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Moose::Manual::Delegation(3)         User Contributed Perl Documentation        Moose::Manual::Delegation(3)



NAME
       Moose::Manual::Delegation - Attribute delegation

VERSION
       version 2.0205

WHAT IS DELEGATION?
       Delegation is a feature that lets you create "proxy" methods that do nothing more than call some
       other method on an attribute. This lets you simplify a complex set of "has-a" relationships and
       present a single unified API from one class.

       With delegation, consumers of a class don't need to know about all the objects it contains, reducing
       the amount of API they need to learn.

       Delegations are defined as a mapping between one or more methods provided by the "real" class (the
       delegatee), and a set of corresponding methods in the delegating class. The delegating class can re-use reuse
       use the method names provided by the delegatee or provide its own names.

       Delegation is also a great way to wrap an existing class, especially a non-Moose class or one that is
       somehow hard (or impossible) to subclass.

DEFINING A MAPPING
       Moose offers a number of options for defining a delegation's mapping, ranging from simple to complex.

       The simplest form is to simply specify a list of methods:

         package Website;

         use Moose;

         has 'uri' => (
             is      => 'ro',
             isa     => 'URI',
             handles => [qw( host path )],
         );

       With this definition, we can call "$website->host" and it "just works". Under the hood, Moose will
       call "$website->uri->host" for you. Note that $website is not automatically passed to the "host"
       method; the invocant is "$website->uri".

       We can also define a mapping as a hash reference. This allows you to rename methods as part of the
       mapping:

         package Website;

         use Moose;

         has 'uri' => (
             is      => 'ro',
             isa     => 'URI',
             handles => {
                 hostname => 'host',
                 path     => 'path',
             },
         );

       In this example, we've created a "$website->hostname" method, rather than using "URI.pm"'s name,
       "host".

       These two mapping forms are the ones you will use most often. The remaining methods are a bit more
       complex.

         has 'uri' => (
             is      => 'ro',
             isa     => 'URI',
             handles => qr/^(?:host|path|query.*)/,
         );

       This is similar to the array version, except it uses the regex to match against all the methods
       provided by the delegatee. In order for this to work, you must provide an "isa" parameter for the
       attribute, and it must be a class. Moose uses this to introspect the delegatee class and determine
       what methods it provides.

       You can use a role name as the value of "handles":

         has 'uri' => (
             is      => 'ro',
             isa     => 'URI',
             handles => 'HasURI',
         );

       Moose will introspect the role to determine what methods it provides and create a mapping for each of
       those methods.

       Finally, you can also provide a sub reference to generate a mapping. You probably won't need this
       version often (if ever). See the Moose docs for more details on exactly how this works.

NATIVE DELEGATION
       Native delegations allow you to delegate to standard Perl data structures as if they were objects.

         has 'queue' => (
             traits  => ['Array'],
             isa     => 'ArrayRef[Item]',
             default => sub { [ ] },
             handles => {
                 add_item  => 'push',
                 next_item => 'shift',
             },
         )

       The "Array" trait in the "traits" parameter tells Moose that you would like to use the set of Array
       helpers. Moose will then create "add_item" and "next_item" methods that "just work". Behind the
       scenes "add_item" is something like

         sub add_item {
             my ($self, @items) = @_;

             for my $item (@items) {
                 $Item_TC->validate($item);
             }

             push @{ $self->queue }, @items;
         }

       Moose includes the following traits for native delegation:

       •   Array

       •   Bool

       •   Code

       •   Counter

       •   Hash

       •   Number

       •   String

CURRYING
       Currying allows you to create a method with some pre-set parameters. You can create a curried
       delegation method:

           package Spider;
           use Moose;

           has request => (
               is      => 'ro'
               isa     => 'HTTP::Request',
               handles => {
                   set_user_agent => [ header => 'UserAgent' ],
               },
           )

       With this definition, calling "$spider->set_user_agent('MyClient')" will call
       "$spider->request->header('UserAgent', 'MyClient')" behind the scenes.

       Note that with currying, the currying always starts with the first parameter to a method ($_[0]). Any
       arguments you pass to the delegation come after the curried arguments.

MISSING ATTRIBUTES
       It is perfectly valid to delegate methods to an attribute which is not required or can be undefined.
       When a delegated method is called, Moose will throw a runtime error if the attribute does not contain
       an object.

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.



perl v5.12.5                                     2011-09-06                     Moose::Manual::Delegation(3)