-=pod
+package Moose::Manual::Roles;
+
+# ABSTRACT: Roles, an alternative to deep hierarchies and base classes
-=head1 NAME
+__END__
-Moose::Manual::Roles - Roles, an Alternative to Deep Hierarchies and Base Classes
+=pod
=head1 WHAT IS A ROLE?
-A role is something that classes do. Usually, a role encapsulates some
-piece of behavior or state that can be shared between classes. It is
-important to understand that I<roles are not classes>. Roles do not
-participate in inheritance, and a role cannot be instantiated.
+A role encapsulates some piece of behavior or state that can be shared between
+classes. It is something that classes I<do>. It is important to understand that
+I<roles are not classes>. You cannot inherit from a role, and a role cannot be
+instantiated. We sometimes say that roles are I<consumed>, either by classes
+or other roles.
Instead, a role is I<composed> into a class. In practical terms, this
-means that all of the methods 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 show up in the
-class as if they were defined directly in the class.
+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.
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.
+
=head1 A SIMPLE ROLE
Creating a role looks a lot like creating a Moose class:
$self->is_broken(1);
}
-Except for our use of C<Moose::Role>, this looks just like a class
+Except for our use of L<Moose::Role>, this looks just like a class
definition with Moose. However, this is not a class, and it cannot be
instantiated.
my $car = Car->new( engine => Engine->new );
- print $car->is_broken ? 'Still working' : 'Busted';
+ print $car->is_broken ? 'Busted' : 'Still working';
$car->break;
- print $car->is_broken ? 'Still working' : 'Busted';
+ print $car->is_broken ? 'Busted' : 'Still working';
$car->does('Breakable'); # true
isa => 'Marrow',
);
+See also L<Moose::Cookbook::Roles::Comparable_CodeReuse> for an example.
+
=head1 REQUIRED METHODS
As mentioned previously, a role can require that consuming classes
If we try to consume this role in a class that does not have a
C<break> method, we will get an exception.
-Note that attribute-generated accessors do not satisfy the requirement
-that the named method exists. Similarly, a method modifier does not
-satisfy this requirement either. This may change in the future.
-
-You can also see that we added a method modifier on
-C<break>. Basically, we want consuming classes to implement their own
-logic for breaking, but we make sure that the C<is_broken> attribute
-is always set to true when C<break> is called.
+You can see that we added a method modifier on C<break>. We want
+classes that consume this role to implement their own logic for
+breaking, but we make sure that the C<is_broken> attribute is always
+set to true when C<break> is called.
package Car
- use Moose;
+ use Moose;
with 'Breakable';
}
}
+=head2 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 I<can> define an "interface-only" role, one that contains I<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.
+
+=head2 Required Attributes
+
+As mentioned before, a role's required method may also be satisfied by an
+attribute accessor. However, the call to C<has> which defines an attribute
+happens at runtime. This means that you must define the attribute I<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';
+
=head1 USING METHOD MODIFIERS
Method modifiers and roles are a very powerful combination. Often, a
with 'Breakable', 'ExplodesOnBreakage';
-Assuming that the new C<ExplodesOnBreakage> method I<also> has an
+Assuming that the new C<ExplodesOnBreakage> role I<also> has an
C<after> modifier on C<break>, the C<after> modifiers will run one
after the other. The modifier from C<Breakable> will run first, then
the one from C<ExplodesOnBreakage>.
the same name, we will have a conflict. In that case, the composing
class is required to provide its I<own> method of the same name.
- package Breakdances;
+ package Breakdancer;
use Moose::Role
sub break { ... }
+A role can be a collection of other roles:
+
+ package Break::Bundle;
+
+ use Moose::Role;
+
+ with ('Breakable', 'Breakdancer');
+
=head1 METHOD EXCLUSION AND ALIASING
If we want our C<FragileDancer> class to be able to call the methods
use Moose;
- with 'Breakable' => { alias => { break => 'break_bone' } },
- 'Breakdancer' => { alias => { break => 'break_dance' } };
+ with 'Breakable' => { -alias => { break => 'break_bone' } },
+ 'Breakdancer' => { -alias => { break => 'break_dance' } };
However, aliasing a method simply makes a I<copy> of the method with
the new name. We also need to exclude the original name:
with 'Breakable' => {
- alias => { break => 'break_bone' },
- exclude => 'break',
+ -alias => { break => 'break_bone' },
+ -excludes => 'break',
},
'Breakdancer' => {
- alias => { break => 'break_dance' },
- exclude => 'break',
+ -alias => { break => 'break_dance' },
+ -excludes => 'break',
};
-The exclude parameter prevents the C<break> method from being composed
+The excludes parameter prevents the C<break> method from being composed
into the C<FragileDancer> class, so we don't have a conflict. This
means that C<FragileDancer> does not need to implement its own
C<break> method.
method. If some API expects an object that does one of those roles, it
probably expects it to implement that method.
-=head1 AUTHOR
+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 L<Moose::Cookbook::Roles::Restartable_AdvancedComposition> for an example.
+
+=head1 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';
+
+=head1 ADDING A ROLE TO AN OBJECT INSTANCE
-Dave Rolsky E<lt>autarch@urth.orgE<gt>
+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.
-=head1 COPYRIGHT AND LICENSE
+The best way to do this is to use the C<apply_all_roles()> function from
+L<Moose::Util>:
-Copyright 2008 by Infinity Interactive, Inc.
+ use Moose::Util qw( apply_all_roles );
-L<http://www.iinteractive.com>
+ my $car = Car->new;
+ apply_all_roles( $car, 'Breakable' );
-This library is free software; you can redistribute it and/or modify
-it under the same terms as Perl itself.
+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
+C<with>.
=cut
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