bionic (3) XML::LibXML::Iterator.3pm.gz

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.