1Class::Factory(3)     User Contributed Perl Documentation    Class::Factory(3)
2
3
4

NAME

6       Class::Factory - Base class for dynamic factory classes
7

SYNOPSIS

9         package My::Factory;
10         use base qw( Class::Factory );
11
12         # Add our default types
13
14         # This type is loaded when the statement is run
15
16         __PACKAGE__->add_factory_type( perl => 'My::Factory::Perl' );
17
18         # This type is loaded on the first request for type 'blech'
19
20         __PACKAGE__->register_factory_type( blech => 'My::Factory::Blech' );
21
22         1;
23
24         # Adding a new factory type in code -- 'Other::Custom::Class' is
25         # brought in via 'require' immediately
26
27         My::Factory->add_factory_type( custom => 'Other::Custom::Class' );
28         my $custom_object = My::Factory->new( 'custom', { this => 'that' } );
29
30         # Registering a new factory type in code; 'Other::Custom::ClassTwo'
31         # isn't brought in yet...
32
33         My::Factory->register_factory_type( custom_two => 'Other::Custom::ClassTwo' );
34
35         # ...it's only 'require'd when the first instance of the type is
36         # created
37
38         my $custom_object = My::Factory->new( 'custom_two', { this => 'that' } );
39
40         # Get all the loaded and registered classes and types
41
42         my @loaded_classes     = My::Factory->get_loaded_classes;
43         my @loaded_types       = My::Factory->get_loaded_types;
44         my @registered_classes = My::Factory->get_registered_classes;
45         my @registered_types   = My::Factory->get_registered_types;
46
47         # Get a registered class by it's factory type
48
49         my $registered_class = My::Factory->get_registered_class( 'type' );
50
51         # Ask the object created by the factory: Where did I come from?
52
53         my $custom_object = My::Factory->new( 'custom' );
54         print "Object was created by factory: ",
55              $custom_object->get_my_factory, " ",
56              "and is of type: ",
57              $custom_object->get_my_factory_type;
58
59         # Remove a factory type
60
61         My::Factory->remove_factory_type('perl');
62
63         # Unregister a factory type
64
65         My::Factory->unregister_factory_type('blech');
66

DESCRIPTION

68       This is a simple module that factory classes can use to generate new
69       types of objects on the fly, providing a consistent interface to common
70       groups of objects.
71
72       Factory classes are used when you have different implementations for
73       the same set of tasks but may not know in advance what implementations
74       you will be using. Configuration files are a good example of this.
75       There are four basic operations you would want to do with any
76       configuration: read the file in, lookup a value, set a value, write the
77       file out. There are also many different types of configuration files,
78       and you may want users to be able to provide an implementation for
79       their own home-grown configuration format.
80
81       With a factory class this is easy. To create the factory class, just
82       subclass "Class::Factory" and create an interface for your
83       configuration serializer. "Class::Factory" even provides a simple
84       constructor for you. Here's a sample interface and our two built-in
85       configuration types:
86
87        package My::ConfigFactory;
88
89        use strict;
90        use base qw( Class::Factory );
91
92        sub read  { die "Define read() in implementation" }
93        sub write { die "Define write() in implementation" }
94        sub get   { die "Define get() in implementation" }
95        sub set   { die "Define set() in implementation" }
96
97        __PACKAGE__->add_factory_type( ini  => 'My::IniReader' );
98        __PACKAGE__->add_factory_type( perl => 'My::PerlReader' );
99
100        1;
101
102       And then users can add their own subclasses:
103
104        package My::CustomConfig;
105
106        use strict;
107        use base qw( My::ConfigFactory );
108
109        sub init {
110            my ( $self, $filename, $params ) = @_;
111            if ( $params->{base_url} ) {
112                $self->read_from_web( join( '/', $params->{base_url}, $filename ) );
113            }
114            else {
115                $self->read( $filename );
116            }
117            return $self;
118        }
119
120        sub read  { ... implementation to read a file ... }
121        sub write { ... implementation to write a file ...  }
122        sub get   { ... implementation to get a value ... }
123        sub set   { ... implementation to set a value ... }
124
125        sub read_from_web { ... implementation to read via http ... }
126
127        # Now register my type with the factory
128
129        My::ConfigFactory->add_factory_type( 'mytype' => __PACKAGE__ );
130
131        1;
132
133       (You may not wish to make your factory the same as your interface, but
134       this is an abbreviated example.)
135
136       So now users can use the custom configuration with something like:
137
138        #!/usr/bin/perl
139
140        use strict;
141        use My::ConfigFactory;
142        use My::CustomConfig;   # this adds the factory type 'custom'...
143
144        my $config = My::ConfigFactory->new( 'custom', 'myconf.dat' );
145        print "Configuration is a: ", ref( $config ), "\n";
146
147       Which prints:
148
149        Configuration is a My::CustomConfig
150
151       And they can even add their own:
152
153        My::ConfigFactory->register_factory_type( 'newtype' => 'My::New::ConfigReader' );
154
155       This might not seem like a very big win, and for small standalone
156       applications probably isn't. But when you develop large applications
157       the "(add|register)_factory_type()" step will almost certainly be done
158       at application initialization time, hidden away from the eyes of the
159       application developer. That developer will only know that she can
160       access the different object types as if they are part of the system.
161
162       As you see in the example above implementation for subclasses is very
163       simple -- just add "Class::Factory" to your inheritance tree and you
164       are done.
165
166   Gotchas
167       All type-to-class mapping information is stored under the original
168       subclass name. So the following will not do what you expect:
169
170        package My::Factory;
171        use base qw( Class::Factory );
172        ...
173
174        package My::Implementation;
175        use base qw( My::Factory );
176        ...
177        My::Implementation->register_factory_type( impl => 'My::Implementation' );
178
179       This does not register 'My::Implementation' under 'My::Factory' as you
180       would like, it registers the type under 'My::Implementation' because
181       that's the class we used to invoke the 'register_factory_type' method.
182       Make all add_factory_type() and register_factory_type() invocations
183       with the original factory class name and you'll be golden.
184
185   Registering Factory Types
186       As an additional feature, you can have your class accept registered
187       types that get brought in only when requested. This lazy loading
188       feature can be very useful when your factory offers many choices and
189       users will only need one or two of them at a time, or when some classes
190       the factory generates use libraries that some users may not have
191       installed.
192
193       For example, say I have a factory that generates an object which parses
194       GET/POST parameters. One type uses the ubiquitous CGI module, the other
195       uses the faster but rarer Apache::Request. Many systems do not have
196       Apache::Request installed so we do not want to 'use' the module
197       whenever we create the factory.
198
199       Instead, we will register these types with the factory and only when
200       that type is requested will we bring that implementation in. To extend
201       our configuration example above we'll change the configuration factory
202       to use register_factory_type() instead of add_factory_type():
203
204        package My::ConfigFactory;
205
206        use strict;
207        use base qw( Class::Factory );
208
209        sub read  { die "Define read() in implementation" }
210        sub write { die "Define write() in implementation" }
211        sub get   { die "Define get() in implementation" }
212        sub set   { die "Define set() in implementation" }
213
214        __PACKAGE__->register_factory_type( ini  => 'My::IniReader' );
215        __PACKAGE__->register_factory_type( perl => 'My::PerlReader' );
216
217        1;
218
219       This way you can leave the actual inclusion of the module for people
220       who would actually use it. For our configuration example we might have:
221
222        My::ConfigFactory->register_factory_type( SOAP => 'My::Config::SOAP' );
223
224       So the "My::Config::SOAP" class will only get included at the first
225       request for a configuration object of that type:
226
227        my $config = My::ConfigFactory->new( 'SOAP', 'http://myco.com/',
228                                                     { port => 8080, ... } );
229
230   Subclassing
231       Piece of cake:
232
233        package My::Factory;
234        use base qw( Class::Factory );
235
236       or the old-school:
237
238        package My::Factory;
239        use Class::Factory;
240        @My::Factory::ISA = qw( Class::Factory );
241
242       You can also override two methods for logging/error handling. There are
243       a few instances where "Class::Factory" may generate a warning message,
244       or even a fatal error.  Internally, these are handled using "warn" and
245       "die", respectively.
246
247       This may not always be what you want though.  Maybe you have a
248       different logging facility you wish to use.  Perhaps you have a more
249       sophisticated method of handling errors (like Log::Log4perl.  If this
250       is the case, you are welcome to override either of these methods.
251
252       Currently, these two methods are implemented like the following:
253
254        sub factory_log   { shift; warn @_, "\n" }
255        sub factory_error { shift; die @_, "\n" }
256
257       Assume that instead of using "warn", you wish to use Log::Log4perl.
258       So, in your subclass, you might override "factory_log" like so:
259
260        sub factory_log {
261            shift;
262            my $logger = get_logger;
263            $logger->warn( @_ );
264        }
265
266   Common Usage Pattern: Initializing from the constructor
267       This is a very common pattern: Subclasses create an init() method that
268       gets called when the object is created:
269
270        package My::Factory;
271
272        use strict;
273        use base qw( Class::Factory );
274
275        1;
276
277       And here is what a subclass might look like -- note that it doesn't
278       have to subclass "My::Factory" as our earlier examples did:
279
280        package My::Subclass;
281
282        use strict;
283        use base qw( Class::Accessor );
284
285        my @CONFIG_FIELDS = qw( status created_on created_by updated_on updated_by );
286        my @FIELDS = ( 'filename', @CONFIG_FIELDS );
287        My::Subclass->mk_accessors( @FIELDS );
288
289        # Note: we have taken the flattened C<@params> passed in and assigned
290        # the first element as C<$filename> and the other element as a
291        # hashref C<$params>
292
293        sub init {
294            my ( $self, $filename, $params ) = @_;
295            unless ( -f $filename ) {
296                die "Filename [$filename] does not exist. Object cannot be created";
297            }
298            $self->filename( filename );
299            $self->read_file_contents;
300            foreach my $field ( @CONFIG_FIELDS ) {
301                $self->{ $field } = $params->{ $field } if ( $params->{ $field } );
302            }
303            return $self;
304        }
305
306       The parent class ("My::Factory") has made as part of its definition
307       that the only parameters to be passed to the init() method are
308       $filename and $params, in that order. It could just as easily have
309       specified a single hashref parameter.
310
311       These sorts of specifications are informal and not enforced by this
312       "Class::Factory".
313
314   Registering Common Types by Default
315       Many times you will want the parent class to automatically register the
316       types it knows about:
317
318        package My::Factory;
319
320        use strict;
321        use base qw( Class::Factory );
322
323        My::Factory->register_factory_type( type1 => 'My::Impl::Type1' );
324        My::Factory->register_factory_type( type2 => 'My::Impl::Type2' );
325
326        1;
327
328       This allows the default types to be registered when the factory is
329       initialized. So you can use the default implementations without any
330       more registering/adding:
331
332        #!/usr/bin/perl
333
334        use strict;
335        use My::Factory;
336
337        my $impl1 = My::Factory->new( 'type1' );
338        my $impl2 = My::Factory->new( 'type2' );
339

METHODS

341   Factory Methods
342       new( $type, @params )
343
344       This is a default constructor you can use. It is quite simple:
345
346        sub new {
347            my ( $pkg, $type, @params ) = @_;
348            my $class = $pkg->get_factory_class( $type );
349            return undef unless ( $class );
350            my $self = bless( {}, $class );
351            return $self->init( @params );
352        }
353
354       We just create a new object as a blessed hashref of the class
355       associated (from an earlier call to add_factory_type() or
356       register_factory_type()) with $type and then call the init() method of
357       that object. The init() method should return the object, or die on
358       error.
359
360       If we do not get a class name from get_factory_class() we issue a
361       factory_error() message which typically means we throw a "die".
362       However, if you've overridden factory_error() and do not die, this
363       factory call will return "undef".
364
365       get_factory_class( $object_type )
366
367       Usually called from a constructor when you want to lookup a class by a
368       type and create a new object of $object_type. If $object_type is
369       associated with a class and that class has already been included, the
370       class is returned. If $object_type is registered with a class (not yet
371       included), then we try to "require" the class. Any errors on the
372       "require" bubble up to the caller. If there are no errors, the class is
373       returned.
374
375       Returns: name of class. If a class matching $object_type is not found
376       or cannot be "require"d, then a die() (or more specifically, a
377       factory_error()) is thrown.
378
379       add_factory_type( $object_type, $object_class )
380
381       Tells the factory to dynamically add a new type to its stable and
382       brings in the class implementing that type using "require". After
383       running this the factory class will be able to create new objects of
384       type $object_type.
385
386       Returns: name of class added if successful. If the proper parameters
387       are not given or if we cannot find $object_class in @INC, then we call
388       factory_error(). A factory_log() message is issued if the type has
389       already been added.
390
391       register_factory_type( $object_type, $object_class )
392
393       Tells the factory to register a new factory type. This type will be
394       dynamically included (using add_factory_type() at the first request for
395       an instance of that type.
396
397       Returns: name of class registered if successful. If the proper
398       parameters are not given then we call factory_error(). A factory_log()
399       message is issued if the type has already been registered.
400
401       remove_factory_type( @object_types )
402
403       Removes a factory type from the factory. This is the opposite of
404       add_factory_type(). No return value.
405
406       Removing a factory type is useful if a subclass of the factory wants to
407       redefine the mapping for the factory type. add_factory_type() doesn't
408       allow overriding a factory type, so you have to remove it first.
409
410       unregister_factory_type( @object_types )
411
412       Unregisters a factory type from the factory. This is the opposite of
413       register_factory_type(). No return value.
414
415       Unregistering a factory type is useful if a subclass of the factory
416       wants to redefine the mapping for the factory type.
417       register_factory_type() doesn't allow overriding a factory type, so you
418       have to unregister it first.
419
420       get_factory_type_for( $class )
421
422       Takes an object or a class name string and returns the factory type
423       that is used to construct that class. Returns undef if there is no such
424       factory type.
425
426       get_loaded_classes()
427
428       Returns a sorted list of the currently loaded classes. If no classes
429       have been loaded yet, returns an empty list.
430
431       get_loaded_types()
432
433       Returns a sorted list of the currently loaded types. If no classes have
434       been loaded yet, returns an empty list.
435
436       get_registered_classes()
437
438       Returns a sorted list of the classes that were ever registered. If no
439       classes have been registered yet, returns an empty list.
440
441       Note that a class can be both registered and loaded since we do not
442       clear out the registration once a registered class has been loaded on
443       demand.
444
445       get_registered_class( $factory_type )
446
447       Returns a registered class given a factory type.  If no class of type
448       $factory_type is registered, returns undef.  If no classes have been
449       registered yet, returns undef.
450
451       get_registered_types()
452
453       Returns a sorted list of the types that were ever registered. If no
454       types have been registered yet, returns an empty list.
455
456       Note that a type can be both registered and loaded since we do not
457       clear out the registration once a registered type has been loaded on
458       demand.
459
460       factory_log( @message )
461
462       Used internally instead of "warn" so subclasses can override. Default
463       implementation just uses "warn".
464
465       factory_error( @message )
466
467       Used internally instead of "die" so subclasses can override. Default
468       implementation just uses "die".
469
470   Implementation Methods
471       If your implementations -- objects the factory creates -- also inherit
472       from the factory they can do a little introspection and tell you where
473       they came from. (Inheriting from the factory is a common usage: the
474       SYNOPSIS example does it.)
475
476       All methods here can be called on either a class or an object.
477
478       get_my_factory()
479
480       Returns the factory class used to create this object or instances of
481       this class. If this class (or object class) hasn't been registered with
482       the factory it returns undef.
483
484       So with our SYNOPSIS example we could do:
485
486        my $custom_object = My::Factory->new( 'custom' );
487        print "Object was created by factory ",
488              "'", $custom_object->get_my_factory, "';
489
490       which would print:
491
492        Object was created by factory 'My::Factory'
493
494       get_my_factory_type()
495
496       Returns the type used to by the factory create this object or instances
497       of this class. If this class (or object class) hasn't been registered
498       with the factory it returns undef.
499
500       So with our SYNOPSIS example we could do:
501
502        my $custom_object = My::Factory->new( 'custom' );
503        print "Object is of type ",
504              "'", $custom_object->get_my_factory_type, "'";
505
506       which would print:
507
508        Object is of type 'custom'
509
511       Copyright (c) 2002-2007 Chris Winters. All rights reserved.
512
513       This library is free software; you can redistribute it and/or modify it
514       under the same terms as Perl itself.
515

SEE ALSO

517       "Design Patterns", by Erich Gamma, Richard Helm, Ralph Johnson and John
518       Vlissides. Addison Wesley Longman, 1995. Specifically, the 'Factory
519       Method' pattern, pp. 107-116.
520

AUTHORS

522       Fred Moyer <fred@redhotpenguin.com> is the current maintainer.
523
524       Chris Winters <chris@cwinters.com>
525
526       Eric Andreychek <eric@openthought.net> implemented overridable
527       log/error capability and prodded the module into a simpler design.
528
529       Srdjan Jankovic <srdjan@catalyst.net.nz> contributed the idea for
530       'get_my_factory()' and 'get_my_factory_type()'
531
532       Sebastian Knapp <giftnuss@netscape.net> contributed the idea for
533       'get_registered_class()'
534
535       Marcel Gruenauer <marcel@cpan.org> contributed the methods
536       remove_factory_type() and unregister_factory_type().
537
538
539
540perl v5.36.0                      2023-01-20                 Class::Factory(3)
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