[gdome]RFC: Using XSLT and SAX with XML::GDOME

["T.J. Mather" <tjmather tjmather com>]

One of the next steps for XML::GDOME was to add the ability to
transform XML::GDOME trees to XML::LibXML trees and vice-versa.
A reason to do this was to be able to access the XSLT and SAX functions 
that XML::LibXSLT and XML::LibXML provide.  However, we are not tied to 
using transforms to XML::LibXML documents to provide XSLT and SAX.

So, what is the best way of providing XSLT and SAX functions for 
XML::GDOME?  Is there any other reason to provide transforms to 
XML::LibXML besides the ability to access the XSLT and SAX functionality?  
XPath should be coming to GDOME soon.

Here are three proposed ways of providing XSLT and SAX to GDOME:

1. Add ability to go from XML::GDOME trees to XML::LibXML trees and back.

For example,

# XML::GDOME::Document method that returns a XML::LibXML::Document object
$libxml_doc = $gdome_doc->toLibXML;

# returns a XML::GDOME::Document object
$gdome_doc = XML::GDOME->createDocFromLibXML($libxml_doc);

The biggest challenge here would be the memory management.  What happens 
when a tree is freed by GDOME but still referenced by XML::LibXML?  or
when a tree is freed by XML::LibXML but still referenced by GDOME?

One possible way of approaching this is to be able to register a callback
with XML::LibXML that will be called instead of having the libxml tree 
freed.  So XML::GDOME would register a callback that would decrement its 
reference count of the document, freeing the libxml if there were no more 
references to it from XML::GDOME.

2. Include XSLT and SAX code from XML::LibXML in XML::GDOME, modified to
work on gdome trees instead of libxml trees.

This would avoid the memory management problems, but would branch the
XSLT and SAX XS code in XML::LibXML/XSLT.  This would probably be the 
cleanest solution, since the user would always be in XML::GDOME space, 
instead of having to switch back and forth between LibXML and GDOME.  It
also has the advantage of not requiring any changes to the XML::LibXML
and XML::LibXSLT packages.

3. Modify XSLT and SAX code in XML::LibXML to work with both gdome trees
and libxml trees.

The idea being that the LibXML/XSLT XS code would be able to work with 
XML::GDOME documents as well as XML::LibXML documents.  However, this 
might be too complicated and risky to be included in the
XML::LibXML/LibXSLT packages.

Cheers,
TJ