绘制了一个正方形，与之对应的八个点，能用上鼠标拖动其中一个点可改变正方形的大小！也能按住鼠标左键拖曳正方形！-Draw a square, eight corresponding points, drag the mouse can be used on a point which can change the size of a square! Hold down the left mouse button can also drag a square!