Moose::Cookbook::ExtenUser:ConMoose::Cookbook::Extending::ExtensionOverview(3)NAMEMoose::Cookbook::Extending::ExtensionOverview - Moose extension
overview
VERSION
version 2.1005
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.
AUTHOR
Moose is maintained by the Moose Cabal, along with the help of many
contributors. See "CABAL" in Moose and "CONTRIBUTORS" in Moose for
details.
COPYRIGHT AND LICENSE
This software is copyright (c) 2013 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 v5.18.1Moose::Cookbook::Extending::ExtensionOverview(3)
