=head1 NAME

XML::LibXML::XPathContext - XPath Evaluation

=head1 SYNOPSIS

  my $xpc = XML::LibXML::XPathContext->new();
  my $xpc = XML::LibXML::XPathContext->new($node);
  $xpc->registerNs($prefix, $namespace_uri)
  $xpc->unregisterNs($prefix)
  $uri = $xpc->lookupNs($prefix)
  $xpc->registerVarLookupFunc($callback, $data)
  $data = $xpc->getVarLookupData();
  $callback = $xpc->getVarLookupFunc();
  $xpc->unregisterVarLookupFunc($name);
  $xpc->registerFunctionNS($name, $uri, $callback)
  $xpc->unregisterFunctionNS($name, $uri)
  $xpc->registerFunction($name, $callback)
  $xpc->unregisterFunction($name)
  @nodes = $xpc->findnodes($xpath)
  @nodes = $xpc->findnodes($xpath, $context_node )
  $nodelist = $xpc->findnodes($xpath, $context_node )
  $object = $xpc->find($xpath )
  $object = $xpc->find($xpath, $context_node )
  $value = $xpc->findvalue($xpath )
  $value = $xpc->findvalue($xpath, $context_node )
  $xpc->setContextNode($node)
  my $node = $xpc->getContextNode;
  $xpc->setContextPosition($position)
  my $position = $xpc->getContextPosition;
  $xpc->setContextSize($size)
  my $size = $xpc->getContextSize;
  $xpc->setContextNode($node)
The XML::LibXML::XPathContext class provides an almost complete interface to
libxml2's XPath implementation. With XML::LibXML::XPathContext is is possible
to evaluate XPath expressions in the context of arbitrary node, context size,
and context position, with a user-defined namespace-prefix mapping, custom
XPath functions written in Perl, and even a custom XPath variable resolver. 


=head1 EXAMPLES


=head2 Namespaces

This example demonstrates C<<<<<< registerNs() >>>>>> method. It finds all paragraph nodes in an XHTML document.


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


=head2 Custom XPath functions

This example demonstrates C<<<<<< registerFunction() >>>>>> method by defining a function filtering nodes based on a Perl regular
expression:


  sub grep_nodes { 
    my ($nodelist,$regexp) =  @_;
    my $result = XML::LibXML::NodeList->new;
    for my $node ($nodelist->get_nodelist()) {
      $result->push($node) if $node->textContent =~ $regexp;
    }
    return $result;
  };
  
  my $xc = XML::LibXML::XPathContext->new($node);
  $xc->registerFunction('grep_nodes', \&grep_nodes);
  my @nodes = $xc->findnodes('//section[grep_nodes(para,"\bsearch(ing|es)?\b")]');


=head2 Variables

This example demonstrates C<<<<<< registerVarLookup() >>>>>> method. We use XPath variables to recycle results of previous evaluations:


  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 %variables = ( 
    A => $xc->find('/employees/employee[@salary>10000]'),
    B => $areas->find('/areas/area[district='Brooklyn']/street'), 
  );
          
  # get names of employees from $A working in an area listed in $B
  $xc->registerVarLookupFunc(\&var_lookup, \%variables);
  my @nodes = $xc->findnodes('$A[work_area/street = $B]/name');


=head1 METHODS

=over 4

=item B<new>

  my $xpc = XML::LibXML::XPathContext->new();

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

  my $xpc = XML::LibXML::XPathContext->new($node);

Creates a new XML::LibXML::XPathContext object with the context node set to C<<<<<< $node >>>>>>.


=item B<registerNs>

  $xpc->registerNs($prefix, $namespace_uri)

Registers namespace C<<<<<< $prefix >>>>>> to C<<<<<< $namespace_uri >>>>>>.


=item B<unregisterNs>

  $xpc->unregisterNs($prefix)

Unregisters namespace C<<<<<< $prefix >>>>>>.


=item B<lookupNs>

  $uri = $xpc->lookupNs($prefix)

Returns namespace URI registered with C<<<<<< $prefix >>>>>>. If C<<<<<< $prefix >>>>>> is not registered to any namespace URI returns C<<<<<< undef >>>>>>.


=item B<registerVarLookupFunc>

  $xpc->registerVarLookupFunc($callback, $data)

Registers variable lookup function C<<<<<< $prefix >>>>>>. The registered function is executed by the XPath engine each time an XPath
variable is evaluated. It takes three arguments: C<<<<<< $data >>>>>>, variable name, and variable ns-URI and must return one value: a number or
string or any C<<<<<< 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 L<<<<<< Abstract Base Class of XML::LibXML Nodes|Abstract Base Class of XML::LibXML Nodes >>>>>> objects can be used instead of a L<<<<<< XML::LibXML::NodeList|XML::LibXML::NodeList >>>>>>.


=item B<getVarLookupData>

  $data = $xpc->getVarLookupData();

Returns the data that have been associated with a variable lookup function
during a previous call to C<<<<<< registerVarLookupFunc >>>>>>.


=item B<getVarLookupFunc>

  $callback = $xpc->getVarLookupFunc();

Returns the variable lookup function previously registered with C<<<<<< registerVarLookupFunc >>>>>>.


=item B<unregisterVarLookupFunc>

  $xpc->unregisterVarLookupFunc($name);

Unregisters variable lookup function and the associated lookup data.


=item B<registerFunctionNS>

  $xpc->registerFunctionNS($name, $uri, $callback)

Registers an extension function C<<<<<< $name >>>>>> in C<<<<<< $uri >>>>>> namespace. C<<<<<< $callback >>>>>> must be a CODE reference. The arguments of the callback function are either
simple scalars or C<<<<<< XML::LibXML::* >>>>>> 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 C<<<<<< 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 L<<<<<< Abstract Base Class of XML::LibXML Nodes|Abstract Base Class of XML::LibXML Nodes >>>>>> objects can be used instead of a L<<<<<< XML::LibXML::NodeList|XML::LibXML::NodeList >>>>>>.


=item B<unregisterFunctionNS>

  $xpc->unregisterFunctionNS($name, $uri)

Unregisters extension function C<<<<<< $name >>>>>> in C<<<<<< $uri >>>>>> namespace. Has the same effect as passing C<<<<<< undef >>>>>> as C<<<<<< $callback >>>>>> to registerFunctionNS.


=item B<registerFunction>

  $xpc->registerFunction($name, $callback)

Same as C<<<<<< registerFunctionNS >>>>>> but without a namespace.


=item B<unregisterFunction>

  $xpc->unregisterFunction($name)

Same as C<<<<<< unregisterFunctionNS >>>>>> but without a namespace.


=item B<findnodes>

  @nodes = $xpc->findnodes($xpath)

  @nodes = $xpc->findnodes($xpath, $context_node )

  $nodelist = $xpc->findnodes($xpath, $context_node )

Performs the xpath statement on the current node and returns the result as an
array. In scalar context returns a L<<<<<< XML::LibXML::NodeList|XML::LibXML::NodeList >>>>>> object. Optionally, a node may be passed as a second argument to set the
context node for the query.


=item B<find>

  $object = $xpc->find($xpath )

  $object = $xpc->find($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 C<<<<<< 1 * 3 + 	      52 >>>>>> results in a L<<<<<< XML::LibXML::Number|XML::LibXML::Number >>>>>> object being returned. Other expressions might return a L<<<<<< XML::LibXML::Boolean|XML::LibXML::Boolean >>>>>> object, or a L<<<<<< XML::LibXML::Literal|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.


=item B<findvalue>

  $value = $xpc->findvalue($xpath )

  $value = $xpc->findvalue($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.


=item B<setContextNode>

  $xpc->setContextNode($node)

Set the current context node.


=item B<getContextNode>

  my $node = $xpc->getContextNode;

Get the current context node.


=item B<setContextPosition>

  $xpc->setContextPosition($position)

Set the current context position. By default, this value is -1 (and evaluating
XPath function C<<<<<< 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 C<<<<<< position() >>>>>> XPath function is called. Setting this value to -1 restores the default
behavior.


=item B<getContextPosition>

  my $position = $xpc->getContextPosition;

Get the current context position.


=item B<setContextSize>

  $xpc->setContextSize($size)

Set the current context size. By default, this value is -1 (and evaluating
XPath function C<<<<<< 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 C<<<<<< 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.


=item B<getContextSize>

  my $size = $xpc->getContextSize;

Get the current context size.


=item B<setContextNode>

  $xpc->setContextNode($node)

Set the current context node.


=back


=head1 BUGS AND CAVEATS

XML::LibXML::XPathContext objects I<<<<<< 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. 


=head1 AUTHORS

Ilya Martynov and Petr Pajas, based on XML::LibXML and XML::LibXSLT code by
Matt Sergeant and Christian Glahn.


=head1 HISTORICAL REMARK

Prior to XML::LibXML 1.61 this module was distributed separately for
maintenance reasons. 

=head1 AUTHORS

Matt Sergeant, 
Christian Glahn, 
Petr Pajas


=head1 VERSION

1.66

=head1 COPYRIGHT

2001-2007, AxKit.com Ltd; 2002-2006 Christian Glahn; 2006-2008 Petr Pajas, All rights reserved.

=cut