=pod

=head1 NAME

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

=head1 INTRODUCTION

Moose provides a powerful introspection API by building on top of
C<Class::MOP>. "MOP" stands for Meta-Object Protocol. In plainer
english, a MOP is an API for examing classes, attributes, methods, and
so on.

In fact, it is C<Class::MOP> that provides many of Moose's core
features, including attributes, before/after/around method modifiers,
and immutability. In most cases, Moose subclasses an existing
C<Class::MOP> class to extend. Moose also adds some entirely new
features, including roles, method augmentation, and types.

It's important to know about C<Class::MOP> so you know what docs to
read. Often times, the introspection method that you're looking for is
defined in a C<Class::MOP> class, rather than Moose itself.

The MOP provides more than just I<read-only> introspection. It also
lets you add attributes, method, apply roles, and much more. In fact,
all of the declarative Moose sugar is simply a thin layer on top of
the MOP API.

The meta API is useful for a lot of things. If you want to write Moose
extensions, you'll definitely need to learn about the meta API. The
introspection methods are also handy if you want to do something like
generate docs or inheritance graphs, or do some other sort of 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.

=head1 GETTING STARTED

The typical entry point to the meta API is a class's metaclass object,
which is a L<Moose::Meta::Class>. This is available by calling the
C<meta> method on a class or object:

  package User;

  use Moose;

  my $meta = __PACKAGE__->meta;

The C<meta> method is added to a class when it uses Moose.

You can also use C<Class::MOP::get_metaclass_by_name($name)> to get a
metaclass object for any class. This is safer than calling C<<
$class->meta >> when you're not sure that the class has a meta method.

If you want a metaclass object for a class that I<isn't> using Moose,
a good idiom to use is:

  my $meta = Class::MOP::Class->initialize($class_name);

The C<< Class::MOP::Class->initialize >> 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 C<Class::MOP::Class>
object. This will work for classes that use Moose, meta API classes,
and classes which don't use Moose at all.

=head1 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 C<get_all_attributes> method is documented in
C<Class::MOP::Class>. It returns a list of L<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 $meth->fully_qualified_name, "\n";
  }

Now we're looping over a list of L<Moose::Meta::Method> objects. Note
that some of these objects may actually be a subclass of
L<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 I<names>, not metaclass
objects.

=head1 CHANGING THE OBJECT

You can also use the metaclass object to change the class directly, by
adding attributes, methods, etc.

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 C<<
$metaclass->make_mutable >>. Once you're done changing it, you can
restore immutability by calling C<< $metaclass->make_immutable >>.

However, the most common use for this part of 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.

As an example, we can add a method to a class:

  $meta->add_method( 'say' => sub { print @_, "\n" } );

Or an attribute:

  $meta->add_attribute(
      name => 'size',
      is   => 'rw',
      isa  => 'Int',
  );

Obviously, this is much more cumbersome than the normal way of
defining methods and attributes, but being able to do this via an API
makes for very powerful extensions.

=head1 GOING FURTHER

We recommend that you take a look at all of the "Meta" and "Extending"
recipes in the L<Moose::Cookbook>. These 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
C<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 L<Moose.pm docs|Moose/SEE ALSO>.

=head1 AUTHOR

Dave Rolsky E<lt>autarch@urth.orgE<gt> and Stevan Little
E<lt>stevan@iinteractive.comE<gt>

=head1 COPYRIGHT AND LICENSE

Copyright 2008 by Infinity Interactive, Inc.

L<http://www.iinteractive.com>

This library is free software; you can redistribute it and/or modify
it under the same terms as Perl itself.

=cut