Using volume rocker when screen is locked

I'm a beginner at programming in general. I don't even know how to begin to search for something开发者_如何学C like this. What i want to do is make a simple text app that counts. Mainly i want to count how many fish i catch. I want to be able to use the volume rocker when the screen is locked to count how many fish i catch. I haven't started on the app yet, so i don't have any coding to post. If someone can point me in the right direction, that would be wonderful. Thanks in advanced.

This code will get started listening for the volume button presses. I imagine if you want to do it while the screen is off you'll have to acquire some kind of wakelock that lets the screen shut off but still keeps your app running.

        @Override
        public boolean onKeyDown(int keyCode, KeyEvent event) {
            if ((keyCode == KeyEvent.KEYCODE_VOLUME_DOWN)){
                //Do something
            }
            return true;
        }