] synchronously.
  ///
  /// Use [startTrackingPointer] to add the routes in the first place.
  @protected
  void stopTrackingPointer(int pointer) {
    if (_trackedPointers.contains(pointer)) {
      GestureBinding.instance.pointerRouter.removeRoute(pointer, handleEvent);
      _trackedPointers.remove(pointer);
      if (_trackedPointers.isEmpty) {
        didStopTrackingLastPointer(pointer);
      }
    }
  }

  /// Stops tracking the pointer associated with the given event if the event is
  /// a [PointerUpEvent] or a [PointerCancelEvent] event.
  @protected
  void stopTrackingIfPointerNoLongerDown(PointerEvent event) {
    if (event is PointerUpEvent || event is PointerCancelEvent || event is PointerPanZoomEndEvent) {
      stopTrackingPointer(event.pointer);
    }
  }
}

/// The possible states of a [PrimaryPointerGestureRecognizer].
///
/// The recognizer advances from [ready] to [possible] when it starts tracking a
/// primary pointer. Where it advances from there depends on how the gesture is
/// resolved for that pointer:
///
///  * If the primary pointer is resolved by the gesture winning the arena, the
///    recognizer stays in the [possible] state as long as it continues to track
///    a pointer.
///  * If the primary pointer is resolved by the gesture being rejected and
///    losing the arena, the recognizer's state advances to [defunct].
///
/// Once the recognizer has stopped tracking any remaining pointers, the
/// recognizer returns to [ready].
enum GestureRecognizerState {
  /// The recognizer is ready to start recognizing a gesture.
  ready,

  /// The sequence of pointer events seen thus far is consistent with the
  /// gesture the recognizer is attempting to recognize but the gesture has not
  /// been accepted definitively.
  possible,

  /// Further pointer events cannot cause this recognizer to recognize the
  /// gesture until the recognizer returns to the [ready] state (typically when
  /// all the pointers the recognizer is tracking are removed from the screen).
  defunct,
}

/// A base class for gesture recognizers that track a single primary pointer.
///
/// Gestures based on this class will stop tracking the gesture if the primary
/// pointer travels beyond [preAcceptSlopTolerance] or [postAcceptSlopTolerance]
/// pixels from the original contact point of the gesture.
///
/// If the [preAcceptSlopTolerance] was breached before the gesture was accepted
/// in the gesture arena, the gesture will be rejected.
abstract class PrimaryPointerGestureRecognizer extends OneSequenceGestureRecognizer {
  /// Initializes the [deadline] field during construction of subclasses.
  ///
  /// {@macro flutter.gestures.GestureRecognizer.supportedDevices}
  PrimaryPointerGestureRecognizer({
    this.deadline,
    this.preAcceptSlopTolerance = kTouchSlop,
    this.postAcceptSlopTolerance = kTouchSlop,
    super.debugOwner,
    super.supportedDevices,
    super.allowedButtonsFilter,
  }) : assert(
         preAcceptSlopTolerance == null || preAcceptSlopTolerance >= 0,
         'The preAcceptSlopTolerance must be positive or null',
       ),
       assert(
         postAcceptSlopTolerance == null || postAcceptSlopTolerance >= 0,
         'The postAcceptSlopTolerance must be positive or null',
       );

  /// If non-null, the recognizer will call [didExceedDeadline] after this
  /// amount of time has elapsed since starting to track the primary pointer.
  ///
  /// The [didExceedDeadline] will not be called if the primary pointer is
  /// accepted, rejected, or all pointers are up or canceled before [deadline].
  final Duration? deadline;

  /// The maximum distance in logical pixels the gesture is allowed to drift
  /// from the initial touch down position before the gesture is accepted.
  ///
  /// Drifting past the allowed slop amount causes the gesture to be rejected.
  ///
  /// Can be null to indicate that the gesture can drift for any distance.
  /// Defaults to 18 logical pixels.
  final double? preAcceptSlopTolerance;

  /// The maximum distance in logical pixels the gesture is allowed to drift
  /// after the gesture has been accepted.
  ///
  /// Drifting past the allowed slop amount causes the gesture to stop tracking
  /// and signaling subsequent callbacks.
  ///
  /// Can be null to indicate that the gesture can drift for any distance.
  /// Defaults to 18 logical pixels.
  final double? postAcceptSlopTolerance;

  /// The current state of the recognizer.
  ///
  /// See [GestureRecognizerState] for a description of the states.
  GestureRecognizerState get state => _state;
  GestureRecognizerState _state = GestureRecognizerState.ready;

  /// The ID of the primary pointer this recognizer is tracking.
  ///
  /// If this recognizer is no longer tracking any pointers, this field holds
  /// the ID of the primary pointer this recognizer was most recently tracking.
  /// This enables the recognizer to know which pointer it was most recently
  /// tracking when [acceptGesture] or [rejectGesture] is called (which may be
  /// called after the recognizer is no longer tracking a pointer if, e.g.
  /// [GestureArenaManager.hold] has been called, or if there are other
  /// recognizers keeping the arena open).
  int? get primaryPointer => _primaryPointer;
  int? _primaryPointer;

  /// The location at which the primary pointer contacted the screen.
  ///
  /// This will only be non-null while this recognizer is tracking at least
  /// one pointer.
  OffsetPair? get initialPosition => _initialPosition;
  OffsetPair? _initialPosition;

  // Whether this pointer is accepted by winning the arena or as defined by
  // a subclass calling acceptGesture.
  bool _gestureAccepted = false;
  Timer? _timer;

  @override
  void addAllowedPointer(PointerDownEvent event) {
    super.addAllowedPointer(event);
    if (state == GestureRecognizerState.ready) {
      _state = GestureRecognizerState.possible;
      _primaryPointer = event.pointer;
      _initialPosition = OffsetPair(local: event.localPosition, global: event.position);
      if (deadline != null) {
        _timer = Timer(deadline!, () => didExceedDeadlineWithEvent(event));
      }
    }
  }

  @override
  void handleNonAllowedPointer(PointerDownEvent event) {
    if (!_gestureAccepted) {
      super.handleNonAllowedPointer(event);
    }
  }

  @override
  void handleEvent(PointerEvent event) {
    assert(state != GestureRecognizerState.ready);
    if (state == GestureRecognizerState.possible && event.pointer == primaryPointer) {
      final bool isPreAcceptSlopPastTolerance =
          !_gestureAccepted &&
          preAcceptSlopTolerance != null &&
          _getGlobalDistance(event) > preAcceptSlopTolerance!;
      final bool isPostAcceptSlopPastTolerance =
          _gestureAccepted &&
          postAcceptSlopTolerance != null &&
          _getGlobalDistance(event) > postAcceptSlopTolerance!;

      if (event is PointerMoveEvent &&
          (isPreAcceptSlopPastTolerance || isPostAcceptSlopPastTolerance)) {
        resolve(GestureDisposition.rejected);
        stopTrackingPointer(primaryPointer!);
      } else {
        handlePrimaryPointer(event);
      }
    }
    stopTrackingIfPointerNoLongerDown(event);
  }

  /// Override to provide behavior for the primary pointer when the gesture is still possible.
  @protected
  void handlePrimaryPointer(PointerEvent event);

  /// Override to be notified when [deadline] is exceeded.
  ///
  /// You must override this method or [didExceedDeadlineWithEvent] if you
  /// supply a [deadline]. Subclasses that override this method must _not_
  /// call `super.didExceedDeadline()`.
  @protected
  void didExceedDeadline() {
    assert(deadline == null);
  }

  /// Same as [didExceedDeadline] but receives the [event] that initiated the
  /// gesture.
  ///
  /// You must override this method or [didExceedDeadline] if you supply a
  /// [deadline]. Subclasses that override this method must _not_ call
  /// `super.didExceedDeadlineWithEvent(event)`.
  @protected
  void didExceedDeadlineWithEvent(PointerDownEvent event) {
    didExceedDeadline();
  }

  @override
  void acceptGesture(int pointer) {
    if (pointer == primaryPointer) {
      _stopTimer();
      _gestureAccepted = true;
    }
  }

  @override
  void rejectGesture(int pointer) {
    if (pointer == primaryPointer && state == GestureRecognizerState.possible) {
      _stopTimer();
      _state = GestureRecognizerState.defunct;
    }
  }

  @override
  void didStopTrackingLastPointer(int pointer) {
    assert(state != GestureRecognizerState.ready);
    _stopTimer();
    _state = GestureRecognizerState.ready;
    _initialPosition = null;
    _gestureAccepted = false;
  }

  @override
  void dispose() {
    _stopTimer();
    super.dispose();
  }

  void _stopTimer() {
    if (_timer != null) {
      _timer!.cancel();
      _timer = null;
    }
  }

  double _getGlobalDistance(PointerEvent event) {
    final Offset offset = event.position - initialPosition!.global;
    return offset.distance;
  }

  @override
  void debugFillProperties(DiagnosticPropertiesBuilder properties) {
    super.debugFillProperties(properties);
    properties.add(EnumProperty<GestureRecognizerState>('state', state));
  }
}

/// A container for a [local] and [global] [Offset] pair.
///
/// Usually, the [global] [Offset] is in the coordinate space of the screen
/// after conversion to logical pixels and the [local] offset is the same
/// [Offset], but transformed to a local coordinate space.
@immutable
class OffsetPair {
  /// Creates a [OffsetPair] combining a [local] and [global] [Offset].
  const OffsetPair({required this.local, required this.global});

  /// Creates a [OffsetPair] from [PointerEvent.localPosition] and
  /// [PointerEvent.position].
  OffsetPair.fromEventPosition(PointerEvent event)
    : local = event.localPosition,
      global = event.position;

  /// Creates a [OffsetPair] from [PointerEvent.localDelta] and
  /// [PointerEvent.delta].
  OffsetPair.fromEventDelta(PointerEvent event) : local = event.localDelta, global = event.delta;

  /// A [OffsetPair] where both [Offset]s are [Offset.zero].
  static const OffsetPair zero = OffsetPair(local: Offset.zero, global: Offset.zero);

  /// The [Offset] in the local coordinate space.
  final Offset local;

  /// The [Offset] in the global coordinate space after conversion to logical
  /// pixels.
  final Offset global;

  /// Adds the `other.global` to [global] and `other.local` to [local].
  OffsetPair operator +(OffsetPair other) {
    return OffsetPair(local: local + other.local, global: global + other.global);
  }

  /// Subtracts the `other.global` from [global] and `other.local` from [local].
  OffsetPair operator -(OffsetPair other) {
    return OffsetPair(local: local - other.local, global: global - other.global);
  }

  @override
  String toString() => '${objectRuntimeType(this, 'OffsetPair')}(local: $local, global: $global)';
}