1 XML Linking Language (XLink) W3C Working Draft - 3/3/1998.

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XML Linking Language (XLink)

W3C Working Draft - 3/3/1998

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Overview of presentation

• Introduction to links

• Locator syntax

• Link Recognition

• Linking Elements

• Extended Link Groups

• Link Behavior

• Attribute Remapping

• Conformance

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What is a Link ?

• Link is an explicit relation between two or more data objects or portions of data objects

• Links are asserted by elements in an XML documents

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Basic Characteristics of Links

• The link is expressed at one of its ends• Users can initiate travel from that end to the other• The link’s effect on windows, frames etc. are

mainly determined by the browsers• The link goes to only one destination

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Influential Standards

• HTML• HyTime• TEI P3 (Text Encoding Initiative Guidelines)

and others.

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Terminology

• Linking Element - An Element that asserts the existence and describes the characteristics of a link

• Locator - Data, provided as part of a link, which identifies a resource

• Resource - Anything reachable by the use of a locator in some linking element

• Traversal - The action of using a link i.e. accessing a resource

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Locator Syntax

• Locator for a resource is provided by means of a URI

• Xpointers can be used in conjunction with the URI, as fragment identifiers or queries, to specify a more precise sub-resource

(A sub-resource is a portion of a resource, pointed to as the precise destination of a link)

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Locator Syntax (contd.)

Locator ::= URI

| Connector (XPointer | Name)

| URI Connector (XPointer | Name

Connector ::= ‘#’ | ‘|’

URI ::= URIchar*

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Link Recognition

• The existence of a link is asserted by a linking element

Example:

<A xml:link=“simple”

href =“http://www.w3.org”>The W3C</A>

Attribute list declaration:

<!ATTLIST A xml:link CDATA

#fixed “simple”>

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Linking Elements

• Xlink defines two types of linking elements– Simple Link – Extended Link

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Linking Elements (contd.)

• Information associated with a link – One or more locators to identify the the remote

resources participating in the link

– Semantics of the link

– Semantics of the remote resource

– Semantics of the local resource, if the link is inline

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Linking Elements (contd.)

• Locators

A link must supply a locator for each remote resource

A locator takes the form of an attribute called href

Example:

<!ENTITY %locator.att

“href CDATA#REQUIRED”

>

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Linking Elements (contd.)

• Link Semantics – Whether the link is inline or out-of-line

Inline Links - A link where the content of the linking element serves as a resource

Out-of-line Links - A link whose content does not serve as one of the links resources

– The role of a link

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Linking Elements (contd.)

• Link Semantics Example<!ENTITY % link-semantics.att

“inline (true|false) ‘true’

role CDATA #IMPLIED”

>

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Linking Elements (contd.)

• Remote Resource Semantics <!ENTITY % remote-resource-semantics.att

“role CDATA #IMPLIED

title CDATA #IMPLIED

show (embed|replace|new) #IMPLIED

actuate (auto|user) #IMPLIED

behavior CDATA #IMPLIED”

>

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Linking Elements (contd.)

• Local Resource Semantics<!ENTITY % local-resource-semantics.att

“content- role CDATA #IMPLIED

content- title CDATA#IMPLIED”

>

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Linking Elements (contd.)

• Simple Link are usually inline and always one-directional.

• Example:

<mylink xml:link=“simple” title = “citation”

href=“http://www.a.com/foo.xml” show=“new”

content-role=“Reference”> as in Smith (1997)

</mylink>

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Linking Elements (contd.)

Example Declaration<!ELEMENT simple ANY>

<!ATTLIST simple

xml:link CDATA #FIXED “simple”

%locator.att;

%remote-resource-semantics.att;

%local-resource-semantics.att;

%simple-link-semantics.att;

>

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Linking Elements (contd.)• Extended Links

– Link Together any number of resources, resulting in multiple targets

– Link to and from resources that cannot contain the links themselves.

– Applying and filtering sets of relevant links on demand

– Enabling other advanced hypermedia capabilities

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Linking Elements (contd.)

• Sample Declaration<!ELEMENT simple ANY>

<!ATTLIST simple

xml:link CDATA #FIXED “extended”

%link-semantics.att;

%local-resource-semantics.att;

>

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Extended Link Groups

• An extended link group element 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

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Extended Link Groups (contd.)• Sample declarations for extended link group and extended

link document elements

<!ELEMENT group (document*)>

<!ATTLIST group

xml:link CDATA #FIXED “group”

steps CDATA #“IMPLIED”

>

<!ELEMENT document EMPTY

<!ATTLIST document

xml:link CDATA #FIXED “document”

%locator.att;

>

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Link Behavior

• Authors can signal certain intentions as to the timing and effects of traversal.

• These intentions expressed along the ‘show’ and ‘actuate’ axis

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Link Behavior (contd.)

• “Show” Axis – The show attribute is used to express a policy

as to the context in which a resource that is traversed is to be displayed or processed.

– It can take on one of three values• embed• replace• new

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Link Behavior (contd.)

• “Actuate” Axis– The actuate attribute is used to express a policy

as to when traversal of a link should occur– It may take on one of two values

• auto• user

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Attribute Remapping

• User chosen attribute names can be mapped to the default names using the attribute xml:attribute

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Conformance

• An element conforms to Xlink if:– The element has an xml:link attribute whose

value is one of the attribute values prescribed by this specification and

– the element and all of its attributes and content adhere to the syntactic requirements imposed by the chosen xml:link attribute value, as prescribed in this specification.