Hyper text differs from the normal text
in that they have hyperlinks. The hyperlinks are identified
by the characteristic blue color underlined text that identifies
hot spots. These hotspots when clicked, will take us to the
Web pages that are specified as links.
Linking or cross-referencing can be done when :
||There is a need to provide
context-sensitive help, for instance, when we navigate
tutorials, it will be easy to understand if elaborations
for some technical terms or external references are provided.
||A file has to be referred
or displayed on clicking the mouse at a particular point of the document.
Link is a functionality that is associated
with a text or an object in a document using markup language.
Providing links enable us to refer to an
object or a file within the same document or a document in
some other location. The object which is referred to, can
be an image file or any other file, or text within the same
document, or a different document. Within a HTML or an XML
document, there may be several links. The active document,
which contains these links, is called the source file.
The linked file or file that has been referred
to is called the destination file or the target.
The target file may be directly opened on
clicking a hotspot or it may in turn lead to some other link.
In simple terms of HTML linking, a link is the association
between a source and the target.
The target may be completely a new HTML page, in which case,
the description of the target (locator) would be a Universal
Resource Identifier (URI). The target may be a named element
within an HTML page. It can be identified using the # symbol,
called a fragment identifier. Then, the fragment identifier
has to be followed by the NAME attribute of the target element.
Both the source and the target file can
be viewed simultaneously. One of the best examples of viewing
both the source and the target simultaneously, is the search
index provided in the HELP menu of any application, wherein
selecting a particular index opens the corresponding document.
The differences between HTML links and XML
links are depicted in the table shown here.
|The linking mechanism is simple.
||The linking mechanism is complex.
links talks about sources and target.
talks about linking elements instead of sources.
|Links are unidirectional,
that is, one link lead the other end of a link in a straight.
||Links can be
bidirectional, so the other end of link could be a source as well as target.
source link leads to the target link.
could refer to resource, which could be a piece
of data, obtained as a result of a database query
or an external link that acts as an intermediary
en route to the final destination XML's X links
allows to specify multiple or group linked locators.
There are six different areas that need
to be addressed while dealing with linking and querying. They are as follows.
XML information set - The World Wide Web
Consortium (W3C)' s document defines what an XML information
set is. It says that XML information set comprises various
pieces of information, which together make up the XML document.
Xlink is W3C' s mechanism for linking to
other resources within an XML document. Xlink also allows
no-XML document to be linked together. XPath is the general
language specification for addressing parts of an XML document framed by W3C' s.
XPointer is W3C' s mechanism for pointing
to a particular location within an XML document.
XML fragment Interchange for transmitting
a part of XML document as per W3C specification.
Querying XML document -XML document can
be queried using XSLT technology recommended by W3C, which
is dealt with in the later part of the session.
The XML information set, or Infoset is a
working draft created by W3C to describe various pieces of
information that together form an XML document. There are
fifteen distinct types of information that forms the XML document.
They are represented in the table as shown.
||Exactly one document information item.
||One or more element information items.
||Attribute information items.
||Processing instruction information items.
||Character information items.
||Reference to skipped entity information items.
||Comment information items.
||A document type declaration information items.
||Entity information items.
||Notation information items.
||Entity start marker information items.
||Entity end marker information items.
||CDATA start marker information items.
||CDATA end marker information items.
||Namespace declaration items.
HTML has two link elements namely, A and
IMG, whereas in XML links, the link elements are identified
by the element attributes.
Any XML element can act as a link element
provided it has the right kind of attributes.
<|ELEMENT CORRELATION ANY> <!ATTLIST
CORRELATION xlink:form CDATA #FIXED value>
The primary attribute that identifies the
XML element as a link is the xlink:form attribute, whose declaration
in an XML DTD, would be as shown . Here the value should be
a locator and not the linking elements. Also the value can
be simple or extended.
XML links work with link elements. The link
elements in turn contain locators.
Locators are in the form of attributes or
other elements that points to specific locations.
In general, a locator is a URI, a fragment identifier, or
a URI combined with a fragment identifier. Locators for XML
documents are extended pointers.
The syntax of locators allows us to use
the two variations as shown here.
URI#fragment - This fetches the whole of the resource identified
by the URI and then extracts the part identified by the fragment identifier.
URI|fragment - The application can decide
how it will process the URI in order to extract the resource.
This could be used to retrieve a particular part of the document.
If the fragment identifier is a character
string, the string is treated as the value of the id attribute
of an XML element. For instance, the locator sample.html#sa2
would point to the element with attribute value of sa2 in
the file sample.html.
XML' s Xlinks are used to establish hyperlinks in XML documents.
The W3C Xlink working draft defines two
categories of links. They are, simple links and extended links.
The xml:link attribute is used for specifying
a link or location term as shown here.
| "locator" | "group" | "document"
The figure here shows the outline classification of XML links.
Copyrights : Layout Galaxy All Rights Reserved
No part of this tutorial may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, electrostatic, magnetic tape, mechanical or otherwise, without prior permission in writing from Layout Galaxy.