class MyDecorView extends FrameLayout {

        Paint paint = new Paint();

        public MyDecorView(Context context) {
            super(context);
            paint.setTextSize(64);
            paint.setColor(Color.WHITE);
        }

        @Override
        protected void dispatchDraw(Canvas canvas) {
            super.dispatchDraw(canvas);
            canvas.drawColor(0x88000000);
            canvas.drawText("draw some text", 100, 200, paint);
        }

        public static void addMeToTheViewTree(Activity activity) {
            MyDecorView mdv = new MyDecorView(activity);
            ViewGroup decorView = (ViewGroup) activity.getWindow().getDecorView();
            View child = decorView.getChildAt(0);
            decorView.removeView(child);
            mdv.addView(child);
            decorView.addView(mdv);
        }
    }