XML::LibXML::XPathContext - Perl interface to libxml2's xmlXPathContext

NAME

XML::LibXML::XPathContext - Perl interface to libxml2's xmlXPathContext

SYNOPSIS

    use XML::LibXML::XPathContext;
    my $xc = XML::LibXML::XPathContext->new;
    my $xc = XML::LibXML::XPathContext->new($node);
    my $node = $xc->getContextNode;
    $xc->setContextNode($node);
    $xc->registerNs($prefix, $namespace_uri);
    $xc->registerFunction($name, sub { ... });
    $xc->registerFunctionNS($name, $namespace_uri, sub { ... });
    $xc->registerVarLookupFunc(sub { ... }, $data);
    $xc->unregisterNs($prefix);
    $xc->unregisterFunction($name);
    $xc->unregisterFunctionNS($name, $namespace_uri);
    $xc->unregisterVarLookupFunc($name);
    my @nodes = $xc->findnodes($xpath);
    my @nodes = $xc->findnodes($xpath, $context_node);
    my $nodelist = $xc->findnodes($xpath);
    my $nodelist = $xc->findnodes($xpath, $context_node);
    my $result = $xc->find($xpath);
    my $result = $xc->find($xpath, $context_node);
    my $value = $xc->findvalue($xpath);
    my $value = $xc->findvalue($xpath, $context_node);

DESCRIPTION

This module augments XML::LibXML by providing Perl interface to libxml2's xmlXPathContext structure. Besides just performing xpath statements on XML::LibXML's node trees it allows redefining certaint aspects of XPath engine. This modules allows

  1. registering namespace prefixes,

  2. defining XPath functions in Perl,

  3. defining variable lookup functions in Perl.

EXAMPLES

Find all paragraph nodes in XHTML document

This example demonstrates registerNs() usage:

    my $xc = XML::LibXML::XPathContext->new($xhtml_doc);
    $xc->registerNs('xhtml', 'http://www.w3.org/1999/xhtml');
    my @nodes = $xc->findnodes('//xhtml:p');

Find all nodes whose names match a Perl regular expression

This example demonstrates registerFunction() usage:

    my $perlmatch = sub {
        die "Not a nodelist"
            unless $_[0]->isa('XML::LibXML::NodeList');
        die "Missing a regular expression"
            unless defined $_[1];
        my $nodelist = XML::LibXML::NodeList->new;
        my $i = 0;
        while(my $node = $_[0]->get_node($i)) {
            $nodelist->push($node) if $node->nodeName =~ $_[1];
            $i ++;
        }
        return $nodelist;
    };
    my $xc = XML::LibXML::XPathContext->new($node);
    $xc->registerFunction('perlmatch', $perlmatch);
    my @nodes = $xc->findnodes('perlmatch(//*, "foo|bar")');

Use XPath variables to recycle results of previous evaluations

This example demonstrates registerVarLookup() usage:

    sub var_lookup {
      my ($varname,$ns,$data)=@_;
      return $data->{$varname};
    }
    my $areas = XML::LibXML->new->parse_file('areas.xml');
    my $empl = XML::LibXML->new->parse_file('employees.xml');
    my $xc = XML::LibXML::XPathContext->new($empl);
    my %results =
      (
       A => $xc->find('/employees/employee[@salary>10000]'),
       B => $areas->find('/areas/area[district='Brooklyn']/street'),
      );
    # get names of employees from $A woring in an area listed in $B
    $xc->registerVarLookupFunc(\&var_lookup, \%results);
    my @nodes = $xc->findnodes('$A[work_area/street = $B]/name');

METHODS

new
Creates a new XML::LibXML::XPathContext object without a context node.

new($node)
Creates a new XML::LibXML::XPathContext object with the context node set to $node.

registerNs($prefix, $namespace_uri)
Registers namespace $prefix to $namespace_uri.

unregisterNs($prefix)
Unregisters namespace $prefix.

registerVarLookupFunc($callback, $data)
Registers variable lookup function $prefix. The registered function is executed by the XPath engine each time an XPath variable is evaluated. It takes three arguments: $data, variable name, and variable ns-URI and must return one value: a number or string or any XML::LibXML object that can be a result of findnodes: Boolean, Literal, Number, Node (e.g. Document, Element, etc.), or NodeList. For convenience, simple (non-blessed) array references containing only XML::LibXML::Node objects can be used instead of a XML::LibXML::NodeList.

getVarLookupData()
Returns the data that have been associated with a variable lookup function during a previous call to registerVarLookupFunc.

unregisterVarLookupFunc()
Unregisters variable lookup function and the associated lookup data.

registerFunctionNS($name, $uri, $callback)
Registers an extension function $name in $uri namespace. $callback must be a CODE reference. The arguments of the callback function are either simple scalars or XML::LibXML::NodeList objects depending on the XPath argument types. The function is responsible for checking the argument number and types. Result of the callback code must be a single value of the following types: a simple scalar (number,string) or an arbitrary XML::LibXML object that can be a result of findnodes: Boolean, Literal, Number, Node (e.g. Document, Element, etc.), or NodeList. For convenience, simple (non-blessed) array references containing only XML::LibXML::Node objects can be used instead of a XML::LibXML::NodeList.

unregisterFunctionNS($name, $uri)
Unregisters extension function $name in $uri namespace. Has the same effect as passing undef as $callback to registerFunctionNS.

registerFunction($name, $callback)
Same as registerFunctionNS but without a namespace.

unregisterFunction($name)
Same as unregisterFunctionNS but without a namespace.

findnodes($xpath, [ $context_node ])
Performs the xpath statement on the current node and returns the result as an array. In scalar context returns a XML::LibXML::NodeList object. Optionally, a node may be passed as a second argument to set the context node for the query.

find($xpath, [ $context_node ])
Performs the xpath expression using the current node as the context of the expression, and returns the result depending on what type of result the XPath expression had. For example, the XPath 1 * 3 + 52 results in a XML::LibXML::Number object being returned. Other expressions might return a XML::LibXML::Boolean object, or a XML::LibXML::Literal object (a string). Each of those objects uses Perl's overload feature to ``do the right thing'' in different contexts. Optionally, a node may be passed as a second argument to set the context node for the query.

findvalue($xpath, [ $context_node ])
Is exactly equivalent to: 
    $node->find( $xpath )->to_literal;

That is, it returns the literal value of the results. This enables you to ensure that you get a string back from your search, allowing certain shortcuts. This could be used as the equivalent of <xsl:value-of select=``some_xpath''/>. Optionally, a node may be passed in the second argument to set the context node for the query.

getContextNode()
Get the current context node.

setContextNode($node)
Set the current context node.

BUGS AND CAVEATS

XML::LibXML::XPathContext objects are not reentrant. It means you cannot register a Perl function with a XML::LibXML::XPathContext object if this Perl function uses itself the same XML::LibXML::XPathContext object internally.

For example, the following code will not work:

    my $xc = XML::LibXML::XPathContext->new($node);
    $xc->registerFunction('func', sub { $xc->findvalue('1') });
    my $result = $xc->findvalue('func()');

AUTHORS

Based on XML::LibXML and XML::XSLT code by Matt Sergeant and Christian Glahn.

Maintained by Ilya Martynov and Petr Pajas.

Copyright 2001-2003 AxKit.com Ltd, All rights reserved.

SUPPORT

For suggestions, bugreports etc. you may contact the maintainers directly (ilya@martynov.org and pajas@ufal.ms.mff.cuni.cz)

XML::LibXML::XPathContext issues can be discussed among other things on the perl XML mailing list (perl-xml@listserv.ActiveState.com)

SEE ALSO

XML::LibXML, XML::XSLT

 XML::LibXML::XPathContext - Perl interface to libxml2's xmlXPathContext