Re: Starting a timer from a worker thread

"Scott McPhillips [MVP]" <org-dot-mvps-at-scottmcp>
Mon, 14 May 2007 11:08:09 -0400
Steph wrote:

Thanks Joe, I shall take a look at Multimedia timers.

However, I have actually implemented this in my code and the timer does
fire, despite not having a message pump. I was originally asking in case I
could not rely on this behaviour, but now I am confused as to why it works at
all. Any ideas?

BTW what are your thoughts on using a boolean flag around the time consuming
function? Is that a bad idea?

Best regards,

 > void CMyDlg::MyThread()
 > {
 > // prepare data - can't use GetPercentComplete here
 > // time consuming function
 > m_blnInTCF = true;
 > m_theDll.TimeConsumingFunction();
 > m_blnInTCF = false;
 > // could post message to kill timer here instead of checking in
 > // tidy data - can't use GetPercentCompelete here
 > }
 > void CMyDlg::OnTimer(UINT nIDEvent)
 > {
 > int intPercent = 0;
 > // check it's our timer
 > if (m_intTimerID == nIDEvent)
 > {
 > // if the thread is running
 > if (WAIT_TIMEOUT == WaitForSingleObject(m_pThread->m_hThread,
0 ))
 > {
 > if (m_blnInTCF)
 > {
 > // update progress bar
 > blnSuccess = m_theDll.GetPercentComplete(&intPercent);
 > // test return value and update progress control
 > }
 > }
 > else
 > {
 > KillTimer(m_intTimerID);
 > }
 > }
 > }

Although you created the timer in the thread, you did it for the dialog
window, which runs exclusively in the main thread. What you have done
is call a CWnd function (SetTimer) in the context of another thread.
This is against the rules of MFC, and it is pointless as well: What your
call actually does is stop your thread until the main thread can handle
the call. So you might as well just start the timer in the main thread.

Ignore the digression into multimedia timers: They might be appropriate
if you needed a timer in the worker thread, where there is no message
pump, but that is not relevant to updating in the main thread. (It
seems Joe didn't understand your intent with the timer.)

The m_blnInTCF looks pointless: It tells you the same thing that
WaitForSingleObject tells you better.

Finally, none of this is relevant to your original problem, that calling
the percent complete function "appears to throw an error." Perhaps you
need thread synchronization (or perhaps not) but you have not told us
much about that problem.

Scott McPhillips [MVP VC++]

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