Currently QML only supports enums as signal and method parameters if they are declared in the same class as the signal or method. This is a limitation inherited from QMetaObject, that doesn't allow QML to determine if the parameter type is an enum, or just some other random type.

The ideal solution would be to extend Q_ENUMS() to support "registering" an enum from another class. For example:

class Foo : public QObject {
    Q_ENUMS(Bar::CustomEnum);

slots:
    void myMethod(Bar::CustomEnum);
};

in this case, all QML really needs is for the Bar::staticMetaObject to appear in the Foo::staticMetaObject's extra data list.