MFC, timers and modal message boxes

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5.00/5 (1 vote)

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Hello to everyone.
I'll try to be as simple as possible.

I created an MFC dialog based application with am unsorted listbox (m_loglist)
I added a timer in the CTimerTestingDlg class. The timer ticks every second.
I also created an int counter with initial value set to 0.
In the OnTimer handler, i add the following code.

C#

void CTimerTestingDlg::OnTimer(UINT_PTR nIDEvent)
{
    // TODO: Add your message handler code here and/or call default
    counter++;

    CString value;
    value.Format(L"%d", counter);

    if (counter==5)
    {
        AfxMessageBox(L"five seconds",MB_OK | MB_SYSTEMMODAL);
        
    }
    m_loglist.AddString(value);

    CDialog::OnTimer(nIDEvent);
}

The output is:
1
2
3
4
(here a message box appears saying "five seconds")
6
7
8
9
5 (I clicked OK so the missing "5" value appears)
10
11
12
..... and so on

Im trying to understand the mechanism that allows this to happen.

The questions are:

1) Since the message box is modal, shouldn't the message pump of MFC be blocked until the response comes? This means, the values of 6 and above shouldnt appear until i press the OK button..

2)Nowhere in the bibliography is mentioned that the AfxMessageBox function call launches a new thread. If we assume that this is the answer to (1), we have an instance of the OnTimer method of the CTimerTestingDlg object left in the middle.(= the one that hasnt typed the value "5" yet due to the message box)
When the timer ticks again, it seems that the OnTimer method of same CTimerTestingDlg object is called again, while the instance above is in the middle! Im i right? if yes, how can this ever be possible?

3) When i click OK, the execution of the OnTimer instance that was left in the middle continues and types the missing "5" value. (after "9" in my example). Where was this OnTimer instance stored? In the stack?

Thank you for your time, im trying to understand what lies beneath here.

Posted 3-Nov-11 5:02am

dlavrantonis

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2 solutions

Solution 1

It looks like the timer launches a different thread, for each call of the OnTimer function.

Posted 3-Nov-11 6:46am

Shmuel Zang

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Chuck O'Toole · Accepted Answer · 2011-11-03T07:03:00

Perfectly understandable and doesn't require magic and/or threads.

The OnTimer() function is an event called upon the receipt of the "WM_TIMER" windows message. This message is dispatched by the MFC provided "message pump". This single WM_TIMER message was removed from the message queue prior to dispatch to OnTimer().

AfxMessageBox() does it's thing and waits for the "OK" button (or whatever) to be clicked. To detect the click, it needs to process windows messages for itself hence it provides it's own "message pump". Even though this message pump is provided by the message box itself, it will process *any* windows message for this application / thread family.

Along comes another "WM_TIMER" message for the next second and is dispatched to the same OnTimer() function.

Generally, when you are processing events, you run the processing to completion before returning to the "dispatch" portion of the message pump so you do not see these artifacts (throw a 10 or 20 second Sleep() in there instead of the message box and you'll see a more predictable sequence). In this case, AfxMessageBox() needed a message pump to complete it's work and you're timer ticks got processed by it.

Judicious use of KillTimer() and SetTimer() should be used if you want to interrupt the flow of ticks for whatever reason.

(Edited to answer another part of your question)

dlavrantonis wrote:
Where was this OnTimer instance stored? In the stack?

Yup, set a breakpoint on the m_loglist.AddString(value); statement *after* the message box appears. You'll get there for some number > 5. When you are there, examine the stack / call frame backwards, you'll eventually find the other call to OnTimer() for number 5. When you click on "OK", all you're doing is returning up through the stack.