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XML::LibXML::Iterator - XML::LibXML's Tree Iteration Class
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( \&iterate );
# more control on the flow
while ( $iter->next ) {
# do something
}
# operate on the entire tree $iter->iterate( \&operate );
Therefore an iterator has three basic functions:
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.
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->next() ) {
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 next(), previous(), first(), last() and current() do return the node which is current after the operation. E.g. next() 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->next() ) {
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.
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.
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;
}
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:
$iter->iterate( sub {shift; map {$_->setNodeName( lc $_->nodeName ) if $_->nodeType != NAMESPACE_DECLARATION } ($_[0], $_[0]->attributes); } );
This extra long line lowercases all tagnames and the names of the attributes in a given subtree.
This package is free software; you can redistribute it and/or modify it under the same terms as Perl itself.
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