OPaint调OnDraw(),OPaint()是消息.OnDraw()是虚函数。
You must override this function to display your view of the document. You can make graphic device interface (GDI) calls using the CDC object pointed to by the pDC parameter. You can select GDI resources, such as pens or fonts, into the device context before drawing and then deselect them afterwards. Often your drawing code can be device-independent; that is, it doesn’t require information about what type of device is displaying the image.