Illegal attempt to connectthe class that is in a different thread that the QML engine

[MaximBozek]

Hi,

I have this class to handle incoming CAN Frames. I want to emit these frames to be printed in a TextArea, but I am getting this error:

QML debugging is enabled. Only use this in a safe environment.
QML thread running
Connected to CAN interface: vcan0
WARNING: QApplication was not created in the main() thread.
qrc:/Depoortere_Visualization_2/main.qml:220:9: QML Connections: Implicitly defined onFoo properties in Connections are deprecated. Use this syntax instead: function onFoo(<arguments>) { ... }
QQmlEngine: Illegal attempt to connect to CANHandler(0x7ffe0ef65080) that is in a different thread than the QML engine QQmlApplicationEngine(0x7fc4b6d42ac0.

I have tried numerous thing to get this working, but ended with the same error each time. I am running this in separate threads to be able to continuously read incoming CAN frames. Anybody have any idea on a fix.

This is my main.cpp:

#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <QQmlContext>
#include <iostream>
#include <thread>
#include "CANHandler.h"
#include "TranslationHandler.h"

int qmlFunction(int argc, char *argv[], CANHandler *canHandler) {
    QGuiApplication app(argc, argv);
    QQmlApplicationEngine engine;

    TranslationHandler transHandler;
    engine.rootContext()->setContextProperty("transHandler", &transHandler);

    engine.rootContext()->setContextProperty("canHandler", canHandler);

    const QUrl url("qrc:/Depoortere_Visualization_2/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);

    if (engine.rootObjects().isEmpty()) {
        std::cerr << "QML failed to load." << std::endl;
        return -1;
    }

    return app.exec();
}

int main(int argc, char *argv[])
{
    CANHandler canHandler;

    std::thread canThread([&]() {
        QObject::connect(&canHandler, &CANHandler::newFrameReceived, [&](const QString &frame) {
            std::cout << frame.toStdString() << std::endl;
        });
        canHandler.mainFunction();
    });

    std::thread qmlThread(qmlFunction, argc, argv, &canHandler);

    if (canThread.joinable()) {
        std::cout << "CAN thread running" << std::endl;
        canThread.join();
    }

    if (qmlThread.joinable()) {
        std::cout << "QML thread running" << std::endl;
        qmlThread.join();
    }

    return 0;
}

[J.Hilk]

[MaximBozek]

@J-Hilk Thanks for the help...

[J.Hilk]

@MaximBozek said in Illegal attempt to connectthe class that is in a different thread that the QML engine:

@J-Hilk Thanks for the help...

You're welcome.

It might look silly, because its answered as a meme, but its generally the problem and solution.

your CANHandler is created as the first thing on the stack. Way later the QGuiApplication is created on the stack of a different thread even.

[MaximBozek]

Yes, but I want these running in different threads. I read and saw some material on Signals and slots, but I haven't really figured out how it works completaly. I think it should be possible with signals and slots.

[Christian Ehrlicher]

Why can't you simply do what @J-Hilk told you to do - create a QGuiApplication first.

[Pl45m4]

@J-Hilk

This masterpiece deserves an extra medal 🤣