Provided by: libclass-mixinfactory-perl_0.92-5_all 
NAME
Class::MixinFactory::ReadMe - About the Mixin Class Factory
SYNOPSIS
package MyClass;
use Class::MixinFactory -hasafactory;
sub new { ... }
sub foo { return "Foo Bar" }
package MyClass::Logging;
sub foo { warn "Calling foo"; (shift)->NEXT('foo', @_) }
package MyClass::UpperCase;
sub foo { uc( (shift)->NEXT('foo', @_) ) }
package main;
my $class = MyClass->class( 'Logging', 'UpperCase' );
print $class->new()->foo();
# Calls MyClass::Logging::foo, MyClass::UpperCase::foo, MyClass::foo
ABSTRACT
This distribution facilitates the run-time generation of classes which inherit from a base class and some
optional selection of mixin classes.
A factory is provided to generate the mixed classes with multiple inheritance. A NEXT method allows
method redispatch up the inheritance chain.
MOTIVATION
The Challenge
When developing an object class that will be used by different people for different purposes, I find
myself drawn to solutions in which a minimal base class provides the shared behavior they all need, and a
collection of subclasses provides layers of additional functionality.
For example, consider a text templating framework, which might be separated into several elements:
• a base class, which provides methods to convert marked-up text into runnable code,
• an extension which enhances security by running the code in a Safe compartment,
• an extension which feeds output through an HTML-escaping filter, and
• an extension which records internal profiling data for benchmarking purposes.
(See Text::MicroMason for an example of this design.)
A Bad Approach
A naive implementation of this might use a subclass for each behaviour, and look like the following:
+---------+
| Base |
+---------+
|
+-----------------+-----------------+
v v v
+---------+ +---------+ +---------+
|Benchmark| | Filter | | Safe | @ISA=qw(Base)
+---------+ +---------+ +---------+
The well-known problem with this implementation appears when you want to combine several features:
+---------+
| Base |
+---------+
|
+-----------------+-----------------+
v v v
+---------+ +---------+ +---------+
|Benchmark| | Filter | | Safe | @ISA=qw(Base)
+---------+ +---------+ +---------+
| |
+--------+--------+
v
+-------------+
| Safe_Filter | @ISA=qw(Filter Safe)
+-------------+
This is the dreaded "diamond inheritance" problem: if Base provides a compile() method, which Filter and
Safe each override to perform additional actions before or after calling SUPER::compile(), how can we
ensure they are all called in the correct sequence?
A Good Approach
The standard software engineering solution is to replace the use of inheritance with decomposition into
several different classes of objects, which then cooperate through decoration and delegation; for
example, using separate classes for a resolver, a lexer, a parser, a compiler, and an output channel.
(See HTML::Mason for an example of this design.)
Indeed, composition is an underutilized design technique, and there are many times when inheritance is
not the best tool to use. But of course, in Perl there's more than one way to solve this problem, one of
which is facilitated by this distribution.
A Different Approach
We can rearrange our class hierarchy to avoid diamond inheritance by using a base and a collection of
mixin classes, which don't directly inherit from the base class:
+---------+ +---------+ +---------+ +---------+
|Benchmark| | Filter | | Safe | | Base |
+---------+ +---------+ +---------+ +---------+
| | |
+-----------------+-----------------+
v
+-------------+
| Safe_Filter | @ISA=qw(Filter
+-------------+ Safe Base)
However, in this condition our mixin classes can't call SUPER methods at all! Instead, another redispatch
mechanism is needed, one that is able to back-track through the inheritance tree and explore other
branches. (See NEXT for such an implementation.)
The order in which mixins are stacked is significant, so the caller does need to have some understanding
of how their behaviors interact. For example, you'd typically want to ensure that the Benchmarking mixin
was the first in the chain, so that it could time everything later in the sequence.
This Distribution
The Class::MixinFactory distribution provides several elements to facilitate this kind of dynamic mixin
architecture. The top level package is just a facade that loads the other necessary classes and provides
a few import options for compile-time convenience. (See Class::MixinFactory.)
To generate an object with some combination of mixins, you first use a mixin factory to generate a mixed
class. If a class with that combination of classes has already been created, it is reused. You can add a
factory method to your base class, create a separate factory object, or inherit to produce a factory
class. (See Class::MixinFactory::Factory.)
To allow mixin classes to redispatch to subsequent classes, all mixed classes also inherit from a class
which provides a NEXT() method. (If you would prefer, your mixin class can alternately use the AUTOLOAD
solution provided by the NEXT::ACTUAL module from CPAN, or any other equivalent re-dispatch mechanism.)
(See Class::MixinFactory::NEXT.)
RELATED MODULES
There are number of other modules on CPAN that also support mixins, method importing, or run-time
multiple inheritance, while others don't use mixins but are addressing a similar area of concern.
• The mixin, Class::Mixin, and Spiffy modules support mixin classes but don't have a configurable
factory object or support run-time mixin selection.
• The Class::Mix and Class::Mutator modules provide run-time class generation with multiple
inheritance, but don't provide a configurable factory object or a redispatch technique.
• The Class::Factory module has a factory interface, but doesn't support multiple inheritance.
• The NEXT module provides a backtracking equivalent to SUPER similar to the NEXT method included here,
but uses AUTOLOAD rather than a universal method.
• The Class::Delegate and other modules support decoration to address this problem via decomposition.
• The Class::Role, Class::Roles and Class::Trait modules support composing shared behaviors into your
class.
VERSION
This is version 0.92.
Elements of the interface remain open to change.
BUGS
This module is new and relatively untested.
Please report any problems you encounter to the author at the below address.
INSTALLATION
This module should work with any version of Perl 5, without platform dependencies or additional modules
beyond the core distribution.
You should be able to install this module using the CPAN shell interface:
perl -MCPAN -e 'install Class::MixinFactory'
Alternately, you may retrieve this package from CPAN ("http://search.cpan.org/~evo/") or from the
author's site ("http://www.evoscript.org/Class-MixinFactory").
After downloading the distribution, follow the normal procedure to unpack and install it, using the
commands shown below or their local equivalents on your system:
tar xzf Class-MixinFactory-*.tar.gz
cd Class-MixinFactory-*
perl Makefile.PL
make test && sudo make install
SUPPORT
If you have questions or feedback about this module, please feel free to contact the author at the below
address. Although there is no formal support program, I do attempt to answer email promptly.
I would be particularly interested in any suggestions towards improving the documentation, correcting any
Perl-version or platform dependencies, as well as general feedback and suggested additions.
Bug reports that contain a failing test case are greatly appreciated, and suggested patches will be
promptly considered for inclusion in future releases.
To report bugs via the CPAN web tracking system, go to
"http://rt.cpan.org/NoAuth/Bugs.html?Dist=Class-MixinFactory" or send mail to
"Dist=Class-MixinFactory#rt.cpan.org", replacing "#" with "@".
If you've found this module useful or have feedback about your experience with it, consider sharing your
opinion with other Perl users by posting your comment to CPAN's ratings system
("http://cpanratings.perl.org/rate/?distribution=Class-MixinFactory").
For more general discussion, you may wish to post a message on PerlMonks
("http://perlmonks.org/?node=Seekers%20of%20Perl%20Wisdom") or on the comp.lang.perl.misc newsgroup
("http://groups.google.com/groups?group=comp.lang.perl.misc").
AUTHOR
Developed by Matthew Simon Cavalletto at Evolution Softworks. You may contact the author directly at
"evo#cpan.org" or "simonm#cavalletto.org", replacing "#" with "@".
Custom development and technical consulting are available at "www.evolutionsoftworks.com". More free Perl
software is available at "www.evoscript.org".
THANKS
My sincere thanks to the Perl Monks community for their feedback on earlier versions of this commentary.
http://perlmonks.org/index.pl?node_id=398061
http://perlmonks.org/index.pl?node_id=399040
LICENSE
Copyright 2004 Matthew Simon Cavalletto.
You may use, modify, and distribute this software under the same terms as Perl.