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<html><head><title>Transformations (Perl and XML)</title><link rel="stylesheet" type="text/css" href="../style/style1.css" /><meta name="DC.Creator" content="Erik T. Ray and Jason McIntosh" /><meta name="DC.Format" content="text/xml" scheme="MIME" /><meta name="DC.Language" content="en-US" /><meta name="DC.Publisher" content="O'Reilly & Associates, Inc." /><meta name="DC.Source" scheme="ISBN" content="059600205XL" /><meta name="DC.Subject.Keyword" content="stuff" /><meta name="DC.Title" content="Perl and XML" /><meta name="DC.Type" content="Text.Monograph" /></head><body bgcolor="#ffffff"><img alt="Book Home" border="0" src="gifs/smbanner.gif" usemap="#banner-map" /><map name="banner-map"><area shape="rect" coords="1,-2,616,66" href="index.htm" alt="Perl & XML" /><area shape="rect" coords="629,-11,726,25" href="jobjects/fsearch.htm" alt="Search this book" /></map><div class="navbar"><table width="684" border="0"><tr><td align="left" valign="top" width="228"><a href="ch02_11.htm"><img alt="Previous" border="0" src="../gifs/txtpreva.gif" /></a></td><td align="center" valign="top" width="228" /><td align="right" valign="top" width="228"><a href="ch03_01.htm"><img alt="Next" border="0" src="../gifs/txtnexta.gif" /></a></td></tr></table></div><h2 class="sect1">2.12. Transformations</h2><p>The last topic we want to introduce is the concept of <a name="INDEX-154" /> <a name="INDEX-155" />transformations. In XML, a<em class="emphasis">transformation</em> is a process of restructuringor converting a document into another form. The W3C recommends alanguage for transforming XML called<a name="INDEX-156" /><a name="INDEX-157" /> XML Stylesheet Language forTransformations (XSLT). It's an incredibly usefuland fun technology to work with.</p><p>Like XML Schema, an XSLT transformation script is an XML document.It's composed of <em class="emphasis">templaterules</em><a name="INDEX-158" />, each of which is an instruction forhow to turn one element type into something else. The term<em class="emphasis">template</em> is often used to mean an example ofhow something should look, with blanks that you should fill in.That's exactly how template rules work: they areexamples of how the final document should be, with the blanks filledin by the XSLT processor.</p><p><a href="ch02_12.htm#perlxml-CHP-2-EX-5">Example 2-5</a> is a rudimentary transformation thatconverts a simple<a name="INDEX-159" />DocBook XML document into an<a name="INDEX-160" />HTMLpage.</p><a name="perlxml-CHP-2-EX-5" /><div class="example"><h4 class="objtitle">Example 2-5. An XSLT transformation document </h4><blockquote><pre class="code"><xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="1.0"><xsl:output method="html"/><!-- RULE FOR BOOK ELEMENT --><xsl:template match="book"> <html> <head> <title><xsl:value-of select="title"/></title> </head> <body> <h1><xsl:value-of select="title"/></h1> <h3>Table of Contents</h3> <xsl:call-template name="toc"/> <xsl:apply-templates select="chapter"/> </body> </html></xsl:template><!-- RULE FOR CHAPTER --><xsl:template match="chapter"> <xsl:apply-templates/></xsl:template><!-- RULE FOR CHAPTER TITLE --><xsl:template match="chapter/title"> <h2> <xsl:text>Chapter </xsl:text> <xsl:number count="chapter" level="any" format="1"/> </h2> <xsl:apply-templates/></xsl:template> <!-- RULE FOR PARA --><xsl:template match="para"> <p><xsl:apply-templates/></p></xsl:template><!-- NAMED RULE: TOC --><xsl:template name="toc"> <xsl:if test="count(chapter)>0"> <xsl:for-each select="chapter"> <xsl:text>Chapter </xsl:text> <xsl:value-of select="position( )"/> <xsl:text>: </xsl:text> <i><xsl:value-of select="title"/></i> <br/> </xsl:for-each> </xsl:if></xsl:template></xsl:stylesheet></pre></blockquote></div><p>First, the XSLT processor reads the stylesheet and creates a table oftemplate rules. Next, it parses the source XML document (the one tobe converted) and traverses it one node at a time. A<em class="emphasis">node</em> is an element, a piece of text, aprocessing instruction, an attribute, or a namespace declaration. Foreach node, the XSLT processor tries to find the <em class="emphasis">bestmatching</em><a name="INDEX-161" /> rule. It applies the rule, outputtingeverything the template says it should, jumping to other rules asnecessary.</p><p><a href="ch02_12.htm#perlxml-CHP-2-EX-6">Example 2-6</a> is a sample document on which you canrun the transformation.</p><a name="perlxml-CHP-2-EX-6" /><div class="example"><h4 class="objtitle">Example 2-6. A document to transform </h4><blockquote><pre class="code"><book> <title>The Blathering Brains</title> <chapter> <title>At the Bazaar</title> <para>What a fantastic day it was. The crates were stacked high with imported goods: dates, bananas, dried meats, fine silks, and more things than I could imagine. As I walked around, savoring the fragrances of cinnamon and cardamom, I almost didn't notice a small booth with a little man selling brains.</para> <para>Brains! Yes, human brains, still quite moist and squishy, swimming in big glass jars full of some greenish fluid.</para> <para>"Would you like a brain, sir?" he asked. "Very reasonable prices. Here is Enrico Fermi's brain for only two dracmas. Or, perhaps, you would prefer Aristotle? Or the great emperor Akhnaten?"</para> <para>I recoiled in horror...</para> </chapter></book></pre></blockquote></div><p>Let's walk through the transformation. </p><ol><li><p>The first element is <tt class="literal"><book></tt>. The bestmatching rule is the first one, because it explicitly matches"book." The template says to outputtags like <tt class="literal"><html></tt>,<tt class="literal"><head></tt>, and<tt class="literal"><title></tt>. Note that these tags are treated asdata markup because they don't have the<tt class="literal">xsl:</tt> namespace prefix.</p></li><li><p>When the processor gets to the XSLT instruction<tt class="literal"><xsl:value-of select="title"/></tt>, it has tofind a <tt class="literal"><title></tt> element that is a child ofthe current element, <tt class="literal"><book></tt>. Then it mustobtain the <em class="emphasis">value</em> of that element, which issimply all the text contained within it. This text is output inside a<tt class="literal"><title></tt> element as the template directs.</p></li><li><p>The processor continues in this way until it gets to the<tt class="literal"><xsl:call-template</tt><tt class="literal">name="toc"/></tt> instruction. If you look at thebottom of the stylesheet, you'll find a templaterule that begins with <tt class="literal"><xsl:templatename="toc"></tt>. This template rule is a <em class="emphasis">namedtemplate</em> and acts like a function call. It assembles atable of contents and returns the text to the calling rule foroutput.</p></li><li><p>Inside the named template is an element called <tt class="literal"><xsl:iftest="count(chapter)>0"></tt>. This element is aconditional statement whose test is whether more than one<tt class="literal"><chapter></tt> is inside the current element(still <tt class="literal"><book></tt>). The test passes, andprocessing continues inside the element.</p></li><li><p>The <tt class="literal"><xsl:for-each select="chapter"></tt>instruction causes the processor to visit each<tt class="literal"><chapter></tt> child element and temporarily makeit the current element while in the body of the<tt class="literal"><xsl:for-each></tt> element. This step isanalogous to a <tt class="literal">foreach( )</tt> loop in Perl. The<tt class="literal"><xsl:value-of select="position( )"/></tt>statement derives the numerical position of each<tt class="literal"><chapter></tt> and outputs it so that the resultdocument reads "Chapter 1,""Chapter 2," and so on.</p></li><li><p>The named template "toc" returnsits text to the calling rule and execution continues. Next, theprocessor receives an <tt class="literal"><xsl:apply-templatesselect="chapter"/></tt> directive. An output of<tt class="literal"><xsl:apply-templates></tt> without any attributesmeans that the processor should then process each of the currentelement's children, making it the current element.However, since a <tt class="literal">select="chapter"</tt> attribute ispresent, only children who are of type<tt class="literal"><chapter></tt> should be processed. After alldescendants have been processed and this instruction returns itstext, it will be output and the rest of the rule will be followeduntil the end.</p></li><li><p>Moving on to the first <tt class="literal"><chapter></tt> element,the processor locates a suitable rule and sees only an<tt class="literal"><xsl:apply-tempaltes/></tt> rule. The rest of theprocessing is pretty easy, as the rules for the remaining elements,<tt class="literal"><title></tt> and <tt class="literal"><para></tt>,are straightforward.</p></li></ol><p>XSLT is a rich language for handling transformations, but oftenleaves something to be desired. It can be slow on large documents,since it has to build an internal representation of the wholedocument before it can do any processing. Its syntax, while aremarkable achievement for XML, is not as expressive and easy to useas Perl. We will explore numerous Perl solutions to some problemsthat XSL could also solve. You'll have to decidewhether you prefer XSLT's simplicity orPerl's power.</p><p>That's our whirlwind tour of XML. Next,we'll jump into the fundamentals of XML processingwith Perl using parsers and basic writers. At this point, you shouldhave a good idea of what XML is used for and howit's used, and you should be able to recognize allthe parts when you see them. If you still have any doubts, stop nowand grab an<a name="INDEX-162" /> <a name="INDEX-163" /> XML<a name="INDEX-164" /> tutorial.</p><hr width="684" align="left" /><div class="navbar"><table width="684" border="0"><tr><td align="left" valign="top" width="228"><a href="ch02_11.htm"><img alt="Previous" border="0" src="../gifs/txtpreva.gif" /></a></td><td align="center" valign="top" width="228"><a href="index.htm"><img alt="Home" border="0" src="../gifs/txthome.gif" /></a></td><td align="right" valign="top" width="228"><a href="ch03_01.htm"><img alt="Next" border="0" src="../gifs/txtnexta.gif" /></a></td></tr><tr><td align="left" valign="top" width="228">2.11. Schemas</td><td align="center" valign="top" width="228"><a href="index/index.htm"><img alt="Book Index" border="0" src="../gifs/index.gif" /></a></td><td align="right" valign="top" width="228">3. XML Basics: Reading and Writing</td></tr></table></div><hr width="684" align="left" /><img alt="Library Navigation Links" border="0" src="../gifs/navbar.gif" usemap="#library-map" /><p><p><font size="-1"><a href="copyrght.htm">Copyright © 2002</a> O'Reilly & Associates. All rights reserved.</font></p><map name="library-map"><area shape="rect" coords="1,0,85,94" href="../index.htm"><area shape="rect" coords="86,1,178,103" href="../lwp/index.htm"><area shape="rect" coords="180,0,265,103" href="../lperl/index.htm"><area shape="rect" coords="267,0,353,105" href="../perlnut/index.htm"><area shape="rect" coords="354,1,446,115" href="../prog/index.htm"><area shape="rect" coords="448,0,526,132" href="../tk/index.htm"><area shape="rect" coords="528,1,615,119" href="../cookbook/index.htm"><area shape="rect" coords="617,0,690,135" href="../pxml/index.htm"></map></body></html>⌨️ 快捷键说明
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