Dynamic cast (interface) class in QML

[Krulle]

Hello all,

what is the best practice to make an C++ interface/base class accessible in QML?

For example (not complete and not right):

class IFoo : public QObject 
{
    explicit IFoo(QObject *parent = nullptr) {}
    ~virtual IFoo() {}
    virtual void overrideMe() = 0;
}

class FooA : public IFoo
{
    FooA(QObject *parent = nullptr) {}
}

class FooB : public IFoo
{
    FooB(QObject *parent = nullptr) {}
}

main:

int main(int argc, char *argv[])
{
    QQmlApplicationEngine engine;
    
    IFoo *foo1 = new FooA;
    IFoo *foo2 = new FooB;
    
    FooB *foo3 = new FooB;

    engine.rootContext()->setContextProperty("foo", foo1); // which type it is in QML? IFoo?
    // or
    engine.rootContext()->setContextProperty("foo", qobject_cast<FooA*>(foo1)); // and
    engine.rootContext()->setContextProperty("foo2", qobject_cast<FooB*>(foo2));

    engine.rootContext()->setContextProperty("foo3", foo3); // FooB
}

Is there a way to cast foo1 in QML? Or is this fundamentally a bad idea and evidence of bad code?

Thank you very much!

[JoeCFD]

I do no think you need any cast. The following code should work.

    auto foo1 = new FooA;
    auto foo2 = new FooB;
    
    engine.rootContext()->setContextProperty("foo1", foo1); // and
    engine.rootContext()->setContextProperty("foo2", foo2);

[Krulle]

You're right.

And how can i use and cast the Base in QML?
Or is this impossible and i have to use the exact type in QML?
What is the best way to represent this in QML then?

What first came to my mind:

Item {
    property QtObject modul: foo1 ? foo1 : foo2
    property bool isFoo1: foo1 ? true : false

    Component.onCompleted: { 
        modul.overrideMe()
    }
}

But this is of course quite bullshit and I have to check each time which type is present.
Are there any other ideas?

[ChrisW67]

I would guess you need to start with QML_INTERFACE and QML_IMPLEMENTS_INTERFACES

[Krulle]

@ChrisW67
Thank you. That is the right input :)

[Krulle]

Hello folks,

I would like to give some feedback.
Basically, it's quite simple: Only the interface class is passed as a ContextProperty, QML then automatically casts the class into the correct type.
This also works very reliably.

int main(int argc, char *argv[])
{
    QQmlApplicationEngine engine;
    
    IFoo *foo = new FooA;

   engine.rootContext()->setContextProperty("iFoo", foo); // in QML it is FooA

}

For differentiation, I return the class name in the interface class with the meta system. I have not found a function in QML that does this reliably (but I also use 5.9).