Why is QThread::finished signal not being emitted?

Question

I've created my own TestService which runs on a separate QThread, but when the MainLoop terminates the QThread::finished signal does not get emitted. I saw a similar question, but the problem was slightly different there because the OP was overloading QThread whereas I simply move my class to the thread.

Note that I do not overload the QThread class, I only overload QObject based on this example: http://mayaposch.wordpress.com/2011/11/01/how-to-really-truly-use-qthreads-the-full-explanation/

Here is my TestService class:

#include <QObject>
#include <QThread>
#include <QMutex>
#include <QWaitCondition>
#include <iostream>

using namespace std;
class TestService: public QObject
{
    Q_OBJECT;
private:
    volatile int _count;
    QWaitCondition _monitor;
    QMutex _mutex;
    QThread* _thread;

public:
    TestService(int numSeconds)
    {
        _count = numSeconds;
        _thread = NULL;
        cout << "TestService()" << endl;
    }

    virtual ~TestService()
    {
        cout << "~TestService()" << endl;
    }

    void Start()
    {
        QMutexLocker locker(&_mutex);
        if(_thread == NULL)
        {
            _thread = new QThread;

            // Move this service to a new thread
            this->moveToThread(_thread);

            // The main loop will be executed when the thread
            // signals that it has started.
            connect(_thread, SIGNAL(started()), this, SLOT(MainLoop()));

            // Make sure that we notify ourselves when the thread
            // is finished in order to correctly clean-up the thread.
            connect(_thread, SIGNAL(finished()), this, SLOT(OnFinished()));

            // The thread will quit when the sercives
            // signals that it's finished.
            connect(this, SIGNAL(Finished()), _thread, SLOT(quit()));

            // The thread will be scheduled for deletion when the 
            // service signals that it's finished
            connect(this, SIGNAL(Finished()), _thread, SLOT(deleteLater()));

            // Start the thread
            _thread->start();
        }
    }

    void Stop()
    {
        _count = 0;
        _monitor.wakeAll();
    }
private slots:
    void MainLoop()
    {
        cout << "MainLoop() Entered" << endl;
        while(_count > 0)
        {
            cout << "T minus " << _count << " seconds." << endl;

            QMutexLocker locker(&_mutex);
            _monitor.wait(&_mutex, 1000);
            _count--;
        }
        cout << "MainLoop() Finished" << endl;
        emit Finished();
    }

    virtual void OnFinished()
    {
        cout << "OnFinished()" << endl;
    }
signals:
    void Finished();
};

Here is the testing code:

void ServiceTest()
{
    cout << "Press q to quit." << endl;
    cout << "Press s to start." << endl;
    cout << "Press t to stop." << endl;
    QSharedPointer<TestService> testService(new TestService(10));
    char in = 'a';
    while( in != 'q' )
    {
        switch(tolower(in))
        {
        case 's':
            testService->Start();
            break;
        case 't':
            testService->Stop();
            break;
        default:
            break;
        }
        cin.get(in);
        in = tolower(in);
    }
}

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    ServiceTest();

    QTimer::singleShot(0, &a, SLOT(quit()));

    return a.exec();
}

The output is:

Press q to quit.
Press s to start.
Press t to stop.
TestService()
s
MainLoop() Entered
T minus 10 seconds.
T minus 9 seconds.
T minus 8 seconds.
t
MainLoop() Finished
q
~TestService()
Press any key to continue . . .

Could anybody explain why is finished not being emitted how I can fix it?

Answer 1

Signal finished() gets emitted of cause, but you don't catch it.

Here:

connect(_thread, SIGNAL(finished()), this, SLOT(OnFinished()));

Qt::QueuedConnection is used, as _thread and this (service) are in different threads.

By the time finished() is emitted, _thread's event loop already finished executing, so signal will not be delivered to the slot.

You can explicitly use Qt::DirectConnection.

EDIT:

QTherad works like this:

QThread::start()
{
    emit started();
    run();
    emit finished();
}

QThread::run()
{
     eventloop->exec();
}

So, by the time finished is emitted, eventloop already stop execution. And as you move service to _thread, service's event loop is _thread event loop.

Note, that QObject itself has no its own event loop. Event loops are created by dialogs, threads and application.

---

Actually I will recommend in your simple case just use QtConcurent::run, as you do not perform actual event processing in the new thread, but just run single function.