The mouse-down event anchors a corner or center of the shape (or its bounding-box), the user drags to stretch out the shape to the desired size and aspect, and the mouse-up event confirms the draw.

• 鼠标按下的事件就确定了形状的角度或中心（或其边界），用户拖动鼠标可以把图形拉伸到期望的大小和外形，鼠标释放事件则确认这次画图完成。