The invalidated areas accumulate in the update region until the region is processed when the next WM_PAINT message occurs or until the region is validated by using the ValidateRect function.The system sends a WM_PAINT message to a window whenever its update region is not empty and there are no other messages in the application queue for that window.If the bErase parameter is TRUE for any part of the update region, the background is erased in the entire region, not just in the specified part.
LPCRECT lpRect,
BOOL bErase = TRUE
);指定要无效重绘的区域，并在OnDraw对无效区域进行判断，只绘制无效的部分。