Qt quit both c++ and UI



  • Hello,

    I have a Qt5 interface that runs both C++ and a UI made with QML.
    At one point, I have to quit the program. Before, what I was doing was Qt.quit() in the QML and everything was ok.

    However, now I have some threads that I must close in the main.cpp before quitting. Instead of doing Qt.quit() in the QML, I want to close everything from my main.cpp such as

    *main.cpp
    loop1.quit();
    loop2.quit();
    quitqt ?
    

    the "quitqt" is what I miss. I tried different things from information I found on forums but the QML UI never quits and continue to run.
    I tried for instance: app.quit() and QApplication.quit()

    What could I do ?

    Thank you very much for your help,

    Alex


  • Lifetime Qt Champion

    Hi,

    What about :

    QGuiApplication app(argc, argv);
    
    // your stuff
    
    int ret = app.exec();
    
    // Stop your threads cleanly
    
    return ret;
    


  • Thank you for your answer. However, the QML UI still remains.
    That seems to be normal with this code. The following is the template code for a new Qt project:

    *main.cpp
    int main(int argc, char *argv[])
    {
        QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
        QGuiApplication app(argc, argv);
    
        QQmlApplicationEngine engine;
        engine.load(QUrl(QLatin1String("qrc:/main.qml")));
    
        return app.exec();
    }
    

    This main.cpp program will run and execute "return app.exec();" and the QML UI will remain.
    In QML, the function "Qt.quit()" closes everythin but I don't know how to do the same through the main.cpp.

    Thank you for your help !

    Alex


  • Lifetime Qt Champion

    If I understand things correctly you would like to be able to quit your threads before ending the application, right ?

    Hence my suggestion to store the return value from app.exec(), then quit your threads and then return the value you stored before.

    Or do you want your threads to quit your application ?



  • Sorry, I think I was not clear.
    I want to quit my threads (this is done ok) and then I want to quit the application.
    The problem is that in the main.cpp, only doing "return app.exec();" does not close my Qt application. The QML UI remains open. I want to close this QML UI. I can do it in the QML code with Qt.Quit() but I would like to do it in my main.cpp instead.

    Thank you very much,

    Alex


  • Lifetime Qt Champion

    Then call qApp->quit() once you're done with your threads.



  • Thank you again for your time.
    However it's still the same, the UI remains...


  • Lifetime Qt Champion

    Can you show the code you are using ?



  • Here is the main.cpp code:

    #include <QGuiApplication>
    #include <QQmlApplicationEngine>
    
    int main(int argc, char *argv[])
    {
        QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
        QGuiApplication app(argc, argv);
    
        QQmlApplicationEngine engine;
        engine.load(QUrl(QLatin1String("qrc:/main.qml")));
    
        qApp->quit();
    
        return app.exec();
    }
    

    The QML code is a simple empty window.

    When I run this main.cpp code, even with "qApp->quit();", the program does not quit (the QML window remains open).

    From this forum, there seems to be other solutions. I tried "exit(1)" which seems to work but I am not sure it exits cleanly.
    http://stackoverflow.com/questions/10038372/how-to-terminate-a-qt-programming-if-initialization-failed

    Thank you again for your time !

    Alex


  • Qt Champions 2016

    @alecs26 said in Qt quit both c++ and UI:

    When I run this main.cpp code, even with "qApp->quit();", the program does not quit (the QML window remains open).

    Then there's something fishy. Either some thread is hanging or your QML code is hanging. As a side note, how do you ensure the threads had quit before returning and how do you start them (there's nothing even remotely resembling this in the C++ code you pasted).

    By the way

    qApp->quit();
    

    does nothing here, there's no event loop running. You could try:

    QMetaObject::invokeMethod(qApp, "quit", Qt::QueuedConnection);
    

    instead.

    I tried "exit(1)" which seems to work but I am not sure it exits cleanly.

    Never do that without a good reason. ::exit's purpose is to terminate the application in extreme circumstances, not whenever one feels like it. For one it will not call any of your object's (app and engine) destructors, which is bad enough by itself and has serious implications.



  • @kshegunov @SGaist
    The code I posted is for a simple program that just launches a qml window. It was to simplify the use case. Even with this simple case, I can't get the QML window to close (except with "exit(1)").
    The QML code is simply:

    import QtQuick 2.7
    import QtQuick.Controls 2.0
    import QtQuick.Layouts 1.0
    
    ApplicationWindow
    {
        visible: true
        width: 640
        height: 480
        title: qsTr("Hello World")
    
    }
    

    Your suggestion:

    QMetaObject::invokeMethod(qApp, "quit", Qt::QueuedConnection);
    

    seems to work. Can you tell me what it does different of qApp->quit ?
    I guess it quits cleanly ?

    Thank you !

    Alex


  • Lifetime Qt Champion

    Calling qApp->quit() before exec() won't have any effect since the event loop isn't even started.


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