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Android GestureDetector

2016/05/27

Quick note about learning GestureDetector and ScaleGestureDetector.

GestureDetector

Android provides GestureDetector class for detecting common gestures.

Steps

  1. Create an instance of GestureDetector
  2. Override View or Activity’s onTouchEvent(MotionEvent event) and pass event to GestureDetector.onTouchEvent(event);
  3. Implement GestureDetector.OnGestureListener callbacks for gestures.
  4. Implement GestureDetector.OnDoubleTapListener callbacks for click gestures.
  5. Or implement GestureDetector.SimpleOnGestureListener callbacks if process only a few gestures above.

Suggest to use SimpleOnGestureListener which only needs to implement the gestures we need.

Gestures

Check SimpleOnGestureListener Api page for the full list of gestures.

The following table shows the gestures of both OnGestureListener and OnDoubleTapListener. The first six are from OnGestureListener and the last three are from OnDoubleTapListener. But SimpleOnGestureListener have all of them.

Gestures Description
onDown() Notified when a tap occurs with the down MotionEvent that triggered it.
onFling() Notified of a fling event when it occurs with the initial on down MotionEvent and the matching up MotionEvent.
onLongPress() Notified when a long press occurs with the initial on down MotionEvent that trigged it.
onScroll() Notified when a scroll occurs with the initial on down MotionEvent and the current move MotionEvent.
onShowPress() The user has performed a down MotionEvent and not performed a move or up yet.
onSingleTapUp() Notified when a tap occurs with the up MotionEvent that triggered it.
onDoubleTap() Notified when a double-tap occurs.
onDoubleTapEvent() Notified when an event within a double-tap gesture occurs, including the down, move, and up events.
onSingleTapConfirmed() Notified when a single-tap occurs. Unlike onSingleTapUp(), it will only be called after the detector is confident that the user’s first tap is not followed by a second tap leading to a double-tap gesture.

Example

public class MainActivity extends Activity { 
    
    private GestureDetectorCompat mDetector; 

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        // Create an instance of GestureDetector
        mDetector = new GestureDetectorCompat(this, new MyGestureListener());
    }

    @Override 
    public boolean onTouchEvent(MotionEvent event){ 

    	// Pass event to GestureDetector.onTouchEvent(event)
        this.mDetector.onTouchEvent(event);
        return super.onTouchEvent(event);
    }
    
    class MyGestureListener extends GestureDetector.SimpleOnGestureListener {
        private static final String DEBUG_TAG = "Gestures"; 
        
        @Override
        public boolean onDown(MotionEvent event) { 
            Log.d(DEBUG_TAG, "Down"); 
            return true;
        }

        @Override
        public boolean onFling(MotionEvent event1, MotionEvent event2, 
                float velocityX, float velocityY) {
            Log.d(DEBUG_TAG, "Fling");
            return true;
        }
    }
}

ScaleGestureDetector

It detects scaling transformation gestures or called pinch gestures for scaling views. (Two fingers)

Steps

  1. Create an instance of ScaleGestureDetector
  2. Override View or Activity’s onTouchEvent(MotionEvent event) and pass event to ScaleGestureDetector.onTouchEvent(event);
  3. Implement ScaleGestureDetector.OnScaleGestureListener callbacks for gestures.
  4. Or implement ScaleGestureDetector.SimpleOnScaleGestureListener() callbacks if process only a few gestures above.

Gestures

Gestures Description
onScaleBegin() Responds to the beginning of a scaling gesture.
onScale() Responds to scaling events for a gesture in progress.
onScaleEnd() Responds to the end of a scale gesture.

Example

public class MainActivity extends Activity {

    private ImageView imageView;
    private float scale = 1f;
    private ScaleGestureDetector detector;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        imageView=(ImageView)findViewById(R.id.imageView);
        detector = new ScaleGestureDetector(this, new ScaleListener());
    }

	public boolean onTouchEvent(MotionEvent event) {
        detector.onTouchEvent(event);
        return super.onTouchEvent(event);
    }

    private class ScaleListener extends ScaleGestureDetector.SimpleOnScaleGestureListener {

        float onScaleBegin = 0;
        float onScaleEnd = 0;

        @Override
        public boolean onScaleBegin(ScaleGestureDetector detector) {

            Toast.makeText(getApplicationContext(), "Scale Begin" ,Toast.LENGTH_SHORT).show();
            onScaleBegin = scale;

            return true;
        }

        @Override
        public boolean onScale(ScaleGestureDetector detector) {

            scale *= detector.getScaleFactor();

            imageView.setScaleX(scale);
            imageView.setScaleY(scale);

            return true;
        }

        @Override
        public void onScaleEnd(ScaleGestureDetector detector) {

            Toast.makeText(getApplicationContext(), "Scale Ended", Toast.LENGTH_SHORT).show();

            onScaleEnd = scale;

            if (onScaleEnd > onScaleBegin){
                Toast.makeText(getApplicationContext(), "Scaled Up by a factor of " + String.valueOf( onScaleEnd / onScaleBegin ), Toast.LENGTH_SHORT).show();
            }

            if (onScaleEnd < onScaleBegin){
                Toast.makeText(getApplicationContext(), "Scaled Down by a factor of " + String.valueOf( onScaleBegin / onScaleEnd ), Toast.LENGTH_SHORT.show();
            }

            super.onScaleEnd(detector);
        }
	}
}

Ref

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