Override the value of an attribute in a derived Ecore-class (EMF)

Question

I'm aware of the fact that I cannot override or redefine attributes as a whole in an ecore-based model when it comes to inheritance. But can I somehow assign a new value to an existing, inherited attribute?

E.g. Class A defines the attribute name which is a string, the default value is set to 'defaultA'. Class B inherits from Class A and name should have the value 'defaultB'.

I tried to just reimplement the attribute with the same name and datatype in Class B, but I cannot create a genmodel from that ("There may not be two features named 'name'").

Isn't it even possible to change the value of an inherited attribute?

Yellowbird

In my experience, these kind of things can be achieved in the generated code. I'm not aware of a way to do this directly in the model (assuming this is what you want?) – Author

Answer 1

No, that's not supported. The field for the feature is declared in the base class and that assigns the default defined for the feature...

Fulbright

Answer 2

Attributes cannot be overridden in plain EMF but there is a workaround through OCL. (Of course, you can generate Java code and implement the attributes as needed but it won't work in the Eclipse instance where you created the meta-model.)

The trick is that operations can be overridden in EMF and through OCL you can implement the operations as needed.

Below is a minimal example (written in OCLinEcore editor) that defines a meta-model consisting of classes A and B. The class A introduces an attribute label which is just redirected to the operation computeLabel(). Class 'B' provides a different implementation for the computeLabel() operation.

package workaround : workaround = 'workaround' {
    class A {
        operation computeLabel() : String {
            body: 'labelA';
        }
        attribute label : String {
            derivation: computeLabel();
        }
    }
    class B extends A {
        operation computeLabel() : String {
            body: 'labelB';
        }
    }
}

You can test it by creating a dynamic instance of class B and opening the B.xmi file in the standard Sample Reflective Ecore Model Editor. You'll then see:

Craiova

Answer 3

This rather hackish alternative is to declare an operation in the subclass with the same name as the feature getter name.

This is a light-weight variation of Viliam Simko's solution.

With this solution you will not have a proper name feature in Class B, but when then name feature in Class A is accessed your getter in Class B will be called instead.

I don't even know if this is really allowed. But it seems to be working with EMF 2.13.0, Xcore 1.5.0.

Example:

class ClassA {
    String name
}

class ClassB extends ClassA {
    op String getName() {
        return "Name B"
    }
}

Adolpho