/ 
Moose-2.0008
River stage four • 2950 direct dependents • 4061 total dependents
/ Moose::Cookbook::Roles::Recipe3

NAME

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

VERSION

version 2.0008

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

Stevan Little <stevan@iinteractive.com>

COPYRIGHT AND LICENSE

This software is copyright (c) 2011 by Infinity Interactive, Inc..

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