Drag Events

Drag events occur when the user moves their finger across the screen of the device, or when they move the mouse while holding its button down.

Multiple drag events can occur at the same time, if the user is using multiple fingers. Such cases will be handled correctly by Flame, and you can even keep track of the events by using their pointerId property.

For those components that you want to respond to drags, add the DragCallbacks mixin.

  • This mixin adds four overridable methods to your component: onDragStart, onDragUpdate, onDragEnd, and onDragCancel. By default, these methods do nothing – they need to be overridden in order to perform any function.

  • In addition, the component must implement the containsLocalPoint() method (already implemented in PositionComponent, so most of the time you don’t need to do anything here) – this method allows Flame to know whether the event occurred within the component or not.

class MyComponent extends PositionComponent with DragCallbacks {
  MyComponent() : super(size: Vector2(180, 120));

   @override
   void onDragStart(DragStartEvent event) {
     // Do something in response to a drag event
   }
}

Demo

In this example you can use drag gestures to either drag star-like shapes across the screen, or to draw curves inside the magenta rectangle.

drag_events.dart
  1import 'dart:math';
  2
  3import 'package:flame/components.dart';
  4import 'package:flame/events.dart';
  5import 'package:flame/game.dart';
  6import 'package:flame/geometry.dart';
  7import 'package:flutter/rendering.dart';
  8
  9class DragEventsGame extends FlameGame {
 10  @override
 11  Future<void> onLoad() async {
 12    addAll([
 13      DragTarget(),
 14      Star(
 15        n: 5,
 16        radius1: 40,
 17        radius2: 20,
 18        sharpness: 0.2,
 19        color: const Color(0xffbae5ad),
 20        position: Vector2(70, 70),
 21      ),
 22      Star(
 23        n: 3,
 24        radius1: 50,
 25        radius2: 40,
 26        sharpness: 0.3,
 27        color: const Color(0xff6ecbe5),
 28        position: Vector2(70, 160),
 29      ),
 30      Star(
 31        n: 12,
 32        radius1: 10,
 33        radius2: 75,
 34        sharpness: 1.3,
 35        color: const Color(0xfff6df6a),
 36        position: Vector2(70, 270),
 37      ),
 38      Star(
 39        n: 10,
 40        radius1: 20,
 41        radius2: 17,
 42        sharpness: 0.85,
 43        color: const Color(0xfff82a4b),
 44        position: Vector2(110, 110),
 45      ),
 46    ]);
 47  }
 48}
 49
 50/// This component is the pink-ish rectangle in the center of the game window.
 51/// It uses the [DragCallbacks] mixin in order to receive drag events.
 52class DragTarget extends PositionComponent with DragCallbacks {
 53  DragTarget() : super(anchor: Anchor.center);
 54
 55  final _rectPaint = Paint()..color = const Color(0x88AC54BF);
 56
 57  /// We will store all current circles into this map, keyed by the `pointerId`
 58  /// of the event that created the circle.
 59  final Map<int, Trail> _trails = {};
 60
 61  @override
 62  void onGameResize(Vector2 size) {
 63    super.onGameResize(size);
 64    this.size = size - Vector2(100, 75);
 65    if (this.size.x < 100 || this.size.y < 100) {
 66      this.size = size * 0.9;
 67    }
 68    position = size / 2;
 69  }
 70
 71  @override
 72  void render(Canvas canvas) {
 73    canvas.drawRect(size.toRect(), _rectPaint);
 74  }
 75
 76  @override
 77  void onDragStart(DragStartEvent event) {
 78    super.onDragStart(event);
 79    final trail = Trail(event.localPosition);
 80    _trails[event.pointerId] = trail;
 81    add(trail);
 82  }
 83
 84  @override
 85  void onDragUpdate(DragUpdateEvent event) {
 86    _trails[event.pointerId]!.addPoint(event.localEndPosition);
 87  }
 88
 89  @override
 90  void onDragEnd(DragEndEvent event) {
 91    super.onDragEnd(event);
 92    _trails.remove(event.pointerId)!.end();
 93  }
 94
 95  @override
 96  void onDragCancel(DragCancelEvent event) {
 97    super.onDragCancel(event);
 98    _trails.remove(event.pointerId)!.cancel();
 99  }
100}
101
102class Trail extends Component {
103  Trail(Vector2 origin)
104      : _paths = [Path()..moveTo(origin.x, origin.y)],
105        _opacities = [1],
106        _lastPoint = origin.clone(),
107        _color =
108            HSLColor.fromAHSL(1, random.nextDouble() * 360, 1, 0.8).toColor();
109
110  final List<Path> _paths;
111  final List<double> _opacities;
112  Color _color;
113  late final _linePaint = Paint()..style = PaintingStyle.stroke;
114  late final _circlePaint = Paint()..color = _color;
115  bool _released = false;
116  double _timer = 0;
117  final _vanishInterval = 0.03;
118  final Vector2 _lastPoint;
119
120  static final random = Random();
121  static const lineWidth = 10.0;
122
123  @override
124  void render(Canvas canvas) {
125    assert(_paths.length == _opacities.length);
126    for (var i = 0; i < _paths.length; i++) {
127      final path = _paths[i];
128      final opacity = _opacities[i];
129      if (opacity > 0) {
130        _linePaint.color = _color.withOpacity(opacity);
131        _linePaint.strokeWidth = lineWidth * opacity;
132        canvas.drawPath(path, _linePaint);
133      }
134    }
135    canvas.drawCircle(
136      _lastPoint.toOffset(),
137      (lineWidth - 2) * _opacities.last + 2,
138      _circlePaint,
139    );
140  }
141
142  @override
143  void update(double dt) {
144    assert(_paths.length == _opacities.length);
145    _timer += dt;
146    while (_timer > _vanishInterval) {
147      _timer -= _vanishInterval;
148      for (var i = 0; i < _paths.length; i++) {
149        _opacities[i] -= 0.01;
150        if (_opacities[i] <= 0) {
151          _paths[i].reset();
152        }
153      }
154      if (!_released) {
155        _paths.add(Path()..moveTo(_lastPoint.x, _lastPoint.y));
156        _opacities.add(1);
157      }
158    }
159    if (_opacities.last < 0) {
160      removeFromParent();
161    }
162  }
163
164  void addPoint(Vector2 point) {
165    if (!point.x.isNaN) {
166      for (final path in _paths) {
167        path.lineTo(point.x, point.y);
168      }
169      _lastPoint.setFrom(point);
170    }
171  }
172
173  void end() => _released = true;
174
175  void cancel() {
176    _released = true;
177    _color = const Color(0xFFFFFFFF);
178  }
179}
180
181class Star extends PositionComponent with DragCallbacks {
182  Star({
183    required int n,
184    required double radius1,
185    required double radius2,
186    required double sharpness,
187    required this.color,
188    super.position,
189  }) {
190    _path = Path()..moveTo(radius1, 0);
191    for (var i = 0; i < n; i++) {
192      final p1 = Vector2(radius2, 0)..rotate(tau / n * (i + sharpness));
193      final p2 = Vector2(radius2, 0)..rotate(tau / n * (i + 1 - sharpness));
194      final p3 = Vector2(radius1, 0)..rotate(tau / n * (i + 1));
195      _path.cubicTo(p1.x, p1.y, p2.x, p2.y, p3.x, p3.y);
196    }
197    _path.close();
198  }
199
200  final Color color;
201  final Paint _paint = Paint();
202  final Paint _borderPaint = Paint()
203    ..color = const Color(0xFFffffff)
204    ..style = PaintingStyle.stroke
205    ..strokeWidth = 3;
206  final _shadowPaint = Paint()
207    ..color = const Color(0xFF000000)
208    ..maskFilter = const MaskFilter.blur(BlurStyle.normal, 4.0);
209  late final Path _path;
210
211  @override
212  bool containsLocalPoint(Vector2 point) {
213    return _path.contains(point.toOffset());
214  }
215
216  @override
217  void render(Canvas canvas) {
218    if (isDragged) {
219      _paint.color = color.withOpacity(0.5);
220      canvas.drawPath(_path, _paint);
221      canvas.drawPath(_path, _borderPaint);
222    } else {
223      _paint.color = color.withOpacity(1);
224      canvas.drawPath(_path, _shadowPaint);
225      canvas.drawPath(_path, _paint);
226    }
227  }
228
229  @override
230  void onDragStart(DragStartEvent event) {
231    super.onDragStart(event);
232    priority = 10;
233  }
234
235  @override
236  void onDragEnd(DragEndEvent event) {
237    super.onDragEnd(event);
238    priority = 0;
239  }
240
241  @override
242  void onDragUpdate(DragUpdateEvent event) {
243    position += event.localDelta;
244  }
245}

Drag anatomy

onDragStart

This is the first event that occurs in a drag sequence. Usually, the event will be delivered to the topmost component at the point of touch with the DragCallbacks mixin. However, by setting the flag event.continuePropagation to true, you can allow the event to propagate to the components below.

The DragStartEvent object associated with this event will contain the coordinate of the point where the event has originated. This point is available in multiple coordinate system: devicePosition is given in the coordinate system of the entire device, canvasPosition is in the coordinate system of the game widget, and localPosition provides the position in the component’s local coordinate system.

Any component that receives onDragStart will later be receiving onDragUpdate and onDragEnd events as well.

onDragUpdate

This event is fired continuously as user drags their finger across the screen. It will not fire if the user is holding their finger still.

The default implementation delivers this event to all the components that received the previous onDragStart with the same pointer id. If the point of touch is still within the component, then event.localPosition will give the position of that point in the local coordinate system. However, if the user moves their finger away from the component, the property event.localPosition will return a point whose coordinates are NaNs. Likewise, the event.renderingTrace in this case will be empty. However, the canvasPosition and devicePosition properties of the event will be valid.

In addition, the DragUpdateEvent will contain delta – the amount the finger has moved since the previous onDragUpdate, or since the onDragStart if this is the first drag-update after a drag- start.

The event.timestamp property measures the time elapsed since the beginning of the drag. It can be used, for example, to compute the speed of the movement.

onDragEnd

This event is fired when the user lifts their finger and thus stops the drag gesture. There is no position associated with this event.

onDragCancel

The precise semantics when this event occurs is not clear, so we provide a default implementation which simply converts this event into an onDragEnd.

Mixins

DragCallbacks

The DragCallbacks mixin can be added to any Component in order for that component to start receiving drag events.

This mixin adds methods onDragStart, onDragUpdate, onDragEnd, and onDragCancel to the component, which by default don’t do anything, but can be overridden to implement any real functionality.

Another crucial detail is that a component will only receive drag events that originate within that component, as judged by the containsLocalPoint() function. The commonly-used PositionComponent class provides such an implementation based on its size property. Thus, if your component derives from a PositionComponent, then make sure that you set its size correctly. If, however, your component derives from the bare Component, then the containsLocalPoint() method must be implemented manually.

If your component is a part of a larger hierarchy, then it will only receive drag events if its ancestors have all implemented the containsLocalPoint correctly.

class MyComponent extends PositionComponent with DragCallbacks {
  MyComponent({super.size});

  final _paint = Paint();
  bool _isDragged = false;

  @override
  void onDragStart(DragStartEvent event) => _isDragged = true;

  @override
  void onDragUpdate(DragUpdateEvent event) => position += event.delta;

  @override
  void onDragEnd(DragEndEvent event) => _isDragged = false;

  @override
  void render(Canvas canvas) {
    _paint.color = _isDragged? Colors.red : Colors.white;
    canvas.drawRect(size.toRect(), _paint);
  }
}