在 msdn 说明中 CWnd::UpdateWindow 不理解的地方?
CWnd::UpdateWindow
Updates the client area by sending a WM_PAINT message if the update region is not empty. The UpdateWindow member function sends a WM_PAINT message directly, bypassing the application queue. If the update region is empty, WM_PAINT is not sent.
其中 update region is empty 到底指的是什么意思?