Linking (XLink, XML Base, XPointer)

1. XML Linking facilities
2. XHTML linking
3. XLink
4. Simple XLink Example
5. Global Attributes
6. Simple link example
7. Simplification via DTD
8. XLink types
9. Example: Student's Workplan
10. Simplification via DTD
11. Simplified: Student's Workplan
12. Student's Workplan (cont.)
13. XPointer Framework
14. XPointer Framework (cont)
15. XPointer Framework (cont)
16. Namespaces
17. Element Scheme
18. Why not XPath
19. XBase

XML Linking facilities

Three recommendations to provide advanced linking facilities:

• XLink: hyperlinking
• XPointer: addressing document fragments
• XBase: provides facilities like the HTML base element

XHTML linking

• An <a> element defines a hyperlink:
  • using URI as its locator technology
  • expressed at one end only
  • hyperlink identifies the other end
  • user can initiate traversal from end where it is defined
  • hyperlink effect on windows, etc, defined by browser
• XLink provides a much richer environment

XLink

• A namespace (usually prefixed as xlink)
• Attributes in that namespace define linking semantics
• Two classes of Linking elements:
  • simple (XHTML like)
  • extended
• XLink processor understands the constructs
• Application defines the behaviour
• XLink not just for browsers

Simple XLink Example

• To put a link into an XML document:
  • no longer limited to A, IMG, etc, as links
  • must declare elements that are links (eg, in the DTD)
• For example, A simple link using an arbitrary element ABC:

<ABC xlink:type="simple" xlink:href="http://www.w3.org/">The W3C</ABC>

Global Attributes

• xlink:href: defines a resource by a URI
• xlink:show
  • embed: like IMG added to document
  • new: starts up a new window
  • replace: overwrites current window
• xlink:actuate
  • onLoad: traversed when existence recognised
  • onRequest: when explicitly requested
  • other and none: application dependent
• xlink:role, xlink:arcrole, and xlink:title
  • no processing semantics defined
  • usage left for implementation (eg, tooltip)

Simple link example

<ABC

  xlink:type    = "simple"

  xlink:href    = "http://www.w3.org/"

  xlink:role    = "w3chome"

  xlink:title   = "W3C Home Page"

  xlink:show    = "replace"

  xlink:actuate = "onRequest">

    The W3C

</ABC>

• Link is simple
• Destination defined by href attribute value
• Replaces current window when link activated
• Happens when requested by user, browser or some other agent
• The example is similar to the usual reference in HTML

Simplification via DTD

• XML can be simplifed by:
  • defaulting role, type, etc, attributes to tag name
  • defaulting type appropriately
• With an appropriate DTD, the example could be reduced to, eg:

<ABC

  xlink:href    = "http://www.w3.org/"

  xlink:role    = "w3chome"

  xlink:title   = "W3C Home Page">

    The W3C

</ABC>

XLink types

simple

Defines a link that is an extension of HTML <a>

extended

Connect any number of resources. It contains:

locator

remote resource participating in an extended link (using URI)

resource

local resource used by the extended link

arc

traversal semantics of the participating resources in an extended link (direction, show, actuate, etc)

title

human readable labels for links

Defines a general “link table”

Example: Student's Workplan

<workplan xlink:title="Nevada Barr's workplan" xlink:type="extended">

  <relation xlink:type="arc" xlink:from="student" xlink:to="course"/>

  <relation xlink:type="arc" xlink:from="student" xlink:to="advisor"/>

  <relation xlink:type="arc" xlink:from="marks"   xlink:to="student"/>

  <data xlink:type="locator" xlink:role="student" 

     xlink:href="http://brunel.ac.uk/sr1003.xml"/>

  <data xlink:type="locator" xlink:role="course"

     xlink:href="http://brunel.ac.uk/cs5011A.xml"/>

  <data xlink:type="locator" xlink:role="course"

     xlink:href="http://brunel.ac.uk/cs5012A.xml"/>

  <data xlink:type="locator" xlink:role="advisor"

     xlink:href="http://brunel.ac.uk/advisor/smith.xml"/>

  <marks xlink:type="resource" xlink:role="marks"

     xlink:label="marks.html">7.8</marks>

</workplan>

Note that

• identification is made based on role
• an arc can be a shorthand for several links

Simplification via DTD

XLink usage in XML can be simplifed by:

• Defaulting role to tag name
• Defaulting type appropriately:
  • relation is of type arc
  • student, course, advisor are of type locator
  • marks is of type resource
  • workplan is of type extended

Simplified: Student's Workplan

 <workplan xlink:title="Nevada Barr's workplan"> 

  <relation xlink:from="student" xlink:to="course"/>

  <relation xlink:from="student" xlink:to="advisor"/>

  <relation xlink:from="marks"   xlink:to="student"/>

  <student xlink:href="http://brunel.ac.uk/sr1003.xml" />

  <course  xlink:href="http://brunel.ac.uk/cs5011A.xml"/>

  <course  xlink:href="http://brunel.ac.uk/cs5012A.xml"/>

  <advisor xlink:href="http://brunel.ac.uk/advisor/smith.xml"/>

  <marks   xlink:label="marks.html">7.8</marks>

 </workplan>

Student's Workplan (cont.)

XPointer Framework

• Goal: to address into and XML document
• In XHTML, one could use:

http://a.b.c#q

• this remains valid!
• A more general framework is defined:

http://a.b.c#scheme1(desc1) scheme2(desc2)

• where scheme1 and scheme2 are various schemes

XPointer Framework (cont)

• W3C defines:
  • a general framework “xpointer framework” to define schemes
  • two specific frameworks: “element scheme” and “namespace scheme”
• XML applications may develop and register their schemes. E.g.:

http://a.b.c#svgView(viewBox(...))

• is specific to SVG

XPointer Framework (cont)

• The framework defines a simple syntax to add #fmwk(...) type constructions to URI-s
• There is a left-to-right rule for cases like #a(...) b(...):
  • the first scheme that identifies something wins
• #id remains as a shorthand

Namespaces

• In general, scheme names have namespaces like XML elements;
• This is where the namespace scheme comes in:

#xmlns(img=http://example.org/image)img:rect(10,10,50,50)

• The “default” namespace is reserved for W3C Recommendations, or registered schemes
  • W3C will soon offer a registering mechanism
• Other XML dialects may define their own schemes

Element Scheme

• Simple shorthand to address elements with their 'id':

http://a.b.c#element(something)

• is equal to:

http://a.b.c#something

• But it is also possible to address the children:

http://a.b.c#element(something/3/1)

• “first grandchild of the third child”
• No full XPath processing at this point

Why not XPath

• Obvious question: what about an XPath scheme?
• There was work on an “xpointer scheme”, as an extension to XPath, but it failed to reach concensus
• There are some unofficial XPath schemes out there
• W3C may pick standardize this soon

XBase

Provides equivalent functionality to HTML BASE element