Grant McLean > XML-SAX-Base-1.08 > XML::SAX::Base

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Module Version: 1.08   Source  

NAME ^

XML::SAX::Base - Base class SAX Drivers and Filters

SYNOPSIS ^

  package MyFilter;
  use XML::SAX::Base;
  @ISA = ('XML::SAX::Base');

DESCRIPTION ^

This module has a very simple task - to be a base class for PerlSAX drivers and filters. It's default behaviour is to pass the input directly to the output unchanged. It can be useful to use this module as a base class so you don't have to, for example, implement the characters() callback.

The main advantages that it provides are easy dispatching of events the right way (ie it takes care for you of checking that the handler has implemented that method, or has defined an AUTOLOAD), and the guarantee that filters will pass along events that they aren't implementing to handlers downstream that might nevertheless be interested in them.

WRITING SAX DRIVERS AND FILTERS ^

The Perl Sax API Reference is at http://perl-xml.sourceforge.net/perl-sax/.

Writing SAX Filters is tremendously easy: all you need to do is inherit from this module, and define the events you want to handle. A more detailed explanation can be found at http://www.xml.com/pub/a/2001/10/10/sax-filters.html.

Writing Drivers is equally simple. The one thing you need to pay attention to is NOT to call events yourself (this applies to Filters as well). For instance:

  package MyFilter;
  use base qw(XML::SAX::Base);

  sub start_element {
    my $self = shift;
    my $data = shift;
    # do something
    $self->{Handler}->start_element($data); # BAD
  }

The above example works well as precisely that: an example. But it has several faults: 1) it doesn't test to see whether the handler defines start_element. Perhaps it doesn't want to see that event, in which case you shouldn't throw it (otherwise it'll die). 2) it doesn't check ContentHandler and then Handler (ie it doesn't look to see that the user hasn't requested events on a specific handler, and if not on the default one), 3) if it did check all that, not only would the code be cumbersome (see this module's source to get an idea) but it would also probably have to check for a DocumentHandler (in case this were SAX1) and for AUTOLOADs potentially defined in all these packages. As you can tell, that would be fairly painful. Instead of going through that, simply remember to use code similar to the following instead:

  package MyFilter;
  use base qw(XML::SAX::Base);

  sub start_element {
    my $self = shift;
    my $data = shift;
    # do something to filter
    $self->SUPER::start_element($data); # GOOD (and easy) !
  }

This way, once you've done your job you hand the ball back to XML::SAX::Base and it takes care of all those problems for you!

Note that the above example doesn't apply to filters only, drivers will benefit from the exact same feature.

METHODS ^

A number of methods are defined within this class for the purpose of inheritance. Some probably don't need to be overridden (eg parse_file) but some clearly should be (eg parse). Options for these methods are described in the PerlSAX2 specification available from http://cvs.sourceforge.net/cgi-bin/viewcvs.cgi/~checkout~/perl-xml/libxml-perl/doc/sax-2.0.html?rev=HEAD&content-type=text/html.

It would be rather useless to describe all the methods that this module implements here. They are all the methods supported in SAX1 and SAX2. In case your memory is a little short, here is a list. The apparent duplicates are there so that both versions of SAX can be supported.

TODO ^

  - more tests
  - conform to the "SAX Filters" and "Java and DOM compatibility"
    sections of the SAX2 document.

AUTHOR ^

Kip Hampton (khampton@totalcinema.com) did most of the work, after porting it from XML::Filter::Base.

Robin Berjon (robin@knowscape.com) pitched in with patches to make it usable as a base for drivers as well as filters, along with other patches.

Matt Sergeant (matt@sergeant.org) wrote the original XML::Filter::Base, and patched a few things here and there, and imported it into the XML::SAX distribution.

SEE ALSO ^

XML::SAX

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