The timeout slot is not fired when the timer is called from a separate thread in the main()

[Stavros Vaionitis]

Hi there,

I have the following code

#include <QtConcurrent/QtConcurrent>
#include <QApplication>
#include <QDateTime>
#include <QDebug>
#include <QObject>
#include <QTimer>

class PrintThreadIdTimer : public QObject
{
    Q_OBJECT
public:
    PrintThreadIdTimer(int interval, QObject *parent = nullptr)
    {
        m_timer = new QTimer(this);
        m_timer->setInterval(interval);
        connect(m_timer, &QTimer::timeout, this, &PrintThreadIdTimer::printThreadIdAndTimeMsecs);
    }

    ~PrintThreadIdTimer()
    {
        disconnect(m_timer, &QTimer::timeout, this, &PrintThreadIdTimer::printThreadIdAndTimeMsecs);

        delete m_timer;
        m_timer = nullptr;
    }

    void start()
    {
        m_timer->start();
    }

    void stop()
    {
        m_timer->stop();
    }

    int interval()
    {
        return m_timer->interval();
    }

public slots:
    void printThreadIdAndTimeMsecs()
    {
        qDebug() << Q_FUNC_INFO << "ThreadId:" << QThread::currentThreadId()
                 << "TimeMsecs:" << QDateTime::currentMSecsSinceEpoch();
    }

private:
    QTimer *m_timer = nullptr;
};

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);

    qDebug() << Q_FUNC_INFO << "Thread:" << QThread::currentThreadId()
             << "Time:" << QDateTime::currentMSecsSinceEpoch();

    auto appRunAndExit = [=]()
    {
        // Just wait a little bit to enter the event loop
        QThread::msleep(1000);

        PrintThreadIdTimer *printThreadTimer = new PrintThreadIdTimer(1000);

        printThreadTimer->start();

        for (int i = 0; i < 3; ++i)
        {
            qDebug() << Q_FUNC_INFO
                     << "ThreadId:" << QThread::currentThreadId()
                     << "interval():" << printThreadTimer->interval()
                     << "Time:" << QDateTime::currentMSecsSinceEpoch();

            QThread::msleep(1000);
        }

        printThreadTimer->stop();

        QCoreApplication::quit();
    };
    QThreadPool::globalInstance()->start(appRunAndExit);

    qDebug() << Q_FUNC_INFO << "-> Thread:" << QThread::currentThreadId()
             << "Time:" << QDateTime::currentMSecsSinceEpoch();

    int out = app.exec();

    qDebug() << Q_FUNC_INFO << "---> Thread:" << QThread::currentThreadId()
             << "Time:" << QDateTime::currentMSecsSinceEpoch();

    return out;
}

#include "cpp-qt6-threaded-timers.moc"

The class PrintThreadIdTimer is just a wrapper class on a QTimer which is calling the printThreadIdAndTimeMsecs when the timeout signal is sent.

Now I wanted to use this class on a simple application, but as far as I know, the QTimer needs an event loop, so I used the QApplication::exec().

I used the lambda function appRunAndExit () in order to create a simple example where I am using the PrintThreadIdTimer and then closing the application. I run this function on a separate thread using QThreadPool::globalInstance()->start().

Now, when I run the above example, it seems that the slot function is not called, and I am not really sure why. The output that I am getting is the following

int main(int, char**) Thread: 0x7f420c7ca880 Time: 1700741341051
int main(int, char**) -> Thread: 0x7f420c7ca880 Time: 1700741341052
main(int, char**)::<lambda()> ThreadId: 0x7f4201f7a640 interval(): 1000 Time: 1700741342052
main(int, char**)::<lambda()> ThreadId: 0x7f4201f7a640 interval(): 1000 Time: 1700741343053
main(int, char**)::<lambda()> ThreadId: 0x7f4201f7a640 interval(): 1000 Time: 1700741344054
int main(int, char**) ---> Thread: 0x7f420c7ca880 Time: 1700741345055

What is the issue in the above code? Do I need to change something, or have I understood something wrong?

I am using Qt 6.2.5.

Let me know if you need more information.

Kind regards,

Stavros

[JonB]

@Stavros-Vaionitis
@jsulm knows better than I. It's not a good idea to do sleepy stuff, but I'm not sure that would be the issue here if the timer never runs.

Your thread needs to run a Qt event loop (itself, not just the one in the main thread) for timers to work. I'm not sure your lambda runnable does this (from QRunnable::run()). QThread::run() does. Perhaps @jsulm or someone would comment on whether this is correct/the issue?

[jsulm]

@Stavros-Vaionitis said in The timeout slot is not fired when the timer is called from a separate thread in the main():

What is the issue in the above code?

The for loop with QThread::msleep(1000) - don't do such things in event driven applications.

[JonB]

@Stavros-Vaionitis
@jsulm knows better than I. It's not a good idea to do sleepy stuff, but I'm not sure that would be the issue here if the timer never runs.

Your thread needs to run a Qt event loop (itself, not just the one in the main thread) for timers to work. I'm not sure your lambda runnable does this (from QRunnable::run()). QThread::run() does. Perhaps @jsulm or someone would comment on whether this is correct/the issue?

[Stavros Vaionitis]

Hi @jsulm,

Thanks for your quick reply. I am aware about the sleep() and the for loop for event driven apps, but this is a quick and dirty example that I wanted to test QTimer with a simple example.

[Christian Ehrlicher]

How should the timer send a signal when there is no eventloop running where it can be created?

[Stavros Vaionitis]

Hi @Christian-Ehrlicher and @JonB,

Thank you for your quick replies and the valuable input. I thought that the main event loop is accessible from all the threads that are created there. I modified my code as follows

#include <QtConcurrent/QtConcurrent>
#include <QApplication>
#include <QDateTime>
#include <QDebug>
#include <QMetaObject>
#include <QObject>
#include <QTimer>

class PrintThreadIdTimer : public QObject
{
    Q_OBJECT
public:
    PrintThreadIdTimer(int interval, QObject *parent = nullptr)
    {
        m_timer = new QTimer(this);
        m_timer->setInterval(interval);
        connect(m_timer, &QTimer::timeout, this, &PrintThreadIdTimer::printThreadIdAndTimeMsecs);
    }

    ~PrintThreadIdTimer()
    {
        disconnect(m_timer, &QTimer::timeout, this, &PrintThreadIdTimer::printThreadIdAndTimeMsecs);

        delete m_timer;
        m_timer = nullptr;
    }

    int interval()
    {
        return m_timer->interval();
    }

public slots:
    void start()
    {
        m_timer->start();
    }

    void stop()
    {
        m_timer->stop();
    }

private slots:
    void printThreadIdAndTimeMsecs()
    {
        qDebug() << Q_FUNC_INFO << "ThreadId:" << QThread::currentThreadId()
                 << "TimeMsecs:" << QDateTime::currentMSecsSinceEpoch();
    }

private:
    QTimer *m_timer = nullptr;
};

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);

    qDebug() << Q_FUNC_INFO << "Thread:" << QThread::currentThreadId()
             << "Time:" << QDateTime::currentMSecsSinceEpoch();

    auto appRunAndExit = []()
    {
        PrintThreadIdTimer *printThreadTimer = new PrintThreadIdTimer(1000);
        QThread *thread = new QThread();

        printThreadTimer->moveToThread(thread);

        QObject::connect(thread, &QThread::started, printThreadTimer, &PrintThreadIdTimer::start);
        QObject::connect(thread, &QThread::finished, printThreadTimer, &PrintThreadIdTimer::stop);

        thread->start();

        for (int i = 0; i < 3; ++i)
        {
            qDebug() << Q_FUNC_INFO
                     << "ThreadId:" << QThread::currentThreadId()
                     << "interval():" << printThreadTimer->interval()
                     << "Time:" << QDateTime::currentMSecsSinceEpoch();

            QThread::msleep(1000);
        }

        thread->quit();
        thread->wait();

        QCoreApplication::quit();
    };
    QThreadPool::globalInstance()->start(appRunAndExit);

    qDebug() << Q_FUNC_INFO << "-> Thread:" << QThread::currentThreadId()
             << "Time:" << QDateTime::currentMSecsSinceEpoch();

    int out = app.exec();

    qDebug() << Q_FUNC_INFO << "---> Thread:" << QThread::currentThreadId()
             << "Time:" << QDateTime::currentMSecsSinceEpoch();

    return out;
}

Inside the lambda function I created another thread which I used it in order to move the PrintThreadIdTimer object there. I modified the class PrintThreadIdTimer in order to have start() and stop() slots to be connected with the started() and finished() signals of the thread.

Now, it's working as expected. I am not sure if there is a better way to do this.

Kind regards,

Stavros

[Christian Ehrlicher]

@Stavros-Vaionitis said in The timeout slot is not fired when the timer is called from a separate thread in the main():

I thought that the main event loop is accessible from all the threads th

Every thread has it's own event loop started inside QThread::run(): https://doc.qt.io/qt-6/qthread.html#details

And @JonB already told you this two posts above.