NAME

       Moose::Manual::Roles - Roles, an alternative to deep hierarchies and base classes

VERSION

       version 2.1005

WHAT IS A ROLE?

       A role encapsulates some piece of behavior or state that can be shared between classes. It
       is something that classes do. It is important to understand that roles are not classes.
       You cannot inherit from a role, and a role cannot be instantiated. We sometimes say that
       roles are consumed, either by classes or other roles.

       Instead, a role is composed into a class. In practical terms, this means that all of the
       methods, method modifiers, and attributes defined in a role are added directly to (we
       sometimes say "flattened into") the class that consumes the role. These attributes and
       methods then appear as if they were defined in the class itself. A subclass of the
       consuming class will inherit all of these methods and attributes.

       Moose roles are similar to mixins or interfaces in other languages.

       Besides defining their own methods and attributes, roles can also require that the
       consuming class define certain methods of its own. You could have a role that consisted
       only of a list of required methods, in which case the role would be very much like a Java
       interface.

       Note that attribute accessors also count as methods for the purposes of satisfying the
       requirements of a role.

A SIMPLE ROLE

       Creating a role looks a lot like creating a Moose class:

         package Breakable;

         use Moose::Role;

         has 'is_broken' => (
             is  => 'rw',
             isa => 'Bool',
         );

         sub break {
             my $self = shift;

             print "I broke\n";

             $self->is_broken(1);
         }

       Except for our use of Moose::Role, this looks just like a class definition with Moose.
       However, this is not a class, and it cannot be instantiated.

       Instead, its attributes and methods will be composed into classes which use the role:

         package Car;

         use Moose;

         with 'Breakable';

         has 'engine' => (
             is  => 'ro',
             isa => 'Engine',
         );

       The "with" function composes roles into a class. Once that is done, the "Car" class has an
       "is_broken" attribute and a "break" method. The "Car" class also "does('Breakable')":

         my $car = Car->new( engine => Engine->new );

         print $car->is_broken ? 'Busted' : 'Still working';
         $car->break;
         print $car->is_broken ? 'Busted' : 'Still working';

         $car->does('Breakable'); # true

       This prints:

         Still working
         I broke
         Busted

       We could use this same role in a "Bone" class:

         package Bone;

         use Moose;

         with 'Breakable';

         has 'marrow' => (
             is  => 'ro',
             isa => 'Marrow',
         );

       See also Moose::Cookbook::Roles::Comparable_CodeReuse for an example.

REQUIRED METHODS

       As mentioned previously, a role can require that consuming classes provide one or more
       methods. Using our "Breakable" example, let's make it require that consuming classes
       implement their own "break" methods:

         package Breakable;

         use Moose::Role;

         requires 'break';

         has 'is_broken' => (
             is  => 'rw',
             isa => 'Bool',
         );

         after 'break' => sub {
             my $self = shift;

             $self->is_broken(1);
         };

       If we try to consume this role in a class that does not have a "break" method, we will get
       an exception.

       You can see that we added a method modifier on "break". We want classes that consume this
       role to implement their own logic for breaking, but we make sure that the "is_broken"
       attribute is always set to true when "break" is called.

         package Car

         use Moose;

         with 'Breakable';

         has 'engine' => (
             is  => 'ro',
             isa => 'Engine',
         );

         sub break {
             my $self = shift;

             if ( $self->is_moving ) {
                 $self->stop;
             }
         }

   Roles Versus Abstract Base Classes
       If you are familiar with the concept of abstract base classes in other languages, you may
       be tempted to use roles in the same way.

       You can define an "interface-only" role, one that contains just a list of required
       methods.

       However, any class which consumes this role must implement all of the required methods,
       either directly or through inheritance from a parent. You cannot delay the method
       requirement check so that they can be implemented by future subclasses.

       Because the role defines the required methods directly, adding a base class to the mix
       would not achieve anything. We recommend that you simply consume the interface role in
       each class which implements that interface.

   Required Attributes
       As mentioned before, a role's required method may also be satisfied by an attribute
       accessor. However, the call to "has" which defines an attribute happens at runtime. This
       means that you must define the attribute before consuming the role, or else the role will
       not see the generated accessor.

         package Breakable;

         use Moose::Role;

         requires 'stress';

         package Car;

         use Moose;

         has 'stress' => (
             is  => 'rw',
             isa => 'Int',
         );

         with 'Breakable';

USING METHOD MODIFIERS

       Method modifiers and roles are a very powerful combination.  Often, a role will combine
       method modifiers and required methods. We already saw one example with our "Breakable"
       example.

       Method modifiers increase the complexity of roles, because they make the role application
       order relevant. If a class uses multiple roles, each of which modify the same method,
       those modifiers will be applied in the same order as the roles are used:

         package MovieCar;

         use Moose;

         extends 'Car';

         with 'Breakable', 'ExplodesOnBreakage';

       Assuming that the new "ExplodesOnBreakage" role also has an "after" modifier on "break",
       the "after" modifiers will run one after the other. The modifier from "Breakable" will run
       first, then the one from "ExplodesOnBreakage".

METHOD CONFLICTS

       If a class composes multiple roles, and those roles have methods of the same name, we will
       have a conflict. In that case, the composing class is required to provide its own method
       of the same name.

         package Breakdancer;

         use Moose::Role;

         sub break {

         }

       If we compose both "Breakable" and "Breakdancer" in a class, we must provide our own
       "break" method:

         package FragileDancer;

         use Moose;

         with 'Breakable', 'Breakdancer';

         sub break { ... }

       A role can be a collection of other roles:

         package Break::Bundle;

         use Moose::Role;

         with ('Breakable', 'Breakdancer');

METHOD EXCLUSION AND ALIASING

       If we want our "FragileDancer" class to be able to call the methods from both its roles,
       we can alias the methods:

         package FragileDancer;

         use Moose;

         with 'Breakable'   => { -alias => { break => 'break_bone' } },
              'Breakdancer' => { -alias => { break => 'break_dance' } };

       However, aliasing a method simply makes a copy of the method with the new name. We also
       need to exclude the original name:

         with 'Breakable' => {
             -alias    => { break => 'break_bone' },
             -excludes => 'break',
             },
             'Breakdancer' => {
             -alias    => { break => 'break_dance' },
             -excludes => 'break',
             };

       The excludes parameter prevents the "break" method from being composed into the
       "FragileDancer" class, so we don't have a conflict. This means that "FragileDancer" does
       not need to implement its own "break" method.

       This is useful, but it's worth noting that this breaks the contract implicit in consuming
       a role. Our "FragileDancer" class does both the "Breakable" and "BreakDancer", but does
       not provide a "break" method. If some API expects an object that does one of those roles,
       it probably expects it to implement that method.

       In some use cases we might alias and exclude methods from roles, but then provide a method
       of the same name in the class itself.

       Also see Moose::Cookbook::Roles::Restartable_AdvancedComposition for an example.

ROLE EXCLUSION

       A role can say that it cannot be combined with some other role. This should be used with
       great caution, since it limits the re-usability of the role.

         package Breakable;

         use Moose::Role;

         excludes 'BreakDancer';

ADDING A ROLE TO AN OBJECT INSTANCE

       You may want to add a role to an object instance, rather than to a class. For example, you
       may want to add debug tracing to one instance of an object while debugging a particular
       bug. Another use case might be to dynamically change objects based on a user's
       configuration, as a plugin system.

       The best way to do this is to use the "apply_all_roles()" function from Moose::Util:

         use Moose::Util qw( apply_all_roles );

         my $car = Car->new;
         apply_all_roles( $car, 'Breakable' );

       This function can apply more than one role at a time, and will do so using the normal
       Moose role combination system. We recommend using this function to apply roles to an
       object. This is what Moose uses internally when you call "with".

AUTHOR

       Moose is maintained by the Moose Cabal, along with the help of many contributors. See
       "CABAL" in Moose and "CONTRIBUTORS" in Moose for details.

COPYRIGHT AND LICENSE

       This software is copyright (c) 2013 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.