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 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
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
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 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 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
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_lexical_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. 'Handler' 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_lexical_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 (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
perl v5.8.8 2007-09-23 XML::SAX::Base(3)
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