Q99999: INFO: Background Processing in an MFC Application

Article: Q99999
Product(s): Microsoft C Compiler
Version(s): winnt:
Operating System(s): 
Keyword(s): kbnokeyword kbMFC kbVC kbVC200 kbVC400 kbGrpDSMFCATL
Last Modified: 21-OCT-2000

-------------------------------------------------------------------------------
The information in this article applies to:

- The Microsoft Foundation Classes (MFC), used with:
   - Microsoft Visual C++ for Windows 
   - Microsoft Visual C++ 32-bit Edition 
-------------------------------------------------------------------------------

SUMMARY
=======

Many applications perform lengthy processing "in the background" during
intervals when the user is not otherwise interacting with the application. In an
application developed for the Microsoft Windows operating system, an application
can perform background processing by splitting a lengthy process into many small
fragments. After processing each fragment, the application yields execution
control to Windows using a PeekMessage() loop.

An application developed with the Microsoft Foundation Class Library can perform
background processing either by using the PeekMessage() loop in the library
code's main message loop or by embedding another PeekMessage() loop in the
application.

NOTE: The remainder of this article, namely the sample code, is specific to
Visual C++ for Windows and Visual C++ 32-bit Edition, version 2.x. For Visual
C++ 32-bit Edition, version 4.0, please refer to the Frequently Asked Question
(FAQ) about MFC "Background Processing in an MFC Application" in the Visual C++
4.0 Books Online.

MORE INFORMATION
================

In an application developed with the Microsoft Foundation Class Library, the
main message loop in the CWinApp class contains a PeekMessage() loop. This loop
calls the CWinApp::OnIdle() function between messages. An application can
process messages in this idle time by overriding the OnIdle() function. For more
information about performing background processing in the OnIdle() function,
please refer to the documentation for the CWinApp::OnIdle() function in the
"Class Libraries Reference" manual.

Another method to perform background processing in an application involves
embedding a PeekMessage() loop in a function. Because a PeekMessage() loop is
very similar to the main message loop, such a loop in an application developed
with the Microsoft Foundation Class Library must perform many of the functions
of the main message loop in the library. The following code fragment
demonstrates writing a PeekMessage() loop that is compatible with the Microsoft
Foundation Class Library:

     while (bDoingBackgroundProcessing)
     {
        while (::PeekMessage(&msg, NULL, 0, 0, PM_REMOVE))
        {
           if (msg.message == WM_QUIT)
           {
              bDoingBackgroundProcessing = FALSE;
              ::PostQuitMessage(nExitCode);
              break;
           }
           if (!AfxGetApp()->PreTranslateMessage(&msg))
           {
              ::TranslateMessage(&msg);
              ::DispatchMessage(&msg);
           }
        }
        AfxGetApp()->OnIdle(0);   // updates user interface
        AfxGetApp()->OnIdle(1);   // frees temporary objects

       // Perform some background processing here
       // When you are done with the background task
       // set bDoingBackgroundProcessing to FALSE
     }

Additional query words:

======================================================================
Keywords          : kbnokeyword kbMFC kbVC kbVC200 kbVC400 kbGrpDSMFCATL 
Technology        : kbAudDeveloper kbMFC
Version           : winnt:
Issue type        : kbinfo

=============================================================================

THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO THE FOREGOING LIMITATION MAY NOT APPLY.

Copyright Microsoft Corporation 1986-2002.