Derived features allows you to compute a feature value based on other feature values or implement virtual features that hides the fact that the value is computed, not stored.
Introduction
It is pretty common that features (attributes and references) in a class are related in such a way that some may be computed from others. E.g. a fullName attribute may be computed from givenName and familyName attributes. This may be modelled using operations, e.g. getFullName(), but by using a derived feature (here: attribute) instead, accessing the value becomes more natural, particularly when using languages like OCL. In addition, you may more easily support setting the derived value, e.g. splitting the full name and setting both the givenName and familyName features. In such a case, you can consider it a virtual feature, in the sense that there is no way of knowing which features are stored and which are computed. This allows you to change the implementation, e.g. move to storing the fullName and deriving givenName and familyName, without affecting the user of the model (instances).
Derived features in Ecore
Ecore supports derived features, both by including snippets of interpreted code (OCL or other interpreted languages) as annotations in the model or by modifying the generated code. In both cases, the feature must be marked as derived, but setting the derived attribute of the EStructuralFeature (EAttribute or EReference). In addition, you should consider the following EStructuralFeature attributes:
- changeable: can this feature be set or just read? A derived feature is typically read-only, so the changeable attribute will be typically be set to false. Note that OCLInEcore uses the readonly keyword representing the inverse value.
- volatile: should this feature's value be stored in the EObject (and a Java field be generated)? Since a derived feature is computed, its value will typically not need to be stored so the volatile attribute will be true. However, if the computation is costly, it may make sense to store the last computed value, and reuse it if it hasn't changed.
- transient: should this value be serialized? Similar to the volatile attribute, the transient attribute is typically set to true, since a computed value needn't be stored in a file.