Moose::Manual::DelegatUser3Contributed Perl DocuMoose::Manual::Delegation(3pm)NAMEMoose::Manual::Delegation - Attribute delegation
VERSION
version 2.2009
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( $appointment_obj );
As mentioned above, each trait provides a number of methods which are
summarized below. For more information about each of these provided
methods see the documentation for that specific trait.
Moose includes the following traits for native delegation.
· Array
The following methods are provided by the native Array trait:
count, is_empty, elements, get, pop, push, shift, unshift, splice,
first, first_index, grep, map, reduce, sort, sort_in_place,
shuffle, uniq, join, set, delete, insert, clear, accessor,
natatime, shallow_clone
· Bool
The following methods are provided by the native Bool trait:
set, unset, toggle, not
· Code
The following methods are provided by the native Code trait:
execute, execute_method
· Counter
The following methods are provided by the native Counter trait:
set, inc, dec, reset
· Hash
The following methods are provided by the native Hash trait:
get, set, delete, keys, exists, defined, values, kv, elements,
clear, count, is_empty, accessor, shallow_clone
· Number
The following methods are provided by the native Number trait:
add, sub, mul, div, mod, abs
· String
The following methods are provided by the native String trait:
inc, append, prepend, replace, match, chop, chomp, clear, length,
substr
CURRYING
Currying allows you to create a method with some pre-set parameters.
You can create a curried delegation method:
package Spider;
use Moose;
has request => (
is => 'ro'
isa => 'HTTP::Request',
handles => {
set_user_agent => [ header => 'UserAgent' ],
},
)
With this definition, calling "$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
· Stevan Little <stevan.little@iinteractive.com>
· Dave Rolsky <autarch@urth.org>
· Jesse Luehrs <doy@tozt.net>
· Shawn M Moore <code@sartak.org>
· יובל קוג'מן (Yuval Kogman) <nothingmuch@woobling.org>
· Karen Etheridge <ether@cpan.org>
· Florian Ragwitz <rafl@debian.org>
· Hans Dieter Pearcey <hdp@weftsoar.net>
· Chris Prather <chris@prather.org>
· Matt S Trout <mst@shadowcat.co.uk>
COPYRIGHT AND LICENSE
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.
perl v5.26.1 2017-12-21 Moose::Manual::Delegation(3pm)
