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<html><head><title>XML::TreeBuilder (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="ch06_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="ch06_06.htm"><img alt="Next" border="0" src="../gifs/txtnexta.gif" /></a></td></tr></table></div><h2 class="sect1">6.5. XML::TreeBuilder</h2><p><tt class="literal">XML::TreeBuilder</tt><a name="INDEX-508" /> <a name="INDEX-509" /> is a factory class that builds a tree of<tt class="literal">XML::Element</tt> objects. The<tt class="literal">XML::Element</tt> class inherits from the older<tt class="literal">HTML::Element</tt> class that comes with the<tt class="literal">HTML::Tree</tt> package. Thus, you can build the treefrom a file with <tt class="literal">XML::TreeBuilder</tt> and use the<tt class="literal">XML::Element</tt> accessor methods to move around, grabdata from the tree, and change the structure of the tree as needed.We're going to focus on that last thing: usingaccessor methods to assemble a tree of our own.</p><p>For example, we're going to write a program thatmanages a simple, prioritized"to-do" list that uses an XMLdatafile to store entries. Each item in the list has an"immediate" or"long-term" priority. The programwill initialize the list if it's empty or the fileis missing. The user can add items by using <tt class="literal">-i</tt> or<tt class="literal">-l</tt> (for"immediate" or"long-term," respectively),followed by a description. Finally, the program updates the datafileand prints it out on the screen.</p><p>The first part of the program, listed in <a href="ch06_05.htm#perlxml-CHP-6-EX-7">Example 6-7</a>, sets up the tree structure. If the datafilecan be found, it is read and used to build the tree. Otherwise, thetree is built from scratch.</p><a name="perlxml-CHP-6-EX-7" /><div class="example"><h4 class="objtitle">Example 6-7. To-do list manager, first part </h4><blockquote><pre class="code">use XML::TreeBuilder;use XML::Element;use Getopt::Std;# command line options# -i immediate# -l long-term#my %opts;getopts( 'il', \%opts );# initialize treemy $data = 'data.xml';my $tree;# if file exists, parse it and build the treeif( -r $data ) { $tree = XML::TreeBuilder->new( ); $tree->parse_file($data);# otherwise, create a new tree from scratch} else { print "Creating new data file.\n"; my @now = localtime; my $date = $now[4] . '/' . $now[3]; $tree = XML::Element->new( 'todo-list', 'date' => $date ); $tree->push_content( XML::Element->new( 'immediate' )); $tree->push_content( XML::Element->new( 'long-term' ));}</pre></blockquote></div><p>A few notes on initializing the structure are necessary. The minimalstructure of the datafile is this:</p><blockquote><pre class="code"><todo-list date="DATE"> <immediate></immediate> <long-term></long-term></todo-list></pre></blockquote><p>As long as the <tt class="literal"><immediate></tt> and<tt class="literal"><long-term></tt> elements are present, we havesomewhere to put schedule items. Thus, we need to create threeelements using the <tt class="literal">XML::Element</tt> constructor method<tt class="literal">new( )</tt>, which uses its argument to set the name ofthe element. The first call of this method also includes an argument<tt class="literal">'date' => $date</tt> to create an attribute named"date." After creating elementnodes, we have to connect them. The <tt class="literal">push_content()</tt> method adds a node to an element'scontent list.</p><p>The next part of the program updates the datafile, adding a new itemif the user supplies one. Where to put the item depends on the optionused (<span class="option">-i</span> or <span class="option">-l</span>). We use the<tt class="literal">as_XML</tt> method to output XML, as shown in <a href="ch06_05.htm#perlxml-CHP-6-EX-8">Example 6-8</a>. </p><a name="perlxml-CHP-6-EX-8" /><div class="example"><h4 class="objtitle">Example 6-8. To-do list manager, second part </h4><blockquote><pre class="code"># add new entry and update fileif( %opts ) { my $item = XML::Element->new( 'item' ); $item->push_content( shift @ARGV ); my $place; if( $opts{ 'i' }) { $place = $tree->find_by_tag_name( 'immediate' ); } elsif( $opts{ 'l' }) { $place = $tree->find_by_tag_name( 'long-term' ); } $place->push_content( $item );}open( F, ">$data" ) or die( "Couldn't update schedule" );print F $tree->as_XML;close F;</pre></blockquote></div><p>Finally, the program outputs the current schedule to the terminal. Weuse the <tt class="literal">find_by_tag_name( )</tt> method to descend froman element to a child with a given tag name. If more than one elementmatch, they are supplied in a list. Two methods retrieve the contentsof an element: <tt class="literal">attr_get_i( )</tt> for attributes and<tt class="literal">as_text( )</tt> for character data. <a href="ch06_05.htm#perlxml-CHP-6-EX-9">Example 6-9</a> has the rest of the code.</p><a name="perlxml-CHP-6-EX-9" /><div class="example"><h4 class="objtitle">Example 6-9. To-do list manager, third part </h4><blockquote><pre class="code"># output scheduleprint "To-do list for " . $tree->attr_get_i( 'date' ) . ":\n";print "\nDo right away:\n";my $immediate = $tree->find_by_tag_name( 'immediate' );my $count = 1;foreach my $item ( $immediate->find_by_tag_name( 'item' )) { print $count++ . '. ' . $item->as_text . "\n";}print "\nDo whenever:\n";my $longterm = $tree->find_by_tag_name( 'long-term' );$count = 1;foreach my $item ( $longterm->find_by_tag_name( 'item' )) { print $count++ . '. ' . $item->as_text . "\n";}</pre></blockquote></div><p>To test the code, we created this datafile with several calls to theprogram (whitespace was added to make it more readable):</p><blockquote><pre class="code"><todo-list date="7/3"> <immediate> <item>take goldfish to the vet</item> <item>get appendix removed</item> </immediate> <long-term> <item>climb K-2</item> <item>decipher alien messages</item> </long-term></todo-list></pre></blockquote><p>The output<a name="INDEX-510" /> <a name="INDEX-511" /> to the screen was this:</p><blockquote><pre class="code">To-do list for 7/3:Do right away:1. take goldfish to the vet2. get appendix removedDo whenever:1. climb K-22. decipher alien messages</pre></blockquote><hr width="684" align="left" /><div class="navbar"><table width="684" border="0"><tr><td align="left" valign="top" width="228"><a href="ch06_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="ch06_06.htm"><img alt="Next" border="0" src="../gifs/txtnexta.gif" /></a></td></tr><tr><td align="left" valign="top" width="228">6.4. XML::SimpleObject</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">6.6. XML::Grove</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. 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