INFO: Background Processing in an MFC Application (99999)

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
   }