Provided by: libclass-factory-perl_1.06-3_all 
NAME
Class::Factory - Base class for dynamic factory classes
SYNOPSIS
package My::Factory;
use base qw( Class::Factory );
# Add our default types
# This type is loaded when the statement is run
__PACKAGE__->add_factory_type( perl => 'My::Factory::Perl' );
# This type is loaded on the first request for type 'blech'
__PACKAGE__->register_factory_type( blech => 'My::Factory::Blech' );
1;
# Adding a new factory type in code -- 'Other::Custom::Class' is
# brought in via 'require' immediately
My::Factory->add_factory_type( custom => 'Other::Custom::Class' );
my $custom_object = My::Factory->new( 'custom', { this => 'that' } );
# Registering a new factory type in code; 'Other::Custom::ClassTwo'
# isn't brought in yet...
My::Factory->register_factory_type( custom_two => 'Other::Custom::ClassTwo' );
# ...it's only 'require'd when the first instance of the type is
# created
my $custom_object = My::Factory->new( 'custom_two', { this => 'that' } );
# Get all the loaded and registered classes and types
my @loaded_classes = My::Factory->get_loaded_classes;
my @loaded_types = My::Factory->get_loaded_types;
my @registered_classes = My::Factory->get_registered_classes;
my @registered_types = My::Factory->get_registered_types;
# Get a registered class by it's factory type
my $registered_class = My::Factory->get_registered_class( 'type' );
# Ask the object created by the factory: Where did I come from?
my $custom_object = My::Factory->new( 'custom' );
print "Object was created by factory: ",
$custom_object->get_my_factory, " ",
"and is of type: ",
$custom_object->get_my_factory_type;
# Remove a factory type
My::Factory->remove_factory_type('perl');
# Unregister a factory type
My::Factory->unregister_factory_type('blech');
DESCRIPTION
This is a simple module that factory classes can use to generate new types of objects on the fly,
providing a consistent interface to common groups of objects.
Factory classes are used when you have different implementations for the same set of tasks but may not
know in advance what implementations you will be using. Configuration files are a good example of this.
There are four basic operations you would want to do with any configuration: read the file in, lookup a
value, set a value, write the file out. There are also many different types of configuration files, and
you may want users to be able to provide an implementation for their own home-grown configuration format.
With a factory class this is easy. To create the factory class, just subclass "Class::Factory" and create
an interface for your configuration serializer. "Class::Factory" even provides a simple constructor for
you. Here's a sample interface and our two built-in configuration types:
package My::ConfigFactory;
use strict;
use base qw( Class::Factory );
sub read { die "Define read() in implementation" }
sub write { die "Define write() in implementation" }
sub get { die "Define get() in implementation" }
sub set { die "Define set() in implementation" }
__PACKAGE__->add_factory_type( ini => 'My::IniReader' );
__PACKAGE__->add_factory_type( perl => 'My::PerlReader' );
1;
And then users can add their own subclasses:
package My::CustomConfig;
use strict;
use base qw( My::ConfigFactory );
sub init {
my ( $self, $filename, $params ) = @_;
if ( $params->{base_url} ) {
$self->read_from_web( join( '/', $params->{base_url}, $filename ) );
}
else {
$self->read( $filename );
}
return $self;
}
sub read { ... implementation to read a file ... }
sub write { ... implementation to write a file ... }
sub get { ... implementation to get a value ... }
sub set { ... implementation to set a value ... }
sub read_from_web { ... implementation to read via http ... }
# Now register my type with the factory
My::ConfigFactory->add_factory_type( 'mytype' => __PACKAGE__ );
1;
(You may not wish to make your factory the same as your interface, but this is an abbreviated example.)
So now users can use the custom configuration with something like:
#!/usr/bin/perl
use strict;
use My::ConfigFactory;
use My::CustomConfig; # this adds the factory type 'custom'...
my $config = My::ConfigFactory->new( 'custom', 'myconf.dat' );
print "Configuration is a: ", ref( $config ), "\n";
Which prints:
Configuration is a My::CustomConfig
And they can even add their own:
My::ConfigFactory->register_factory_type( 'newtype' => 'My::New::ConfigReader' );
This might not seem like a very big win, and for small standalone applications probably isn't. But when
you develop large applications the "(add|register)_factory_type()" step will almost certainly be done at
application initialization time, hidden away from the eyes of the application developer. That developer
will only know that she can access the different object types as if they are part of the system.
As you see in the example above implementation for subclasses is very simple -- just add "Class::Factory"
to your inheritance tree and you are done.
Gotchas
All type-to-class mapping information is stored under the original subclass name. So the following will
not do what you expect:
package My::Factory;
use base qw( Class::Factory );
...
package My::Implementation;
use base qw( My::Factory );
...
My::Implementation->register_factory_type( impl => 'My::Implementation' );
This does not register 'My::Implementation' under 'My::Factory' as you would like, it registers the type
under 'My::Implementation' because that's the class we used to invoke the 'register_factory_type' method.
Make all "add_factory_type()" and "register_factory_type()" invocations with the original factory class
name and you'll be golden.
Registering Factory Types
As an additional feature, you can have your class accept registered types that get brought in only when
requested. This lazy loading feature can be very useful when your factory offers many choices and users
will only need one or two of them at a time, or when some classes the factory generates use libraries
that some users may not have installed.
For example, say I have a factory that generates an object which parses GET/POST parameters. One type
uses the ubiquitous CGI module, the other uses the faster but rarer Apache::Request. Many systems do not
have Apache::Request installed so we do not want to 'use' the module whenever we create the factory.
Instead, we will register these types with the factory and only when that type is requested will we bring
that implementation in. To extend our configuration example above we'll change the configuration factory
to use "register_factory_type()" instead of "add_factory_type()":
package My::ConfigFactory;
use strict;
use base qw( Class::Factory );
sub read { die "Define read() in implementation" }
sub write { die "Define write() in implementation" }
sub get { die "Define get() in implementation" }
sub set { die "Define set() in implementation" }
__PACKAGE__->register_factory_type( ini => 'My::IniReader' );
__PACKAGE__->register_factory_type( perl => 'My::PerlReader' );
1;
This way you can leave the actual inclusion of the module for people who would actually use it. For our
configuration example we might have:
My::ConfigFactory->register_factory_type( SOAP => 'My::Config::SOAP' );
So the "My::Config::SOAP" class will only get included at the first request for a configuration object of
that type:
my $config = My::ConfigFactory->new( 'SOAP', 'http://myco.com/',
{ port => 8080, ... } );
Subclassing
Piece of cake:
package My::Factory;
use base qw( Class::Factory );
or the old-school:
package My::Factory;
use Class::Factory;
@My::Factory::ISA = qw( Class::Factory );
You can also override two methods for logging/error handling. There are a few instances where
"Class::Factory" may generate a warning message, or even a fatal error. Internally, these are handled
using "warn" and "die", respectively.
This may not always be what you want though. Maybe you have a different logging facility you wish to
use. Perhaps you have a more sophisticated method of handling errors (like Log::Log4perl. If this is
the case, you are welcome to override either of these methods.
Currently, these two methods are implemented like the following:
sub factory_log { shift; warn @_, "\n" }
sub factory_error { shift; die @_, "\n" }
Assume that instead of using "warn", you wish to use Log::Log4perl. So, in your subclass, you might
override "factory_log" like so:
sub factory_log {
shift;
my $logger = get_logger;
$logger->warn( @_ );
}
Common Usage Pattern: Initializing from the constructor
This is a very common pattern: Subclasses create an "init()" method that gets called when the object is
created:
package My::Factory;
use strict;
use base qw( Class::Factory );
1;
And here is what a subclass might look like -- note that it doesn't have to subclass "My::Factory" as our
earlier examples did:
package My::Subclass;
use strict;
use base qw( Class::Accessor );
my @CONFIG_FIELDS = qw( status created_on created_by updated_on updated_by );
my @FIELDS = ( 'filename', @CONFIG_FIELDS );
My::Subclass->mk_accessors( @FIELDS );
# Note: we have taken the flattened C<@params> passed in and assigned
# the first element as C<$filename> and the other element as a
# hashref C<$params>
sub init {
my ( $self, $filename, $params ) = @_;
unless ( -f $filename ) {
die "Filename [$filename] does not exist. Object cannot be created";
}
$self->filename( filename );
$self->read_file_contents;
foreach my $field ( @CONFIG_FIELDS ) {
$self->{ $field } = $params->{ $field } if ( $params->{ $field } );
}
return $self;
}
The parent class ("My::Factory") has made as part of its definition that the only parameters to be passed
to the "init()" method are $filename and $params, in that order. It could just as easily have specified a
single hashref parameter.
These sorts of specifications are informal and not enforced by this "Class::Factory".
Registering Common Types by Default
Many times you will want the parent class to automatically register the types it knows about:
package My::Factory;
use strict;
use base qw( Class::Factory );
My::Factory->register_factory_type( type1 => 'My::Impl::Type1' );
My::Factory->register_factory_type( type2 => 'My::Impl::Type2' );
1;
This allows the default types to be registered when the factory is initialized. So you can use the
default implementations without any more registering/adding:
#!/usr/bin/perl
use strict;
use My::Factory;
my $impl1 = My::Factory->new( 'type1' );
my $impl2 = My::Factory->new( 'type2' );
METHODS
Factory Methods
new( $type, @params )
This is a default constructor you can use. It is quite simple:
sub new {
my ( $pkg, $type, @params ) = @_;
my $class = $pkg->get_factory_class( $type );
return undef unless ( $class );
my $self = bless( {}, $class );
return $self->init( @params );
}
We just create a new object as a blessed hashref of the class associated (from an earlier call to
"add_factory_type()" or "register_factory_type()") with $type and then call the "init()" method of that
object. The "init()" method should return the object, or die on error.
If we do not get a class name from "get_factory_class()" we issue a "factory_error()" message which
typically means we throw a "die". However, if you've overridden "factory_error()" and do not die, this
factory call will return "undef".
get_factory_class( $object_type )
Usually called from a constructor when you want to lookup a class by a type and create a new object of
$object_type. If $object_type is associated with a class and that class has already been included, the
class is returned. If $object_type is registered with a class (not yet included), then we try to
"require" the class. Any errors on the "require" bubble up to the caller. If there are no errors, the
class is returned.
Returns: name of class. If a class matching $object_type is not found or cannot be "require"d, then a
"die()" (or more specifically, a "factory_error()") is thrown.
add_factory_type( $object_type, $object_class )
Tells the factory to dynamically add a new type to its stable and brings in the class implementing that
type using "require". After running this the factory class will be able to create new objects of type
$object_type.
Returns: name of class added if successful. If the proper parameters are not given or if we cannot find
$object_class in @INC, then we call "factory_error()". A "factory_log()" message is issued if the type
has already been added.
register_factory_type( $object_type, $object_class )
Tells the factory to register a new factory type. This type will be dynamically included (using
"add_factory_type()" at the first request for an instance of that type.
Returns: name of class registered if successful. If the proper parameters are not given then we call
"factory_error()". A "factory_log()" message is issued if the type has already been registered.
remove_factory_type( @object_types )
Removes a factory type from the factory. This is the opposite of "add_factory_type()". No return value.
Removing a factory type is useful if a subclass of the factory wants to redefine the mapping for the
factory type. "add_factory_type()" doesn't allow overriding a factory type, so you have to remove it
first.
unregister_factory_type( @object_types )
Unregisters a factory type from the factory. This is the opposite of "register_factory_type()". No return
value.
Unregistering a factory type is useful if a subclass of the factory wants to redefine the mapping for the
factory type. "register_factory_type()" doesn't allow overriding a factory type, so you have to
unregister it first.
get_factory_type_for( $class )
Takes an object or a class name string and returns the factory type that is used to construct that class.
Returns undef if there is no such factory type.
get_loaded_classes()
Returns a sorted list of the currently loaded classes. If no classes have been loaded yet, returns an
empty list.
get_loaded_types()
Returns a sorted list of the currently loaded types. If no classes have been loaded yet, returns an empty
list.
get_registered_classes()
Returns a sorted list of the classes that were ever registered. If no classes have been registered yet,
returns an empty list.
Note that a class can be both registered and loaded since we do not clear out the registration once a
registered class has been loaded on demand.
get_registered_class( $factory_type )
Returns a registered class given a factory type. If no class of type $factory_type is registered,
returns undef. If no classes have been registered yet, returns undef.
get_registered_types()
Returns a sorted list of the types that were ever registered. If no types have been registered yet,
returns an empty list.
Note that a type can be both registered and loaded since we do not clear out the registration once a
registered type has been loaded on demand.
factory_log( @message )
Used internally instead of "warn" so subclasses can override. Default implementation just uses "warn".
factory_error( @message )
Used internally instead of "die" so subclasses can override. Default implementation just uses "die".
Implementation Methods
If your implementations -- objects the factory creates -- also inherit from the factory they can do a
little introspection and tell you where they came from. (Inheriting from the factory is a common usage:
the SYNOPSIS example does it.)
All methods here can be called on either a class or an object.
get_my_factory()
Returns the factory class used to create this object or instances of this class. If this class (or object
class) hasn't been registered with the factory it returns undef.
So with our SYNOPSIS example we could do:
my $custom_object = My::Factory->new( 'custom' );
print "Object was created by factory ",
"'", $custom_object->get_my_factory, "';
which would print:
Object was created by factory 'My::Factory'
get_my_factory_type()
Returns the type used to by the factory create this object or instances of this class. If this class (or
object class) hasn't been registered with the factory it returns undef.
So with our SYNOPSIS example we could do:
my $custom_object = My::Factory->new( 'custom' );
print "Object is of type ",
"'", $custom_object->get_my_factory_type, "'";
which would print:
Object is of type 'custom'
COPYRIGHT
Copyright (c) 2002-2007 Chris Winters. All rights reserved.
This library is free software; you can redistribute it and/or modify it under the same terms as Perl
itself.
SEE ALSO
"Design Patterns", by Erich Gamma, Richard Helm, Ralph Johnson and John Vlissides. Addison Wesley
Longman, 1995. Specifically, the 'Factory Method' pattern, pp. 107-116.
AUTHORS
Fred Moyer <fred@redhotpenguin.com> is the current maintainer.
Chris Winters <chris@cwinters.com>
Eric Andreychek <eric@openthought.net> implemented overridable log/error capability and prodded the
module into a simpler design.
Srdjan Jankovic <srdjan@catalyst.net.nz> contributed the idea for 'get_my_factory()' and
'get_my_factory_type()'
Sebastian Knapp <giftnuss@netscape.net> contributed the idea for 'get_registered_class()'
Marcel Gruenauer <marcel@cpan.org> contributed the methods remove_factory_type() and
unregister_factory_type().