Provided by: libmoose-perl_2.2206-1_amd64 
NAME
Moose::Cookbook::Extending::ExtensionOverview - Moose extension overview
VERSION
version 2.2206
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.
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 Moose::Exporter and 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.
PARTS OF Moose YOU CAN EXTEND
The types of things you might want to do in Moose extensions fall into a few broad
categories.
Metaclass Extensions
One way of extending Moose is by extending one or more Moose metaclasses. For example, in
Moose::Cookbook::Meta::Table_MetaclassTrait we saw a metaclass role that added a "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 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
Moose::Util::MetaRole module.
Providing Sugar Functions
As part of a metaclass extension, you may also want to provide some sugar functions, just
like Moose.pm does. Moose provides a helper module called Moose::Exporter that makes this
much simpler. We will be use Moose::Exporter in several of the extension recipes.
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, 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 Moose::Meta::Instance,
Moose::Meta::Method::Constructor, and Moose::Meta::Method::Destructor objects.
The Moose::Util::MetaRole module lets you apply roles to the base object class, as well as
the meta classes just mentioned.
Providing a Role
Some extensions come in the form of a role for you to consume. The
MooseX::Object::Pluggable extension is a great example of this. In fact, despite the
"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 "with" sugar:
package MyApp::User;
use Moose;
with 'My::Role';
Don't use "MooseX" in the name for such packages.
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 MooseX::Types::URI and
MooseX::Types::DateTime distributions are two good examples of how this works. These both
build on top of the MooseX::Types extension.
ROLES VS TRAITS VS SUBCLASSES
It is important to understand that 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 "MooseX::Embiggen::Meta::Attribute::Role::Big".
See Moose::Cookbook::Meta::Labeled_AttributeTrait and
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.
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.
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.
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 Moose::Exporter. However, in this case, you will use
Moose::Util::MetaRole to apply all of your roles. The advantage of using this module is
that it preserves any subclassing or roles already applied to the user's metaclasses. This
means that your extension is cooperative by default, and consumers of your extension can
easily use it with other role-based extensions. Most uses of Moose::Util::MetaRole can be
handled by Moose::Exporter directly; see the 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 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.
Providing Sugar
With 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;
$meta->embiggen(@_);
}
And then the consumer of your extension can use your "embiggen" sub:
package Consumer;
use Moose;
use MooseX::Embiggen;
extends 'Thing';
embiggen ...;
This can be combined with metaclass and base class roles quite easily.
More advanced extensions
Providing your extension simply as a set of traits that gets applied to the appropriate
metaobjects is easy, but sometimes not sufficient. For instance, sometimes you need to
supply not just a base object role, but an actual base object class (due to needing to
interact with existing systems that only provide a base class). To write extensions like
this, you will need to provide a custom "init_meta" method in your exporter. For instance:
package MooseX::Embiggen;
use Moose::Exporter;
my ($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, "init_meta" was generated for you, but here you must override it
in order to add additional functionality. Some differences to note:
"build_import_methods" instead of "setup_import_methods"
"build_import_methods" simply returns the "import", "unimport", and "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 Moose::Exporter. The
"build_import_methods" sub also takes an additional "install" parameter, which tells
it to just go ahead and install these methods (since we don't need to modify them).
"sub init_meta"
Next, we must write our "init_meta" wrapper. The important things to remember are that
it is called as a method, and that %options needs to be passed through to the existing
implementation. We call the base implementation by using the $init_meta subroutine
reference that was returned by "build_import_methods" earlier.
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 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 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 "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 "use Moose; use MooseX::Embiggen;" is not affected regardless.
This is just one example of what can be done with a custom "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 "import" method.
LEGACY EXTENSION MECHANISMS
Before the existence of Moose::Exporter and 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 metaclass.pm, Moose::Policy (which uses metaclass.pm under the
hood), and various hacks to do what 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.
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 Moose::Exporter and
Moose::Util::MetaRole as well.
AUTHORS
• Stevan Little <stevan@cpan.org>
• Dave Rolsky <autarch@urth.org>
• Jesse Luehrs <doy@cpan.org>
• Shawn M Moore <sartak@cpan.org>
• יובל קוג'מן (Yuval Kogman) <nothingmuch@woobling.org>
• Karen Etheridge <ether@cpan.org>
• Florian Ragwitz <rafl@debian.org>
• Hans Dieter Pearcey <hdp@cpan.org>
• Chris Prather <chris@prather.org>
• Matt S Trout <mstrout@cpan.org>
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 v5.36.0 202Moose::Cookbook::Extending::ExtensionOverview(3pm)