Santos Apps

I’m currently working on a game that will require multiple touch points to be tracked, and found that there wasn’t any good posts or documentation that explains how to do this. So, I’d thought I’d share my knowledge.

Android keeps track of touch points based on pointer ID’s. The MotionEvent gives you access to everything you need to capture any number of touch points. The most important thing to understand is:

Pointer index is NOT the pointer ID. Pointer ID and pointer index mean two different things.

The index is used only when the event contains information for more than one touch point, which is typically only when the action is ACTION_MOVE. ACTION_DOWN, ACTION_POINTER_DOWN, ACTION_UP and ACTION_POINTER_UP only ever contain information for ONE touch point. When capturing ACTION_MOVE, you need to call motionEvent.getPointerId(index); to find the pointer ID. You should use the pointer ID to keep track of touch events over a period of time.

Another very important thing to note is that event.getActionMasked() == ACTION_DOWN is only true for the FIRST touch point. You must use ACTION_POINTER_DOWN to capture additional touch points. The same goes for ACTION_UP and ACTION_POINTER_UP.

Below is a sample application I built for tracking multiple touch points. It uses a HashMap to keep track of the pointer ID’s.

Screenshot

Android Multi Touch Example Application

Code

public class MultiTouchView extends View implements View.OnTouchListener {

	private Drawable shape;

	HashMap<Integer, PointF> touchPoints = new HashMap<Integer, PointF>();

	protected Drawable getShape() {
		if (shape == null) {
			shape = getContext().getResources().getDrawable(
					R.drawable.finger_indicator);
		}
		return shape;
	}

	public MultiTouchView(Context context) {
		super(context);
		setOnTouchListener(this);

	}

	@Override
	public boolean onTouch(View v, MotionEvent event) {
		int action = event.getActionMasked();

		switch (action) {
		case MotionEvent.ACTION_DOWN:
		case MotionEvent.ACTION_POINTER_DOWN:
			captureDown(event);
			break;
		case MotionEvent.ACTION_UP:
		case MotionEvent.ACTION_POINTER_UP:
			captureUp(event);
			break;
		case MotionEvent.ACTION_MOVE:
			capturePointerMoves(event);
			break;
		}
		invalidate();
		return true;
	}

	private void captureDown(MotionEvent event) {
		int index = event.getActionIndex();
		int id = event.getPointerId(index);
		touchPoints.put(id, new PointF(event.getX(index), event.getY(index)));
	}

	private void captureUp(MotionEvent event) {
		int index = event.getActionIndex();
		int id = event.getPointerId(index);
		touchPoints.remove(id);
	}

	private void capturePointerMoves(MotionEvent event) {
		int length = event.getPointerCount();
		int id;
		for (int i = 0; i < length; i++) {
			id = event.getPointerId(i);
			try {
				touchPoints.get(id).set(event.getX(i), event.getY(i));
			} catch (IndexOutOfBoundsException e) {
			}
		}
	}

	@Override
	protected void onDraw(Canvas canvas) {

		if (touchPoints.size() > 0) {
			int radius = 80; // Size of finger indicator
			
			Iterator<Integer> iterator = touchPoints.keySet().iterator();
			while (iterator.hasNext()) {
				int key = iterator.next();
				
				PointF point = touchPoints.get(key);
				int x = (int) point.x;
				int y = (int) point.y;
				
				Rect bounds = new Rect();
				bounds.set(x - radius, y - radius, x + radius, y + radius);
				
				getShape().setBounds(bounds);
				getShape().draw(canvas);

				Paint paint = new Paint();
				paint.setColor(Color.WHITE);
				paint.setTextSize(48);
				canvas.drawText(String.valueOf(key), x - 12, y + 12, paint);
			}
			
		}
		

		super.onDraw(canvas);
	}

}

This example application will draw a red circle with the pointer ID in the middle for every touch point it captures. It will follow the touch points as they move and remove them when a finger is lifted off the screen.

I hope this well help people out trying to do multi touch on Android!
Comments are welcome!