产生一个大小为20*20缓存的图形对象。在获取图形设备对象后开始对图形对象进行填充，使得在内存中的图形对象是由四个黑白相间的小方格组成。-Produces a cache size of 20* 20 graphic objects. Graphics device to access the graphics object after the object is populated, so the graphics object in memory is composed of four small black and white squares.