1Moose::Manual::MOP(3) User Contributed Perl DocumentationMoose::Manual::MOP(3)
2
6 Moose::Manual::MOP - The Moose (and Class::MOP) meta API
7
9 Moose provides a powerful introspection API built on top of
10 "Class::MOP". "MOP" stands for Meta-Object Protocol. In plainer
11 English, a MOP is an API for performing introspection on classes,
12 attributes, methods, and so on.
13 In fact, it is "Class::MOP" that provides many of Moose's core
15 features, including attributes, before/after/around method modifiers,
16 and immutability. In most cases, Moose takes an existing "Class::MOP"
17 class and subclasses it to add additional features. Moose also adds
18 some entirely new features of its own, such as roles, the augment
19 modifier, and types.
20 If you're interested in the MOP, it's important to know about
22 "Class::MOP" so you know what docs to read. Often, the introspection
23 method that you're looking for is defined in a "Class::MOP" class,
24 rather than Moose itself.
25 The MOP provides more than just read-only introspection. It also lets
27 you add attributes and methods, apply roles, and much more. In fact,
28 all of the declarative Moose sugar is simply a thin layer on top of the
29 MOP API.
30 If you want to write Moose extensions, you'll need to learn some of the
32 MOP API. The introspection methods are also handy if you want to
33 generate docs or inheritance graphs, or do some other runtime
34 reflection.
35 This document is not a complete reference for the meta API. We're just
37 going to cover some of the highlights, and give you a sense of how it
38 all works. To really understand it, you'll have to read a lot of other
39 docs, and possibly even dig into the Moose guts a bit.
40
42 The usual entry point to the meta API is through a class's metaclass
43 object, which is a Moose::Meta::Class. This is available by calling the
44 "meta" method on a class or object:
45 package User;
47 use Moose;
49 my $meta = __PACKAGE__->meta;
51 The "meta" method is added to a class when it uses Moose.
53 You can also use "Class::MOP::Class->initialize($name)" to get a
55 metaclass object for any class. This is safer than calling
56 "$class->meta" when you're not sure that the class has a meta method.
57 The "Class::MOP::Class->initialize" constructor will return an existing
59 metaclass if one has already been created (via Moose or some other
60 means). If it hasn't, it will return a new "Class::MOP::Class" object.
61 This will work for classes that use Moose, meta API classes, and
62 classes which don't use Moose at all.
63
65 The metaclass object can tell you about a class's attributes, methods,
66 roles, parents, and more. For example, to look at all of the class's
67 attributes:
68 for my $attr ( $meta->get_all_attributes ) {
70 print $attr->name, "\n";
71 }
72 The "get_all_attributes" method is documented in "Class::MOP::Class".
74 For Moose-using classes, it returns a list of Moose::Meta::Attribute
75 objects for attributes defined in the class and its parents.
76 You can also get a list of methods:
78 for my $method ( $meta->get_all_methods ) {
80 print $method->fully_qualified_name, "\n";
81 }
82 Now we're looping over a list of Moose::Meta::Method objects. Note that
84 some of these objects may actually be a subclass of
85 Moose::Meta::Method, as Moose uses different classes to represent
86 wrapped methods, delegation methods, constructors, etc.
87 We can look at a class's parent classes and subclasses:
89 for my $class ( $meta->linearized_isa ) {
91 print "$class\n";
92 }
93 for my $subclass ( $meta->subclasses ) {
95 print "$subclass\n";
96 }
97 Note that both these methods return class names, not metaclass objects.
99
101 The metaclass object can change the class directly, by adding
102 attributes, methods, etc.
103 As an example, we can add a method to a class:
105 $meta->add_method( 'say' => sub { print @_, "\n" } );
107 Or an attribute:
109 $meta->add_attribute( 'size' => { is => 'rw', isa => 'Int' } );
111 Obviously, this is much more cumbersome than using Perl syntax or Moose
113 sugar for defining methods and attributes, but this API allows for very
114 powerful extensions.
115 You might remember that we've talked about making classes immutable
117 elsewhere in the manual. This is a good practice. However, once a class
118 is immutable, calling any of these update methods will throw an
119 exception.
120 You can make a class mutable again simply by calling
122 "$meta->make_mutable". Once you're done changing it, you can restore
123 immutability by calling "$meta->make_immutable".
124 However, the most common use for this part of the meta API is as part
126 of Moose extensions. These extensions should assume that they are being
127 run before you make a class immutable.
128
130 If you're interested in extending Moose, we recommend reading all of
131 the "Meta" and "Extending" recipes in the Moose::Cookbook. Those
132 recipes show various practical applications of the MOP.
133 If you'd like to write your own extensions, one of the best ways to
135 learn more about this is to look at other similar extensions to see how
136 they work. You'll probably also need to read various API docs,
137 including the docs for the various Moose::Meta::* classes and the
138 "Class::MOP" distribution.
139 Finally, we welcome questions on the Moose mailing list and IRC.
141 Information on the mailing list, IRC, and more references can be found
142 in the Moose.pm docs.
143
145 Dave Rolsky <autarch@urth.org> and Stevan Little
146 <stevan@iinteractive.com>
147
149 Copyright 2009 by Infinity Interactive, Inc.
150 <http://www.iinteractive.com>
152 This library is free software; you can redistribute it and/or modify it
154 under the same terms as Perl itself.
155perl v5.12.2 2010-08-21 Moose::Manual::MOP(3)