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Unable to change the qml property from c++

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

    Hi,

    You should also provide:

    • your QML code.
    • the Qt version you are using
    • the platform you are running on

    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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    • KazuoAsanoK Offline
      KazuoAsanoK Offline
      KazuoAsano
      Qt Champions 2018
      wrote on last edited by
      #3

      In additional informations,
      I try shot code. It's fine in Qt5.12 linux x64.
      Is there a difference in those?

      main.cpp

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

      main.qml

      import QtQuick 2.9
      import QtQuick.Controls 2.2
      
      ApplicationWindow {
        visible: true
        width: 100
        height: 100
        title: qsTr("test")
      
        Rectangle {
          objectName: "colorBox"
      
          width: parent.width
          height: parent.height
          anchors.fill: parent
      
          color: "blue"
        }
      }
      
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      • dheerendraD Offline
        dheerendraD Offline
        dheerendra
        Qt Champions 2022
        wrote on last edited by dheerendra
        #4

        What ever u tried will work only if the object is created and displayed. I am not sure about your first sample. Second sample shows your object is created in c++ and not displayed anywhere. I suspect that your qml object is not displayed. So you have nothing to visually check.

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

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        • J Offline
          J Offline
          JennyAug13
          wrote on last edited by
          #5

          Unable to visually check, but the debug statements after setProperty line of code are executed successfully, will this link helps in doing visual change? https://stackoverflow.com/questions/51824477/changing-qml-object-value-from-c

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          • J Offline
            J Offline
            JennyAug13
            wrote on last edited by JennyAug13
            #6

            Yes, the property is succesfully changed but cannot visualize it in the view. Will setting context property help in visual change of the color property in my qml file? In my case, it is not like passing a data from model to qml file but should pass a color change to the qml file. Will this article helps?

            http://doc.qt.io/qt-5/qtqml-cppintegration-contextproperties.html

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            • dheerendraD Offline
              dheerendraD Offline
              dheerendra
              Qt Champions 2022
              wrote on last edited by
              #7

              There is nothing like passing property to qml file. Qml file is component. Please note what u r changing is object property. So you have to have object displayed.
              Question for you - how are you telling property is not changed ?

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

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              • J Offline
                J Offline
                JennyAug13
                wrote on last edited by
                #8

                I have my object being displayed already in my qml file as

                objectName: "colorBox"
                
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                • dheerendraD Offline
                  dheerendraD Offline
                  dheerendra
                  Qt Champions 2022
                  wrote on last edited by
                  #9

                  Show me your qml code. Also in the first code sample what happened ? Did it come to if condition ?

                  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
                    #10
                    This post is deleted!
                    1 Reply Last reply
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                    • dheerendraD Offline
                      dheerendraD Offline
                      dheerendra
                      Qt Champions 2022
                      wrote on last edited by dheerendra
                      #11

                      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 ?

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

                      J 1 Reply Last reply
                      0
                      • 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
                              0
                              • 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

                                        1 Reply Last reply
                                        1
                                        • 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.

                                          1 Reply Last reply
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