Gesture Detection

Touch screens lend themselves well to gesture based input. A gesture could be a pinch with two fingers to indicate the desire to zoom, a tap or double tap, a long press and so on.

libGDX provides a GestureDetector (source) that lets you detect the following gestures:

  • touchDown: A user touches the screen.
  • longPress: A user touches the screen for some time.
  • tap: A user touches the screen and lifts the finger again. The finger must not move outside a specified square area around the initial touch position for a tap to be registered. Multiple consecutive taps will be detected if the user performs taps within a specified time interval.
  • pan: A user drags a finger across the screen. The detector will report the current touch coordinates as well as the delta between the current and previous touch positions. Useful to implement camera panning in 2D.
  • panStop: Called when no longer panning.
  • fling: A user dragged the finger across the screen, then lifted it. Useful to implement swipe gestures.
  • zoom: A user places two fingers on the screen and moves them together/apart. The detector will report both the initial and current distance between fingers in pixels. Useful to implement camera zooming.
  • pinch: Similar to zoom. The detector will report the initial and current finger positions instead of the distance. Useful to implement camera zooming and more sophisticated gestures such as rotation.

A GestureDetector is an event handler in disguise. To listen for gestures, one has to implement the GestureListener interface and pass it to the constructor of the GestureDetector. The detector is then set as an InputProcessor, either on an InputMultiplexer or as the main InputProcessor:

  1. public class MyGestureListener implements GestureListener{
  3.        @Override
  4.        public boolean touchDown(float x, float y, int pointer, int button) {
  6.            return false;
  7.        }
  9.     @Override
  10.     public boolean tap(float x, float y, int count, int button) {
  12.         return false;
  13.     }
  15.     @Override
  16.     public boolean longPress(float x, float y) {
  18.         return false;
  19.     }
  21.     @Override
  22.     public boolean fling(float velocityX, float velocityY, int button) {
  24.         return false;
  25.     }
  27.     @Override
  28.     public boolean pan(float x, float y, float deltaX, float deltaY) {
  30.         return false;
  31.     }
  33.     @Override
  34.     public boolean panStop(float x, float y, int pointer, int button) {
  36.         return false;
  37.     }
  39.        @Override
  40.        public boolean zoom (float originalDistance, float currentDistance){
  42.        return false;
  43.        }
  45.        @Override
  46.        public boolean pinch (Vector2 initialFirstPointer, Vector2 initialSecondPointer, Vector2 firstPointer, Vector2 secondPointer){
  48.        return false;
  49.        }
  50.        @Override
  51.     public void pinchStop () {
  52.     }
  53. }
  1. Gdx.input.setInputProcessor(new GestureDetector(new MyGestureListener()));

The GestureListener can signal whether it consumed the event or wants it to be passed on to the next InputProcessor by returning either true or false respectively from its methods.

As with the events reported to a normal InputProcessor, the respective methods will be called right before the call to ApplicationListener.render() on the rendering thread.

The GestureDetector also has a second constructor that allows it to specify various parameters for gesture detection. Please refer to the Javadocs) for more information.

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