NAME¶
Moose::Manual::MOP - The Moose (and Class::MOP) meta API
VERSION¶
version 2.0603
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.
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::*" and "Class::MOP::*" classes.
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¶
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) 2012 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.