QML Singleton C++ provider slot that takes variable number of arguments

Question:

As everyone knows, in JavaScript all functions can take any number of arguments. In Qt you can make QObject, whose methods accessible from QML, having marked it by Q_INVOKABLE. For example:

class myObj: public QObject
{
    Q_OBJECT

//...

public slots:
    Q_INVOKABLE QJSValue myFunction(QJSValue value);

//...

};

And then you can call it from JS:

(function(){
    //like this:
    var result = myObj.myFunction("test");
    //but also like this:
    var result2 = myObj.myFunction(1,2,3,4,5); //,6,7,8, ..., 9998, 9999
})();

So, how to handle variable number of parameters on C++ side? On JS side we have "arguments" object. Is there any analog for Q_INVOKABLE methods?

Answer:

It's looks like this is impossible for every QObject, but you can wrap your singleton C++ backend SLOT with custom JS script, which will join together multiple arguments to single array. This can be done through the qmlRegisterSingletonType method callback, which provides access to the QJSEngine instance.

QJSValue ApiProvider::initSingletonType(QQmlEngine *qmlEngine, QJSEngine *jsEngine) {
    qDebug() << "ApiProvider initSingletonType";
    Q_UNUSED(qmlEngine);
    QJSValue instance=jsEngine->newQObject(new ApiProvider(jsEngine));
    QJSValue restProvider=jsEngine->evaluate(
        "(function (instance) {" \
        "   instance.createElement=function(type,...child){" \
        "       return instance.createElementInternal(type,child);" \
        "   }; " \
        "   return instance;" \
        "})"
    );
    if (restProvider.isError()) {
        qCritical() << restProvider.toString();
        return QJSValue();
    } else {
        QJSValue result=restProvider.call({instance});
        if (result.isError()) {
            qCritical() << result.toString();
            return QJSValue();
        } else {
            return instance;
        }
    }
}

...


qmlRegisterSingletonType(
    "com.tripolskypetr.quitejs",
    1, 0,
    "Api",
    ApiProvider::initSingletonType
);

This application implements a conditional jsx factory, where each element can have an arbitrary number of descendants in rest parameters