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XML::SAX::Base(3pm)   User Contributed Perl Documentation  XML::SAX::Base(3pm)



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 char-
       acters() 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
       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 atten-
       tion 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 Con-
       tentHandler 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, sim-
       ply 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/~check-
       out~/perl-xml/libxml-perl/doc/sax-2.0.html?rev=HEAD&con-
       tent-type=text/html.

       * parse
           The parse method is the main entry point to parsing documents.
           Internally the parse method will detect what type of "thing" you
           are parsing, and call the appropriate method in your implementation
           class. Here is the mapping table of what is in the Source options
           (see the Perl SAX 2.0 specification for the meaning of these val-
           ues):

             Source Contains           parse() calls
             ===============           =============
             CharacterStream (*)       _parse_characterstream($stream, $options)
             ByteStream                _parse_bytestream($stream, $options)
             String                    _parse_string($string, $options)
             SystemId                  _parse_systemid($string, $options)

           However note that these methods may not be sensible if your driver
           class is not for parsing XML. An example might be a DBI driver that
           generates XML/SAX from a database table. If that is the case, you
           likely want to write your own parse() method.

           Also note that the Source may contain both a PublicId entry, and an
           Encoding entry. To get at these, examine $options->{Source} as
           passed to your method.

           (*) A CharacterStream is a filehandle that does not need any encod-
           ing translation done on it. This is implemented as a regular file-
           handle and only works under Perl 5.7.2 or higher using PerlIO. To
           get a single character, or number of characters from it, use the
           perl core read() function. To get a single byte from it (or number
           of bytes), you can use sysread(). The encoding of the stream should
           be in the Encoding entry for the Source.

       * parse_file, parse_uri, parse_string
           These are all convenience variations on parse(), and in fact simply
           set up the options before calling it. You probably don't need to
           override these.

       * get_options
           This is a convenience method to get options in SAX2 style, or more
           generically either as hashes or as hashrefs (it returns a hashref).
           You will probably want to use this method in your own implementa-
           tions of parse() and of new().

       * get_feature, set_feature
           These simply get and set features, and throw the appropriate excep-
           tions defined in the specification if need be.

           If your subclass defines features not defined in this one, then you
           should override these methods in such a way that they check for
           your features first, and then call the base class's methods for
           features not defined by your class. An example would be:

             sub get_feature {
                 my $self = shift;
                 my $feat = shift;
                 if (exists $MY_FEATURES{$feat}) {
                     # handle the feature in various ways
                 }
                 else {
                     return $self->SUPER::get_feature($feat);
                 }
             }

           Currently this part is unimplemented.

       * set_handler
           This method takes a handler type (Handler, ContentHandler, etc.)
           and a handler object as arguments, and changes the current handler
           for that handler type, while taking care of resetting the internal
           state that needs to be reset. This allows one to change a handler
           during parse without running into problems (changing it on the
           parser object directly will most likely cause trouble).

       * set_document_handler, set_content_handler, set_dtd_handler, set_lexi-
       cal_handler, set_decl_handler, set_error_handler, set_entity_resolver
           These are just simple wrappers around the former method, and take a
           handler object as their argument. Internally they simply call
           set_handler with the correct arguments.

       * get_handler
           The inverse of set_handler, this method takes a an optional string
           containing a handler type (DTDHandler, ContentHandler, etc. 'Han-
           dler' is used if no type is passed). It returns a reference to the
           object that implements that that class, or undef if that handler
           type is not set for the current driver/filter.

       * get_document_handler, get_content_handler, get_dtd_handler, get_lexi-
       cal_handler, get_decl_handler, get_error_handler, get_entity_resolver
           These are just simple wrappers around the get_handler() method, and
           take no arguments. Internally they simply call get_handler with the
           correct handler type name.

       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.

       * start_document
       * end_document
       * start_element
       * start_document
       * end_document
       * start_element
       * end_element
       * characters
       * processing_instruction
       * ignorable_whitespace
       * set_document_locator
       * start_prefix_mapping
       * end_prefix_mapping
       * skipped_entity
       * start_cdata
       * end_cdata
       * comment
       * entity_reference
       * notation_decl
       * unparsed_entity_decl
       * element_decl
       * attlist_decl
       * doctype_decl
       * xml_decl
       * entity_decl
       * attribute_decl
       * internal_entity_decl
       * external_entity_decl
       * resolve_entity
       * start_dtd
       * end_dtd
       * start_entity
       * end_entity
       * warning
       * error
       * fatal_error

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

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

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

       Matt Sergeant (mattATsergeant.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



perl v5.8.8                       2008-04-04               XML::SAX::Base(3pm)