XML Linking Language (XLink)

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

2. Overview of presentation • Introduction to links • Locator syntax • Link Recognition • Linking Elements • Extended Link Groups • Link Behavior • Attribute Remapping • Conformance

3. 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

4. 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

5. Influential Standards • HTML • HyTime • TEI P3 (Text Encoding Initiative Guidelines) and others.

6. 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

7. 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)

8. Locator Syntax (contd.) Locator ::= URI | Connector (XPointer | Name) | URI Connector (XPointer | Name Connector ::= ‘#’ | ‘|’ URI ::= URIchar*

9. 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”>

10. Linking Elements • Xlink defines two types of linking elements • Simple Link • Extended Link

11. 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

12. 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” >

13. 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

14. Linking Elements (contd.) • Link Semantics Example <!ENTITY % link-semantics.att “inline (true|false) ‘true’ role CDATA #IMPLIED” >

15. 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” >

16. Linking Elements (contd.) • Local Resource Semantics <!ENTITY % local-resource-semantics.att “content- role CDATA #IMPLIED content- title CDATA #IMPLIED” >

17. 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>

18. 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; >

19. 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

20. Linking Elements (contd.) • Sample Declaration <!ELEMENT simple ANY> <!ATTLIST simple xml:link CDATA #FIXED “extended” %link-semantics.att; %local-resource-semantics.att; >

21. 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

22. 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; >

23. Link Behavior • Authors can signal certain intentions as to the timing and effects of traversal. • These intentions expressed along the ‘show’ and ‘actuate’ axis

24. 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

25. 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

26. Attribute Remapping • User chosen attribute names can be mapped to the default names using the attribute xml:attribute

27. 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.