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  4. Cannot connect MyClass::data(QVariant) to (null)::updateData(QVariant)
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Cannot connect MyClass::data(QVariant) to (null)::updateData(QVariant)

Scheduled Pinned Locked Moved QML and Qt Quick
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  • p3c0P Offline
    p3c0P Offline
    p3c0
    Moderators
    wrote on last edited by
    #5

    2nd Attempt will not work since Window requires QQmlApplicationEngine and not QQuickView to load it.

    Have a look at "this":http://qt-project.org/doc/qt-5/qtqml-cppintegration-interactqmlfromcpp.html#invoking-qml-methods for an example.

    Now instead of using QQmlEngine directly you can use more convenient QQmlApplicationEngine:
    @
    QQmlApplicationEngine engine;
    engine.load(QUrl(QStringLiteral("qrc:///main.qml")));

    QObject *rootObject= engine.rootObjects().first();
    qDebug() << rootObject->objectName();

    QVariant returnedValue;
    QVariant msg = "Hello from C++";

    QMetaObject::invokeMethod(rootObject, "updateData",
    Q_RETURN_ARG(QVariant, returnedValue),
    Q_ARG(QVariant, msg));

    qDebug() << "QML function returned:" << returnedValue.toString();
    @

    157

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    0
    • H Offline
      H Offline
      houmingc
      wrote on last edited by
      #6

      i am running Qt on Ubuntu not windows

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      0
      • p3c0P Offline
        p3c0P Offline
        p3c0
        Moderators
        wrote on last edited by
        #7

        It's Window not Windows. Window here refers to the root element which you have used in the QML file.
        I think you must start with the basics of QML first.

        157

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        • H Offline
          H Offline
          houmingc
          wrote on last edited by
          #8

          Thanks, QMetaObject::InvokeMethod works better than signals/slot for C++ invoking QML. From same document, QML invoking C++, i could not compile, below is my code. Can help?
          @
          -----------------header file myclass----------------
          #include <QDebug>

          class MyClass: public QObject
          {
          Q_OBJECT
          public slots:
          void cppSlot(const QString &msg){
          qDebug()<<"Called the c++ slot with message"<<msg;
          }
          };

          -----------------cpp file----------------
          #include <QApplication>
          #include <QQmlApplicationEngine>
          #include <QQuickView>
          #include <QDebug>
          #include "myclass.h"

          int main(int argc, char *argv[])
          {
          QGuiApplication app(argc, argv);

          QQuickView view("qrc:///main.qml");
          QObject *item= view.rootObject();
          
          MyClass myClass;
          QObject::connect(item,SIGNAL(qmlSignal(QString)),&myClass,SLOT(cppSlot(QString)));
          
          return app.exec&#40;&#41;;
          

          }

          -----------------qml file----------------
          import QtQuick 2.2
          import QtQuick.Controls 1.1

          ApplicationWindow {
          id: item
          visible: true
          width: 640
          height: 480
          title: qsTr("Hello World")

          signal qmlSignal(string msg)
          

          MouseArea{
          anchors.fill:parent
          onClicked: item.qmlSignal("Hello from QML")
          }
          }
          @

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          0
          • p3c0P Offline
            p3c0P Offline
            p3c0
            Moderators
            wrote on last edited by
            #9

            What error do you get during compiling?

            157

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            • p3c0P Offline
              p3c0P Offline
              p3c0
              Moderators
              wrote on last edited by
              #10

              To use ApplicationWindow/Window you will need to use QQmlApplicationEngine and not QQuickView. The error should be at runtime not compiletime.

              157

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              • H Offline
                H Offline
                houmingc
                wrote on last edited by
                #11

                error at QQuickView and QObject statement.

                @

                no matching function for call to 'QQuickView::QQuickView(const char[16])'
                cannot convert 'QQuickItem' to 'QObject *' in initialization.
                @

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                • p3c0P Offline
                  p3c0P Offline
                  p3c0
                  Moderators
                  wrote on last edited by
                  #12

                  Use QQmlApplicationEngine for loading ApplicationWindow/Window.
                  @
                  QQmlApplicationEngine engine;
                  engine.load(QUrl(QStringLiteral("qrc:///main.qml")));
                  QObject *rootObject= engine.rootObjects().first();
                  QObject::connect(rootObject,SIGNAL(qmlSignal(QString)),
                  &myClass,SLOT(cppSlot(QString)));
                  @

                  Above should work.

                  157

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                  • H Offline
                    H Offline
                    houmingc
                    wrote on last edited by
                    #13

                    insert statement 'MyClass myclass;' before QObject::connect.
                    2 error encounter:
                    undefined reference to 'vtable for MyClass'
                    ld returned 1 exit status

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                    • p3c0P Offline
                      p3c0P Offline
                      p3c0
                      Moderators
                      wrote on last edited by
                      #14

                      Run qmake and then Rebuild.

                      157

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                      • H Offline
                        H Offline
                        houmingc
                        wrote on last edited by
                        #15

                        I created a new header file for class 'MyClass'
                        Now my error is
                        QQmlApplicationEngine failed to load component
                        Assert:"!isEmpty()" in file /include/QtCore/qlist.h
                        The program has unexpectedly finished.

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                        0
                        • p3c0P Offline
                          p3c0P Offline
                          p3c0
                          Moderators
                          wrote on last edited by
                          #16

                          I don't know. You must have misplaced something somewhere. Better would be to create a new project with that class and qml file. Use the code that i posted above to launch the qml file. It is not a very complicated code.

                          157

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                          • H Offline
                            H Offline
                            houmingc
                            wrote on last edited by
                            #17

                            Thanks very much.

                            1 Reply Last reply
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