Question

In our application we have an MFC dialog that opens and processes some data needed to display. Hence the processing is initiated lazily inside OnPaint() event. The processing
takes some time (about 20-30 sec), and is solely algorithmic (no UI API called).
What happens is that sometimes after the processing is finished the window “disappears”. We found out that it’s brought behind the parent window (main frame).

Calling SetForegroundWindow() right after processing has finished, or performing the processing in a non-UI thread (letting OnPaint() finish in reasonable time) has eliminated the problem.

Is this issue known? Is there any defined time limit for OnPaint() execution?

Thanks,

Alex.

Answer 1

http://msdn.microsoft.com/en-us/library/01c9aaty%28v=vs.80%29.aspx

The WM_PAINT message is sent when the UpdateWindow or RedrawWindow
member function is called.

A window may receive internal paint messages as a result of calling
the RedrawWindow member function with the RDW_INTERNALPAINT flag set.
In this case, the window may not have an update region. An application
should call the GetUpdateRect member function to determine whether the
window has an update region. If GetUpdateRect returns 0, the
application should not call the BeginPaint and EndPaint member
functions.

It is an application’s responsibility to check for any necessary
internal repainting or updating by looking at its internal data
structures for each WM_PAINT message because a WM_PAINT message may
have been caused by both an invalid area and a call to the
RedrawWindow member function with the RDW_INTERNALPAINT flag set.

An internal WM_PAINT message is sent only once by Windows. After an
internal WM_PAINT message is sent to a window by the UpdateWindow
member function, no further WM_PAINT messages will be sent or posted
until the window is invalidated or until the RedrawWindow member
function is called again with the RDW_INTERNALPAINT flag set.