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Qt quit both c++ and UI

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  • SGaistS Offline
    SGaistS Offline
    SGaist
    Lifetime Qt Champion
    wrote on last edited by
    #2

    Hi,

    What about :

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

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    • A Offline
      A Offline
      alecs26
      wrote on last edited by
      #3

      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

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      • SGaistS Offline
        SGaistS Offline
        SGaist
        Lifetime Qt Champion
        wrote on last edited by
        #4

        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 ?

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        Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

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        • A Offline
          A Offline
          alecs26
          wrote on last edited by
          #5

          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

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          • SGaistS Offline
            SGaistS Offline
            SGaist
            Lifetime Qt Champion
            wrote on last edited by
            #6

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

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            Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

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            • A Offline
              A Offline
              alecs26
              wrote on last edited by
              #7

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

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              • SGaistS Offline
                SGaistS Offline
                SGaist
                Lifetime Qt Champion
                wrote on last edited by
                #8

                Can you show the code you are using ?

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                Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

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                • A Offline
                  A Offline
                  alecs26
                  wrote on last edited by
                  #9

                  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

                  kshegunovK 1 Reply Last reply
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                  • A alecs26

                    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

                    kshegunovK Offline
                    kshegunovK Offline
                    kshegunov
                    Moderators
                    wrote on last edited by kshegunov
                    #10

                    @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.

                    Read and abide by the Qt Code of Conduct

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                    • A Offline
                      A Offline
                      alecs26
                      wrote on last edited by
                      #11

                      @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

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                      • SGaistS Offline
                        SGaistS Offline
                        SGaist
                        Lifetime Qt Champion
                        wrote on last edited by
                        #12

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

                        Interested in AI ? www.idiap.ch
                        Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

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