I have what I'm calling a C++ "service" who's interface I want to expose to QML. I'm trying to use QQmlContext's setContextProperty to link the object into the QML and connect to it from a QML Connections block.

QML isn't complaining with a reference error as it did previously when I hadn't registered the service in the QML context:

qrc:/main.qml:13: ReferenceError: service is not defined

So, QML seems to find the service object now, however the QML slot javascript function is not getting invoked. I see this in Qt Creator:

Debugging starts
QML debugging is enabled. Only use this in a safe environment.
QML Debugger: Waiting for connection on port 62301...
Calling the clbk signal
Debugging has finished

There should be an In onClbk message per console.log("In onClbk"); I know that I can use QMetaObject::invokeMethod to invoke a QML object's function directly, but I am trying to have a little looser coupling through the use of signals and slots.

I would like to avoid creating a QQuickItem and instantiating the service in the QML, if at all possible.

Unfortunately, the boilerplate code is legion and this is my SSCCE.

Here is a zip file of all the project directory as created through Qt Creator 5.4.

main.cpp

int main(int argc, char *argv[])
{
    QGuiApplication app(argc, argv);
    QQmlApplicationEngine engine;
    auto rc = engine.rootContext();
    auto service = new Service();
    engine.rootContext()->setContextProperty(QStringLiteral("service"), service);
    engine.load(QUrl(QStringLiteral("qrc:/main.qml")));

    // invoke the trigger arbitrarily
    QTimer *timer = new QTimer();
    timer->setSingleShot(true);
    QObject::connect(timer, SIGNAL(timeout()), service, SLOT(trigger_clbk()));
    timer->start(4000);
    return app.exec();
}

service.h

class Service : public QQuickItem {
    Q_OBJECT

public:
    virtual ~Service(){}
signals:
    void clbk();
public slots:
    void trigger_clbk() {
        qDebug()<<"Calling the clbk signal";
        clbk();
    }
};

main.qml

import QtQuick 2.4
import QtQuick.Window 2.2

Window {
    visible: true
    MainForm {
        anchors.fill: parent
        mouseArea.onClicked: {
            Qt.quit();
        }
        // subscribe to a signal
        Connections {
            target: service
            onClbk: function(){
                console.log("In onClbk");
            }
        }
    }
}

Main.ui.qml

import QtQuick 2.3

Rectangle {
    property alias mouseArea: mouseArea

    width: 360
    height: 360

    MouseArea {
        id: mouseArea
        anchors.fill: parent
    }

    Text {
        anchors.centerIn: parent
        text: "Hello World"
    }
}