Dynamically call methods from a scripting engine using QMetaMethod and QVariant.

French Paragon

Hi, I'm wrinting a python api for one of my app using pybind11 (which is the best choice for me, even if it is sometimes not easy to integrate it with some Qt paradigms).

I want to let the programmer using my api to interact with a certain family of qobjects (the editable items) using the meta object system through python.

For the properties it's easy. I just had to wrote a method to convert pybind11::handle to QVariant, treating the cases that matter for my software (strings, numbers and nested editable items) and vice versa.

But for some use cases I need invokable methods (for examples creating a new nested editable item and getting back its attributed id). I found this thread that describe how we can cheat with QGenericReturnArgument, QGenericArgument and QVariant to invoke a method once we know its signature, have the corresponding QVariants converted from the corresponding pybind11::handle (provided in a tuple): Qt invoke method with QVariant.

So I can write something like that:

pybind11::object EditableItemWrapper::invokeMethod(std::string m_name, py::tuple args) {
     QMetaMethod m = getMeth(m_name); //simplified

     do_checks(); //see if arguments matches both in numbers and types.

     int narg = m.parameterCount();

     switch(narg) {
     case 0:
           //do the business here.
           void v = NULL;
           const char* name = QMetaType::type(QMetaType(m.returnType()).metaObject().className());
           QVariant retVal (m.returnType(), v);

           bool sucess = invokeMethod(wrapped_obj(), m_name.c_str(), QGenericReturnArgument(name, retVal.data());
     ....
     case n:
           //repeat cleverly for each n up to a reasonable maximal under Q_METAMETHOD_INVOKE_MAX_ARGS
           //use the same tric for input values, creating the appropritate QVariant and giving the data() ptr to QGenericArgument
     }
}

But, the doc says its discouraged to work like that... Why ? (For example there is this limitation on the number of parameters, but it's a needed evil, isn't it ?)

Ideally I'd like to invoke my method given a name, A QVariantList as parameter and get back a QVariant and a status code as return, isn't there something to do it already ?

And is there any other solution ? What do you think ?

Thanks in advance for your answers.

Laurent