SAMPLE: Using a TimerProc Function in MFC Application

WINDOWS             | WINDOWS NT

The information in this article applies to:

• The Microsoft Foundation Classes (MFC), included with:

      - Microsoft Visual C++ for Windows, versions 1.0, 1.5, 1.51, 1.52
      - Microsoft Visual C++, 32-bit Edition, version 1.0, 2.0, 2.1, and
        4.0
  

SUMMARY

The TMRPROC sample is designed to demonstrate how to use the SetTimer() API in an MFC Application in such a way that the TimerProc() callback function (a member of a C++ class) is used instead of handling WM_TIMER messages. There are two makefiles included:

• TMRPROC.MAK, generated by Visual C++ version 1.5.
• TMRPRC32.MAK, generated by Visual C++ version 2.0.

• Microsoft Download Service (MSDL)

        Dial (206) 936-6735 to connect to MSDL
        Download TMRPROC.EXE (size: 36624 bytes) 
  

• Internet (anonymous FTP)

        ftp ftp.microsoft.com
        Change to the SOFTLIB\MSLFILES directory
        Get TMRPROC.EXE (size: 36624 bytes) 
  

MORE INFORMATION

The challenge of using callback functions as C++ class members can be approached in several different ways, depending on the circumstances surrounding the callback. In "Calling All Members:..." (see References below), four different approaches are mentioned. They are:

• Not accessing any class members from within the static callback.
• Storing the "this" pointer to the object in a static member.
• Passing the "this" pointer to the callback function via a parameter supplied by APIs that use callbacks.
• Storing a list of "this" pointers mapped to the list of objects servicing the callback in a static structure.

TMRPROC.CPP and .H

Storing the "this" pointer to the object in a static member is demonstrated in the CWinApp-derived class, CTmrprocApp.

There is no window associated with this class, so the output used to indicate the timer event is done via OutputDebugString(). The pThis variable is a static member variable that is initialized in the constructor to contain the "this" pointer for the class. The TimerProc() callback is simply a static member function of type "void CALLBACK EXPORT." It is static, so no "this" pointer is passed in. For this reason, the pThis member variable is used to store the CTmrprocApp's "this" pointer. Whenever TimerProc() gets called, the current instance of CTmrprocApp can be referenced through the CTmrprocApp::pThis member.

This is the simplest case. There are three functions added to the CTmrprocApp class to accomplish this objective - TimerProc(), CTmrprocApp::OnAppsettimer(), and CTmrprocApp::OnAppkilltimer(). The later two are the menu handlers that allow the user to start and stop the timer.

TMRPRVW.CPP and .H

Storing a list of "this" pointers mapped to the list of objects servicing the callback in a static structure is implemented in the view class, CTmrprocView.

In this case, a little more work is required, but not much. Because a timer needs to be set for each instance of the view class, a scheme must be implemented to associate each timer ID with a corresponding view object. The view object is identified by its "this" pointer and the timer by its timer ID (passed back from SetTimer()). The view class associates these and stores them in a CMapWordToPtr static member variable. This way, when each view's TimerProc() is called, it can first look up its "this" pointer in the pointer/ID map, and then update its own members.

There are several functions needed to start, stop, and reset the count on the timer. The count is just a running count of the number of times TimerProc() has been called for the current view. The following are the start, stop, and reset functions:

   CTmrprocView::OnTimerStart()    - Start timer in current view.
   CTmrprocView::OnTimerStop()     - Stop timer in current view.
   CTmrprocView::OnTimerReset()    - Reset count for current view.

   CTmrprocView::OnTimerAllStart() - Start timers for all views.
   CTmrprocView::OnTimerAllStop()  - Stop timers for all views.
   CTmrprocView::OnTimerAllReset() - Reset timer count for all views.

REFERENCES

MSDN Technical Articles, C/C++ Articles, "Calling All Members: Member Functions As Callbacks," by Dale Rogerson.