Invalidates the client area within the given rectangle by adding that rectangle to the CWnd update region.
The invalidated rectangle, along with all other areas in the update region, is marked for painting when the next WM_PAINT message is sent. The invalidated areas accumulate in the update region until the region is processed when the next WM_PAINT call occurs, or until the region is validated by the ValidateRect or ValidateRgn member function.
The bErase parameter specifies whether the background within the update area is to be erased when the update region is processed. If bErase is TRUE, the background is erased when the BeginPaint member function is called; if bErase is FALSE, the background remains unchanged. If bErase is TRUE for any part of the update region, the background in the entire region is erased, not just in the given part.
Windows sends a WM_PAINT message whenever the CWnd update region is not empty and there are no other messages in the application queue for that window.