grab window's focus. Keyboard and scroll events will be forwarded
        to the sprite who has the focus. Check the 'focused' property of sprite
        in the on-render event to decide how to render it (say, add an outline
        when focused=true)

def grab_focus(self):
        """grab window's focus. Keyboard and scroll events will be forwarded
        to the sprite who has the focus. Check the 'focused' property of sprite
        in the on-render event to decide how to render it (say, add an outline
        when focused=true)"""
        scene = self.get_scene()
        if scene and scene._focus_sprite != self:
            scene._focus_sprite = self