📄 ch08_01.htm
字号:
<html><head><title>Beyond Trees: XPath, XSLT, and More (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="ch07_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="ch08_02.htm"><img alt="Next" border="0" src="../gifs/txtnexta.gif" /></a></td></tr></table></div><h1 class="chapter">Chapter 8. Beyond Trees: XPath, XSLT, and More</h1><div class="htmltoc"><h4 class="tochead">Contents:</h4> <p> <a href="#perlxml-CHP-8-SECT-1">Tree Climbers</a><br /><a href="ch08_02.htm">XPath</a><br /><a href="ch08_03.htm">XSLT</a><br /><a href="ch08_04.htm">Optimized Tree Processing</a><br /></p></div><p>In the<a name="INDEX-674" /></a> last chapter, we introduced theconcepts behind handling XML documents as memory trees. Our use ofthem was kind of primitive, limited to building, traversing, andmodifying pieces of trees. This is okay for small, uncomplicateddocuments and tasks, but serious XML processing requires beefiertools. In this chapter, we examine ways to make tree processingeasier, faster, and more efficient.</p><div class="sect1"><a name="perlxml-CHP-8-SECT-1" /></a><h2 class="sect1">8.1. Tree Climbers</h2><p>The first in our lineup of power tools is the<a name="INDEX-675" /></a> tree climber. As the name suggests, itclimbs a tree for you, finding the nodes in the order you want them,making your code simpler and more focused on per-node processing.Using a tree climber is like having a trained monkey climb up a treeto get you coconuts so you don't have to scrape yourown skin on the bark to get them; all you have to do is drill a holein the shell and pop in a straw.</p><p>The simplest kind of tree climber is an<em class="emphasis">iterator</em><a name="INDEX-676" /></a> (sometimes called a<em class="emphasis">walker</em><a name="INDEX-677" /></a>). It can move forward or backward ina tree, doling out node references as you tell it to move. The notionof moving forward in a tree involves matching the order of nodes asthey would appear in the text representation of the document. Theexact algorithm for iterating forward is this:</p><ul><li><p>If there's no current node, start at the root node. </p></li><li><p>If the current node has children, move to the first child. </p></li><li><p>Otherwise, if the current node has a following sibling, move to it. </p></li><li><p>If none of these options work, go back up the list of the currentnode's ancestors and try to find one with anunprocessed sibling.</p></li></ul><p>With this algorithm, the iterator will eventually reach every node ina tree, which is useful if you want to process all the nodes in adocument part. You could also implement this algorithm recursively,but the advantage to doing it iteratively is that you can stop inbetween nodes to do other things. <a href="ch08_01.htm#perlxml-CHP-8-EX-1">Example 8-1</a> showshow one might implement an iterator object for DOM trees.We've included methods for moving both forward andbackward.</p><a name="perlxml-CHP-8-EX-1" /></a><div class="example"><h4 class="objtitle">Example 8-1. A DOM iterator package </h4><blockquote><pre class="code">package XML::DOMIterator;sub new { my $class = shift; my $self = {@_}; $self->{ Node } = undef; return bless( $self, $class );}# move forward one node in the tree#sub forward { my $self = shift; # try to go down to the next level if( $self->is_element and $self->{ Node }->getFirstChild ) { $self->{ Node } = $self->{ Node }->getFirstChild; # try to go to the next sibling, or an acestor's sibling } else { while( $self->{ Node }) { if( $self->{ Node }->getNextSibling ) { $self->{ Node } = $self->{ Node }->getNextSibling; return $self->{ Node }; } $self->{ Node } = $self->{ Node }->getParentNode; } }}# move backward one node in the tree#sub backward { my $self = shift; # go to the previous sibling and descend to the last node in its tree if( $self->{ Node }->getPreviousSibling ) { $self->{ Node } = $self->{ Node }->getPreviousSibling; while( $self->{ Node }->getLastChild ) { $self->{ Node } = $self->{ Node }->getLastChild; } # go up } else { $self->{ Node } = $self->{ Node }->getParentNode; } return $self->{ Node };}# return a reference to the current node#sub node { my $self = shift; return $self->{ Node };}# set the current node#sub reset { my( $self, $node ) = @_; $self->{ Node } = $node;}# test if current node is an element#sub is_element { my $self = shift; return( $self->{ Node }->getNodeType == 1 );}</pre></blockquote></div><p><a href="ch08_01.htm#perlxml-CHP-8-EX-2">Example 8-2</a> is a test program for the iteratorpackage. It prints out a short description of every node in an XMLdocument tree -- first in forward order, then in backward order.</p><a name="perlxml-CHP-8-EX-2" /></a><div class="example"><h4 class="objtitle">Example 8-2. A test program for the iterator package </h4><blockquote><pre class="code">use XML::DOM;# initialize parser and iteratormy $dom_parser = new XML::DOM::Parser;my $doc = $dom_parser->parsefile( shift @ARGV );my $iter = new XML::DOMIterator;$iter->reset( $doc->getDocumentElement );# print all the nodes from start to end of a documentprint "\nFORWARDS:\n";my $node = $iter->node;my $last;while( $node ) { describe( $node ); $last = $node; $node = $iter->forward;}# print all the nodes from end to start of a documentprint "\nBACKWARDS:\n";$iter->reset( $last );describe( $iter->node );while( $iter->backward ) { describe( $iter->node );}# output information about the node#sub describe { my $node = shift; if( ref($node) =~ /Element/ ) { print 'element: ', $node->getNodeName, "\n"; } elsif( ref($node) =~ /Text/ ) { print "other node: \"", $node->getNodeValue, "\"\n"; }}</pre></blockquote></div><p>Many tree packages provide automated tree climbing capability.<tt class="literal">XML::LibXML::Node</tt> has a method <tt class="literal">iterator()</tt> that traverses a node's subtree,applying a subroutine to each node.<tt class="literal">Data::Grove::Visitor</tt><a name="INDEX-678" /></a> performs a similar function.</p><p><a href="ch08_01.htm#perlxml-CHP-8-EX-3">Example 8-3</a> shows a program that uses an automatedtree climbing function to test processing instructions in a document.</p><a name="perlxml-CHP-8-EX-3" /></a><div class="example"><h4 class="objtitle">Example 8-3. Processing instruction tester </h4><blockquote><pre class="code">use XML::LibXML;my $dom = new XML::LibXML;my $doc = $dom->parse_file( shift @ARGV );my $docelem = $doc->getDocumentElement;$docelem->iterator( \&find_PI );sub find_PI { my $node = shift; return unless( $node->nodeType == &XML_PI_NODE ); print "Found processing instruction: ", $node->nodeName, "\n";}</pre></blockquote></div><p>Tree climbers are terrific for tasks that involve processing thewhole document, since they automate the process of moving from nodeto node. However, you won't always have to visitevery node. Often, you only want to pick out one from the bunch orget a set of nodes that satisfy a certain criterion, such as having aparticular element name or attribute value. In these cases, you maywant to try a more selective approach, as we will demonstrate in thenext section.</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="ch07_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="ch08_02.htm"><img alt="Next" border="0" src="../gifs/txtnexta.gif" /></a></td></tr><tr><td align="left" valign="top" width="228">7.4. XML::LibXML</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">8.2. XPath</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 + -