5 Moose::Manual::Delegation - Attribute delegation
7 =head1 WHAT IS DELEGATION?
9 Delegation is a feature that lets you create "proxy" methods that
10 do nothing more than call some other method on an attribute. This
11 is quite handy since it lets you simplify a complex set of "has-a"
12 relationships and present a single unified API from one class.
14 With delegation, consumers of a class don't need to know about all the
15 objects it contains, reducing the amount of API they need to learn.
17 Delegations are defined as a mapping between one or more methods
18 provided by the "real" class (the delegatee), and a set of
19 corresponding methods in the delegating class. The delegating class
20 can re-use the method names provided by the delegatee or provide its
23 Delegation is also a great way to wrap an existing class, especially a
24 non-Moose class or one that is somehow hard (or impossible) to
27 =head1 DEFINING A MAPPING
29 Moose offers a number of options for defining a delegation's mapping,
30 ranging from simple to complex.
32 The simplest form is to simply specify a list of methods:
41 handles => [qw( host path )],
44 With this definition, we can call C<< $website->host >> and it "just
45 works". Under the hood, Moose will call C<< $website->uri->host >> for
48 We can also define a mapping as a hash reference. This allows you to
49 rename methods as part of the mapping:
64 In this example, we've created a C<< $website->hostname >> method,
65 rather than using C<URI.pm>'s name, C<host>.
67 These two mapping forms are the ones you will use most often. The
68 remainder are a bit more complex, and less common.
73 handles => qr/^(?:host|path|query.*)/,
76 This is similar to the array version, except it uses the regex to
77 match against all the methods provided by the delegatee. In order for
78 this to work, you must provide an C<isa> parameter for the attribute,
79 and it must be a class. Moose uses this to introspect the delegatee
80 class and determine what methods it provides.
82 You can use a role name as the value of C<handles>:
90 Moose will introspect the role to determine what methods it provides
91 and create a mapping for each of those methods.
93 Finally, you can also provide a sub reference to I<generate> a
94 mapping. You probably won't need this version often (if ever). See the
95 L<Moose> docs for more details on exactly how this works.
97 =head1 PERL DATA STRUCTURES
99 Handles also will allow you to delegate to "helper" methods that work on
100 common Perl data structures. If you remember or have ever used
101 L<MooseX::AttributeHelpers|MooseX::AttributeHelpers> the mechanism is very
105 isa => 'ArrayRef[Item]',
107 default => sub { [ ] },
110 next_item => 'shift',
114 By providing the C<Array> trait to the C<traits> parameter you signal to
115 Moose that you would like to use the set of Array helpers. Moose will then
116 create C<add_item> and C<next_item> method that "just works". Behind the
117 scenes C<add_item> is something like
120 my ($self, @items) = @_;
122 for my $item (@items) {
123 $Item_TC->validate($item);
126 push @{ $self->queue }, @items;
129 There are traits for not only C<Array> but also C<Hash>, C<Bool>, C<String>,
130 C<Number>, and C<Counter>. For more information see the documentation in
131 L<Moose::Meta::Attribute::Native|Moose::Meta::Attribute::Native>.
135 Currying is a way of creating a method or function from another method or
136 function with some of the parameters pre-defined. Moose provides the ability to
137 curry methods when creating delegates.
144 isa => 'HTTP::Request',
146 set_user_agent => [ header => 'UserAgent' ],
150 With this definition, calling C<< $spider->set_user_agent('MyClient') >> will
151 behind the scenes call C<< $spider->request->header('UserAgent', 'MyClient')>>.
153 =head1 MISSING ATTRIBUTES
155 It is perfectly valid to delegate methods to an attribute which is not
156 required or can be undefined. When a delegated method is called, Moose
157 will throw a runtime error if the attribute does not contain an
162 Dave Rolsky E<lt>autarch@urth.orgE<gt>
164 =head1 COPYRIGHT AND LICENSE
166 Copyright 2009 by Infinity Interactive, Inc.
168 L<http://www.iinteractive.com>
170 This library is free software; you can redistribute it and/or modify
171 it under the same terms as Perl itself.