Creating a system overlay window (always on top)

I’m trying to create an always-op-top button/clickable-image
which stays on top of all the windows all the time.

The proof of
concept is

• here – Smart Taskbar (on AppBrain)
• and here V1.4.0 Sidebar style SWKey – Button savior (on xda-developers)

I have been successful and have a running service now. The service
displays some text on top left corner of screen all the time while
user can freely interact with rest of apps in normal manner.

What I’m
doing is subclass ViewGroup and add it to root window manager with
flag TYPE_SYSTEM_OVERLAY. Now I want to add a button/clickable-image
in place of this text which can receive touch events on itself. I
tried overriding “onTouchEvent” for the whole ViewGroup but it does
not receive any event.

How can I receive events only on certain parts
of my always-on-top view group? Kindly suggest.

public class HUD extends Service {
    HUDView mView;

    @Override
    public IBinder onBind(Intent intent) {
        return null;
    }

    @Override
    public void onCreate() {
        super.onCreate();
        Toast.makeText(getBaseContext(),"onCreate", Toast.LENGTH_LONG).show();
        mView = new HUDView(this);
        WindowManager.LayoutParams params = new WindowManager.LayoutParams(
                WindowManager.LayoutParams.WRAP_CONTENT,
                WindowManager.LayoutParams.WRAP_CONTENT,
                WindowManager.LayoutParams.TYPE_SYSTEM_OVERLAY,
                0,
//              WindowManager.LayoutParams.FLAG_NOT_FOCUSABLE
//                      | WindowManager.LayoutParams.FLAG_NOT_TOUCHABLE,
                PixelFormat.TRANSLUCENT);
        params.gravity = Gravity.RIGHT | Gravity.TOP;
        params.setTitle("Load Average");
        WindowManager wm = (WindowManager) getSystemService(WINDOW_SERVICE);
        wm.addView(mView, params);
    }

    @Override
    public void onDestroy() {
        super.onDestroy();
        Toast.makeText(getBaseContext(),"onDestroy", Toast.LENGTH_LONG).show();
        if(mView != null)
        {
            ((WindowManager) getSystemService(WINDOW_SERVICE)).removeView(mView);
            mView = null;
        }
    }
}

class HUDView extends ViewGroup {
    private Paint mLoadPaint;

    public HUDView(Context context) {
        super(context);
        Toast.makeText(getContext(),"HUDView", Toast.LENGTH_LONG).show();

        mLoadPaint = new Paint();
        mLoadPaint.setAntiAlias(true);
        mLoadPaint.setTextSize(10);
        mLoadPaint.setARGB(255, 255, 0, 0);
    }

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        canvas.drawText("Hello World", 5, 15, mLoadPaint);
    }

    @Override
    protected void onLayout(boolean arg0, int arg1, int arg2, int arg3, int arg4) {
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        //return super.onTouchEvent(event);
        Toast.makeText(getContext(),"onTouchEvent", Toast.LENGTH_LONG).show();
        return true;
    }
}