Why don't my signals fire slots without Qt::DirectConnection? - c++

Question

I have a main class that launches a thread that performs a single action. I am trying to catch when the thread operations finish.
The main class:
// .h
class MainClass : public QObject
{
Q_OBJECT
public:
QThread thread;
// ...
public slots:
void onFinish();
}
// .cpp
void MainClass::startThread()
{
thread = new QThread();
worker = new Worker();
worker->moveToThread(thread);
connect(thread, SIGNAL(started()), worker, SLOT(process()));
connect(worker, SIGNAL(finished()), thread, SLOT(quit()));
connect(worker, SIGNAL(finished()), worker, SLOT(deleteLater()));
connect(thread, SIGNAL(finished()), thread, SLOT(deleteLater()));
connect(worker, SIGNAL(finished()), this, SLOT(onFinish()));
// connect(worker, SIGNAL(finished()), this, SLOT(onFinish()), Qt::DirectConnection);
}
void MainClass::onFinish()
{
std::cout << "Finished!" << std::endl << std::flush;
}
The Worker class for the thread:
// .h
class Worker : public QObject
{
Q_OBJECT
public:
explicit Worker(QObject *parent = 0);
~Worker();
public slots:
void process();
signals:
void finished();
}
// .cpp
void Worker::process()
{
// ...Do stuff...
emit finished();
}
Executing startThread does not print "Finished!". Why?
I have noticed that if I add Qt::DirectConnection to the line that connects to onFinish() (like in the commented line), the message is printed. But what can I do if I want to execute the onFinish() actions in the thread of the main class?
Edit 1
In addition, it seems that the finished() -> quit() connect is also not working, because if I call thread->isFinished() or thread->isRunning(), after sleeping in the main thread to be sure that the thread task is completed, I obtain false and true, respectively.
Edit 2
Since it may also be relevant, here is the main.cpp:
int _tmain(int argc, _TCHAR* argv[])
{
QCoreApplication a(argc, argv);
std::unique_ptr <MainClass> mc = std::make_unique <MainClass>();
mc->startThread();
mc->thread->wait();
return a.exec();
}

Answer 1

On a first look I would say that the order of connections is important. The deleteLater() slots should always be the last methods.
connect(thread, SIGNAL(started()), worker, SLOT(process()));
connect(worker, SIGNAL(finished()), this, SLOT(onFinish()));
connect(worker, SIGNAL(finished()), thread, SLOT(quit()));
connect(worker, SIGNAL(finished()), worker, SLOT(deleteLater()));
connect(thread, SIGNAL(finished()), thread, SLOT(deleteLater()));
Sitenode - QRunnable
You might consider to use QRunnable instead, which is designed for short running tasks/jobs. You can combine them with QObject or QFuture/QFutureWatcher to get notified as soon as they a finished.
Update 1
Your main-event loop never starts and it looks like you want to exit the application when the task finishes. I don't think this is a nice implementation but these modifcations should work for you:
int _tmain(int argc, _TCHAR* argv[])
{
QCoreApplication a(argc, argv);
std::unique_ptr<MainClass> mc = std::make_unique<MainClass>();
mc->startThread();
// do not wait!
// let the main-event-loop handle events -> a.exec()
// and quit() application by signal/slot
return a.exec();
}
void MainClass::startThread()
{
thread = new QThread();
worker = new Worker();
worker->moveToThread(thread);
connect(thread, SIGNAL(started()), worker, SLOT(process()));
connect(worker, SIGNAL(finished()), this, SLOT(onFinish()));
connect(worker, SIGNAL(finished()), thread, SLOT(quit()));
connect(worker, SIGNAL(finished()), worker, SLOT(deleteLater()));
connect(thread, SIGNAL(finished()), thread, SLOT(deleteLater()));
connect(thread, SIGNAL(finished()), qApp, SLOT(quit()));
}