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7.6. Empty Content Models
Empty content models are elements that can only accept attributes. W3C XML Schema does not include any special support for empty content models, which can be considered either complex content models without elements or simple content models with a value restricted to the null string.
7.6.1. Creation of Empty Content Models
W3C XML Schema considers empty content models to be the intersection between complex content models (in the case in which no compositors are specified) and simple content models (in the case in which no text nodes are expected, which W3C XML Schema handles as if an empty text node was found). We will, therefore, be able to choose between the two methods to create an empty content model. Where we extended our title element to become mixed content, we carefully avoided adding empty elements, such as the HTML img or br. Let's see how we could define a br element with its id and class attributes using both methods.
7.6.1.1. As simple content models
This is done by defining a simple type that can only accept the empty string as a value. Strictly speaking, empty content models do not accept any whitespace between their start and end tags. Since we want to control this, we must use a datatype that does not alter the whitespaces, i.e., xs:string. Our empty content model is then derived by extension from this simple type:
<xs:simpleType name="empty">
<xs:restriction base="xs:string">
<xs:enumeration value=""/>
</xs:restriction>
</xs:simpleType>
<xs:element name="br">
<xs:complexType>
<xs:simpleContent>
<xs:extension base="empty">
<xs:attribute name="id" type="xs:ID"/>
<xs:attribute name="class" type="xs:NMTOKEN"/>
</xs:extension>
</xs:simpleContent>
</xs:complexType>
</xs:element>7.6.1.2. As complex content models
The other (more straightforward) way to do this is to create a complex content model without any subelements:
<xs:element name="br">
<xs:complexType>
<xs:attribute name="id" type="xs:ID"/>
<xs:attribute name="class" type="xs:NMTOKEN"/>
</xs:complexType>
</xs:element>7.6.2. Derivation of Empty Content Models
Each of the two empty content types keeps the derivation methods of its content model (simple or complex). The main difference between these two methods is essentially a matter of which derivations may be applied on the base type and what effect it will have.
7.6.2.1. Derivation by extension
If we try to remember and compare what we've learned about deriving complex and simple contents by extension, we can see that both allow addition of new attributes to the complex type. However, while we can add new subelements to complex content, we cannot change the type of the text node for a simple content model. Thus, this is the first difference between the two methods: when the empty content model is built on a simple type, it will not be possible to add anything other than attributes, while if it is built on top of a complex type, it will be possible to extend it to accept elements.
7.6.2.2. Derivation by restriction
At first glance, it seems that there are fewer differences here. The restriction methods of both simple and complex contents allow the restriction the scope of the attributes; restricting the content, which is already empty, doesn't seem to be very interesting. It's time, though, to remember what we've learned about a simple type derivation facet, which actually extends the set of valid instance documents! The "empty" simple type that we created to derive our empty simple content model has a base type equal to xs:string. When this simple type is derived through xs:whiteSpace, the result may be an expansion of the sets of valid instance structures. In our case, setting xs:whiteSpace to "collapse" has the effect of accepting any sequence of whitespaces between the start and closing tags. This new type is not "empty," strictly speaking, but may be useful for some (if not for most) applications that are normalizing the whitespaces and do not make any difference between these two cases. Such a derivation can be done on the simple content complex type like this:
<xs:simpleType name="empty">
<xs:restriction base="xs:string">
<xs:enumeration value=""/>
</xs:restriction>
</xs:simpleType>
<xs:complexType name="emptyBr">
<xs:simpleContent>
<xs:extension base="empty">
<xs:attribute name="id" type="xs:ID"/>
<xs:attribute name="class" type="xs:NMTOKEN"/>
</xs:extension>
</xs:simpleContent>
</xs:complexType>
<xs:complexType name="allmostEmptyBr">
<xs:simpleContent>
<xs:restriction base="emptyBr">
<xs:whiteSpace value="collapse"/>
<xs:attribute name="id" type="xs:ID"/>
<xs:attribute name="class" type="xs:NMTOKEN"/>
</xs:restriction>
</xs:simpleContent>
</xs:complexType>7.6.3. Simple or Complex Content Models for Empty Content Models?
As we have seen, choosing a simple or complex type doesn't make an awful lot of difference, except for extensibility. If we want to keep the possibility of adding subelements by derivation in the content model, we'd better choose an empty complex content model. However, if we want to be able to accept whitespaces in a derived type, an empty simple content model is a better bet.
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