What could be the possible cause of Qtimer failure?

[John Van]

I encountered an issue with QTimer where it failed to work properly in one part of my program, even though it functioned correctly elsewhere.

	QEventLoop loop2;
	QTimer::singleShot(500, [&]() {loop2.quit(); });
	loop2.exec();

I was using an event loop to pause the program for a period, but when the timer expired, it didn’t quit as expected, causing the program to hang at that point. I eventually resolved the issue by replacing it with QThread::msleep(500);.

[ChrisW67]

where it failed to work properly in one part of my program, even though it functioned correctly elsewhere.

What is the difference between these places? Would I be correct in guessing that the working instances are in the main thread, and those that do not behave as expected are in other threads?

Just out of interest, is there a reason you did not use the more direct:

QTimer::singleShot(500, &loop2, &QEventLoop::quit);

BTW, this kind of blocking behaviour is usually an anti-pattern in Qt programs.

[J.Hilk]

@John-Van said in What could be the possible cause of Qtimer failure?:

QThread::msleep

this is prime example what will cause timers to "fail"

[JonB]

@John-Van
As both the others have said.

If you want to know how your code is behaving why don't you put a qDebug() in the slot? the you would know whether the timer did not expire/call the slot or whether the loop did not quit.

[John Van]

@ChrisW67
“this kind of blocking behaviour is usually an anti-pattern in Qt programs.”
would you provide a detailed explanation？

[jsulm]

@John-Van said in What could be the possible cause of Qtimer failure?:

would you provide a detailed explanation？

Never block using long lasting loops or local event loops. This is usually not needed in event driven applications. Long lasting calculations should be done in other threads.