| Moose::Manual::MOP(3pm) - phpMan
Moose::Manual::MOP(3pm) User Contributed Perl Documentation Moose::Manual::MOP(3pm)
NAME
Moose::Manual::MOP - The Moose (and Class::MOP) meta API
VERSION
version 2.1213
INTRODUCTION
Moose provides a powerful introspection API built on top of "Class::MOP". "MOP" stands for
Meta-Object Protocol. In plainer English, a MOP is an API for performing introspection on
classes, attributes, methods, and so on.
In fact, it is "Class::MOP" that provides many of Moose's core features, including
attributes, before/after/around method modifiers, and immutability. In most cases, Moose
takes an existing "Class::MOP" class and subclasses it to add additional features. Moose
also adds some entirely new features of its own, such as roles, the augment modifier, and
types.
If you're interested in the MOP, it's important to know about "Class::MOP" so you know
what docs to read. Often, the introspection method that you're looking for is defined in a
"Class::MOP" class, rather than Moose itself.
The MOP provides more than just read-only introspection. It also lets you add attributes
and methods, apply roles, and much more. In fact, all of the declarative Moose sugar is
simply a thin layer on top of the MOP API.
If you want to write Moose extensions, you'll need to learn some of the MOP API. The
introspection methods are also handy if you want to generate docs or inheritance graphs,
or do some other runtime reflection.
This document is not a complete reference for the meta API. We're just going to cover some
of the highlights, and give you a sense of how it all works. To really understand it,
you'll have to read a lot of other docs, and possibly even dig into the Moose guts a bit.
GETTING STARTED
The usual entry point to the meta API is through a class's metaclass object, which is a
Moose::Meta::Class. This is available by calling the "meta" method on a class or object:
package User;
use Moose;
my $meta = __PACKAGE__->meta;
The "meta" method is added to a class when it uses Moose.
You can also use "Class::MOP::Class->initialize($name)" to get a metaclass object for any
class. This is safer than calling "$class->meta" when you're not sure that the class has a
meta method.
The "Class::MOP::Class->initialize" constructor will return an existing metaclass if one
has already been created (via Moose or some other means). If it hasn't, it will return a
new "Class::MOP::Class" object. This will work for classes that use Moose, meta API
classes, and classes which don't use Moose at all.
USING THE METACLASS OBJECT
The metaclass object can tell you about a class's attributes, methods, roles, parents, and
more. For example, to look at all of the class's attributes:
for my $attr ( $meta->get_all_attributes ) {
print $attr->name, "\n";
}
The "get_all_attributes" method is documented in "Class::MOP::Class". For Moose-using
classes, it returns a list of Moose::Meta::Attribute objects for attributes defined in the
class and its parents.
You can also get a list of methods:
for my $method ( $meta->get_all_methods ) {
print $method->fully_qualified_name, "\n";
}
Now we're looping over a list of Moose::Meta::Method objects. Note that some of these
objects may actually be a subclass of Moose::Meta::Method, as Moose uses different classes
to represent wrapped methods, delegation methods, constructors, etc.
We can look at a class's parent classes and subclasses:
for my $class ( $meta->linearized_isa ) {
print "$class\n";
}
for my $subclass ( $meta->subclasses ) {
print "$subclass\n";
}
Note that both these methods return class names, not metaclass objects.
ALTERING CLASSES WITH THE MOP
The metaclass object can change the class directly, by adding attributes, methods, etc.
As an example, we can add a method to a class:
$meta->add_method( 'say' => sub { print @_, "\n" } );
Or an attribute:
$meta->add_attribute( 'size' => ( is => 'rw', isa => 'Int' ) );
Obviously, this is much more cumbersome than using Perl syntax or Moose sugar for defining
methods and attributes, but this API allows for very powerful extensions.
You might remember that we've talked about making classes immutable elsewhere in the
manual. This is a good practice. However, once a class is immutable, calling any of these
update methods will throw an exception.
You can make a class mutable again simply by calling "$meta->make_mutable". Once you're
done changing it, you can restore immutability by calling "$meta->make_immutable".
However, the most common use for this part of the meta API is as part of Moose extensions.
These extensions should assume that they are being run before you make a class immutable.
GOING FURTHER
If you're interested in extending Moose, we recommend reading all of the "Meta" and
"Extending" recipes in the Moose::Cookbook. Those recipes show various practical
applications of the MOP.
If you'd like to write your own extensions, one of the best ways to learn more about this
is to look at other similar extensions to see how they work. You'll probably also need to
read various API docs, including the docs for the various "Moose::Meta::*" and
"Class::MOP::*" classes.
Finally, we welcome questions on the Moose mailing list and IRC. Information on the
mailing list, IRC, and more references can be found in the Moose.pm docs.
AUTHORS
· Stevan Little <stevan.little AT iinteractive.com>
· Dave Rolsky <autarch AT urth.org>
· Jesse Luehrs <doy AT tozt.net>
· Shawn M Moore <code AT sartak.org>
· XXXX XXX'XX (Yuval Kogman) <nothingmuch AT woobling.org>
· Karen Etheridge <ether AT cpan.org>
· Florian Ragwitz <rafl AT debian.org>
· Hans Dieter Pearcey <hdp AT weftsoar.net>
· Chris Prather <chris AT prather.org>
· Matt S Trout <mst AT shadowcat.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 v5.20.1 2014-09-25 Moose::Manual::MOP(3pm)
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