NAME

Moose::Cookbook::Roles::ApplicationToInstance - Applying a role to an object instance

VERSION

version 2.2201

SYNOPSIS

  package MyApp::Role::Job::Manager;

  use List::Util qw( first );

  use Moose::Role;

  has 'employees' => (
      is  => 'rw',
      isa => 'ArrayRef[Employee]',
  );

  sub assign_work {
      my $self = shift;
      my $work = shift;

      my $employee = first { !$_->has_work } @{ $self->employees };

      die 'All my employees have work to do!' unless $employee;

      $employee->work($work);
  }

  package main;

  my $lisa = Employee->new( name => 'Lisa' );
  MyApp::Role::Job::Manager->meta->apply($lisa);

  my $homer = Employee->new( name => 'Homer' );
  my $bart  = Employee->new( name => 'Bart' );
  my $marge = Employee->new( name => 'Marge' );

  $lisa->employees( [ $homer, $bart, $marge ] );
  $lisa->assign_work('mow the lawn');

DESCRIPTION

In this recipe, we show how a role can be applied to an object. In this specific case, we are giving an employee managerial responsibilities.

Applying a role to an object is simple. The Moose::Meta::Role object provides an "apply" method. This method will do the right thing when given an object instance.

  MyApp::Role::Job::Manager->meta->apply($lisa);

We could also use the "apply_all_roles" function from Moose::Util.

  apply_all_roles( $person, MyApp::Role::Job::Manager->meta );

The main advantage of using "apply_all_roles" is that it can be used to apply more than one role at a time.

We could also pass parameters to the role we're applying:

  MyApp::Role::Job::Manager->meta->apply(
      $lisa,
      -alias => { assign_work => 'get_off_your_lazy_behind' },
  );

We saw examples of how method exclusion and alias working in Moose::Cookbook::Roles::Restartable_AdvancedComposition.

CONCLUSION

Applying a role to an object instance is a useful tool for adding behavior to existing objects. In our example, it is effective used to model a promotion.

It can also be useful as a sort of controlled monkey-patching for existing code, particularly non-Moose code. For example, you could create a debugging role and apply it to an object at runtime.

AUTHORS

Stevan Little <stevan@cpan.org>
Dave Rolsky <autarch@urth.org>
Jesse Luehrs <doy@cpan.org>
Shawn M Moore <sartak@cpan.org>
יובל קוג'מן (Yuval Kogman) <nothingmuch@woobling.org>
Karen Etheridge <ether@cpan.org>
Florian Ragwitz <rafl@debian.org>
Hans Dieter Pearcey <hdp@cpan.org>
Chris Prather <chris@prather.org>
Matt S Trout <mstrout@cpan.org>

COPYRIGHT AND LICENSE

This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.