2.4 Attribute Declarations

2.4.1 Overview

Attribute declarations are made with XSC:AttDef elements nested inside of the element declaration. The data type of an attribute is defined with a sub-element, as is a declaration of whether or not it is required and a possible default value.

<!ELEMENT XSC:AttDef (XSC:Doc?, (XSC:CData | XSC:ID | XSC:IDRef | XSC:IDRefs | XSC:Entity | XSC:Entities | XSC:Nmtoken | XSC:Nmtokens | XSC:NotationType | XSC:EnumerationType),( XSC:Required | XSC:Implied | XSC:Fixed | XSC:AttValue))>
<!-- Name is the name of the attribute -->
<!ATTLIST XSC:AttDef
name NMTOKEN #REQUIRED >

<!-- XSC:CData through XSC:EnumerationValue identify attribute types -->
<!ELEMENT XSC:CData EMPTY>
<!ELEMENT XSC:ID EMPTY>
<!ELEMENT XSC:IDRef EMPTY>
<!ELEMENT XSC:IDRefs EMPTY>
<!ELEMENT XSC:Entity EMPTY>
<!ELEMENT XSC:Entities EMPTY>
<!ELEMENT XSC:Nmtoken EMPTY>
<!ELEMENT XSC:Nmtokens EMPTY>
<!ELEMENT XSC:NotationType (NotationValue+)>
<!ELEMENT XSC:NotationValue EMPTY>
<!ATTLIST XSC:NotationValue
Notation NMTOKEN #REQUIRED>
<!ELEMENT XSC:EnumerationType (EnumerationValue+)>
<!ELEMENT XSC:EnumerationValue (Doc?)>
<!ATTLIST XSC:EnumerationValue
Value NMTOKEN #REQUIRED>

<!--XSC:Required through XSC:AttValue identify attribute declarations and defaults-->
<!ELEMENT XSC:Required EMPTY>
<!ELEMENT XSC:Implied EMPTY>
<!ELEMENT XSC:Fixed EMPTY>
<!ATTLIST XSC:Fixed
Value CDATA #REQUIRED>
<!ELEMENT XSC:AttValue EMPTY>
<!ATTLIST XSC:AttValue
Value CDATA #REQUIRED>

In XSchema 1.0, an attribute declaration (XSC:AttDef element) must be nested within the element declaration (XSC:ElementDecl element) for the element to which the attribute belongs. The name attribute of the XSC:AttDef element provides the name by which the attribute will be identified. A nested declaration is shown below.

<XSC:ElementDecl name="Species">
...additionalElementInformation...
<XSC:AttDef name="status">
...additionalAttributeInformation...
</XSC:AttDef>
</XSC:ElementDecl>

This declares an element with the name Species that has an attribute named status.

Merely naming an attribute is inadequate. Attribute declarations must identify data types and provide information about whether the attribute is required. Both sets of information are declared using subelements. The simplest attribute declaration possible identifies an attribute as containing character data (CData) and allows the attribute to be optional, as shown below.

<XSC:AttDef name="sampleAttribute">
<XSC:CData/>
<XSC:Implied/>
</XSC:AttDef>

2.4.2 Attribute Types

XSchema 1.0 provides equivalents for all of the XML 1.0 DTD attribute types. Most of them are declared using empty elements, though notations and enumerated values are identified with nested sub-elements. All of the examples in this section will use the Implied attribute default (and therefore the XSC:Implied element.)

The CData attribute type is one of the most common, permitting an attribute to contain character data as defined by the XML 1.0 specification. If the Species element were to contain an attribute providing the Latin name of the species, the declaration would look like the following.

<XSC:ElementDecl name="Species">
...additionalElementInformation...
<XSC:AttDef name="Latin">
<XSC:CData/>
<XSC:Implied/>
</XSC:AttDef>
</XSC:ElementDecl>

This attribute would then be available for use in instances of the Species element:

<Species Latin="Passerina cyanea">...additionalContent...</Species>

The XSC:ID attribute type is used to uniquely identify elements in a document for application processing. XSC:IDRef and XSC:IDRefs attribute types are used to refer to a single ID value in the same document or multiple ID values in the same document, separated by whitespace, respectively. These attribute declarations should be use with the same constraints as apply to ID, IDREF, and IDREFS attribute types in XML 1.0.

The Entity and Entities attribute types identify the names of unparsed entities. The use of these attribute types should be made with the same constraints as apply to the ENTITY and ENTITIES attribute types in XML 1.0. If a document is checked directly against an XSchema without a conversion to a DTD, information regarding unparsed entities must be available from the parser for these attribute types to be meaninful.

The Nmtoken and Nmtokens attribute types are used to declare attributes that must contain information conforming to the Nmtoken and Nmtokens productions in XML 1.0.

The NotationType and EnumeratedType attribute types are more complex, requiring nested elements to identify their possible content. These two declarations use similar syntax, but the allowed values of NotationType declarations must match the Notations declared elsewhere in the XSchema document.

If the status attribute of the Species element were to allow the values of extinct, endangered, protected, and non-threatened, an appropriate enumerated type declaration would look like:

<XSC:ElementDecl name="Species">
...additionalElementInformation...
<XSC:AttDef name="status">
<XSC:EnumerationType>
<XSC:EnumerationValue Value="extinct"/>
<XSC:EnumerationValue Value="endangered"/>
<XSC:EnumerationValue Value="protected"/>
<XSC:EnumerationValue Value="non-threatened"/>
</XSC:EnumerationType>
<XSC:Implied/>
</XSC:AttDef>
</XSC:ElementDecl>

A Species element created conforming to this declaration might look like:

<Species status="extinct">...additionalContentAboutDodos...</Species>

2.4.3 Attribute Defaults

XSchema requires attribute declarations to provide information about the default value of a given attribute. XSchema provides for the four cases supported by XML 1.0: #REQUIRED, #IMPLIED, #FIXED AttValue, and AttValue. Each of these cases is supported by a particular element in the XSchema syntax. There may be only one default value declaration per attribute.

Required attributes (identified in XML 1.0 by #REQUIRED) are identified by the use of the XSC:Required element. For instance, if the Latin attribute described above was required by the Species element, the XSC:AttDef element would contain an XSC:Required element:

<XSC:ElementDecl name="Species">
...additionalElementInformation...
<XSC:AttDef name="Latin">
<XSC:CData/>
<XSC:Required/>
</XSC:AttDef>
</XSC:ElementDecl>

Optional attributes (identified in XML 1.0 by #IMPLIED) are identified by the use of the XSC:Implied element. Implied indicates that there is no default value provided, and also that no value is required. If the Latin attribute is optional, the XSC:AttDef element would contain an XSC:Implied element:

<XSC:ElementDecl name="Species">
...additionalElementInformation...
<XSC:AttDef name="Latin">
<XSC:CData/>
<XSC:Implied/>
</XSC:AttDef>
</XSC:ElementDecl>

Fixed attributes (identified in XML 1.0 by #FIXED AttValue) are identified through the use of the XSC:Fixed element, which also contains the fixed value for the attribute. Attributes declared using fixed value cannot declare a different value for that attribute. Fixed effectively hard codes attribute values into particular elements. If the Species element had a planet attribute, an XSC:Fixed element would identify the fixed nature of the attribute and provide the value.

<XSC:ElementDecl name="Species">
...additionalElementInformation...
<XSC:AttDef name="planet">
<XSC:CData/>
<XSC:Fixed Value="Earth"/>
</XSC:AttDef>
</XSC:ElementDecl>

Attributes may also be provided with a default value that may be overridden by other declarations. These default values are identified through the use of the AttValue element. The status attribute of species elements described above would be an appropriate target for such a default value, especially if most species being described fell into a particular category:

<XSC:ElementDecl name="Species">
...additionalElementInformation...
<XSC:AttDef name="status">
<XSC:EnumerationType>
<XSC:EnumerationValue Value="extinct"/>
<XSC:EnumerationValue Value="endangered"/>
<XSC:EnumerationValue Value="protected"/>
<XSC:EnumerationValue Value="non-threatened"/>
</XSC:EnumerationType>
<XSC:AttValue Value="non-threatened"/>
</XSC:AttDef>
</XSC:ElementDecl>

Any type of default (required, fixed, etc.) may be used with any attribute type, though default values should always correspond to acceptable values for the attribute type.