Why doesn't invalidate(rect) work?

Question

Android

Why does the following code redraw the entire canvas and not the Rect specified?

@Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        if(rect != null) {
            canvas.drawRect(rect, paint);
        }
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {

        switch(event.getAction()){
            case MotionEvent.ACTION_DOWN:
                float posX = event.getX();
                float posY = event.getY();

                int size = 50;
                int x = (int) Math.floor(posX / size) * size;
                int y = (int) Math.floor(posY / size) * size;

                rect = new Rect(x, y, x + size, y + size);
                invalidate(rect);
                break;
        }
        return true;
    }

Answer 1

In my opinion, you need to add canvas.clipRect