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

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.

 import 'dart:async';
 import 'dart:collection';
 import 'dart:ui' show Offset;

 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 'pointer_router.dart' show PointerRouter;

 /// 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 T RecognizerCallback<T>();

 /// 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 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).
   GestureRecognizer({ this.debugOwner });

   /// 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;

   /// 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.
   void addPointer(PointerDownEvent event);

   /// 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() { }

   /// 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
   T invokeCallback<T>(String name, RecognizerCallback<T> callback, { String debugReport() }) {
     assert(callback != null);
     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 == true ? " $report" : "" }');
         }
         return true;
       }());
       result = callback();
     } catch (exception, stack) {
       FlutterError.reportError(new FlutterErrorDetails(
         exception: exception,
         stack: stack,
         library: 'gesture',
         context: 'while handling a gesture',
         informationCollector: (StringBuffer information) {
           information.writeln('Handler: $name');
           information.writeln('Recognizer:');
           information.writeln('  $this');
         }
       ));
     }
     return result;
   }

   @override
   void debugFillProperties(DiagnosticPropertiesBuilder properties) {
     super.debugFillProperties(properties);
     properties.add(new 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.
   OneSequenceGestureRecognizer({ Object debugOwner }) : super(debugOwner: debugOwner);

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

   /// Called when a pointer event is 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 = new List<GestureArenaEntry>.from(_entries.values);
     _entries.clear();
     for (GestureArenaEntry entry in localEntries)
       entry.resolve(disposition);
   }

   @override
   void dispose() {
     resolve(GestureDisposition.rejected);
     for (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) {
     if (_team != null)
       return _team.add(pointer, this);
     return GestureBinding.instance.gestureArena.add(pointer, this);
   }

   /// Causes events related to the given pointer ID to be routed to this recognizer.
   ///
   /// The pointer events are delivered to [handleEvent].
   ///
   /// Use [stopTrackingPointer] to remove the route added by this function.
   @protected
   void startTrackingPointer(int pointer) {
     GestureBinding.instance.pointerRouter.addRoute(pointer, handleEvent);
     _trackedPointers.add(pointer);
     assert(!_entries.containsValue(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)
       stopTrackingPointer(event.pointer);
   }
 }

 /// The possible states of a [PrimaryPointerGestureRecognizer].
 ///
 /// The recognizer advances from [ready] to [possible] when starts tracking a
 /// primary pointer. When the primary pointer is resolve (either accepted or
 /// or rejected), the recognizers 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 reject the gesture if the primary pointer
 /// travels beyond [kTouchSlop] pixels from the original contact point.
 abstract class PrimaryPointerGestureRecognizer extends OneSequenceGestureRecognizer {
   /// Initializes the [deadline] field during construction of subclasses.
   PrimaryPointerGestureRecognizer({
     this.deadline,
     Object debugOwner,
   }) : super(debugOwner: debugOwner);

   /// If non-null, the recognizer will call [didExceedDeadline] after this
   /// amount of time has elapsed since starting to track the primary pointer.
   final Duration deadline;

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

   /// The ID of the primary pointer this recognizer is tracking.
   int primaryPointer;

   /// The global location at which the primary pointer contacted the screen.
   Offset initialPosition;

   Timer _timer;

   @override
   void addPointer(PointerDownEvent event) {
     startTrackingPointer(event.pointer);
     if (state == GestureRecognizerState.ready) {
       state = GestureRecognizerState.possible;
       primaryPointer = event.pointer;
       initialPosition = event.position;
       if (deadline != null)
         _timer = new Timer(deadline, didExceedDeadline);
     }
   }

   @override
   void handleEvent(PointerEvent event) {
     assert(state != GestureRecognizerState.ready);
     if (state == GestureRecognizerState.possible && event.pointer == primaryPointer) {
       // TODO(abarth): Maybe factor the slop handling out into a separate class?
       if (event is PointerMoveEvent && _getDistance(event) > kTouchSlop) {
         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 if you supply a [deadline].
   @protected
   void didExceedDeadline() {
     assert(deadline == null);
   }

   @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;
   }

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

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

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

   @override
   void debugFillProperties(DiagnosticPropertiesBuilder properties) {
     super.debugFillProperties(properties);
     properties.add(new EnumProperty<GestureRecognizerState>('state', state));
   }
 }
	// Copyright 2015 The Chromium Authors. All rights reserved.
	// Use of this source code is governed by a BSD-style license that can be
	// found in the LICENSE file.

	import 'dart:async';
	import 'dart:collection';
	import 'dart:ui' show Offset;

	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 'pointer_router.dart' show PointerRouter;

	/// 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 T RecognizerCallback<T>();

	/// 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 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).
	GestureRecognizer({ this.debugOwner });

	/// 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;

	/// 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.
	void addPointer(PointerDownEvent event);

	/// 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() { }

	/// 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
	T invokeCallback<T>(String name, RecognizerCallback<T> callback, { String debugReport() }) {
	assert(callback != null);
	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 == true ? " $report" : "" }');
	}
	return true;
	}());
	result = callback();
	} catch (exception, stack) {
	FlutterError.reportError(new FlutterErrorDetails(
	exception: exception,
	stack: stack,
	library: 'gesture',
	context: 'while handling a gesture',
	informationCollector: (StringBuffer information) {
	information.writeln('Handler: $name');
	information.writeln('Recognizer:');
	information.writeln(' $this');
	}
	));
	}
	return result;
	}

	@override
	void debugFillProperties(DiagnosticPropertiesBuilder properties) {
	super.debugFillProperties(properties);
	properties.add(new 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.
	OneSequenceGestureRecognizer({ Object debugOwner }) : super(debugOwner: debugOwner);

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

	/// Called when a pointer event is 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 = new List<GestureArenaEntry>.from(_entries.values);
	_entries.clear();
	for (GestureArenaEntry entry in localEntries)
	entry.resolve(disposition);
	}

	@override
	void dispose() {
	resolve(GestureDisposition.rejected);
	for (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) {
	if (_team != null)
	return _team.add(pointer, this);
	return GestureBinding.instance.gestureArena.add(pointer, this);
	}

	/// Causes events related to the given pointer ID to be routed to this recognizer.
	///
	/// The pointer events are delivered to [handleEvent].
	///
	/// Use [stopTrackingPointer] to remove the route added by this function.
	@protected
	void startTrackingPointer(int pointer) {
	GestureBinding.instance.pointerRouter.addRoute(pointer, handleEvent);
	_trackedPointers.add(pointer);
	assert(!_entries.containsValue(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)
	stopTrackingPointer(event.pointer);
	}
	}

	/// The possible states of a [PrimaryPointerGestureRecognizer].
	///
	/// The recognizer advances from [ready] to [possible] when starts tracking a
	/// primary pointer. When the primary pointer is resolve (either accepted or
	/// or rejected), the recognizers 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 reject the gesture if the primary pointer
	/// travels beyond [kTouchSlop] pixels from the original contact point.
	abstract class PrimaryPointerGestureRecognizer extends OneSequenceGestureRecognizer {
	/// Initializes the [deadline] field during construction of subclasses.
	PrimaryPointerGestureRecognizer({
	this.deadline,
	Object debugOwner,
	}) : super(debugOwner: debugOwner);

	/// If non-null, the recognizer will call [didExceedDeadline] after this
	/// amount of time has elapsed since starting to track the primary pointer.
	final Duration deadline;

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

	/// The ID of the primary pointer this recognizer is tracking.
	int primaryPointer;

	/// The global location at which the primary pointer contacted the screen.
	Offset initialPosition;

	Timer _timer;

	@override
	void addPointer(PointerDownEvent event) {
	startTrackingPointer(event.pointer);
	if (state == GestureRecognizerState.ready) {
	state = GestureRecognizerState.possible;
	primaryPointer = event.pointer;
	initialPosition = event.position;
	if (deadline != null)
	_timer = new Timer(deadline, didExceedDeadline);
	}
	}

	@override
	void handleEvent(PointerEvent event) {
	assert(state != GestureRecognizerState.ready);
	if (state == GestureRecognizerState.possible && event.pointer == primaryPointer) {
	// TODO(abarth): Maybe factor the slop handling out into a separate class?
	if (event is PointerMoveEvent && _getDistance(event) > kTouchSlop) {
	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 if you supply a [deadline].
	@protected
	void didExceedDeadline() {
	assert(deadline == null);
	}

	@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;
	}

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

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

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

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