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Moose-2.0002
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/ Moose::Manual::MOP

NAME

Moose::Manual::MOP - The Moose (and Class::MOP) meta API

VERSION

version 2.0002

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::* classes and the Class::MOP distribution.

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.

AUTHOR

Stevan Little <stevan@iinteractive.com>

COPYRIGHT AND LICENSE

This software is copyright (c) 2011 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.