class DragDropSprite(MouseClickSprite):
    def __init__(self):
        MouseClickSprite.__init__(self)
        self.dragging = False
    
    def is_clicked(self, event):
        if self.rect.collidepoint(event.pos) and event.button == 1:
            return True
        return False
    
    def clicked(self, event):
        if event.type == pygame.MOUSEBUTTONDOWN:
            self.dragging = True
            self.drag_offset = (self.position[0] - event.pos[0],
                                self.position[1] - event.pos[1])
        else:
            self.dragging = False
            
    def update(self):
        if self.dragging:
            self.position = (Mouse.get_pos()[0] + self.drag_offset[0],
                             Mouse.get_pos()[1] + self.drag_offset[1])