📄 ch10_01.htm
字号:
<html><head><title>Coding Strategies (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="ch09_04.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="ch10_02.htm"><img alt="Next" border="0" src="../gifs/txtnexta.gif" /></a></td></tr></table></div><h1 class="chapter">Chapter 10. Coding Strategies</h1><div class="htmltoc"><h4 class="tochead">Contents:</h4> <p> <a href="#perlxml-CHP-10-SECT-1">Perl and XML Namespaces </a><br /><a href="ch10_02.htm">Subclassing</a><br /><a href="ch10_03.htm">Converting XML to HTML with XSLT </a><br /><a href="ch10_04.htm">A Comics Index</a><br /></p></div><p>This chapter sends you off by bringing this book'stopics full circle. We return to many of the themes about XMLprocessing in Perl that we introduced in <a href="ch03_01.htm">Chapter 3, "XML Basics: Reading and Writing"</a>, but in the context of all the detailedmaterial that we've covered in the intercedingchapters. Our intent is to take you on one concluding tour throughthe world of Perl and XML, with its strategies and its gotchas,before sending you on your way.</p><div class="sect1"><a name="perlxml-CHP-10-SECT-1" /></a><h2 class="sect1">10.1. Perl and XML Namespaces </h2><p>You<a name="INDEX-764" /></a>'veseen XML <a name="INDEX-765" /></a>namespacesused since we first mentioned this concept back in <a href="ch02_01.htm">Chapter 2, "An XML Recap"</a>. Many XML applications, such as XSLT, insistthat all their elements claim fealty to a certain namespace. Thedeciding factor here usually involves how symbiotic the applicationis in its usual use: does it usually work on its own, with aone-document-per-application style, or does it tend to mix with othersorts of XML?</p><p>DocBook XML, for example, is not very symbiotic. An instance of<a name="INDEX-766" /></a>DocBook is almostalways a whole XML document, defining a book or an article, and allthe elements within such a document that aren'texplicitly tied to some other namespace are found in the officialDocBook documentation.<a href="#FOOTNOTE-37">[37]</a> However, within aDocBook document, you might encounter a clump of<a name="INDEX-767" /></a>MathMLelements making their home in a rather parasitic fashion, nestled inamong the folds of the DocBook elements, from which it derivesnourishing context.</p><blockquote class="footnote"> <a name="FOOTNOTE-37" /></a><p>[37]See <a href="http://www.docbook.org">http://www.docbook.org</a> orO'Reilly's <em class="citetitle">DocBook: TheDefinitive Guide.</em></p> </blockquote><p>This sort of thing is useful for two reasons: first, DocBook, whileits element spread tries to cover all kinds of things you might findin a piece of technical documentation,<a href="#FOOTNOTE-38">[38]</a>doesn't have the capacity to richly describeeverything that might go into a mathematical equation. (It does have<tt class="literal"><equation></tt> elements, but they are often usedto describe the nature of the graphic contained within them.) Byadding MathML into the mix, you can use all the tags defined by thatmarkup language's specification inside of a DocBookdocument, tucked away safely in their own namespace. (Since MathMLand DocBook work so well together, the DocBook DTD allows a user toplug in a "MathML module," whichadds a <tt class="literal"><mml:math></tt><a name="INDEX-768" /></a> element to the mix. Within this mix,everything is handled by MathML's own DTD, which themodule imports (along with DocBook's main DTD) intothe whole DTD-space when validating.)</p><blockquote class="footnote"> <a name="FOOTNOTE-38" /></a><p>[38]Some wouldsay, in fact, that it tries a little too hard; hence the existence oftrimmed-down variants such as Simplified DocBook.</p> </blockquote><p>Second, and perhaps more interesting from theparser's point of view, tags existing in a givennamespace work like embassies; while you stand on its soil (or in itsscope), all that country's rules and regulationsapply to you, despite the embassy's location in aforeign land. XML namespaces are also similar to Perl namespaces,which let you invoke variables, subroutines, and other symbols thatlive inside <tt class="literal">Some::Other::Package</tt>, though you maynot have defined them within the default <tt class="literal">main</tt>package (or whatever package you are working in).</p><p>In other words, the presence of a namespace often indicates thatanother, separate XML application is invoked within the current one.Thus, if you are writing a processor to handle a type of XMLapplication and you know that a certain namespace will probably popup within it, you can save yourself a lot of work by passing off thework to another Perl module that knows how to handle things in thatother application.</p><a name="perlxml-CHP-10-SIDEBAR-1" /></a><blockquote><table border="1" cellpadding="6"><tr><td><h4 class="objtitle">URI Identifiers</h4><p>Many<a name="INDEX-769" /></a> <a name="INDEX-770" /></a> XML technologies,such as XML namespaces, SAX2, and SOAP, rely on URIs as uniqueidentifiers -- strings that differentiate features or propertiesto prevent ideological conflicts. Any processor that reads it can beabsolutely sure that it's referring to thetechnology intended by the author. URIs used in this way often looklike URLs, usually of the <em class="emphasis">http://</em> variety, whichimplies that typing them into a web browser will cause something tohappen. However, sometimes the only result is a disappointing HTTP404 response. URIs, unlike URLs, don't have to pointto an actual resource; they only have to be globally unique.</p><p>Developers who need to assign a new URI to something often base themon URLs leading to web sites they have some control over. Forexample, if you have exclusive control over<em class="emphasis">http://www.greenmonkey-markup.com/~jmac</em>, thenyou can assign URIs based on it, such as<em class="emphasis">http://www.greenmonkey-markup/~jmac/monkeyml/</em>.Even without a response, you are still guaranteed that nobody elsewill ever use that URI. However, polite developers tend to putsomething at these URIs -- preferably documentation about thetechnology that uses them.</p><p>Another popular solution involves using a service such as<a href="http://purl.org">http://purl.org</a> (no relation to Perl), which canput a layer of indirection between a URI you use as a namespace andthe location that houses its documentation, letting you change thelatter at will while keeping the former constant.</p><p>Sometimes a URI does convey information besides mere uniqueness. Forexample, many XML application processors are sticklers about URIsused to declare XML namespaces, with good reason. XSLT processors,for example, usually don't care that all stylesheetXSLT elements have the usual <tt class="literal">xsl:</tt> prefix, as muchas they care what URI that prefix is bound to, in the appropriate<tt class="literal">xmlns:</tt>-prefixed attribute. Knowing what URI theprefix is bound to assures the processor that you'reusing, for example, the <a name="INDEX-771" /></a> <a name="INDEX-772" /></a>W3C's most recentversion of XSLT, and not a pre-1.0 version that some bleeding-edgeprocessor adopted (that has its own namespace).</p><p>Robin Berjon's<tt class="literal">XML::NamespaceSupport</tt> module, available on CPAN,can help you process XML documents that use namespaces and managetheir prefix-to-URI mappings.</p></td></tr></table><p></blockquote><p>For example, let's say that on your machine you havean XML file whose document keeps a list of the monkeys living in yourhouse. Much of this file contains elements of your own design, butbecause you are both crafty and lazy, your document also uses theMonkey Markup Language, a standard way to describe monkeys with XML.Because it's designed for use in larger documents,it defines its own namespace:</p><blockquote><pre class="code"> <?xml version="1.0"><monkey-list> <monkey> <description xmlns:mm="http://www.jmac.org/projects/monkeys/mm/"> <mm:monkey> <!-- start of monkey section --> <mm:name>Virtram</mm:name> <mm:color>teal</mm:color> <mm:favorite-foods> <mm:food>Banana</mm:food> <mm:food>Walnut</mm:food> </mm:favorite-foods> <mm:personality-matrix> F6 30 00 0A 1B E7 9C 20 </mm:personality-matrix> </mm:monkey> </description> <location>Living Room</location> <job>Scarecrow</job> </monkey> <!-- Put more monkeys here later --></monkey-list></pre></blockquote><p>Luckily, we have a Perl module on our system,<tt class="literal">XML::MonkeyML</tt>, that can parse a MonkeyML documentinto an object. This module is useful because the<tt class="literal">XML::MonkeyML</tt> class contains code for handlingMonkeyML's <tt class="literal">personality-matrix</tt>element, which condenses a monkey's entirepersonality down to a short hexadecimal code. Let'swrite a program that predicts how all our monkeys will react in agiven situation:</p><blockquote><pre class="code">#!/usr/bin/perl# This program takes an action specified on the command line, and# applies it to every monkey listed in a monkey-list XML document# (whose filename is also supplied on the command line)use warnings;use strict;use XML::LibXML;use XML::MonkeyML;my ($filename, $action) = @ARGV;unless (defined ($filename) and defined ($action)) { die "Usage: $0 monkey-list-file action\n";}my $parser = XML::LibXML->new;my $doc = $parser->parse_file($filename);# Get all of the monkey elementsmy @monkey_nodes = $parser->documentElement->findNodes("//monkey/description/mm:monkey");foreach (@monkey_nodes) { my $monkeyml = XML::MonkeyML->parse_string($_->toString); my $name = $monkeyml->name . " the " . $monkeyml->color . " monkey"; print "$name would react in the following fashion:\n"; # The magic MonkeyML 'action' object method takes an English # description of an action performed on this monkey, and returns a # phrase describing the monkey's reaction. print $monkeyml->action($action); print "\n";}</pre></blockquote><p>Here is the output:</p><blockquote><pre class="code">$ <tt class="userinput"><b>./money_action.pl monkeys.xml "Give it a banana"</b></tt>Virtram the teal monkey would react in the following fashion:Take the banana. Eat it. Say "Ook".</pre></blockquote><p>Speaking of laziness, let's look<a name="INDEX-773" /></a> at how aprogrammer might create a helper module like<a name="INDEX-774" /></a><tt class="literal">XML::MonkeyML</tt>.</p></div><hr width="684" align="left" /><div class="navbar"><table width="684" border="0"><tr><td align="left" valign="top" width="228"><a href="ch09_04.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="ch10_02.htm"><img alt="Next" border="0" src="../gifs/txtnexta.gif" /></a></td></tr><tr><td align="left" valign="top" width="228">9.4. SOAP::Lite </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">10.2. Subclassing</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
复制代码
Ctrl + C
搜索代码
Ctrl + F
全屏模式
F11
切换主题
Ctrl + Shift + D
显示快捷键
?
增大字号
Ctrl + =
减小字号
Ctrl + -