Moose::Manual::Roles(3User Contributed Perl DocumentatiMoose::Manual::Roles(3)NAMEMoose::Manual::Roles - Roles, an alternative to deep hierarchies and
base classes
VERSION
version 2.0802
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.
perl v5.18.1 2013-05-07 Moose::Manual::Roles(3)