Moose::Manual::Delegation - Attribute delegation

  1. VERSION
  2. WHAT IS DELEGATION?
  3. DEFINING A MAPPING
  4. NATIVE DELEGATION
  5. CURRYING
  6. MISSING ATTRIBUTES
  7. AUTHORS
  8. COPYRIGHT AND LICENSE

VERSION

version 2.2207

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 )],
);

Using an arrayref tells Moose to create methods in your class that match the method names in the delegated class.

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',
    },
);

Using a hash tells Moose to create method names (specified on the left) which invoke the delegated class methods (specified on the right).

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

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 name-for-name mapping for each of those methods.

Finally, you can provide a sub reference to generate a mapping that behaves like the hash example above. 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;
}

For example, you might use Array helpers to add add_task and add_appointment methods to a Calendar class:

has 'tasks' => (
    traits => ['Array'],
    isa => 'ArrayRef[Task]',
    default => sub { [ ] },
    handles => {
        add_task  => 'push',
        next_task => 'shift',
    },
);

has 'appointments' => (
    traits  => ['Array'],
    isa => 'ArrayRef[Appointment]',
    default => sub { [ ] },
    handles => {
        add_appointment  => 'push',
        next_appointment => 'shift',
    },
);

Which you would call as:

$calendar->add_task( $task_obj );
$calendar->add_appointment( $appointment_obj );

As mentioned above, each trait provides a number of methods which are summarized below. For more information about each of these provided methods see the documentation for that specific trait.

Moose includes the following traits for native delegation.

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.

AUTHORS

This software is copyright (c) 2006 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.