Drag Events¶

Note

This document describes the new drag events API. The old (legacy) approach, which is still supported, is described in Gesture Input.

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 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);
  }
}

HasDraggablesBridge¶

This marker mixin can be used to indicate that the game has both the “new-style” components that use the DragCallbacks mixin, and the “old-style” components that use the Draggable mixin. In effect, every drag event will be propagated twice through the system: first trying to reach the components with DragCallbacks mixin, and then components with Draggable.

class MyGame extends FlameGame with HasDraggablesBridge {
  // ...
}

The purpose of this mixin is to ease the transition from the old event delivery system to the new one. With this mixin, you can transition your Draggable components into using DragCallbacks one by one, verifying that your game continues to work at every step.

Use of this mixin for any new project is highly discouraged.