OnPaint function在接收到系統所發出的WM_PAINT就會執行到(是因為MFC是用message pass的方式)
Windows會發送此訊息
From Embedded Visual C++ Version 4.0
An internal WM_PAINT message is sent only once by Windows
此外也可以透過UpdateWindow去呼叫到OnPaint
After an internal WM_PAINT message is sent to a window by the UpdateWindow method,no further WM_PAINT messages will be sent or posted until the window is invalidated or until the RedrawWindow method is called again with the RDW_INTERNALPAINT flag set.
此外
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也有對Onpaint, Invalidate, Updatewindows做一番解釋
