Qt Signal Slot connection issue

Question

Trying to create 2 objects (clients) living in different threads and communicating with 1 object(server) living on the main thread. However I'm not seeing any activity on the server object. The main goal of this code is to see how the signal slot will behave when 2 signals coming from 2 different threads are communicating with a single object on the main thread.

objects.cpp:

    #include "objects.h"

    Objects::Objects(QObject* parent): QObject (parent)
    {
    }

    void Objects::printData(const QString& data)
    {
        qDebug() << data;
    }

    void Objects::process(const QString& data)
    {
        emit sendData(data+":processed");
    }

    Q_NORETURN void Objects::run()
    {
        while (true) {
            //qDebug() << "hit" << m_rate;
            emit sendData("test"+QString::number(m_rate));
            QThread::sleep(m_rate);
        }
    }

objects.h:

    #ifndef OBJECTS_H
    #define OBJECTS_H
    #include <QObject>
    #include <QDebug>
    #include <QThread>

    class Objects: public QObject
    {
        Q_OBJECT
    public:
        explicit Objects(QObject* parent = Q_NULLPTR);

    signals:
        void sendData(const QString&);

    public slots:
        void printData(const QString&);
        void process(const QString&);
        void run();

    public:
        ulong m_rate;

    };

    #endif // OBJECTS_H

main.cpp:

#include <QCoreApplication>
#include "objects.h"

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

    Objects ser;
    Objects cl1;
    Objects cl2;
    cl1.m_rate = 1;
    cl2.m_rate = 2;

    QThread cl1_t;
    QThread cl2_t;

    QObject::connect(&cl1_t, &QThread::started, &cl1, &Objects::run);
    QObject::connect(&cl2_t, &QThread::started, &cl2, &Objects::run);


    QObject::connect(&cl1, &Objects::sendData, &ser, &Objects::process);
    QObject::connect(&cl2, &Objects::sendData, &ser, &Objects::process);

    QObject::connect(&ser, &Objects::sendData, &cl1, &Objects::printData);
    QObject::connect(&ser, &Objects::sendData, &cl2, &Objects::printData);

    cl1.moveToThread(&cl1_t);
    cl2.moveToThread(&cl2_t);

    cl1_t.start();
    cl2_t.start();

    return a.exec();
}

Why don't you use a QTimer with m_rate as the interval instead of the while loop? That way you won't block the event loop, you won't need the QThread::sleep, and your slots will be processed properly. — thuga

Unknown Unknown · Accepted Answer · 2019-02-08T07:57:18

Looks like your void run() function-member blocks Qt internal event queue. Just add QCoreApplication::processEvents(); in to your void run() and it will solve your problem.

void Objects::run()
{
    while (true) {
        qDebug() << "hit" << m_rate;
        emit sendData("test" + QString::number(m_rate));
        QThread::sleep(m_rate);
        QCoreApplication::processEvents();
    }
}

UPD: I will suggest you to read THIS wiki article for more detailed explanation on your problem.

Qt Signal Slot connection issue

1 Answers