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15.5 Element Attributes
The final piece of the DTD puzzle involves attributes.
You know attributes: they are the name/value pairs included with tags
in your documents that control the behavior and appearance of those
tags. To define attributes and their allowed values within an XML
DTD, use the <!ATTLIST> directive:
<!ATTLIST element attributes>
The element is the name of the element to which
the attributes apply. The attributes are a list
of attribute declarations for the element. Each attribute declaration
in this list consists of an attribute name, its type, and its default
value, if any.
15.5.1 Attribute Values
Attribute values can be of several types,
each denoted in an attribute definition
with one of the following keywords:
CDATA indicates that the attribute value is a
character or string of characters. This is the attribute type you
would use to specify URLs or other arbitrary user data. For example,
the src attribute of the
<img> tag in HTML has a value of
CDATA.
ID indicates that the attribute value is a unique
identifier within the scope of the document. This attribute type is
used with an attribute, such as the HTML id
attribute, whose value defines an ID within the document, as
discussed in the Section B.1 in Appendix B.
IDREF or IDREFS indicates that
the attribute accepts an ID defined elsewhere in the document via an
attribute of type ID. The ID
type is used when defining IDs; IDREF and
IDREFS are used when referencing a single ID and a
list of IDs, respectively.
ENTITY or ENTITIES indicates
that the attribute accepts the name or list of names of unparsed
general entities defined elsewhere in the DTD. The definition and use
of unparsed general entities is covered in Section 15.3.2.
NMTOKEN or NMTOKENS indicates
that the attribute accepts a valid XML name or list of names. These
names are given to the processing application as the value of the
attribute. How they are used is determined by the application.
In addition to these keyword-based types, you can create an
enumerated type by listing the specific values allowed with this
attribute. To create an enumerated type, list the allowed values,
separated by vertical bars and enclosed in parentheses, as the type
of the attribute. For example, here is how the
method attribute for the
<form> tag is defined in the HTML DTD:
method (get|post) "get"
The method attribute accepts one of two values,
either get or post;
get is the default value if nothing is specified
in the document tag.
15.5.2 Required and Default Attributes
After you
define the name and type of an
attribute, you must specify how the XML processor should handle
default or required values for the attribute. You do this by
supplying one of four values after the attribute type.
If you use the #REQUIRED keyword, the associated
attribute must always be provided when the element is used in a
document. Within the XHTML DTD, the src attribute
of the <img> tag is required, since an image
tag makes no sense without an image to display.
The #IMPLIED keyword means that the attribute may
be used but is not required and that no default value is associated
with the attribute. If it is not supplied by the document author, the
attribute has no value when the XML processor handles the element.
For the <img> tag, the
width and height attributes are
implied, since the browser derives sizing information from the image
itself if these attributes are not specified.
If you specify a value, it then becomes the default value for that
attribute. If the user does not specify a value for the attribute,
the XML processor inserts the default value (the value specified in
the DTD).
If you precede the default value with the keyword
#FIXED, the value is not only the default value
for the attribute, it is the only value that can
be used with that attribute if it is specified.
For example, examine the attribute list for the
form element, taken (and abridged) from the HTML
DTD:
<!ATTLIST form
action
CDATA #REQUIRED
method
(get|post) "get"
enctype
CDATA "application/x-www-form-urlencoded"
onsubmit
CDATA #IMPLIED
onreset
CDATA #IMPLIED
accept
CDATA #IMPLIED
accept-charset
CDATA #IMPLIED
>
This example associates seven attributes with the
form element. The action
attribute is required and accepts a character string value. The
method attribute has one of two values, either
get or post.
get is the default, so if the document author
doesn't include the method
attribute in the form tag,
method=get is assumed automatically by the XML
parser.
The enctype attribute for the
form element accepts a character string value and,
if not specified, defaults to a value of
application/x-www-form-urlencoded. The remaining
attributes all accept character strings, are not required, and have
no default values if they are not specified.
If you look at the attribute list for the form
element in the HTML DTD, you'll see that it does not
exactly match our example. That's because
we've modified our example to show the types of the
attributes after any parameter entities have been expanded. In the
actual HTML DTD, the attribute types are provided as parameter
entities whose names give a hint of the kinds of values the attribute
expects. For example, the type of the action
attribute is %URI;, which elsewhere in the DTD is
defined to be CDATA. By using this style, the DTD
author lets you know that the string value for this attribute should
be a URL, not just any old string. Similarly, the type of the
onsubmit and onreset attributes
is given as %Script. This is a hint that the
character string value should name a script to be executed when the
form is submitted or reset.
