XML Schema

Chapter 12. Creating More Building Blocks Using Object-Oriented Features

We have already seen many features that have been borrowed from object-oriented languages. In this chapter, we will see substitution groups (similar to subclasses), abstract elements and datatypes (similar to abstract classes), and final datatypes (similar to final classes).

Substitution Groups

In many cases, a vocabulary needs the ability to accept a variety of different content models. We have two options: we can try to do it using a single generic element name, or we can define a schema smart enough to deal with the possible content model. Since we cannot define multiple different content models for the same element (because of the Consistent Declaration Rule), we can either use xsi:type attributes in the instance documents, or we can define a content model wide enough to accommodate all the possibilities. Such a model would likely be wide enough to also accept combinations that we do not want.

The easiest solution for accommodating different types with W3C XML Schema is to use a different element name for each case. We already saw that the xs:choice(outside a group) compositor allows us to build such constructs where a node in an instance document can accept an element chosen in a list. However, this list is fixed in the complex type definition. We have also seen that this list cannot be extended, since the rules for complex type derivations by extension do not allow it. Substitution groups offer a flexible ...