QTimer not firing in a thread

Question

I have an Qt5 c++ app with 2 threads, thread A is started when the main program starts up. The start method of thread A runs successfully.

So far so good. Next, in the main program I send a signal to Thread A to start a QTimer, which it does - but that timer never expires!

Thread B handles tcp connections. When I initiate a telnet connection to my app, thread B fires up and suddenly I see my Qtimer from thread A expiring at normal intervals.

Why is the QTimer from thread A not expiring until thread B starts?

I suspect my threads are getting messed up. note the last section of code below products this:

thread of this:  QThread(0x200fe00)  
thread of timer:  QThread(0x1fff470)

Which suggest my worker object (this), is in a different thread from my timer object. This timer thread address is actually the MAIN thread. Why? I'm confused.

Suggestions?

---

In my main app I create and start my thread like this:

QThread * MyControllerThread = new QThread(this);

if (MyControllerThread) {

    TheController *worker = new TheController(MyControllerThread);

    if (worker) {
        connect(MyControllerThread, SIGNAL(started()), worker, SLOT(start()));
        connect(MyControllerThread, SIGNAL(finished()), worker, SLOT(deleteLater()));
        connect(MyControllerThread, SIGNAL(finished()), MyControllerThread, SLOT(deleteLater()));
        worker->moveToThread(MyControllerThread);  
        MyControllerThread->start();  
    } 

and in my main app I emit a signal to the new thread:

    emit sig_startlocalpeer(Types::EActionLocalServiceStart);  // Move the local peer to standby mode to start remote tests

which runs a slot in my thread (TheController object):

connect(&m_remotetestintervaltimer,SIGNAL(timeout()),this,SLOT(expiredRemoteTestIntervalTimer()));
m_remotetestintervaltimer.setTimerType(Qt::VeryCoarseTimer);
m_remotetestintervaltimer.start(REMOTETEST_TIMER_INTERVAL);  // Wait between ticks
qDebug() << "thread of this: " << this->thread();
qDebug() << "thread of timer: " << m_remotetestintervaltimer.thread();

Answer 1

Well, it's not a Qt5 bug, it's more an inaccurate understanding of Qt's thread spirit.

In Qt, you have two ways to implement a thread which are using or not an even loop. Here is just a small visual example.

No event loop

myMethodCalledInANewThread
{
    do{ ... }while(...);
}

With an event loop

myMethodCalledInANewThread
{
    [...]
    exec();
}

(Of course you can mix a do/while with an even loop but stay simple).

In QTimer's doc, you can read:

In multithreaded applications, you can use QTimer in any thread that has an event loop. [...] Qt uses the timer's thread affinity to determine which thread will emit the timeout() signal. Because of this, you must start and stop the timer in its thread; it is not possible to start a timer from another thread.

So i'm pretty sure you don't have a second event loop in your second thread and that's why you have the behaviour you described.

To give you some tips to be totally clear with thread using Qt, I suggest you to read:

• QThread doc: http://qt-project.org/doc/qt-5/qthread.html
• QTimer doc: http://qt-project.org/doc/qt-5/QTimer.html

and a very good article about how QThread implementation is misunderstandood by a lot of user. - You're doing it wrong: http://blog.qt.digia.com/blog/2010/06/17/youre-doing-it-wrong/

I hope it will help ;)

Answer 2

The best answer seems to be a combination of RobbieE and Kuba:

You have to explicitly set the parent of the member variable in constructor. The parent-child feature is a Qt thing that exists among classes derived from QObject, it is not a feature of C++.

I never knew this - I assumed that when an object was created, its members variables automatically had their parent set to the object. Good to know!!