Check in C++ whether signal in QML file exists.

[ChrisTof]

Read this post, but it is not helpful: Signal exists
I wonder whether it is possible to detect whether signal in QML files exists before connecting it in C++.
I create a qml component in c++ and try to connect a signal between qml and c++:

QQmlComponent component(qmlEngine, QUrl::fromLocalFile("example.qml"));
    if(component.isError()) {
        return;
    }

    QQmlContext* rootItemContext = QQmlEngine::contextForObject(rootWindow);
    QQuickItem *object = qobject_cast<QQuickItem*>(component.beginCreate(rootItemContext));
    if(object != nullptr) {
        QQmlEngine::setObjectOwnership(object, QQmlEngine::CppOwnership);
        object->setParentItem(rootWindow->contentItem());
        object->setParent(qmlEngine);

        //if(SIGNAL(generateEvent(QVariant, QString) exists)
        QObject::connect(object, SIGNAL(generateEvent(QVariant, QString)), this, SLOT(receiveEventFromQml(QVariant, QString)));

        component.completeCreate();
}

Is it possible to check (during run-time) if signal in qml file exists before connecting it?

[ChrisTof]

Thank you guys, that is working solution for me:

int signalExists = object->metaObject()->indexOfSignal(QMetaObject::normalizedSignature("generateEvent(QVariant, QString)"));
if(signalExists != -1)
    QObject::connect(object, SIGNAL(generateEvent(QVariant, QString)), this, SLOT(receiveEventFromQml(QVariant, QString)));

I had to pass full signature with arguments: (QVariant, QString), without them signal was not found.

@J-Hilk
Yes, it is a working solution but I want to check if the signal exists before connecting, because I want to get rid of warning in case the signal does not exist.

[dheerendra]

You can check from Metaobject. You can see the method indexOfSignal(..).
Also i'm not sure why do you want to do this.

[KroMignon]

@ChrisTof said in Check in C++ whether signal in QML file exists.:

Is it possible to check (during run-time) if signal in qml file exists before connecting it?

You can check this QMetaObject, like this:

 index = object->metaObject()->indexOfSlot(QMetaObject::normalizedSignature("generateEvent"));
    if (index == -1) {
        qWarning("Wrong slot name!");
    }
    else {
        // do connect...
    }

[J.Hilk]

@ChrisTof said in Check in C++ whether signal in QML file exists.:

QObject::connect(object, SIGNAL(generateEvent(QVariant, QString)), this, SLOT(receiveEventFromQml(QVariant, QString)));

you're already using Qt4 Syntax so the connection will be created at run time, simply store the return value of QObject::connect and check if it was successful

QMetaObject::Connection c = QObject::connect(object, SIGNAL(generateEvent(QVariant, QString)), this, SLOT(receiveEventFromQml(QVariant, QString)));
if(c){
   //Successful connected
}

[ChrisTof]

Thank you guys, that is working solution for me:

int signalExists = object->metaObject()->indexOfSignal(QMetaObject::normalizedSignature("generateEvent(QVariant, QString)"));
if(signalExists != -1)
    QObject::connect(object, SIGNAL(generateEvent(QVariant, QString)), this, SLOT(receiveEventFromQml(QVariant, QString)));

I had to pass full signature with arguments: (QVariant, QString), without them signal was not found.

@J-Hilk
Yes, it is a working solution but I want to check if the signal exists before connecting, because I want to get rid of warning in case the signal does not exist.

[Pablo J. Rogina]

@ChrisTof said in Check in C++ whether signal in QML file exists.:

I want to get rid of warning in case the signal does not exist.

If you're using Qt 5, go and use the signal & slot new syntax. You'll have error checking at compile-time for free...

Counter a, b;
QObject::connect(&a, &Counter::valueChanged, &b, &Counter::setValue);

[J.Hilk]

@Pablo-J-Rogina
I don't think Qt5 Syntax works on signals that you define in the QML file only with signals defined in the underlaying c++ class.

However, I'm not entirely sure! 🤷‍♂️