XSchema Spec - Attribute Declarations (Section 2.4), Draft 6
SimonStL at classic.msn.com
Wed Aug 19 02:11:12 BST 1998
This section is large and has absorbed significant changes.
1) It has been updated for namespace handling.
2) The AttDefs are now contained in an AttGroup container element.
3) The attributes used to identify #REQUIRED, #FIXED, #IMPLIED, and default
values for attributes have been reduced to two from three. Many thanks to
Jeni Tennison for her insightful analysis on this, and to Ron Bourret for
helping develop it.
Hopefully, we're about there.
As always, a prettier HTML version of this will be posted shortly at
Dynamic HTML: A Primer / XML: A Primer / Cookies
2.4 Attribute Declarations
Attribute declarations are made with AttDef elements nested inside of AttGroup
container elements. AttGroup elements may be nested inside of ElementDecl
element declarations or XSchema elements. The type of an attribute is defined
with an attribute, as is a declaration of whether or not it is required and a
possible default value. Values for enumerated types are provided with
<!ELEMENT AttGroup (Doc?, More?, AttDef*)>
Name NMTOKEN #IMPLIED
Element NMTOKEN #IMPLIED
id ID #IMPLIED
prefix NMTOKEN #IMPLIED
ns CDATA #IMPLIED>
<!ELEMENT AttDef (Doc?, More?, EnumerationValue*)>
Name NMTOKEN #REQUIRED
Type (CData |
Required (Yes | No) "No"
AttValue CDATA #IMPLIED>
<!ELEMENT EnumerationValue (Doc?, More?)>
Value CDATA #REQUIRED>
In XSchema 1.0, attribute declarations (AttDef elements inside of an AttGroup
element) may be nested within the element declaration (ElementDecl element)
for the element to which the attribute belongs, or outside of that structure
in an XSchema element. If the AttGroup element appears nested inside an
ElementDecl element, the Element attribute must be ignored. Similarly, if an
AttDef element appears nested inside an AttGroup element, its Element
attribute must be ignored in favor of that of the AttGroup. If an AttGroup or
AttDef element appears nested under the XSchema element, the Element attribute
may contain a name token corresponding to the Name attribute of the element to
which this attribute applies. If the Element attribute is missing, that AttDef
or AttGroup declaration may only be used by reference.
AttGroup elements are container elements. All of their attributes, except for
id, apply to the child AttDef elements and may be overridden by the attributes
of those child elements. The Name attribute of the AttGroup element is
provided for convenient reference within the XSchema. This attribute should be
ignored in conversions to DTDs.
The Name attribute of the AttDef element provides the name by which the
attribute will be identified. Attribute names must be unique within the
element in which they are declared. A nested declaration is shown below.
<AttDef Name="status" ...additionalAttributeInformation.../>
This declares an element with the name Species that has an attribute named
status. If the status attribute was declared outside of the Species element
declaration, the declarations would appear as shown below.
<AttDef Name="status" Element="Species" ...additionalAttributeInformation.../>
Merely naming an attribute may be adequate. Attribute declarations may
identify types and provide information about whether the attribute is
required. By default, attributes will be assumed to contain character data
(CData), not be required, and have no default value. This information is
declared using additional attributes. The simplest attribute declaration
possible identifies an attribute as containing character data (CData) and
allows the attribute to be optional, as shown below.
Applications may also use the id attribute to provide unique identifiers for
attribute declarations using values that are unique within the XSchema.
The prefix attribute identifies the prefix that will be applied to this
attribute during conversion to DTDs. The ns attribute identifies the URI which
functions as the namespace name for this attributes. Namespace processing is
covered further in Section 3.0, "XSchema and Namespaces".
2.4.2 Attribute Types
XSchema 1.0 provides equivalents for all of the XML 1.0 DTD attribute types.
All of them are declared using attribute values within the AttDef 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 could look like the following. (The Type attribute could
actually be omitted in this case, as CData is the default type.)
<AttDef name="Latin" Type="CData"/>
This attribute would then be available for use in instances of the Species
<Species Latin="Passerina cyanea">...additionalContent...</Species>
The ID attribute type is used to uniquely identify elements in a document for
application processing. IDRef and 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
must be used 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 must 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 meaningful.
The Nmtoken and Nmtokens attribute types are used to declare attributes that
must contain information conforming to the Nmtoken and Nmtokens productions in
The Notation and Enumerated attribute types are more complex, requiring
Enumeration subelements to identify their possible content. These two
declarations use similar syntax, but the allowed values of Notation
declarations must match the Notations declared elsewhere in the XSchema
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:
<AttDef Name="status" Type="Enumerated">
A Species element created conforming to this declaration might look like:
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,
though they are expressed as choices between required and not required with an
optional default value. There may be only one default value declaration per
Required attributes (identified in XML 1.0 by #REQUIRED) are identified by
assigning the value "Yes" to the Required attribute of an AttDef element. For
instance, if the Latin attribute described above was required by the Species
element, the AttDef element would contain a Required attribute with a value of
<AttDef name="Latin" Required="Yes"/>
Optional attributes (identified in XML 1.0 by #IMPLIED) are identified
assigning the value "No" to the Required attribute of an AttDef element and
not assigning a value to the AttValue attribute. Implied indicates that there
is no default value provided, and also that no value is required. If the Latin
attribute is optional, the AttDef element would contain a "No" value for the
Required attribute. (Note that this is the default status and the Required
declaration does not need to be made explicitly.)
<AttDef name="Latin" Required="No"/>
Fixed attributes (identified in XML 1.0 by #FIXED AttValue) are identified
through the use of the Required attribute in combination with the AttValue
attribute, which must contain 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 Required attribute has a value of "Yes", and the AttValue
attribute is present, the attribute value should be treated as a #FIXED value
in XML 1.0.
For example, to declare a planet attribute for the Species element, a Required
attribute given the value of "Yes" would identify the fixed nature of the
attribute and the AttValue attribute would provide the value.
<AttDef Name="planet" Required="Yes" AttValue="Earth"/>
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 attribute. 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:
<AttDef Name="status" Type="Enumerated" Enumeration="extinct
endangered protected non-threatened" AttValue="non-threatened"/>
Any default (required, fixed, etc.) may be used with any attribute type,
though default values should always correspond to acceptable values for the
2.4.4 Combinations of Types, Defaults, and Default Values
This notation also permits the declaration of certain attributes (IDs with
defaults, for instance) that are prohibited by the standard XML 1.0 DTD
syntax. Developers who use these combinations should test that their documents
will behave as expected in DTD-only environments as well as XSchema
environments. Additional processing of document instances may be necessary to
produce normalized-for-DTD use documents if they included such attributes as
default values. The attribute type should always be considered more important
than its default values in XSchema to DTD conversion.
The table below summarizes the possible combinations of XSchema attribute
defaults and their XML 1.0 DTD equivalents.
Required AttValue XML 1.0 Equivalent
Yes <value> #FIXED <value>
Yes -- #REQUIRED
No <value> AttValue
No -- #IMPLIED
(-- indicates an empty or undeclared value)
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