How to declare metatypes in QML

Pieter Cardoen

Dear

I'm facing a problem with metatypes in QML. I have defined a custom metatype in c++:

class Topic
{
Q_GADGET
Q_PROPERTY(bool valid ...)
}
Q_DECLARE_METATYPE(Topic)

In main, I have included this:

qRegisterMetaType<Topic>("Topic");

When propagating these elements from c++ to QML, everything is working. I can access the property.

I however have a scenario where I want to declare such an object in qml and transmit it to the c++. For that, I want to declare a variable of type "Topic" and use it in a Q_INVOKABLE function. I however can't find a way to declare such a "Topic" object in qml.

How do I do this?

Bob64

@Pieter-Cardoen if I understand you correctly, I don't think it will be possible to do what you want to do. The C++ can only ever know about types declared in C++ at compile time.

Pieter Cardoen

Dear @Bob64

The topic goes about using the metatype in qml/javascript. I want to do this kind of things:

MouseArea{
    onClicked: {
        Topic t // Declare variable -> this is what I want to do
        t.valid = false
        // use t in a Q_INVOKABLE function
    }
}

GrecKo

Sadly this is not possible for the moment : https://bugreports.qt.io/browse/QTBUG-72223

What you can do instead is have a c++ factory Q_INVOKABLE function returning a gadget:

MouseArea{
    onClicked: {
        let t = cppBackend.createTopic();
        t.valid = false
        // use t in a Q_INVOKABLE function
    }
}