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Posted 21 Jul 2002

OnIdle() implementation for CDialog based app´s

, 21 Jul 2002
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Simple implementation for the OnIdle function in the CDialog derived classes


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);
            wParam = 0;
    } else
        resValue = CDialog::WindowProc(message, 
        wParam, lParam);

    QueueStatus = GetQueueStatus(QS_ALLINPUT);

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

    return resValue;


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 ;).


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About the Author

Emilio Guijarro
Netherlands Netherlands
Emilio is a Computer Engineer currently working as software engineer in embedded systems.

Main interests are C/C++ programming, algorithmics, compilers, embedded systems, cryptography, and operating systems.

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Comments and Discussions

GeneralMy vote of 5 Pin
JunfengGuo19-Oct-15 13:28
memberJunfengGuo19-Oct-15 13:28 
GeneralThanks, that was handy Pin
Yusuf X16-Jan-07 10:58
memberYusuf X16-Jan-07 10:58 
GeneralUse CDialog's WM_TIMER Pin
Sanj Gunetileke9-Jun-05 7:26
sussSanj Gunetileke9-Jun-05 7:26 
Generalenable and disable of this OnIdle trick during the execution of the program Pin
gehrti28-Apr-05 23:52
membergehrti28-Apr-05 23:52 
GeneralLittle out of topic, but important Pin
ppppvvvv3-Sep-03 5:21
memberppppvvvv3-Sep-03 5:21 
GeneralA slight issue Pin
Nishant S1-Aug-02 14:59
editorNishant S1-Aug-02 14:59 
GeneralRe: A slight issue Pin
<font color=2-Aug-02 9:11
suss2-Aug-02 9:11 
GeneralRe: A slight issue Pin
John Hind26-Oct-15 2:24
memberJohn Hind26-Oct-15 2:24 
QuestionWhat about WM_KICKIDLE? Pin
George23-Jul-02 16:47
memberGeorge23-Jul-02 16:47 
AnswerRe: What about WM_KICKIDLE? Pin
Emilio Guijarro24-Jul-02 9:44
memberEmilio Guijarro24-Jul-02 9:44 
QuestionWhat about WM_IDLEUPDATECMDUI? Pin
Neville Franks22-Jul-02 11:24
memberNeville Franks22-Jul-02 11:24 
AnswerRe: What about WM_IDLEUPDATECMDUI? Pin
Emilio Guijarro23-Jul-02 7:33
memberEmilio Guijarro23-Jul-02 7:33 
GeneralRe: What about WM_IDLEUPDATECMDUI? Pin
Neville Franks23-Jul-02 15:09
memberNeville Franks23-Jul-02 15:09 
GeneralRe: What about WM_IDLEUPDATECMDUI? Pin
Emilio Guijarro24-Jul-02 9:42
memberEmilio Guijarro24-Jul-02 9:42 
AnswerRe: What about WM_IDLEUPDATECMDUI? Pin
Jiang Sheng22-Mar-04 18:20
memberJiang Sheng22-Mar-04 18:20 

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