QThread finished() emitting fails if called inside window closing - c++

Question

I have done an application with some threads. Everything seems to work ok if I call my stopConsumer inside a keypressedEvent. But If I call it inside a destructor of closeEvent.. it fails.
My QThread class that has a run method like this one:
void Consumer::run()
{
forever {
// do something something
// do something something
// do something something
//-------------------------------- check for abort
abortMutex.lock();
if(abort) {
abortMutex.unlock();
qDebug() << "abort..";
break;
} abortMutex.unlock();
//-------------------------------- check for abort
}
qDebug() << "Consumer > emit finished()";
emit finished();
}
void Consumer::stopConsume() {
abortMutex.lock();
abort = true;
abortMutex.unlock();
}
and a method in the MainWindow:
void initConsumers()
{
consumer1 = new Consumer(....);
connect(consumer1, SIGNAL(finished()),
this, SLOT(deleteConsumer()));
consumer1->start();
}
void stopConsumer() {
if(consumer1!=NULL) {
qDebug() << "stopConsumer";
consumer1->stopConsume();
}
}
If I have a keypressed that calls stopConsumer.. it's ok, deleteConsumer is reached.
If I call stopConsumer inside the MainWindow destructor or inside a MainWindow closeEvent.. the slot deleteConsumer is never reached!
Any ideas?

Answer 1

Given that the Consumer class and your MainWindow have different thread affinities, the call you make to connect inside initConsumers() is likely using a Qt::QueuedConnection, which means that the deleteConsumer() slot won't get called immediately.
If you would like to ensure that the consumer gets deleted from the destructor of your main window (or equivalently, from a close event), one possible solution is to call stopConsume() on the consumer, then wait until the thread is no longer running (see http://qt-project.org/doc/qt-5.1/qtcore/qthread.html#isRunning), then call deleteConsumer() directly.
Update
Here's an example of what I described above:
consumer1->stopConsume();
consumer1->wait();
deleteConsumer();
It's not advisable to switch the connection type to Qt:DirectConnection since that will cause the deleteConsumer() function to be called from the body of Consumer::run(), which will likely crash your application.

Answer 2

Part of the problem here is that you're deriving from QThread, which is not how it is supposed to be used. You can read about why deriving from QThread is wrong here.
Instead, what you should be doing is deriving your class from QObject, creating a QThread object and moving the derived QObject instance to that thread.
class Consumer : public QObject
{
...
signals:
void finished();
private slots:
void run();
}
QThread pThread = new QThread;
Consumer pObject = new Consumer;
// move the pObject to the thread
pObject->moveToThread(pThread);
You can then control the thread with signals and slots.
// assuming you've added a run slot function to the Consumer class
connect(pThread, SIGNAL(started()), pObject, SLOT(run()));
connect(pObject, SIGNAL(finished()), pThread, SLOT(quit()));
connect(pObject, SIGNAL(finished()), pObject, SLOT(deleteLater()));
// Note the thread cleans itself up here, but if the app is quitting,
// waiting on the thread to finish may be required instead
connect(pThread, SIGNAL(finished()), pThread, SLOT(deleteLater()));
And start the thread: -
pThread->start();
Used this way, it also enables multiple objects to be moved to a single new thread, rather than creating a new thread per object instance.