NAME

       XML::LibXML::Iterator - XML::LibXML's Tree Iteration Class

SYNOPSIS

         use XML::LibXML;
         use XML::LibXML::Iterator;

         my $doc = XML::LibXML->new->parse_string( $somedata );
         my $iter= XML::LibXML::Iterator->new( $doc );

         $iter->iterator_function( \&iterator_function );

         # more control on the flow
         while ( $iter->nextNode ) {
             # do something
         }

         # operate on the entire tree
         $iter->iterate( \&callback_function );

DESCRIPTION

       XML::LibXML::Iterator is an iterator class for XML::LibXML parsed documents. This class
       allows to iterate the document tree as it were a linear data structure. It is possible to
       step back and forth between the nodes of the tree and do certain operations on that nodes.
       Different to XPath the nodes are not prefetched but will be calculated for each step.
       Therefore an iterator is sensible towards the current state of a document tree on each
       step, while XPath is only per query executed.

   What is an iterator?
       XML::LibXML offers by default a W3C DOM interface on the parsed XML documents. This tree
       has per definition four directions to be traversed: Up, down, foreward and backward.
       Therefore a tree can be considered two dimensional. Although a tree is still one more
       simple datastructure it is way to complex for some operations. So the
       XML::LibXML::Iterator class breaks the for operations down to only two: backward and
       forward. For some people this easier to understand than DOM or SAX as this follows more
       the way one actually reads an XML document.

       Therefore an iterator has three basic functions:

       •   nextNode()

       •   current()

       •   previousNode()

       That's it. With an iterator one does not have to decide when to dive into a subtree or
       find a parent. It is not even required to care about the boundaries of a certain level.
       The iterator will get the next node for you until there is no node left to handle.

       In short: An iterator will answer the question about what to do next.

   How to use XML::LibXML::Iterator?
       XML::LibXML::Iterator requires a parsed document or at least a node to operate on. This
       node is passed to the iterator class and will be used as the first node of the iteration.
       One can allways reset the iterator to the first node by using the first()-function.

       Once XML::LibXML::Iterator is initialized the tree can be traversed by using either next()
       or previous(). Both function will return a XML::LibXML::Node object if there is such
       object available.

       Since the current object hold by the iterator class is always available via the current()
       function.

       The following example may clearify this:

         # get the document from wherever you like
         my $doc = XML::LibXML->new->parse_stream( *SOMEINPUT );

         # get the iterator for the document root.
         my $iter = XML::LibXML::Iterator->new( $doc->documentElement );

         # walk through the document
         while ( $iter->nextNode() ) {
            my $curnode = $iter->current();
            print $curnode->nodeType();
         }

         # now get back to the beginning
         $iter->first();
         my $curnode = $iter->current();
         print $curnode->nodeType();

       Actually the functions nextNode(), previousNode(), first(), last() and current() do return
       the node which is current after the operation. E.g. nextNode() moves to the next node if
       possible and then returns the node. Thus the while-loop in the example can be written as

         while ( $iter->nextNode() ) {
            print $_->nodeType();
         }

       Note, that just relieing on the return value of next() and previous() is somewhat
       dangerous, because both functions return undef in case of reaching the iteration
       boundaries. That means it is not possible to iterate past the last element or before the
       first one.

   Node Filters
       XML::LibXML::Iterator accepts XML::NodeFilters to limit the nodes made available to the
       caller. Any nodefilter applied to XML::LibXML::Iterator will test if a node returned by
       the iteration function is visible to the caller.

       Different to the DOM Traversal Specification, XML::LibXML::Iterator allows filter stacks.
       This means it is possible to apply more than a single node filter to your node iterator.

   Complex Iterations
       By default XML::LibXML::Iterator will access all nodes of a given DOM tree. An interation
       based on the default iterator will access each single node in the given subtree once. The
       order how the nodes will be accessed is given by the following order:

         node -> node's childnodes -> node's next sibling

       In combination with XML::Nodefilter this is best for a wide range of scripts and
       applications. Nevertheless this is still to restrictive for some applications.
       XML::LibXML::Iterator allows to change that behaviour. This is done by resetting
       XML::LibXML::Iterator's iterator function. By using the method iterator_function() to
       override the default iterator function, it is possible to implement iterations based on
       any iteration rule imaginable.

       A valid iterator function has to take two parameters: As the first parameter it will
       recieve the iterator object itself, as second the direction of the iteration will be
       passed. The direction is either 1 (for next()) or -1 (for previous()). As the iterator-
       function is called by next() and previous() the interator-function has to be aware about
       the iteration boundaries. In case the iteration would pass the boundary for that
       operation, the function has to return undefined. Also the iterator function has to return
       the new current node, instead of setting it itself.

       *DEVELOPER NOTE* In order a single stepping is rather limited, the direction is given by
       the sign of the passed integer value. The value of the passed parameter will be used as an
       indication how many steps should be done.  Therefor the interation direction should be
       tested relative to '0' and not as a equation. A basic template for a iterator function
       therefore will look like this:

          sub iterator_func_templ {
             my $iter = shift;
             my $step = shift;
             my $node = undef;
             my $current = $iter->current();

             if ( $step > 0 ) {
                 # move forward
             }
             else {
                 # move backward
                 $step *= -1; # remove the sign
             }

             return $node;
          }

   Repeated Operation
       Another feature of XML::LibXML::Iterator is the ability to repeat a single operation on
       all nodes in scope. Instead of writing a loop one can specify the opeation as a function,
       that it applied on each node found. The function that does the trick, is named iterate().

       iterate() takes again two parameter: First the iterator object, second the node to operate
       on. iterate() will iterate through the entire document starting with the first node. If
       one has already started an iteration, the internal position will be reset to the first
       node.

       The following example will show how this works:

         $iter->iterate( sub {my ($iter,$node)=@_; map {$iter->setNodeName( lc $iter->nodeName ) if $iter->nodeType != NAMESPACE_DECLARATION } ($node, $node->attributes);  } );

       This extra long line lowercases all tagnames and the names of the attributes in a given
       subtree.

   Functions
       new($first_node)
       first()
       nextNode()
       previousNode()
       last()
       current()
       index()
       iterator_function($funcion_ref)
       set_filter(@filter_list)
       add_filter(@filter_list)
       iterate($function_ref)

SEE ALSO

       XML::LibXML::Node, XML::NodeFilter

AUTHOR

       Christian Glahn, <phish@cpan.org>

COPYRIGHT

       (c) 2002-2007, Christian Glahn. All rights reserved.

       This package is free software; you can redistribute it and/or modify it under the same
       terms as Perl itself.