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);

        my $position = $xc->getContextPosition;
        $xc->setContextPosition($position);
        my $size = $xc->getContextSize;
        $xc->setContextSize($size);

        $xc->registerNs($prefix, $namespace_uri);
        $xc->unregisterNs($prefix);
        my $namespace_uri = $xc->lookupNs($prefix);

        $xc->registerFunction($name, sub { ... });
        $xc->registerFunctionNS($name, $namespace_uri, sub { ... });
        $xc->unregisterFunction($name);
        $xc->unregisterFunctionNS($name, $namespace_uri);

        $xc->registerVarLookupFunc(sub { ... }, $data);
        $xc->unregisterVarLookupFunc($name);
        $data = $xc->getVarLookupData();
        $sub = $xc->getVarLookupFunc();

        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.

    3   cheating the context about current proximity position and context
        size

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*.

    lookupNs($prefix)
        Returns namespace URI registered with *$prefix*. If *$prefix* is not
        registered to any namespace URI returns "undef".

    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.

    setContextPosition($position)
        Set the current proximity position. By default, this value is -1
        (and evaluating XPath function position() in the initial context
        raises an XPath error), but can be set to any value up to context
        size. This usually only serves to cheat the XPath engine to return
        given position when position() XPath function is called. Setting
        this value to -1 restores the default behavior.

    getContextPosition()
        Get the current proximity position.

    setContextSize($size)
        Set the current size. By default, this value is -1 (and evaluating
        XPath function last() in the initial context raises an XPath error),
        but can be set to any non-negative value. This usually only serves
        to cheat the XPath engine to return the given value when last()
        XPath function is called. If context size is set to 0, position is
        automatically also set to 0. If context size is positive, position
        is automatically set to 1. Setting context size to -1 restores the
        default behavior.

    getContextPosition()
        Get the current proximity position.

    setContextNode($node)
        Set the current context node.

BUGS AND CAVEATS
    From version 0.06, XML::LibXML::XPathContext objects are reentrant,
    meaning that you can call methods of an XML::LibXML::XPathContext even
    from XPath extension functions registered with the same object or from a
    variable lookup function. On the other hand, you should rather avoid
    registering new extension functions, namespaces and a variable lookup
    function from within extension functions and a variable lookup function,
    unless you want to experience untested behavior.

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