[Qt-interest] QThread and QTimer signal problems

[Donal]

Hi Tony,

Thanks very much for the response. You are right, this is a very common
problem. After more research I stumbled upon explanations of why you must
create the objects in the run method.

The way I am implementing timeouts isn't ideal. I create a Timer on the
thread when its run is called (start()), then connect the timeout signal to
a method using the Qt::DirectConnection parameter and then terminate the
thread in this method.
Outside of the thread, I check the isRunning() bool method of QThread. I
really don't think this is an ideal solution but for now it seems to work
fine.

This app won't have much user interaction when its running and won't be in a
window, more full screen so I'll have a look at performance issues later on.


Cheers.

Regards,
Donal

On Sat, Feb 14, 2009 at 12:04 AM, Tony Rietwyk <
tony.rietwyk at rightsoft.com.au> wrote:

>  Donal wrote:
>
>  Hi,
>
> I am doing serial communication and basically I want to set up a time out
> facility.
> Here would be the pseudocode code in main thread is:
>
> myTimerThread->startTimer(300 msecs)
> while (readCommport(charBuff) && !myTimerThread->hasTimedOut());
> myTimerThread->stopTimer();
>
> So I've created a timeout thread that creates a QTimer object in its
> constructor with single shot.
> I then connect the timeout signal to a slot I've created, "timedOut()".
> This timedOut slot changes a bool attribute "hasTimedOut".
> In the run method of my thread, I've placed it in an event loop using
> QThread::exec();
> I start the timer by calling a member method of my timer thread.
> So in my while in the main thread I keep calling a function hasTimedOut()
> that returns hasTimedOut variable.
> The problem is, the timeout() signal from the QTimer never emits a signal.
>
> I've placed qDebugs everywhere, including the hasTimedOut function and it
> does get called but the bool attribute never gets changed as the slot is
> never called by QTimer.
>
> I've placed a delay in the readCommport function to force it to time out
> but the QTimer never seems to emit the signal.
>
> I came across an old post that claimed that you could only use QTimer in
> the main thread, but looking at the docs, it says you can place it in any
> event loop'd thread.
>
> Am I doing something fundamentally wrong here? If not I'll post some code
> later.
>
> Thanks.
>
> Donal
>
>
> Hi Donal,
>
> This is a common problem with Qt threads - you have to create the objects
> used by the thread in the run method - not in the thread's constructor.
>
> Do you see any debug warnings from Qt when the application runs?
>
> Also note that QTimer relies on the main event loop - so it cannot be used
> for time critical operations (at least on Windows), since the timer (even in
> a different thread) will not fire when the user drags the window around, or
> resizes it. You need to use platform specific non-event timers in these
> cases.
>
> Hope that helps,
>
> Tony.
>
>
>
>
>
>
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