C++ return to QML

[SPlatten]

I have a C++ API function that looks up a particular string to find an object, it returns a reference to the QML. What can I return to the QML if the reference doesn't correspond to any found object ?

[kshegunov]

@SPlatten said in C++ return to QML:

Can anyone point / help me to a good example / tutorial on using QVariant to construct an object that can be returned to QML ?

For your type to be accessible in QML it is either a primitive, a gadget (a.k.a. a value type) or a QObject derived class. No other options are allowed. If it's a primitive - it's directly accessible. If it's a gadget then it must qualify for a metatype and has to have the Q_GADGET macro, (usually) have it registered with qRegisterMetatype if it's a parameter for signal/slot connections, and most importantly be registered with the QML engine with QML_VALUE_TYPE.
https://doc.qt.io/qt-6/qtqml-cppintegration-definetypes.html

QObject derived types simply have the Q_OBJECT macro and the QML_ELEMENT annotation.

Either way you should declare the available properties with Q_PROPERTY.

[SPlatten]

I believe I've solved the issue, I've added a local static to the function that is called by the QML:

staledata_t& staledata_t::isRegistered(QString strUDT_) {
    staledata_t* pobjData(staledata_t::pFindTag(strUDT_));
    static staledata_t invalid;
    if ( pobjData != nullptr ) {
        return *pobjData;
    }
    return invalid;
}

The invalid variable will only be valid if it has all the required parameters set-up. That works nicely, the next issue, is how can I return an object to the QML that it can interpret? Is JSON an option ?

[JoeCFD]

@SPlatten if you want to pass the object back to QML to do something, why can not you call the object inside staledata_t to do what the object does inside QML?

Also isRegistered() has to be static?

[SPlatten]

@JoeCFD , thank you, no isRegistered isn't static, prototype:

Q_INVOKABLE staledata_t& isRegistered(QString strUDT_);

I think I can create a custom type using QVariant, will look into this further. BTW I said pass not parse, I don't need to manage the passed back object in C++, just return the found data.

[SPlatten]

Can anyone point / help me to a good example / tutorial on using QVariant to construct an object that can be returned to QML ?

[kshegunov]

@SPlatten said in C++ return to QML:

Can anyone point / help me to a good example / tutorial on using QVariant to construct an object that can be returned to QML ?

For your type to be accessible in QML it is either a primitive, a gadget (a.k.a. a value type) or a QObject derived class. No other options are allowed. If it's a primitive - it's directly accessible. If it's a gadget then it must qualify for a metatype and has to have the Q_GADGET macro, (usually) have it registered with qRegisterMetatype if it's a parameter for signal/slot connections, and most importantly be registered with the QML engine with QML_VALUE_TYPE.
https://doc.qt.io/qt-6/qtqml-cppintegration-definetypes.html

QObject derived types simply have the Q_OBJECT macro and the QML_ELEMENT annotation.

Either way you should declare the available properties with Q_PROPERTY.