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Unable to change the qml property from c++

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  • dheerendraD dheerendra

    Are you loading this qml in main.cpp ? Can you tell me what is the meaning of the following statement in your code ?
    QObject object = QObject::findChild <QObject>("colorBox");

    For which object you are trying to find a child ?

    @KazuoAsano has already given the sample example. Did you compare with your program ?

    J Offline
    J Offline
    JennyAug13
    wrote on last edited by
    #12

    @dheerendra Yes I am loading the qml in my main.cpp but it is in the second case. In the first case iam finding the child from another class in cpp function.

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    • dheerendraD Offline
      dheerendraD Offline
      dheerendra
      Qt Champions 2022
      wrote on last edited by
      #13

      If you are finding a object, then it should work. Please note you are loading item object. Item does not have color property. So nothing works. In fact setProperty(...) method should fail. Check the return value of setProperty.

      if (!obj->setProperty("color","blue")) {
          qWarning() << " Failed to set the color Property" <<endl;
          return;
      }
      

      Dheerendra
      @Community Service
      Certified Qt Specialist
      http://www.pthinks.com

      J 1 Reply Last reply
      0
      • dheerendraD dheerendra

        If you are finding a object, then it should work. Please note you are loading item object. Item does not have color property. So nothing works. In fact setProperty(...) method should fail. Check the return value of setProperty.

        if (!obj->setProperty("color","blue")) {
            qWarning() << " Failed to set the color Property" <<endl;
            return;
        }
        
        J Offline
        J Offline
        JennyAug13
        wrote on last edited by
        #14

        @dheerendra The program is crashing during this check

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        • dheerendraD Offline
          dheerendraD Offline
          dheerendra
          Qt Champions 2022
          wrote on last edited by
          #15

          I suspect that obj is null. Please check your program. Please check the sample given by @KazuoAsano & compare your program.

          Dheerendra
          @Community Service
          Certified Qt Specialist
          http://www.pthinks.com

          J 1 Reply Last reply
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          • dheerendraD dheerendra

            I suspect that obj is null. Please check your program. Please check the sample given by @KazuoAsano & compare your program.

            J Offline
            J Offline
            JennyAug13
            wrote on last edited by
            #16

            @dheerendra Yes, the object is null. it is not finding the child when i used QQml engine and not QQmlApplication engine from my main.cpp file.

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            • dheerendraD Offline
              dheerendraD Offline
              dheerendra
              Qt Champions 2022
              wrote on last edited by
              #17

              Please show the main.cpp & how are you loading the qml file. Based on this we can suggest. I'm assuming that you are loading the qml which has 'colorBox' object.

              Dheerendra
              @Community Service
              Certified Qt Specialist
              http://www.pthinks.com

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              • J Offline
                J Offline
                JennyAug13
                wrote on last edited by JennyAug13
                #18

                I tried in this way the same way said by @KazuoAsano, i placed a rectangle in main.qml file and from main.cpp tried to set the property color as red and it remained in blue and here is my code.

                main.qml file

                Gui {
                 id: MainView;
                
                    objectName: "MainView"
                
                    Rectangle{
                        objectName: "colorBox"
                        width: 200
                        height: 100
                        anchors.horizontalCenter: parent.horizontalCenter
                        anchors.verticalCenter: parent.verticalCenter
                        //anchors.centerIn: parent.Center
                        color: "blue"
                    }
                    Loader {
                

                Here is my main.cpp file

                 QQmlApplicationEngine engine;
                    engine.load(QUrl(QStringLiteral("qrc:/QML/main.qml")));
                    if (engine.rootObjects().isEmpty())
                        return -1;
                
                    QObject *rootObject = engine.rootObjects().first();
                    QObject *object = rootObject->findChild<QObject*>("colorBox");
                    if(object)
                    {
                        QColor color(Qt::red);
                        object->setProperty("color",color);
                        qDebug("Change red color");
                    }
                
                

                The colorBox remained in the blue color.

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                • J Offline
                  J Offline
                  JennyAug13
                  wrote on last edited by JennyAug13
                  #19

                  Yes, setProperty will fail because, it is inside an item. And also when i try to print the object name and children count, it shows correct count for the children and also root object name. The actual children count is displayed as 2 with the

                  children().count();
                  

                  whereas when i try to find the object names

                  using

                  QList<QObject*> list = rootObject->findChildren<QObject *>();
                      qDebug("list = %d",list.count());
                       int i;
                       for(i=0;i<list.count();i++)
                       {
                          QObject *obj = list[i];
                          qDebug() << "Object" << i << obj->objectName();
                       }
                  

                  it displays list count as 81 and among some object names some are empty and object 28 is my colorBox, but unable to set the color property from c++ file.

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                  • dheerendraD Offline
                    dheerendraD Offline
                    dheerendra
                    Qt Champions 2022
                    wrote on last edited by
                    #20

                    So it means that you are have object called a color box. So you should get the reference of this object when you do findChild. If you are getting this you should be able to set the color. When you say it does not work, what do you mean ? Your object is Item. Can you check whether value is set to color. I'm not sure what are you doing with this property in Item after wards.

                    Dheerendra
                    @Community Service
                    Certified Qt Specialist
                    http://www.pthinks.com

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                    • KazuoAsanoK Offline
                      KazuoAsanoK Offline
                      KazuoAsano
                      Qt Champions 2018
                      wrote on last edited by
                      #21

                      I seem to it is a difficult problem...
                      I wrote code based on this forum thread information,again,

                      https://github.com/sazus/QtForum_topic_98263

                      Can you check the expected behavior of the software?
                      After that you confirm a difference in those and Let's solve this issue with everyone.

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                      • GrecKoG Offline
                        GrecKoG Offline
                        GrecKo
                        Qt Champions 2018
                        wrote on last edited by
                        #22

                        An alternative would be to not do that, it's bad practice and not very maintainable.
                        Some links to coroborate my claim :

                        • https://doc.qt.io/qt-5/qtquick-bestpractices.html#interacting-with-qml-from-c
                        • http://doc.qt.io/qt-5/qtqml-cppintegration-overview.html#interacting-with-qml-objects-from-c
                        • https://youtu.be/vzs5VPTf4QQ?t=23m20s

                        A proper alternative would be to set a QObject as a context property and expose properties in it.

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                        • J Offline
                          J Offline
                          JennyAug13
                          wrote on last edited by JennyAug13
                          #23

                          Yes thankyou, i found a way. But not through qml but tried from two c++ files.

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