NAME¶
Moose::Manual::Delegation - Attribute delegation
VERSION¶
version 2.1213
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>
- •
- XXXX XXX'XX (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.