Full Screen Caller Pictures & Full Screen Caller ID

So I've been searching for some time on how to either replace the in call screen or put something on top of it....the only solution I have found thus far is to make my own ROM, which doesn't make sense for such a small portion of an OS. however I found these two apps (Full Screen Caller Pictures and Full Screen Caller ID) that ar开发者_StackOverflow中文版e able to put a contact image and button on top of the standard in call screen.

I don't know exactly what either of these developers have done, but hope someone on the forums does.

Any ideas?

extend BroadcastReceiver class, set to boot, and then add the listener onReceive

TelephonyManager telephonymanager = (TelephonyManager) context.getSystemService(Context.TELEPHONY_SERVICE);
        PhoneStateListener listener = new Listener();
        telephonymanager.listen(listener, PhoneStateListener.LISTEN_CALL_STATE);

code of Listener:

class Listener extends PhoneStateListener {

        @Override
        public void onCallStateChanged(int state, String incomingNumber) {
            // TODO Auto-generated method stub
            switch (state) {

                case TelephonyManager.CALL_STATE_IDLE:
                    break;

                case TelephonyManager.CALL_STATE_OFFHOOK:
                    break;


                case TelephonyManager.CALL_STATE_RINGING:
                    // do sth 
                    break;
            }
            super.onCallStateChanged(state, incomingNumber);
        }
    }