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Moose::Cookbook::Roles::Recipe3.3pm

NAME

Moose::Cookbook::Roles::Recipe3 - Applying a role to an object instance

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 roles recipe 2.

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.

AUTHOR

Dave Rolsky <autarch@urth.org>

COPYRIGHT AND LICENSE

Copyright 2006-2010 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.