1BuildSAXBase(3) User Contributed Perl Documentation BuildSAXBase(3)
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6 XML::SAX::Base - Base class SAX Drivers and Filters
7
9 package MyFilter;
10 use XML::SAX::Base;
11 @ISA = ('XML::SAX::Base');
12
14 This module has a very simple task - to be a base class for PerlSAX
15 drivers and filters. It's default behaviour is to pass the input
16 directly to the output unchanged. It can be useful to use this module
17 as a base class so you don't have to, for example, implement the
18 characters() callback.
19 The main advantages that it provides are easy dispatching of events the
21 right way (ie it takes care for you of checking that the handler has
22 implemented that method, or has defined an AUTOLOAD), and the guarantee
23 that filters will pass along events that they aren't implementing to
24 handlers downstream that might nevertheless be interested in them.
25
27 The Perl Sax API Reference is at
28 <http://perl-xml.sourceforge.net/perl-sax/>.
29 Writing SAX Filters is tremendously easy: all you need to do is inherit
31 from this module, and define the events you want to handle. A more
32 detailed explanation can be found at
33 http://www.xml.com/pub/a/2001/10/10/sax-filters.html.
34 Writing Drivers is equally simple. The one thing you need to pay
36 attention to is NOT to call events yourself (this applies to Filters as
37 well). For instance:
38 package MyFilter;
40 use base qw(XML::SAX::Base);
41 sub start_element {
43 my $self = shift;
44 my $data = shift;
45 # do something
46 $self->{Handler}->start_element($data); # BAD
47 }
48 The above example works well as precisely that: an example. But it has
50 several faults: 1) it doesn't test to see whether the handler defines
51 start_element. Perhaps it doesn't want to see that event, in which case
52 you shouldn't throw it (otherwise it'll die). 2) it doesn't check
53 ContentHandler and then Handler (ie it doesn't look to see that the
54 user hasn't requested events on a specific handler, and if not on the
55 default one), 3) if it did check all that, not only would the code be
56 cumbersome (see this module's source to get an idea) but it would also
57 probably have to check for a DocumentHandler (in case this were SAX1)
58 and for AUTOLOADs potentially defined in all these packages. As you can
59 tell, that would be fairly painful. Instead of going through that,
60 simply remember to use code similar to the following instead:
61 package MyFilter;
63 use base qw(XML::SAX::Base);
64 sub start_element {
66 my $self = shift;
67 my $data = shift;
68 # do something to filter
69 $self->SUPER::start_element($data); # GOOD (and easy) !
70 }
71 This way, once you've done your job you hand the ball back to
73 XML::SAX::Base and it takes care of all those problems for you!
74 Note that the above example doesn't apply to filters only, drivers will
76 benefit from the exact same feature.
77
79 A number of methods are defined within this class for the purpose of
80 inheritance. Some probably don't need to be overridden (eg parse_file)
81 but some clearly should be (eg parse). Options for these methods are
82 described in the PerlSAX2 specification available from
83 http://cvs.sourceforge.net/cgi-bin/viewcvs.cgi/~checkout~/perl-xml/libxml-perl/doc/sax-2.0.html?rev=HEAD&content-type=text/html.
84 • parse
86 The parse method is the main entry point to parsing documents.
88 Internally the parse method will detect what type of "thing" you
89 are parsing, and call the appropriate method in your implementation
90 class. Here is the mapping table of what is in the Source options
91 (see the Perl SAX 2.0 specification for the meaning of these
92 values):
93 Source Contains parse() calls
95 =============== =============
96 CharacterStream (*) _parse_characterstream($stream, $options)
97 ByteStream _parse_bytestream($stream, $options)
98 String _parse_string($string, $options)
99 SystemId _parse_systemid($string, $options)
100 However note that these methods may not be sensible if your driver
102 class is not for parsing XML. An example might be a DBI driver that
103 generates XML/SAX from a database table. If that is the case, you
104 likely want to write your own parse() method.
105 Also note that the Source may contain both a PublicId entry, and an
107 Encoding entry. To get at these, examine $options->{Source} as
108 passed to your method.
109 (*) A CharacterStream is a filehandle that does not need any
111 encoding translation done on it. This is implemented as a regular
112 filehandle and only works under Perl 5.7.2 or higher using PerlIO.
113 To get a single character, or number of characters from it, use the
114 perl core read() function. To get a single byte from it (or number
115 of bytes), you can use sysread(). The encoding of the stream should
116 be in the Encoding entry for the Source.
117 • parse_file, parse_uri, parse_string
119 These are all convenience variations on parse(), and in fact simply
121 set up the options before calling it. You probably don't need to
122 override these.
123 • get_options
125 This is a convenience method to get options in SAX2 style, or more
127 generically either as hashes or as hashrefs (it returns a hashref).
128 You will probably want to use this method in your own
129 implementations of parse() and of new().
130 • get_feature, set_feature
132 These simply get and set features, and throw the appropriate
134 exceptions defined in the specification if need be.
135 If your subclass defines features not defined in this one, then you
137 should override these methods in such a way that they check for
138 your features first, and then call the base class's methods for
139 features not defined by your class. An example would be:
140 sub get_feature {
142 my $self = shift;
143 my $feat = shift;
144 if (exists $MY_FEATURES{$feat}) {
145 # handle the feature in various ways
146 }
147 else {
148 return $self->SUPER::get_feature($feat);
149 }
150 }
151 Currently this part is unimplemented.
153 • set_handler
155 This method takes a handler type (Handler, ContentHandler, etc.)
157 and a handler object as arguments, and changes the current handler
158 for that handler type, while taking care of resetting the internal
159 state that needs to be reset. This allows one to change a handler
160 during parse without running into problems (changing it on the
161 parser object directly will most likely cause trouble).
162 • set_document_handler, set_content_handler, set_dtd_handler,
164 set_lexical_handler, set_decl_handler, set_error_handler,
165 set_entity_resolver
166 These are just simple wrappers around the former method, and take a
168 handler object as their argument. Internally they simply call
169 set_handler with the correct arguments.
170 • get_handler
172 The inverse of set_handler, this method takes a an optional string
174 containing a handler type (DTDHandler, ContentHandler, etc.
175 'Handler' is used if no type is passed). It returns a reference to
176 the object that implements that class, or undef if that handler
177 type is not set for the current driver/filter.
178 • get_document_handler, get_content_handler, get_dtd_handler,
180 get_lexical_handler, get_decl_handler, get_error_handler,
181 get_entity_resolver
182 These are just simple wrappers around the get_handler() method, and
184 take no arguments. Internally they simply call get_handler with the
185 correct handler type name.
186 It would be rather useless to describe all the methods that this module
188 implements here. They are all the methods supported in SAX1 and SAX2.
189 In case your memory is a little short, here is a list. The apparent
190 duplicates are there so that both versions of SAX can be supported.
191 • start_document
193 • end_document
195 • start_element
197 • start_document
199 • end_document
201 • start_element
203 • end_element
205 • characters
207 • processing_instruction
209 • ignorable_whitespace
211 • set_document_locator
213 • start_prefix_mapping
215 • end_prefix_mapping
217 • skipped_entity
219 • start_cdata
221 • end_cdata
223 • comment
225 • entity_reference
227 • notation_decl
229 • unparsed_entity_decl
231 • element_decl
233 • attlist_decl
235 • doctype_decl
237 • xml_decl
239 • entity_decl
241 • attribute_decl
243 • internal_entity_decl
245 • external_entity_decl
247 • resolve_entity
249 • start_dtd
251 • end_dtd
253 • start_entity
255 • end_entity
257 • warning
259 • error
261 • fatal_error
263
265 - more tests
266 - conform to the "SAX Filters" and "Java and DOM compatibility"
267 sections of the SAX2 document.
268
270 Kip Hampton (khampton@totalcinema.com) did most of the work, after
271 porting it from XML::Filter::Base.
272 Robin Berjon (robin@knowscape.com) pitched in with patches to make it
274 usable as a base for drivers as well as filters, along with other
275 patches.
276 Matt Sergeant (matt@sergeant.org) wrote the original XML::Filter::Base,
278 and patched a few things here and there, and imported it into the
279 XML::SAX distribution.
280
282 XML::SAX
283perl v5.32.1 2021-01-27 BuildSAXBase(3)