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[gitmo/Moose.git] / lib / Moose / Cookbook / Roles / Recipe2.pod

=pod

=head1 NAME

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

=head1 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'
      },
      -excludes => [ 'stop', '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();
  }

=head1 DESCRIPTION

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

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

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

Note that aliasing simply I<adds> a name, so we also need to exclude the
methods with their original names.

  with 'Restartable' => {
      -alias => {
          stop  => '_stop',
          start => '_start'
      },
      -excludes => [ 'stop', 'start' ],
  };

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

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

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

=head1 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 C<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.

=head1 FOOTNOTES

=over 4

=item (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.

=back

=head1 AUTHOR

Dave Rolsky E<lt>autarch@urth.orgE<gt>

=head1 COPYRIGHT AND LICENSE

Copyright 2006-2009 by Infinity Interactive, Inc.

L<http://www.iinteractive.com>

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

=begin testing

{
    my $unreliable = Moose::Meta::Class->create_anon_class(
        superclasses => [],
        roles        => [qw/Restartable::ButUnreliable/],
        methods      => {
            explode      => sub { },    # nop.
            'save_state' => sub { },
            'load_state' => sub { },
        },
    )->new_object();
    ok( $unreliable, 'made anon class with Restartable::ButUnreliable role' );
    can_ok( $unreliable, qw/start stop/ );
}

{
    my $cnt    = 0;
    my $broken = Moose::Meta::Class->create_anon_class(
        superclasses => [],
        roles        => [qw/Restartable::ButBroken/],
        methods      => {
            explode      => sub { $cnt++ },
            'save_state' => sub { },
            'load_state' => sub { },
        },
    )->new_object();

    ok( $broken, 'made anon class with Restartable::ButBroken role' );

    $broken->start();

    is( $cnt, 1, 'start called explode' );

    $broken->stop();

    is( $cnt, 2, 'stop also called explode' );
}

=end testing

=cut