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! 🤷‍♂️