MSDN上如是说:
The client area is marked for painting when the next WM_PAINT message occurs. The region can also be validated before a WM_PAINT message occurs by the ValidateRect method.
由上可知它和 UpdateWindow区别在于UpdateWindow的作用是使窗口立即重绘。调用Invalidate等函数后窗口不会立即重绘,这是由于WM_PAINT消息的优先级很低,它需要等消息队列中的其它消息发送完后才能被处理。调用UpdateWindow函数可使WM_PAINT被直接发送到目标窗口,从而导致窗口立即重绘。