OnIdle() implementation for CDialog based app�s

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Introduction

A very common and powerful programming tool for MFC applications is the idle time processing, which allows the programmer to be noticed when there aren't pending messages for the current thread. I was surprised when coding a dialog based application I found that this wonderful advantage was not available, due to the synchronous nature of the DoModal() method. Now the target is implementing the same functionality on a CDialog derived class.

The WM_IDLE loop trick

The code I made to solve this problem is really simple, it uses the GetQueueStatus function to check the message queue state. If there are not pending messages, it posts the WM_IDLE message to the current window, allowing it to call OnIdle(). Of course, all this code is implemented in an overridden version of WindowProc().

Emulating the behaviour of the lCount parameter (used to calculate how many idle time has passed since the last false return of OnIdle()) is self-explained in the following source code.

LRESULT CMyDialog::WindowProc(UINT message, 
                WPARAM wParam, LPARAM lParam)
{
    DWORD QueueStatus;
    LRESULT resValue = 0;
    bool OnIdleRetVal = true;

    if(message == WM_IDLE) {
        OnIdleRetVal = OnIdle((UINT)wParam);
        if(!OnIdleRetVal)
            wParam = 0;
    } else
        resValue = CDialog::WindowProc(message, 
        wParam, lParam);

    QueueStatus = GetQueueStatus(QS_ALLINPUT);

    if(HIWORD(QueueStatus) == 0)
        PostMessage(WM_IDLE, 
            wParam + (OnIdleRetVal ? 1 : 0), 0);

    return resValue;
}

Conclusion

Of course, OnIdle() should be declared and implemented in the class. The working mechanism, tips and tricks of the OnIdle function are explained on the CWinApp documentation part of the MSDN Library, so they will be not explained here for saving time and bytes ;).