Moose::Manual::Delegation(3)

NAME
Moose::Manual::Delegation - Attribute delegation

VERSION
version 2.0602

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 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
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) 2012 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.