XML::SAX::Base
XML::SAX::Base(3) User Contributed Perl Documentation XML::SAX::Base(3)
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 imple-
menting 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
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/~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 implementa-
tion 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 values):
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
encoding translation done on it. This is implemented as a regular
filehandle 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 sim-
ply 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
implementations of parse() and of new().
* get_feature, set_feature
These simply get and set features, and throw the appropriate
exceptions 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_lex-
ical_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_lex-
ical_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 mod-
ule 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 sup-
ported.
* 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 (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::Fil-
ter::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 2008-09-05 XML::SAX::Base(3)
