Connection is not processed in thread

Devoo

Hi,
For some reason connection is not processed in thread.
Thread contains an infinite loop with QCoreApplication::processEvents();

Minimal code reproduction.
main.cpp

#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <thread>
#include "objectinthread.h"



int main(int argc, char *argv[])
{
    QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);

    QGuiApplication app(argc, argv);

    QQmlApplicationEngine engine;
    const QUrl url(QStringLiteral("qrc:/main.qml"));
    QObject::connect(&engine, &QQmlApplicationEngine::objectCreated,
                     &app, [url](QObject *obj, const QUrl &objUrl) {
        if (!obj && url == objUrl)
            QCoreApplication::exit(-1);
    }, Qt::QueuedConnection);
    engine.load(url);


    auto t = std::thread([]()
    {
        ObjectInThread objectInThread;
        while(1)
        {
            QCoreApplication::processEvents();
            std::this_thread::yield();
        }
    });
    t.detach();

    return app.exec();
}

objectinthread.H:

#pragma once
#include <QCoreApplication>
#include <QObject>
#include <QDebug>
#include <thread>
#include <chrono>

class ObjectInThread2 :
    public QObject
{
    Q_OBJECT
public:

public slots:
    void test()
    {
        qDebug() << "success";
    }
};

class ObjectInThread :
    public QObject
{
    Q_OBJECT
public:
    ObjectInThread()
    {
        auto t = std::thread([this]()
        {
            ObjectInThread2 objectInThread2;
            connect
            (
                this,
                &ObjectInThread::emitSignal,
                &objectInThread2,
                &ObjectInThread2::test
            );
            while (1)
            {
                std::this_thread::sleep_for(std::chrono::seconds(1));
                QCoreApplication::processEvents();
                qDebug() << "processEvents";
            }
        });
        t.detach();
        while (1)
        {
            std::this_thread::sleep_for(std::chrono::seconds(5));
            qDebug() << "emit";
            emit emitSignal();
        }
    }

signals:
    void emitSignal();

};

Output:

processEvents
processEvents
processEvents
processEvents
processEvents
emit
processEvents
processEvents
processEvents

Slot test() is never used.
Any idea how to fix it?

JonB

@Devoo
You may well know more than I do about this, but if this is to work at all across threads should you be using QThread and its support rather than std::thread?

Devoo

The same problem with QThread.

Christian Ehrlicher

@Devoo said in Connection is not processed in thread:

With Qthread the same issue.

Please take a look at the QThread documentation. Esp. about the thread-local eventloop. You're not calling / executing it so it will not work.