QMetaObject::invokeMethod call QML method from C++, with more than 10 arguments

Developer123

I'm wondering how can I call QMetaObject::invokeMethod with many arguments.

I know I can do like this for multiple arguments:

QMetaObject::invokeMethod(
    myobject, 
    "methodname",
    Q_ARG(QVariant, 123),
    Q_ARG(QVariant, 123),
    Q_ARG(QVariant, 123),
    Q_ARG(QVariant, 123),
    Q_ARG(QVariant, 123),
    Q_ARG(QVariant, 123) ...
);

My problem is that the function I'm trying to call has many (11) fields, and the signature for QMetaObject::invokeMethod is:

bool QMetaObject::invokeMethod(QObject *obj, const char *member, Qt::ConnectionType type, QGenericReturnArgument ret, QGenericArgument val0 = QGenericArgument(nullptr), QGenericArgument val1 = QGenericArgument(), QGenericArgument val2 = QGenericArgument(), QGenericArgument val3 = QGenericArgument(), QGenericArgument val4 = QGenericArgument(), QGenericArgument val5 = QGenericArgument(), QGenericArgument val6 = QGenericArgument(), QGenericArgument val7 = QGenericArgument(), QGenericArgument val8 = QGenericArgument(), QGenericArgument val9 = QGenericArgument())

Therefore only up to 10 arguments are possible. Is there a way to circumvent this without splitting the method into 2?

jsulm

@Developer123 said in QMetaObject::invokeMethod call QML method from C++, with more than 10 arguments:

without splitting the method into 2?

Why would you need to "split" the method? You can simply put at least some of the parameters into a struct and pass this struct as parameter. It is anyway bad design to have so many parameters in a function/method.

Developer123

@jsulm Ok that's a good idea, do you know how can I do that? I tried passing a custom struct, such as:

MyStruct {
  int val1;
  int val2;
};

MyStruct  params;
auto result = QMetaObject::invokeMethod(
    object, 
    "mymethod",
    Q_ARG(QVariant, params)

But I get: no instance of constructor "QArgument<T>::QArgument [with T=QVariant]" matches the argument list.

I'm using QVariant in the Q_ARG() call because I want to access this variable in the QML side as a javascript object, not sure if it would be possible using Q_ARG(MyStruct , params), even though that does compile...

jsulm

@Developer123 said in QMetaObject::invokeMethod call QML method from C++, with more than 10 arguments:

not sure if it would be possible using

Did you try?
You probably will need to register MyStruct, see https://doc.qt.io/qt-6/custom-types.html

Developer123

@jsulm Yep, tried and got error:

QMetaObject::invokeMethod: No such method SomeQObject_QMLTYPE_15::myMethod(MyStruct)
Candidates are:
    myMethod(QVariant,QVariant,QVariant,QVariant,QVariant,QVariant,QVariant,QVariant,QVariant,QVariant,QVariant)

Also, I tried Q_DECLARE_METATYPE(MyStruct); on the header file containing the struct definition of MyStruct, same error... I guess I need to declare the class with the constructor, destructor, etc, instead of a struct, but I'm pretty sure that won't work out of the box.

Edit: I managed to send the QVariant doing:

MyStruct mystruct;
QVariant params;
params.setValue(mystruct);
QMetaObject::invokeMethod(
    qobject, 
    "myMethod",
    Q_ARG(QVariant, params)
);

Now I'm stuck trying to access the values of MyStruct on QML. When I print params on QML, I get:

qml: QVariant(MyStruct, )

J.Hilk

@Developer123 add the Q_GADGET macro to your struct, make those public access function Q_INVOKABLE or, (probably better) define Q_PROPERTY(ies)

Q_PROPERTY(int val1 MEMBER m_val1 CONSTANT FINAL)

IIRC this will require a qRegisterMetaType of your struct

Developer123

@J-Hilk Hello, I followed the steps exactly as defined here (they seem to correspond to your answer exactly).

I got some link errors. Then I cleaned the build and then it compiled and worked successfully. Thanks.

But I want to point out it was a lot of work to get this to work, I wish QT was simpler...

SimonSchroeder

@Developer123 said in QMetaObject::invokeMethod call QML method from C++, with more than 10 arguments:

I wish QT was simpler...

Well, language interoperability (and C++ and QML are two different languages) is usually complicated at first. Once you have figured out how to interoperate it'll get a lot easier.