Question

So I was following along with a 2D graphics tutorial (those interested, its the one @ http://droidnova.com/), and I got to a point where I’ve been experiencing constant crashes in my onTouchEvent() callback. I’m still trying to track the bug, but while trying to debug the issue I noticed that my onTouchEvent callback gets called twice for each event.

For rerference

@Override
    public boolean onTouchEvent(MotionEvent event) 
    {
        int action = event.getAction();
        synchronized (getHolder()) {
            if (action == MotionEvent.ACTION_DOWN) {
                if (gameObjects.size() < 2) {
                    currentlySelected = new GameObject(BitmapFactory.decodeResource(getResources(), R.drawable.icon));
                    currentlySelected.getCoordinates().setX((int) event.getX());
                    currentlySelected.getCoordinates().setY((int) event.getY());
                } else {
                    for (GameObject gameObject: gameObjects) {
                        if (gameObject.inBoundingBox(event.getX(), event.getY())) {
                            currentlySelected = gameObject;
                            break;
                        }
                    }

                    if (currentlySelected != null) {
                        // Remember to remove it from the list so that we don't waste time updating it in updatePhysics twice.
                        gameObjects.remove(currentlySelected);
                    }
                }
            } else if (action == MotionEvent.ACTION_UP && (currentlySelected != null)) {
                gameObjects.add(currentlySelected);
                currentlySelected = null;
            } else if (action == MotionEvent.ACTION_MOVE) {
                currentlySelected.getCoordinates().setX((int) event.getX());
                currentlySelected.getCoordinates().setY((int) event.getY());
            }
        }

No when I put a breakpoint on the first line, and run it in debug mode, when I touch the screen the callback gets called with the following action’s…

1. action = 0 (MotionEvent.ACTION_DOWN)
2. action = 0 (MotionEvent.ACTION_DOWN)
3. action = 1 (MotionEvent.ACTION_MOVE)
4. action = 1 (MotionEvent.ACTION_MOVE)
5. action = 2 (MotionEvent.ACTION_UP)
6. action = 2 (MotionEvent.ACTION_UP)

for each action, the second one gets out at the synchronized call. Anyone know the reason for this behavior?

Answer 1

As it turns out, the problem was with the line

synchronized (getHolder()) {

It should really be

SurfaceHolder h = getHolder()
syncronized(h) {

That seemed to solve it for me.