Doing More With XML Schemas (part 4) - Being Selective

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The previous examples have demonstrated how a schema designer can force a document author to qualify an XML document with appropriate namespaces. However, based on what we've seen thus far, this is an all-or-nothing proposition - either every element is qualified or every element is unqualified - which is not very practical for real-world use.

Fortunately, the XML Schema specification also provides a "form" attribute for each element and attribute definition, which allows you to specify qualification rules on a case-by-case basis. Consider the following example, which demonstrates:

<?xml version="1.0" encoding="UTF-8"?>
<xsd:schema targetNamespace="http://www.somedomain.com/ns/sw/"
xmlns:sw="http://www.somedomain.com/ns/sw/"
xmlns:xsd="http://www.w3.org/2001/XMLSchema">
<!-- 
define a complex type -->
<xsd:complexType name="starWarsEntity">
<xsd:sequence>
<xsd:element 
name="name" type="xsd:string"
form="qualified"/>
<xsd:element name="species" 
type="xsd:string"
form="unqualified"/>
<xsd:element name="language" type="xsd:string"
form="unqualified"/>
<xsd:element 
name="home" type="xsd:string"
form="qualified"/>
</xsd:sequence>
</xsd:complexType>
<!-- 
define the root element and its contents -->
<xsd:element name="gallery">
<xsd:complexType>
<xsd:sequence>
<xsd:element 
name="character"
type="sw:starWarsEntity" maxOccurs="unbounded" form="qualified"/>
</xsd:sequence>
</xsd:complexType>
</xsd:element>
</xsd:schema>

As you can see, I've introduced the "form" attribute into the element definitions above. Like its schema-level cousins, this attribute too takes two values: "qualified" and "unqualified". In the example above, the <character>, <name> and <home> elements are all to be qualified with namespaces; the others may remain unqualified.

And here's an XML document instance conforming to the schema above:

<?xml version="1.0" encoding="UTF-8"?>
<sw:gallery xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:sw="http://www.somedomain.com/ns/sw/"
xsi:schemaLocation="http://www.somedomain.com/ns/sw/starwars.xsd">
<sw:character>
<sw:name>Luke 
Skywalker</sw:name>
<species>Human</species>
<language>Basic</language>
<sw:home>Tatooine</sw:home>
</sw:character>
<sw:character>
<sw:name>Chewbacca</sw:name>
<species>Wookie</species>
<language>Shyriiwook</language>
<sw:home>Kashyyyk</sw:home>
</sw:character>
<sw:character>
<sw:name>Chief 
Chirpa</sw:name>
<species>Ewok</species>
<language>Ewok</language>
<sw:home>Endor</sw:home>
</sw:character>
</sw:gallery>

The XML Schema author can thus selectively enforce XML element qualification - useful if only some of the elements are likely to clash with others.

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