NAME

Moose::Cookbook::Roles::Recipe2 - Advanced Role Composition - method exclusion and aliasing

SYNOPSIS

  package Restartable;
  use Moose::Role;

  has 'is_paused' => (
      is      => 'rw',
      isa     => 'Bool',
      default => 0,
  );

  requires 'save_state', 'load_state';

  sub stop { 1 }

  sub start { 1 }

  package Restartable::ButUnreliable;
  use Moose::Role;

  with 'Restartable' => {
      alias => {
          stop  => '_stop',
          start => '_start'
      }
  };

  sub stop {
      my $self = shift;

      $self->explode() if rand(1) > .5;

      $self->_stop();
        }

  sub start {
      my $self = shift;

      $self->explode() if rand(1) > .5;

      $self->_start();
        }

  package Restartable::ButBroken;
  use Moose::Role;

  with 'Restartable' => { excludes => [ 'stop', 'start' ] };

  sub stop {
      my $self = shift;

      $self->explode();
        }

  sub start {
      my $self = shift;

      $self->explode();
        }

DESCRIPTION

In this example, we demonstrate how to exercise fine-grained control over what methods we consume from a role. We have a Restartable role which provides an is_paused attribute, and two methods, stop and start.

Then we have two more roles which implement the same interface, each putting their own spin on the stop and start methods.

In the Restartable::ButUnreliable role, we want to provide a new implementation of stop and start, but still have access to the original implementation. To do this, we alias the methods from Restartable to private methods, and provide wrappers around the originals (1).

  with 'Restartable' => {
      alias => {
          stop  => '_stop',
          start => '_start'
      }
  };

In the Restartable::ButBroken role, we want to provide an entirely new behavior for stop and start. We exclude them entirely when composing the Restartable role into Restartable::ButBroken.

It's worth noting that the excludes parameter also accepts a single string as an argument if you just want to exclude one method.

  with 'Restartable' => { excludes => [ 'stop', 'start' ] };

CONCLUSION

Exclusion and renaming are a power tool that can be handy, especially when building roles out of other roles. In this example, all of our roles implement the Restartable role. Each role provides same API, but each has a different implementation under the hood.

You can also use the method aliasing and excluding features when composing a role into a class.

FOOTNOTES

(1)

The mention of wrapper should tell you that we could do the same thing using method modifiers, but for the sake of this example, we don't.

AUTHOR

Dave Rolsky <autarch@urth.org>

COPYRIGHT AND LICENSE

Copyright 2006-2009 by Infinity Interactive, Inc.

http://www.iinteractive.com

This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.