Tài liệu XML by Example- P4 pdf

Figure 5.6: Walking down the input treeFollowing the Processor Let’s follow the XSL processor for the first few templates in the style sheet.After loading the style sheet and the source

The syntax for XML paths is similar to file paths XML paths start fromthe root of the document and list elements along the way Elements are separated by the “/” character

The root of the document is “/” The root is a node that sits before the level element It represents the document as a whole

top-The following four paths match respectively the title of the article(<title>XML Style Sheets</title>), the keywords of the article, the top-most article element, and all sections in the article Note that the last pathmatches several elements in the source tree

/article/title /article/keywords /article

/article/section

T I P

Note that “ / ” points to the immediate children of a node Therefore, /article/title

selects the main title of the article (XML Style Sheets), not all the titles below the cle element It won’t select the section titles.

arti-To select all the descendants from a node, use the “ // ” sequence /article//title

selects all the titles in the article It selects the main title and the section titles

In the style sheet, most paths don’t start at the root XSL has the notion ofcurrent element Paths in the match attribute can be relative to the currentelement

Again, this is similar to the file system Double-clicking the accessories

folder in the c:\program filesfolder moves to c:\program files\

accessoriesfolder, not to c:\accessories

If the current element is an article, then titlematches /article/title

but if the current article is a section, titlematches one of the /article/ section/titles

To match any element, use the wildcard character “*” The path /article/*

matches any direct descendant from article, such as title, keywords, and

so on

It is possible to combine paths in a match with the “|” character, such as

title | pmatches title or pelements

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It matches <url protocol=”mailto”>bmarchal@pineapplesoft.com</url>

that has a protocol attribute with the value “mailto” but it does not match

<url>http://www.w3.org/Style</url> The more generic urlpath wouldmatch the later element

url[@protocol]matches URL elements that have a protocol attribute,

no matter what its value is It would match the <url protocol=”http”>www.w3.org/Style</url>but it would not match

<url>http://www.w3.org/Style</url>

Matching Text and Functions

Functions restrict paths to specific elements The following two paths areidentical and select the text of the title of the second section in the docu-ment (Styling)

/article/section[position()=2]/title/text() /article/section[2]/title/text()

Most functions can also take a path as an argument For example,

count(//title)returns the number of title elements in the document.Table 5.1 lists some of the most common functions

Table 5.1: Most common XSL functions

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last() returns the position of the last node in the current node set

starts-with() returns true if the first argument starts with the second

</xsl:functions>

C A U T I O N

Be aware that this element was still very much in flux in the draft we used to prepare this chapter

Deeper in the Tree

After loading the style sheet, the XSL processor loads the source document.Next, it walks through the source document from root to leaf nodes At eachstep it attempts to match the current node against a template

If there is a match, the processor generates the nodes in the resulting tree.When it encounters xsl:apply-templates, it moves to the children of thecurrent node and repeats the process; that is, it attempts to match themagainst a template

In other words, xsl:apply-templatesis a recursive call to the style sheet

A recursive approach is natural to manipulate trees You might have nized a deep-first search algorithm Figure 5.6 illustrates how it works

recog-137Basic XSLT

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Figure 5.6: Walking down the input tree

Following the Processor

Let’s follow the XSL processor for the first few templates in the style sheet.After loading the style sheet and the source document the processor posi-tions itself at the root of the source document It looks for a template thatmatches the root and it immediately finds

When it encounters xsl:appy-templates, the processor moves to the firstchild of the current node The first child of the root is the top-level element

or the article element

The style sheet defines no templates for the article but can match template

against a built-in template Built-in templates are not defined in the style

sheet They are predefined by the processor

The first and only child of title is a text node The style sheet has no rule tomatch text but there is another built-in template that copies the text in theresulting tree.

<xsl:template match=”text()”>

<xsl:value-of select=”.”/>

</xsl:template>

The title’s text has no children so the processor cannot go to the next level

It backtracks to the article element and moves to the next child: the dateelement This element matches the last template

<xsl:template match=”abstract | date | keywords | copyright”/>

This template generates no output in the resulting tree and stops ing for the current element

process-The processor backtracks again to article and processes its other children:copyright, abstract, keywords, and section Copyright, abstract, and key-words match the same rule as abstract and generate no output in theresulting tree

139Basic XSLT

The section element, however, matches the default template and the sor moves to its children, title, and pelements The processor continues tomatch rules with nodes until it has exhausted all the nodes in the originaldocument.

proces-Creating Nodes in the Resulting Tree

Sometimes it is useful to compute the value or the name of new nodes Thefollowing template creates an HTML anchor element that points to theURL The anchor has two attributes The first one, TARGET, is specifieddirectly in the template However, the processor computes the secondattribute, HREF, when it applies the rule

Table 5.2 lists other XSL elements that compute nodes in the resulting tree

Table 5.2: XSL elements to create new objects

xsl:processing-instruction creates a processing instruction

inserting a variable

alternatives

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PRIORITYThere are rules to prioritize templates Without going into too many details,templates with more specific paths take precedence over less specific tem-plates In the following example, the first template has a higher prioritythan the second template because it matches an element with a specificattribute.

Supporting a Different Medium

Recall that my original problem is to provide both an HTML and a text sion of the document We have seen how to automatically create an HTMLversion document, now it’s time to look at the text version

ver-Text Conversion

C A U T I O N

Text conversion stretches the concept of XML to XML conversion; therefore, you have to

be careful in writing the style sheet.

Listing 5.4 is the text style sheet It is very similar to the previous stylesheet except that it inserts only text nodes, no XML elements, in the resulting tree

141Supporting a Different Medium

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<?xml version=”1.0” encoding=”ISO-8859-1”?>

<xsl:stylesheet xmlns:xsl=”http://www.w3.org/1999/XSL/Transform/”>

=== XML Style Sheets ===

Send comments and suggestions to <bmarchal@pineapplesoft.com>.

*** Styling ***

Style sheets are inherited from SGML, an XML ancestor Style sheets originated

➥in publishing and document management applications XSL is XML’s standard style

➥sheet, see [http://www.w3.org/Style].

*** How XSL Works ***

An XSL style sheet is a set of rules where each rule specifies how to format

➥certain elements in the document To continue the example from the previous

➥section, the style sheets have rules for title, paragraphs and keywords With XSL, these rules are powerful enough not only to format the document

➥but also to reorganize it, e.g by moving the title to the front page or

➥extracting the list of keywords This can lead to exciting applications of XSL

➥outside the realm of traditional publishing For example, XSL can be used to

➥convert documents between the company-specific markup and a standard one.

*** The Added Flexibility of Style Sheets ***

Style sheets are separated from documents Therefore one document can have more

➥than one style sheet and, conversely, one style sheet can be shared amongst

➥several documents.

This means that a document can be rendered differently depending on the media or

➥the audience For example, a “managerial” style sheet may present a summary

➥view of a document that highlights key elements but a “clerical” style sheet

➥may display more detailed information

143Supporting a Different Medium

O U T P U T

Customized Views

Currently, most people access the Web through a browser on a Windows

PC Some people use Macintoshes, others use UNIX workstations This willchange in the future as more people turn to specialized devices AlreadyWebTV has achieved some success with a browser in a TV set

Mobile phones and PDAs, such as the popular PalmPilot, will be ingly used for Web browsing Ever tried surfing on a PalmPilot? It workssurprisingly well but, on the small screen, many Web sites are not readableenough

increas-One solution to address the specific needs of smaller devices might be touse XHTML, an XML simplified version of HTML XHTML is based onHTML but it has an XML syntax (as opposed to an SGML syntax) It is alsodesigned to be modular as it is expected smaller devices will implementonly a subset of the recommendation

According to the W3C, these new platforms might account for up to 75% ofWeb viewing by the year 2002 What can you do about it? Will you have tomaintain several versions of your Web site: one for existing Web browsersand one for each new device with its own subset?

XSL to the rescue! It will be easy to manage the diversity of browsers andplatforms by maintaining the document source in XML and by converting

to the appropriate XHTML subset with XSLT In essence, this is how Imanage the e-zine Figure 5.7 illustrates how this works

Figure 5.7: Maintain one XML document and convert it to the appropriate

markup language.

Where to Apply the Style Sheet

So far, we have converted the XML documents before publishing them Theclient never sees XML; it manipulates only HTML

Today, this is the realistic option because few users have an XML-enabled

browser such as Internet Explorer 5.0 or a beta version of Mozilla 5.0

(Mozilla is the open source version of Netscape Communicator)

Furthermore, the XSL recommendation is not final yet so implementations

of XSL processors are not always compatible with one another

Yet, if your users have XML-enabled browsers, it is possible to send themraw XML documents and style sheets The browser dynamically applies thestyle sheets and renders the documents Figure 5.8 contrasts the twooptions

145Where to Apply the Style Sheet

Figure 5.8: Style sheets on the server or on the client

Internet Explorer 5.0

C A U T I O N

Because XSL is still in draft, browser implementations are not compatible The material

in this section works with Internet Explorer 5.0, which implements an early draft of XSL and is not compatible with the current draft, much less with the future recommenda- tion.

The processing instruction xml-stylesheetassociates a style sheet with thecurrent document It takes two parameters, an href to the style sheet andthe type of the style sheet (text/xsl, in this case)

<?xml-stylesheet href=”simple-ie5.xsl” type=”text/xsl”?>

Listing 5.6 is the XML document with the appropriate processing tion for Internet Explorer 5.0

instruc-Listing 5.6: The XML Document Prepared for Internet Explorer 5.0

<?xml version=”1.0”?>

<?xml-stylesheet href=”simple-ie5.xsl” type=”text/xsl”?>

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continues

<article fname=”19990101_xsl”>

<title>XML Style Sheets</title>

<date>January 1999</date>

<copyright>1999, Benoît Marchal</copyright>

<abstract>Style sheets add flexibility to document viewing.</abstract>

<keywords>XML, XSL, style sheet, publishing, web</keywords>

<p>Style sheets are inherited from SGML, an XML ancestor Style sheets

➥originated in publishing and document management applications XSL is XML’s

➥standard style sheet, see <url>http://www.w3.org/Style</url>.</p>

</section>

<section>

<title>How XSL Works</title>

<p>An XSL style sheet is a set of rules where each rule specifies how to format

➥certain elements in the document To continue the example from the previous

➥section, the style sheets have rules for title, paragraphs and keywords.</p>

<p>With XSL, these rules are powerful enough not only to format the document

➥but also to reorganize it, e.g by moving the title to the front page or

➥extracting the list of keywords This can lead to exciting applications of XSL

➥outside the realm of traditional publishing For example, XSL can be used to

➥convert documents between the company-specific markup and a standard one.</p>

</section>

<section>

<title>The Added Flexibility of Style Sheets</title>

<p>Style sheets are separated from documents Therefore one document can have

➥more than one style sheet and, conversely, one style sheet can be shared

➥amongst several documents.</p>

<p>This means that a document can be rendered differently depending on the media

➥or the audience For example, a “managerial” style sheet may present a summary

➥view of a document that highlights key elements but a “clerical” style sheet

➥may display more detailed information.</p>

</section>

</article>

Furthermore, the style sheet must be adapted to the older version of XSLthat Internet Explorer supports Listing 5.7 is the adapted style sheet.Figure 5.9 shows the result in Internet Explorer

Listing 5.7: XSLT Style Sheet for Internet Explorer 5.0

<?xml version=”1.0” encoding=”ISO-8859-1”?>

<xsl:stylesheet xmlns:xsl=”http://www.w3.org/TR/WD-xsl”

continues

Figure 5.9: Internet Explorer 5.0 renders XML.

Changes to the Style Sheet

The style sheet has been adapted in two places First, the XSL namespacepoints to an earlier version of XSL

<xsl:stylesheet xmlns:xsl=”http://www.w3.org/TR/WD-xsl”

Advanced XSLT

XSLT is a powerful transformation mechanism So far, we have only used asubset of it Our resulting document follows a structure that is close to theoriginal document Elements might have been added or removed from thetree but they are not reorganized

Yet, it is often useful to reorganize completely the source document Forexample, we might want to create a table of contents at the beginning ofthe document

This is possible with the xsl:value-ofelement xsl:value-ofinserts trary elements from the source tree anywhere in the resulting tree

arbi-Listing 5.8 is a more sophisticated style sheet that, among other things,creates a table of contents

Listing 5.8: A More Powerful XSLT Style Sheet

149Advanced XSLT

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continues

You can use LotusXSL to apply this style sheet It generates the HTML ument in Listing 5.9 Figure 5.10 shows the result in a browser.

doc-Listing 5.9: The Resulting HTML Document

<!DOCTYPE html PUBLIC “-//W3C//DTD HTML 4.0 Transitional//EN”>

<HTML>

<HEAD>

<TITLE>XML Style Sheets ( January 1999 )</TITLE>

<META name=”keywords” content=”XML, XSL, style sheet, publishing, web”>

<LI><A href=”#N-634270327”>How XSL Works</A></LI>

<LI><A href=”#N-653406839”>The Added Flexibility of Style Sheets</A></LI>

</UL>

<P>Send comments and suggestions to <A href=”mailto:bmarchal@pineapplesoft.com”>bmarchal@pineapplesoft.com</A>.</P>

<P><I><A name=”N-614609527”>Styling</A></I></P>

<P>Style sheets are inherited from SGML, an XML ancestor Style sheets

➥originated in publishing and document management applications XSL is XML’s

➥standard style sheet, see <A target=”_blank”

➥href=”http://www.w3.org/Style”>http://www.w3.org/Style</A>.</P><P><I><A name=

➥”N-634270327”>How XSL Works</A></I></P>

<P>An XSL style sheet is a set of rules where each rule specifies how to format

➥certain elements in the document To continue the example from the previous

➥section, the style sheets have rules for title, paragraphs and keywords.</P>

<P>With XSL, these rules are powerful enough not only to format the document

➥but also to reorganize it, e.g by moving the title to the front page or

➥extracting the list of keywords This can lead to exciting applications of XSL

➥outside the realm of traditional publishing For example, XSL can be used to

➥convert documents between the company-specific markup and a standard one.</P>

<P><I><A name=”N-653406839”>The Added Flexibility of Style Sheets</A></I></P>

<P>Style sheets are separated from documents Therefore one document can have

➥more than one style sheet and, conversely, one style sheet can be shared

➥amongst several documents.</P>

<P>This means that a document can be rendered differently depending on the media

➥or the audience For example, a “managerial” style sheet may present a summary

➥view of a document that highlights key elements but a “clerical” style sheet

➥may display more detailed information.</P>

<P>Copyright ©1999, Benoît Marchal</P></BODY></HTML>

O U T P U T

Figure 5.10: The resulting HTML document in a browser

Declaring HTML Entities in a Style Sheet

This style sheet has an internal DTD to declare the copyentity—an HTML entity HTML has many entities that XML does not recognize

<!DOCTYPE xsl:stylesheet [

<!ENTITY copy “&#0169;”>

]>

Reorganizing the Source Tree

The list of keywords must appear in an HTMLMETAelement The followingexample extracts the keywords from the source tree, with the xsl:value-of

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Calling a Template

When the same styling instructions are used at different places, groupthem in a named template For example, titles appear in the HTML titleand in the body of the document

xsl:includemust be a direct child of xsl:stylesheet, it cannot appear in

xsl:templatefor example

Repetitions

Sometimes a path points to several elements For example, article/ section/titlepoints to the three section titles To loop over the elements,use xsl:for-each The following rule builds a table of contents with sectiontitles:

<LI><A href=”#N-634270327”>How XSL Works</A></LI>

<LI><A href=”#N-653406839”>The Added Flexibility of Style Sheets</A></LI>

xsl:for-eachhas a select attribute so it needs a fully qualified path.

However, within the loop, the current element is the selection that

xsl:value-ofretrieves through the “.” path

The template also introduces the generate-id()function The functionreturns a unique identifier for the current node

Using XSLT to Extract Information

As the various examples in this chapter illustrate, XSLT is a very powerfuland flexible mechanism that serves many purposes

Indeed XSLT is not limited to styling It also can be used to extract mation from XML documents

infor-Imagine I need to generate an index of articles The XSLT solution is a step process In the first step, a style sheet extracts useful information fromthe documents Extracting information can be thought of as transforming alarge XML document into a smaller one

two-The first step creates as many extract documents as there are originals.The next step is to merge all the extracts in one listing It is then a simpleissue to convert the listing in HTML Figure 5.11 illustrates the process

155Using XSLT to Extract Information

Figure 5.11: How using XSL can extract information from XML documents

A N E W S T A N D A R D

The W3C works on a new standard XQL, the XML Query Language, that will offer a

better solution to this problem XQL can query multiple documents stored in an XML database.

XQL will use paths similar or identical to XSLT so it will be familiar Because it works across several documents, XQL is really designed for XML databases XML databases store documents in binary format to provide faster access.

To experiment with XQL without buying a database, you can download the GMD-IPSI XQL engine from xml.darmstadt.gmd.de/xml The engine is written in Java but it has a command-line interface.

Listing 5.10 is a style sheet to extract the URL, the title, the abstract, andthe date of an article document.

Listing 5.10: Style Sheet to Extract Data

<?xml version=”1.0” encoding=”ISO-8859-1”?>

<xsl:stylesheet xmlns:xsl=”http://www.w3.org/1999/XSL/Transform/”>

➥for %%0 in (%files%) do %xslprocessor% -in %%0.xml -out %%0.xtr

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-xsl extract.xsl copy opening.tag index.xml for %%0 in (%files%) do copy index.xml /a + %%0.xtr

➥/a index.xml /a copy index.xml + closing.tag index.xml

➥offering from Sun Jini extends Java towards distributed computing in novative

➥ways In particular, Jini builds on the concept of “spontaneous

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continues

What’s Next

In this chapter, you learned how to use the transformation half of XSL Thenext chapter is dedicated to styling XML directly, no conversion required,with CSS and XSLFO

The combination of XSLT and CSS gives you total control over how yourdocument is displayed

159What's Next