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This is a W3C Working Draft for review by W3C members and other interested parties. It is a draft document and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use W3C Working Drafts as reference material or to cite them as other than "work in progress". A list of current W3C working drafts can be found at http://www.w3.org/TR.
This work is part of the W3C XML Activity (for current status, see http://www.w3.org/MarkUp/XML/Activity ). For information about the XPointer language which is expected to be used with XLink, see http://www.w3.org/TR/WD-xptr.
See http://www.w3.org/TR/NOTE-xlink-principles for additional background on the design principles informing XLink.
This document specifies constructs that may be inserted into XML resources to describe links between objects. It uses XML syntax to create structures that can describe the simple unidirectional hyperlinks of today's HTML as well as more sophisticated multi-ended and typed links.
This document specifies constructs that may be inserted into XML resources to describe links between objects. A link, as the term is used here, is an explicit relationship between two or more data objects or portions of data objects. This specification is concerned with the syntax used to assert link existence and describe link characteristics. Implicit (unasserted) relationships, for example that of one word to the next or that of a word in a text to its entry in an on-line dictionary are obviously important, but outside its scope.
Links are asserted by elements
contained in XML
documents. The simplest case is very like an HTML A
link, and has these characteristics:
A
element in some document)A links normally replaces the current
view, perhaps with a user option to open a new window.While this set of characteristics is already very powerful and obviously has proven itself highly useful and effective, each of these assumptions also limits the range of hypertext functionality. The linking model defined here provides ways to create links that go beyond each of these specific characteristics, thus providing features previously available mostly in dedicated hypermedia systems.
Following is a summary of the design principles governing XLink:
Three standards have been especially influential:
Many other linking systems have also informed this design, especially Dexter, FRESS, MicroCosm, and InterMedia.
The following basic terms apply in this document.
A, HyTime clink, and TEI XREF
are all examples of inline links.
The formal grammar for locators is given using a simple Extended Backus-Naur Form (EBNF) location, as described in the XML specification.
The locator for a resource is typically provided by means of a Uniform Resource Identifier, or URI. XPointers can be used in conjunction with the URI structure, as fragment identifiers or queries, to specify a more precise sub-resource. XPointers can be used in conjunction with URIs to specify a more precise sub-resource.
A locator generally contains a URI,
as described in IETF RFCs [IETF RFC 1738] and [IETF RFC 1808].
As these RFCs state, the URI may include a trailing query (marked
by a leading "?"), and be followed by a "#" and
a fragment identifier, with the query interpreted by the host
providing the indicated resource, and the interpretation of the fragment identifier
dependent on the data type of the indicated resource.
In order to locate XML documents and portions of documents, a locator value may contain either a URI or a fragment identifier, or both. Any fragment identifier for pointing into XML must be an XPointer.
Special syntax may be used to request the use of particular processing models in accessing the locator's resource. This is designed to reflect the realities of network operation, where it may or may not be desirable to exercise fine control over the distribution of work between local and remote processors.
| Locator | ||||||||||||||||||||
|
In this discussion, the term designated resource refers to the resource which an entire locator serves to locate. The following rules apply:
Connector is followed directly
by a Name, the
Name is shorthand for the XPointer "id(Name)"; that is,
the sub-resource is the element in the containing resource that has an XML ID attribute whose value matches the
Name. This shorthand is to encourage use of the robust id
addressing mode.#", this signals an intent that
the containing resource is to be fetched as a whole from the host that provides
it, and that the XPointer processing to extract the sub-resource is to be
performed on the client, that is to say on the same system where the linking
element is recognized and processed.|", no intent is signaled as to
what processing model is to be used for accessing the designated resource.
Note that by definition, a URI includes an optional query component.
In the case where the URI contains a query (to be interpreted by the server), information providers and authors of server software are urged to use queries as follows:
| Query | ||||
|
The existence of a link is asserted by
a linking element. Linking elements must
be recognized reliably by application software in order to provide appropriate
display and behavior. There are several ways link recognition could be accomplished:
for example, reserving element type names, reserving attributes, or leaving
the matter of recognition entirely up to stylesheets and application software.
Reserving attributes provides a balance between giving users control of their
own markup language design and keeping the important structural fact "is a
link" explicit within documents. Therefore, XLink linking-related elements
are recognized based on the use of a designated
attribute named xml:link. Possible values are
simple and extended (which identify linking elements),
as well as locator, group, and document
(which identify other related types of elements). An element in whose start-tag
such an attribute appears is to be treated as an element of the indicated
XLink type as dictated by this specification. For example:
<A xml:link="simple" href="http://www.w3.org/">The W3C</A> |
Note: Subject to definitions to be developed in related standards, the methods described in "7. Attribute Remapping" may be used to rename the reserved attribute.
There are two mechanisms that may be used to associate the xml:link
and xml:attributes attributes with a linking element.
The simplest is to provide the attribute explicitly in a start-tag. A less
verbose method is to use XML's facilities for declaring
default attribute values. For example, the following attribute-list
declaration would indicate that all instances of the A element
in the current document are XLink simple links:
<!ATTLIST A xml:link CDATA #FIXED "simple"> |
XLink defines two types of linking element:
Both kinds of links can have various types of information associated with them.
The following information can be associated with a link and its resources:
This information is supplied in the form of attributes on linking elements. In the following sections, parameter entities are used to group these attributes.
A locator string identifies a participating resource. A link must supply a locator for each remote resource.
A locator takes the form of an attribute called href. Following
is a sample declaration of this attribute, enclosed in a locator.att
parameter entity.
<!ENTITY % locator.att |
The following semantic information can be provided for a link:
If the link is inline, its content counts as a
local resource of the link. (However, any locator subelements inside
the linking element are not considered part of the local resource;
they are simply part of the linking element machinery.) If the link is out-of-line,
its content does not count as a local resource. Every link is either inline
or out-of-line.
The inline status of a link is indicated with an attribute called
inline. It can have the value true (the default) or
false.
Links express various kinds of conceptual relationships between the data
objects or portions they connect, in terms of significance to the author and
user. Some links may be criticisms, others add support or background, while
still others might provide access to demographic information about a data
object (its author's name, version number, etc), or to navigational tools
such as index, glossary, and summary.
To indicate the part that a link plays in representing information, a link
author can optionally provide a string identifying the link's role. The role
is indicated with an attribute called role.
(Note that each resource participating in a link may also be given its
own role, as described in "4.1.3 Remote Resource Semantics".)
Following are sample declarations of these attributes, enclosed in a
link-semantics.att parameter entity.
<!ENTITY % link-semantics.att |
Because simple links have an attribute called role that has
a different function, they cannot have a role attribute for link
semantics. Following is a simple-link-semantics.att parameter
entity declaration for use in simple linking elements.
<!ENTITY % simple-link-semantics.att |
The following semantic information can be provided for the remote resources of a link:
(Note that a link as a whole may also be given its own role, as described
in "4.1.2 Link Semantics".)
A link author can optionally provide role information in an attribute called
role.
A link author can optionally provide title information in an attribute
called title. XLink does not require that application software
make any particular use of title information.
A link author can optionally use attributes called show and
actuate to communicate general policies concerning the traversal behavior
of the link. The show attribute can have one of the values
new, replace, and embed; the actuate
attribute can have one of the values auto and
user.
A link author can also optionally use an attribute called behavior
to communicate detailed instructions for traversal behavior. The contents,
format, and meaning of this attribute are unconstrained.
(See "6. Link Behavior" for more information on the behavior-related
attributes.)
Following are sample declarations of these attributes, enclosed in a
remote-resource-semantics.att parameter entity.
<!ENTITY % remote-resource-semantics.att |
The following semantic information can be provided for the local resource of a link, if the link is inline:
(Note that a link as a whole may also be given its own role, as described
in "4.1.2 Link Semantics".)
A link author can optionally provide role information in an attribute called
content-role.
A link author can optionally provide title information in an attribute
called content-title. XLink does not require that application
software make any particular use of title information.
Following are sample declarations of these attributes, enclosed in a
local-resource-semantics.att parameter entity.
<!ENTITY % local-resource-semantics.att |
Simple links can be used for purposes that approximate the functionality
of a basic HTML A link, but they can also support a limited amount
of additional functionality. Simple links have only one locator and thus,
for convenience, combine the functions of a linking element and a locator
into a single element. As a result of this combination, the simple
linking element offers both a locator attribute and all the link and resource
semantic attributes.
Following is a sample declaration for a simple link, showing all the possible
XLink-related attributes it may have (using the parameter entities provided
in "4.1 Information Associated with Links"). The xml:link attribute value
for a simple link must be simple.
<!ELEMENT simple ANY> |
There are no constraints on the contents of a simple linking element. In
the sample declaration above, it is given a content model of ANY
to indicate that any content model or declared content is acceptable. In
a valid document, every element that is significant to XLink must still conform
to the constraints expressed in its governing DTD.
Following is an example of a simple link:
<mylink xml:link="simple" title="Citation" |
This example mylink element might have the following element
and attribute-list declarations:
<!ELEMENT mylink (#PCDATA)> |
Note that it is meaningful to have an out-of-line simple link, although such links are uncommon. They are called "one-ended" and are typically used to associate discrete semantic properties with locations. The properties might be expressed by attributes on the link, the link's element type name, or in some other way, and are not considered full-fledged resources of the link. Most out-of-line links are extended links, as these have a far wider range of uses.
An extended link differs from a simple link in that it can connect any number of resources, not just one local resource (optionally) and one remote resource, and in that extended links are more often out-of-line than simple links.
The additional capabilities of extended links are required for:
Application software might provide traversal among all of a link's participating resources (subject to semantic constraints outside the scope of this specification) and might signal the fact that a given resource or sub-resource participates in one or more links when it is displayed (even though there is no markup at exactly that point to signal it).
A linking element for an extended link contains a series of child elements that serve as locators. Because an extended link can have more than one remote resource, it separates out linking itself from the mechanisms used to locate each resource (whereas a simple link combines the two).
The linking element itself retains those attributes relevant to the link
as a whole and to its local resource, if any. Following is a sample declaration
for an extended link (using the parameter entities provided in "4.1 Information Associated with Links").
The xml:link attribute value for an extended link must be
extended.
<!ELEMENT extended ANY> |
Attributes relevant to remote resources are expressed on the corresponding
contained locator elements. Each remote resource can have its own semantics
in relation to the link as a whole. Following is a sample declaration for
a locator element, showing all the possible XLink-related attributes it may
have (using the parameter entities provided in "4.1 Information Associated with Links").
The xml:link attribute value for a locator element must be
locator.
<!ELEMENT locator ANY> |
Following is an example of an out-of-line extended link:
<commentary xml:link="extended" inline="false"> |
For convenience, defaults for the semantic attributes on locator elements can be specified on the linking element that contains them. If any such attribute is omitted from a locator element, the value provided on the containing linking element is to be used. Following is a sample declaration for an extended link (using the parameter entities provided in "4.1 Information Associated with Links") showing all the possible XLink-related attributes it may have, including the remote resource semantic attributes.
<!ELEMENT extended ANY> |
The content of a linking element typically consists only of locator elements;
however, the declaration as ANY indicates that any other content
may be added. (In a valid document, every element that is significant to XLink
must still conform to the constraints expressed in its governing DTD.) Only
locator elements that are direct children of the linking element define resources
linked by that linking element.
A key issue with out-of-line extended links is how linking application software can manage and find them, particularly when they are stored in completely separate documents from those in which their participating resources appear. XLink provides a mechanism for identifying relevant link-containing documents, which is discussed in "5. Extended Link Groups".
Hyperlinked documents are often best processed in groups rather than one at a time. If it is desired to highlight resources to advertise that traversal can be initiated, and if at the same time out-of-line links are being used, it may be an absolute requirement to read other documents to find these links and discover where the resources are.
In these cases, an extended link group element, a special kind of extended link, may be used to store a list of links to other documents that together constitute an interlinked group. Each such document is identified by means of an extended link document element, a special kind of locator element.
Following are sample declarations for extended link group and extended
link document elements, showing all the possible XLink-related attributes
they may have (using the parameter entities provided in "4.1 Information Associated with Links").
The xml:link attribute value for an extended link group element
must be group, and the value for an extended link document element
must be document.
<!ELEMENT group (document*)> |
The steps attribute may be used by an author to help deal
with the situation where an extended link group directs application software
to locate another document, which proves to contain an extended link group
of its own. There is a potential for infinite regress, and yet there are situations
where processing several levels of extended link groups is useful. The
steps attribute should have a numeric value that serves as a hint from
the author to any link processor as to how many steps of extended link group
processing should be undertaken. It does not have any normative effect.
For example, should a group of documents be organized with a single "hub"
document containing all the out-of-line links, it might make sense for each
non-hub document to contain an extended link group containing only one reference
to the hub document. In this case, the best value for steps would
be 2.
Link formatting and link behavior are inextricably connected. In general, formatting involves the appearance or treatment of the link prior to any user action, such as choice of font, color, icons, and other devices to show that a link is present. Behavior focuses on what happens when the link is traversed, such as opening, closing, or scrolling windows or panes; displaying the data from various resources in various ways; testing, authenticating, or logging user and context information; or executing various programs.
XLink does not provide mechanisms for controlling link formatting because it is considered to fall into the domain of stylesheets. Link behavior should ideally also be determined by rules based on link types, resource roles, user circumstances, and other factors. However, XLink does provide a few very general behavior mechanisms because they are commonly considered to reflect major or invariant semantics of link types.
The mechanism that XLink provides allows link authors to signal certain
intentions as to the timing and effects of traversal. Such intentions can
be expressed along two axes, labeled show and actuate.
These are used to express policies rather than mechanisms
; any link-processing application software is free to devise its own
mechanisms, best suited to the user environment and processing mode, to implement
the requested policies.
In many cases, much finer control over the details of traversal behavior,
of the type that existing hypertext software typically provides, will be desired.
Such fine control of link behavior is outside the scope of this specification.
However, the behavior attribute is provided as a standard place
for authors to provide, and in which application software may look, for detailed
behavioral instructions.
The show attribute
is used to express a policy as to the context in which a resource
that is traversed to should be displayed or processed. It may take one of
three values:
embedreplacenewThe actuate attribute is used to express a policy as to when
traversal of a link should occur. It may take one of two values:
autouserEach combination of the show and actuate attributes
is meaningful. Perhaps the least obvious is show="replace" combined
with actuate="auto"; this could be used in "forwarding" type
applications, where when one anchor is display, the other(s) are to replace
it without user intervention. Since XLink provides only the most general semantics
for links, details of presentation, such as a time delay or beep before forwarding,
can be specified on a per-application basis using a style language.
XLink provides many attributes that can be attached to linking elements
to describe various aspects of links, and each has a default name. It may
be desired to use existing elements in XML documents as linking elements,
but such elements might already have attributes whose names conflict with
those described in this document. To avoid collisions, user-chosen attribute
names can be mapped to the default names using the xml:attributes
attribute.
This attribute must contain an even number of white-space-separated names,
which are treated as pairs. In each pair, the first name must be one of the
default XLink names (role, href, title,
show, inline, content-role, content-title
, actuate, behavior, steps).
The second name, when recognized in the document, will be treated as though
it were playing the role assigned to the first. For example, consider a DTD
with the following declaration:
<!ELEMENT TEXT-BOOK ANY> |
If it were desired to use this as a simple link, it would be necessary to remap a couple of attributes. This could be accomplished in the internal subset:
<!ATTLIST TEXT-BOOK |
Then in the document, the following would be recognized as a simple link:
|
An element conforms to XLink if:
xml:link attribute whose value is
one of the attribute values prescribed by this specification, andxml:link attribute value,
as prescribed in this specification.Note that conformance is assessed at the level of individual elements, rather than whole XML documents, because XLink and non-XLink linking mechanisms may be used side by side in any one document.
An application conforms to XLink if it interprets XLink-conforming elements according to all required semantics prescribed by this specification and, for any optional semantics it chooses to support, supports them in the way prescribed.
The simple title mechanism described in this draft is insufficient to cope with internationalization or the use of multimedia in link titles. A future version will provide a mechanism for the use of structured link titles.