# PODNAME: Moose::Cookbook::Extending::ExtensionOverview
# ABSTRACT: Moose extension overview

=pod

=head1 DESCRIPTION

Moose provides several ways in which extensions can hook into Moose
and change its behavior. Moose also has a lot of behavior that can be
changed. This recipe will provide an overview of each extension method
and give you some recommendations on what tools to use.

If you haven't yet read the recipes on metaclasses, go read those
first. You can't write Moose extensions without understanding the
metaclasses, and those recipes also demonstrate some basic extension
mechanisms, such as metaclass subclasses and traits.

=head2 Playing Nice With Others

One of the goals of this overview is to help you build extensions that
cooperate well with other extensions. This is especially important if
you plan to release your extension to CPAN.

Moose comes with several modules that exist to help your write
cooperative extensions. These are L<Moose::Exporter> and
L<Moose::Util::MetaRole>. By using these two modules, you will ensure
that your extension works with both the Moose core features and any
other CPAN extension using those modules.

=head1 PARTS OF Moose YOU CAN EXTEND

The types of things you might want to do in Moose extensions fall into
a few broad categories.

=head2 Metaclass Extensions

One way of extending Moose is by extending one or more Moose
metaclasses. For example, in L<Moose::Cookbook::Meta::Table_MetaclassTrait> we saw
a metaclass role that added a C<table> attribute to the
metaclass. If you were writing an ORM, this would be a logical
extension.

Many of the Moose extensions on CPAN work by providing an attribute
metaclass role. For example, the L<MooseX::Aliases> module
provides an attribute metaclass trait that lets you specify aliases
to install for methods and attribute accessors.

A metaclass extension can be packaged as a role/trait or a subclass. If you
can, we recommend using traits instead of subclasses, since it's much easier
to combine disparate traits than it is to combine a bunch of subclasses.

When your extensions are implemented as roles, you can apply them with
the L<Moose::Util::MetaRole> module.

=head2 Providing Sugar Functions

As part of a metaclass extension, you may also want to provide some
sugar functions, just like L<Moose.pm|Moose> does. Moose provides a
helper module called L<Moose::Exporter> that makes this much
simpler. We will be use L<Moose::Exporter> in several of the extension
recipes.

=head2 Object Class Extensions

Another common Moose extension technique is to change the default object
class's behavior. As with metaclass extensions, this can be done with a
role/trait or with a subclass. For example, L<MooseX::StrictConstructor>
extension applies a trait that makes the constructor reject arguments which
don't match its attributes.

Object class extensions often include metaclass extensions as well. In
particular, if you want your object extension to work when a class is
made immutable, you may need to modify the behavior of some or all of the
L<Moose::Meta::Instance>, L<Moose::Meta::Method::Constructor>, and
L<Moose::Meta::Method::Destructor> objects.

The L<Moose::Util::MetaRole> module lets you apply roles to the base
object class, as well as the meta classes just mentioned.

=head2 Providing a Role

Some extensions come in the form of a role for you to consume. The
L<MooseX::Object::Pluggable> extension is a great example of this. In
fact, despite the C<MooseX> name, it does not actually change anything
about Moose's behavior. Instead, it is just a role that an object
which wants to be pluggable can consume.

If you are implementing this sort of extension, you don't need to do
anything special. You simply create a role and document that it should
be used via the normal C<with> sugar:

   package MyApp::User;

   use Moose;

   with 'My::Role';

Don't use "MooseX" in the name for such packages.

=head2 New Types

Another common Moose extension is a new type for the Moose type
system. In this case, you simply create a type in your module. When
people load your module, the type is created, and they can refer to it
by name after that. The L<MooseX::Types::URI> and
L<MooseX::Types::DateTime> distributions are two good examples of how
this works. These both build on top of the L<MooseX::Types> extension.

=head1 ROLES VS TRAITS VS SUBCLASSES

It is important to understand that B<roles and traits are the same thing>. A
trait is simply a role applied to a instance. The only thing that may
distinguish the two is that a trait can be packaged in a way that lets Moose
resolve a short name to a class name. In other words, with a trait, the caller
can refer to it by a short name like "Big", and Moose will resolve it to a
class like C<MooseX::Embiggen::Meta::Attribute::Role::Big>.

See L<Moose::Cookbook::Meta::Labeled_AttributeTrait> and
L<Moose::Cookbook::Meta::Table_MetaclassTrait> for examples of traits in
action. In particular, both of these recipes demonstrate the trait resolution
mechanism.

Implementing an extension as a (set of) metaclass or base object
role(s) will make your extension more cooperative. It is hard for an
end-user to effectively combine together multiple metaclass
subclasses, but it is very easy to combine roles.

=head1 USING YOUR EXTENSION

There are a number of ways in which an extension can be applied. In
some cases you can provide multiple ways of consuming your extension.

=head2 Extensions as Metaclass Traits

If your extension is available as a trait, you can ask end users to
simply specify it in a list of traits. Currently, this only works for
(class) metaclass and attribute metaclass traits:

  use Moose -traits => [ 'Big', 'Blue' ];

  has 'animal' => (
      traits => [ 'Big', 'Blue' ],
      ...
  );

If your extension applies to any other metaclass, or the object base
class, you cannot use the trait mechanism.

The benefit of the trait mechanism is that is very easy to see where a
trait is applied in the code, and consumers have fine-grained control
over what the trait applies to. This is especially true for attribute
traits, where you can apply the trait to just one attribute in a
class.

=head2 Extensions as Metaclass (and Base Object) Roles

Implementing your extensions as metaclass roles makes your extensions
easy to apply, and cooperative with other role-based extensions for
metaclasses.

Just as with a subclass, you will probably want to package your
extensions for consumption with a single module that uses
L<Moose::Exporter>. However, in this case, you will use
L<Moose::Util::MetaRole> to apply all of your roles. The advantage of
using this module is that I<it preserves any subclassing or roles
already applied to the user's metaclasses>. This means that your
extension is cooperative I<by default>, and consumers of your
extension can easily use it with other role-based extensions. Most
uses of L<Moose::Util::MetaRole> can be handled by L<Moose::Exporter>
directly; see the L<Moose::Exporter> docs.

  package MooseX::Embiggen;

  use Moose::Exporter;

  use MooseX::Embiggen::Role::Meta::Class;
  use MooseX::Embiggen::Role::Meta::Attribute;
  use MooseX::Embiggen::Role::Meta::Method::Constructor;
  use MooseX::Embiggen::Role::Object;

  Moose::Exporter->setup_import_methods(
      class_metaroles => {
          class     => ['MooseX::Embiggen::Role::Meta::Class'],
          attribute => ['MooseX::Embiggen::Role::Meta::Attribute'],
          constructor =>
              ['MooseX::Embiggen::Role::Meta::Method::Constructor'],
      },
      base_class_roles => ['MooseX::Embiggen::Role::Object'],
  );

As you can see from this example, you can use L<Moose::Util::MetaRole>
to apply roles to any metaclass, as well as the base object class. If
some other extension has already applied its own roles, they will be
preserved when your extension applies its roles, and vice versa.

=head2 Providing Sugar

With L<Moose::Exporter>, you can also export your own sugar functions:

  package MooseX::Embiggen;

  use Moose::Exporter;

  Moose::Exporter->setup_import_methods(
      with_meta       => ['embiggen'],
      class_metaroles => {
          class => ['MooseX::Embiggen::Role::Meta::Class'],
      },
  );

  sub embiggen {
      my $meta = shift;
      $meta->embiggen(@_);
  }

And then the consumer of your extension can use your C<embiggen> sub:

  package Consumer;

  use Moose;
  use MooseX::Embiggen;

  extends 'Thing';

  embiggen ...;

This can be combined with metaclass and base class roles quite easily.

=head2 More advanced extensions

Providing your extension simply as a set of traits that gets applied to the
appropriate metaobjects is easy, but sometimes not sufficient. For instance,
sometimes you need to supply not just a base object role, but an actual base
object class (due to needing to interact with existing systems that only
provide a base class). To write extensions like this, you will need to provide
a custom C<init_meta> method in your exporter. For instance:

  package MooseX::Embiggen;

  use Moose::Exporter;

  my ($import, $unimport, $init_meta) = Moose::Exporter->build_import_methods(
      install         => ['import', 'unimport'],
      with_meta       => ['embiggen'],
      class_metaroles => {
          class => ['MooseX::Embiggen::Role::Meta::Class'],
      },
  );

  sub embiggen {
      my $meta = shift;
      $meta->embiggen(@_);
  }

  sub init_meta {
      my $package = shift;
      my %options = @_;
      if (my $meta = Class::MOP::class_of($options{for_class})) {
          if ($meta->isa('Class::MOP::Class')) {
              my @supers = $meta->superclasses;
              $meta->superclasses('MooseX::Embiggen::Base::Class')
                  if @supers == 1 && $supers[0] eq 'Moose::Object';
          }
      }
      $package->$init_meta(%options);
  }

In the previous examples, C<init_meta> was generated for you, but here you must
override it in order to add additional functionality. Some differences to note:

=over 4

=item C<build_import_methods> instead of C<setup_import_methods>

C<build_import_methods> simply returns the C<import>, C<unimport>, and
C<init_meta> methods, rather than installing them under the appropriate names.
This way, you can write your own methods which wrap the functionality provided
by L<Moose::Exporter>.  The C<build_import_methods> sub also takes an
additional C<install> parameter, which tells it to just go ahead and install
these methods (since we don't need to modify them).

=item C<sub init_meta>

Next, we must write our C<init_meta> wrapper. The important things to remember
are that it is called as a method, and that C<%options> needs to be passed
through to the existing implementation. We call the base implementation by
using the C<$init_meta> subroutine reference that was returned by
C<build_import_methods> earlier.

=item Additional implementation

This extension sets a different default base object class. To do so, it first
checks to see if it's being applied to a class, and then checks to see if
L<Moose::Object> is that class's only superclass, and if so, replaces that with
the superclass that this extension requires.

Note that two extensions that do this same thing will not work together
properly (the second extension to be loaded won't see L<Moose::Object> as the
base object, since it has already been overridden). This is why using a base
object role is recommended for the general case.

This C<init_meta> also works defensively, by only applying its functionality if
a metaclass already exists. This makes sure it doesn't break with legacy
extensions which override the metaclass directly (and so must be the first
extension to initialize the metaclass). This is likely not necessary, since
almost no extensions work this way anymore, but just provides an additional
level of protection. The common case of C<use Moose; use MooseX::Embiggen;>
is not affected regardless.

=back

This is just one example of what can be done with a custom C<init_meta> method.
It can also be used for preventing an extension from being applied to a role,
doing other kinds of validation on the class being applied to, or pretty much
anything that would otherwise be done in an C<import> method.

=head1 LEGACY EXTENSION MECHANISMS

Before the existence of L<Moose::Exporter> and
L<Moose::Util::MetaRole>, there were a number of other ways to extend
Moose. In general, these methods were less cooperative, and only
worked well with a single extension.

These methods include L<metaclass.pm|metaclass>, L<Moose::Policy>
(which uses L<metaclass.pm|metaclass> under the hood), and various
hacks to do what L<Moose::Exporter> does. Please do not use these for
your own extensions.

Note that if you write a cooperative extension, it should cooperate
with older extensions, though older extensions generally do not
cooperate with each other.

=head1 CONCLUSION

If you can write your extension as one or more metaclass and base
object roles, please consider doing so. Make sure to read the docs for
L<Moose::Exporter> and L<Moose::Util::MetaRole> as well.

=cut
