Calling a QML function from C++

Question

I'm developing a Blackberry 10 mobile application using the BB Native SDK.

I need to call a QML function into my C++ class. I searched a lot on this but I only found the possibility to call C++ into QML not the inverse. You can check this: QML and C++ integration

Can anyone help me on this?

This is the QML code specifying the function that I need to call which add a marker into my mapview:

Container {
    id: pinContainer
    objectName: "pinContObject"
        ...

        function addPin(lat, lon, name, address) {
            var marker = pin.createObject();
            marker.lat = lat;
            marker.lon = lon;
            ...
        }
}

Arriola

Answer 1

Thats what signals and slots are for. You can use the QML Connections for connecting arbitrary signals to arbitrary slots in QML.

http://qt-project.org/doc/qt-4.8/qml-connections.html

  Container {
            id: pinContainer
            objectName: "pinContObject"
            ...

            function addPin(lat, lon, name, address) {
                var marker = pin.createObject();
                marker.lat = lat;
                marker.lon = lon;
                ...
            }

            Connections {
                target: backend
                onDoAddPin: { 
                  addPin(latitude, longitude,name, address)
                }
            }
     }

and in C++ backend, all you have to do is

class Backend: public QObject {
signals:
    void doAddPin(float latitude, float longitude, QString name, QString address);

 ........
 void callAddPinInQML(){
     emit doAddPin( 12.34, 45.67, "hello", "world");
 }
}

Marianamariand

Really greatfull for your answer !!! but how to pass parameters in c++ and affect it to the qml function parameters ? – Arriola

Updated the example to pass function parameters via. signals. The javascript function/variable scoping rules apply if there are any namespace collisions of the variables with the function params of the signals. Good Luck :) – Marianamariand

Thx ^_^ !! But unfortunately, when I try to run the application in my BB device, it doesn't want to load; I checked and I found that there is a problem when I add the connection property in the QML file. I'm really confused :/ – Arriola

Answer 2

MyItem.qml

import QtQuick 2.0
  Item {
    function myQmlFunction(msg) {
      console.log("Got message:", msg)
      return "some return value"
  }
}

main.cpp

QQmlEngine engine;
QQmlComponent component(&engine, "MyItem.qml");
QObject *object = component.create();

QVariant returnedValue;
QVariant msg = "Hello from C++";
QMetaObject::invokeMethod(object, "myQmlFunction",
    Q_RETURN_ARG(QVariant, returnedValue),
    Q_ARG(QVariant, msg));

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

This should works. Here i am calling a QML function from my C++ class.

Hyaline

This is exactly the code which can be found at Qt's documentation: [doc.qt.io/qt-5/… – Pessa

Qt's documentation is good enough but people don't have time to read them. They need specific info quickly that i provided. – Hyaline

You should still give credits, though. Or even provide the link to the docs – Icarus

Answer 3

From Qt documentation:

http://qt-project.org/doc/qt-5/qtqml-cppintegration-interactqmlfromcpp.html#invoking-qml-methods

Calling QMetaObject::invokeMethod one will call proper slot, no matter if it is defined in C++ or QML.

Plainsman

Answer 4

there is another way using binding, when the value of some x value is changed on Cpp side and it is binded on the qml side, the onchange will be called on qml side. This solution is easier and more direct than connecting signal-to-signal as noticed here, however if you want to transfer data within the signal I recommend to use signal-signal connection

ex:

qml side

    import com.drOmranConsulting.backend 1.0

    BackEnd {
        id: _backend
    }

Page1Form {
     objectName: "page1box"

        /* property alias recchange: _backend.rec_x
           signal rec_xChanged is transmitted from C++
           when rec_x is newly set. To receive this signal
           an alias is used in qml side to the value rec_x
           the binding will be updated upon signal emitting
        */
        property alias recXchange: _backend.rec_x

    onRecXchangeChanged:
        {
           console.log("signal received")
           console.log(" the x coordinate is changed on cpp side to " + _backend.getRecX() )
           _rect.x=_backend.getRecX()
        }
     }

backend.h

signals:
    void rec_xChanged();            /* cpp emits signal rec_xChanged             */
                                    /* when rec_x value has been changed         */
                                    /* in QML: value is aliased using            */
                                    /* property alias recchange: _backend.rec_x  */
                                    /* detecing signal at onRecchangeChanged     */

in main.cpp

//Backend class Initialization
backend Backend;
qmlRegisterType<backend>("com.drOmranConsulting.backend",1,0,"BackEnd");

in backend.cpp

emit rec_xChanged();

Skin