qthread - eventloop - in non-qt app

[ctarsoaga]

Hi

I try to use a simple thread

class MyThread : public QThread
{
Q_OBJECT
private:
  virtual void run()
  {
    QTimer timer;
    QObject::connect(&timer, SIGNAL(timeout), this, SLOT(onTimer()));
    timer.start(1000);
    QThread::exec();
  }
private slots:
   void onTimer() {....}
};

class MyThread : public QThread
{
Q_OBJECT
private:
  virtual void run()
  {
    QTimer timer;
    QObject::connect(&timer, SIGNAL(timeout), this, SLOT(onTimer()));
    timer.start(1000);
    QThread::exec();
  }
private slots:
   void onTimer() {....}
};

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in an application that does not have a QCoreApplication event loop in the main thread, e.g.

void main()
{
   QCoreApplication app;
 
   //no app.exec() here, we use our custom loop
 
   forever()... {}
}

void main()
{
   QCoreApplication app;

   //no app.exec() here, we use our custom loop

   forever()... {}
}

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I start an instance of MyThread but I never receive any timer events.
I am not able to add app.exec() in the main thread so I added a QEventLoop somewhere and then I started to receive my timer events.
But I don't understand what is going on: I expected to have the timer events working since I do have an event loop inside my thread.

I cannot check now, but it just came to me that maybe the onTimer() slot gets called on a different thread.
Can that be the problem?

Can somebody please explain?
Thanks a lot

[yeye_olive]

The line

QObject::connect(&timer, SIGNAL(timeout), this, SLOT(onTimer()));

QObject::connect(&timer, SIGNAL(timeout), this, SLOT(onTimer()));

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connects the timer's signal to a slot of the QThread object itself. The QThread object lives in the main thread, not in the thread it manages. Therefore the connection behaves as a queued connection, and the slot will only be executed if you start the event loop in the main thread.

You could instead move the slot to a new object, e.g. MyObject, that lives in the worker thread, e.g.:

class MyObject : public QObject {
Q_OBJECT
/* ... */
public slots:
  void onTimer();
}
 
/* ... */
 
virtual void MyThread::run() {
  QTimer timer;
  MyObject myObject;
  QObject::connect(&timer, SIGNAL(timeout), &myObject, SLOT(onTimer()));
  timer.start(1000);
  QThread::exec();
}

class MyObject : public QObject {
Q_OBJECT
/* ... */
public slots:
  void onTimer();
}

/* ... */

virtual void MyThread::run() {
  QTimer timer;
  MyObject myObject;
  QObject::connect(&timer, SIGNAL(timeout), &myObject, SLOT(onTimer()));
  timer.start(1000);
  QThread::exec();
}

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