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1 | |
2 | =pod |
3 | |
4 | =head1 NAME |
5 | |
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6 | Moose::Cookbook::Extending::Recipe1 - Moose extension overview |
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7 | |
8 | =head1 DESCRIPTION |
9 | |
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10 | Moose provides several ways in which extensions can hook into Moose |
11 | and change its behavior. Moose also has a lot of behavior that can be |
12 | changed. This recipe will provide an overview of each extension method |
13 | and give you some recommendations on what tools to use. |
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14 | |
15 | If you haven't yet read the recipes on metaclasses, go read those |
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16 | first. You can't write Moose extensions without understanding the |
17 | metaclasses, and those recipes also demonstrate some basic extension |
18 | mechanisms, such as metaclass subclasses and traits. |
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19 | |
20 | =head2 Playing Nice With Others |
21 | |
22 | One of the goals of this overview is to help you build extensions that |
23 | cooperate well with other extensions. This is especially important if |
24 | you plan to release your extension to CPAN. |
25 | |
26 | Moose comes with several modules that exist to help your write |
27 | cooperative extensions. These are L<Moose::Exporter> and |
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28 | L<Moose::Util::MetaRole>. By using these two modules, you will ensure |
29 | that your extension works with both the Moose core features and any |
30 | other CPAN extension using those modules. |
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31 | |
32 | =head1 PARTS OF Moose YOU CAN EXTEND |
33 | |
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34 | The types of things you might want to do in Moose extensions fall into |
35 | a few broad categories. |
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36 | |
37 | =head2 Metaclass Extensions |
38 | |
39 | One way of extending Moose is by extending one or more Moose |
40 | metaclasses. For example, in L<Moose::Cookbook::Meta::Recipe4> we saw |
41 | a metaclass subclass that added a C<table> attribute to the |
42 | metaclass. If you were writing an ORM, this would be a logical |
43 | extension. |
44 | |
45 | Many of the Moose extensions on CPAN work by providing an attribute |
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46 | metaclass extension. For example, the L<MooseX::AttributeHelpers> |
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47 | distribution provides a new attribute metaclass that lets you delegate |
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48 | behavior to a non-object attribute (a hashref or simple number). |
49 | |
50 | A metaclass extension can be packaged as a subclass or a |
51 | role/trait. If you can, we recommend using traits instead of |
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52 | subclasses, since it's much easier to combine disparate traits than it |
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53 | is to combine a bunch of subclasses. |
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54 | |
55 | When your extensions are implemented as roles, you can apply them with |
56 | the L<Moose::Util::MetaRole> module. |
57 | |
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58 | =head2 Providing Sugar Functions |
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59 | |
60 | As part of a metaclass extension, you may also want to provide some |
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61 | sugar functions, just like L<Moose.pm|Moose> does. Moose provides a |
62 | helper module called L<Moose::Exporter> that makes this much |
63 | simpler. We will be use L<Moose::Exporter> in several of the extension |
64 | recipes. |
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65 | |
66 | =head2 Object Class Extensions |
67 | |
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68 | Another common Moose extension technique is to change the default |
69 | object class's behavior. For example, the L<MooseX::Singleton> |
70 | extension changes the behavior of your objects so that they are |
71 | singletons. The L<MooseX::StrictConstructor> extension makes the |
72 | constructor reject arguments which don't match its attributes. |
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73 | |
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74 | Object class extensions often include metaclass extensions as well. In |
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75 | particular, if you want your object extension to work when a class is |
76 | made immutable, you may need to extend some or all of the |
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77 | L<Moose::Meta::Instance>, L<Moose::Meta::Method::Constructor>, and |
78 | L<Moose::Meta::Method::Destructor> objects. |
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79 | |
80 | The L<Moose::Util::MetaRole> module lets you apply roles to the base |
81 | object class, as well as the meta classes just mentioned. |
82 | |
83 | =head2 Providing a Role |
84 | |
85 | Some extensions come in the form of a role for you to consume. The |
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86 | L<MooseX::Object::Pluggable> extension is a great example of this. In |
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87 | fact, despite the C<MooseX> name, it does not actually change anything |
88 | about Moose's behavior. Instead, it is just a role that an object |
89 | which wants to be pluggable can consume. |
90 | |
91 | If you are implementing this sort of extension, you don't need to do |
92 | anything special. You simply create a role and document that it should |
93 | be used via the normal C<with> sugar: |
94 | |
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95 | package MyApp::User; |
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96 | |
97 | use Moose; |
98 | |
99 | with 'MooseX::My::Role'; |
100 | |
101 | =head2 New Types |
102 | |
103 | Another common Moose extension is a new type for the Moose type |
104 | system. In this case, you simply create a type in your module. When |
105 | people load your module, the type is created, and they can refer to it |
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106 | by name after that. The L<MooseX::Types::URI> and |
107 | L<MooseX::Types::DateTime> distributions are two good examples of how |
108 | this works. These both build on top of the L<MooseX::Types> extension. |
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109 | |
110 | =head1 ROLES VS TRAITS VS SUBCLASSES |
111 | |
112 | It is important to understand that B<roles and traits are the same |
113 | thing>. A role can be used as a trait, and a trait is a role. The only |
114 | thing that distinguishes the two is that a trait is packaged in a way |
115 | that lets Moose resolve a short name to a class name. In other words, |
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116 | with a trait, the caller can refer to it by a short name like "Big", |
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117 | and Moose will resolve it to a class like |
118 | C<MooseX::Embiggen::Meta::Attribute::Role::Big>. |
119 | |
120 | See L<Moose::Cookbook::Meta::Recipe3> and |
121 | L<Moose::Cookbook::Meta::Recipe5> for examples of traits in action. In |
122 | particular, both of these recipes demonstrate the trait resolution |
123 | mechanism. |
124 | |
125 | Implementing an extension as a (set of) metaclass or base object |
126 | role(s) will make your extension more cooperative. It is hard for an |
127 | end-user to effectively combine together multiple metaclass |
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128 | subclasses, but it is very easy to combine roles. |
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129 | |
130 | =head1 USING YOUR EXTENSION |
131 | |
132 | There are a number of ways in which an extension can be applied. In |
133 | some cases you can provide multiple ways of consuming your extension. |
134 | |
135 | =head2 Extensions as Metaclass Traits |
136 | |
137 | If your extension is available as a trait, you can ask end users to |
138 | simply specify it in a list of traits. Currently, this only works for |
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139 | (class) metaclass and attribute metaclass traits: |
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140 | |
141 | use Moose -traits => [ 'Big', 'Blue' ]; |
142 | |
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143 | has 'animal' => ( |
144 | traits => [ 'Big', 'Blue' ], |
145 | ... |
146 | ); |
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147 | |
148 | If your extension applies to any other metaclass, or the object base |
149 | class, you cannot use the trait mechanism. |
150 | |
151 | The benefit of the trait mechanism is that is very easy to see where a |
152 | trait is applied in the code, and consumers have fine-grained control |
153 | over what the trait applies to. This is especially true for attribute |
154 | traits, where you can apply the trait to just one attribute in a |
155 | class. |
156 | |
157 | =head2 Extensions as Metaclass (and Base Object) Subclasses |
158 | |
159 | Moose does not provide any simple APIs for consumers to use a subclass |
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160 | extension, except for attribute metaclasses. The attribute declaration |
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161 | options include a C<metaclass> option a consumer of your extension can |
162 | use to specify your subclass. |
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163 | |
164 | This is one reason why implementing an extension as a subclass can be |
165 | a poor choice. However, you can force the use of certain subclasses at |
166 | import time by calling C<< Moose->init_meta >> for the caller, and |
167 | providing an alternate metaclass or base object class. |
168 | |
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169 | If you do want to do this, you should look at using L<Moose::Exporter> |
170 | to re-export the L<Moose.pm|Moose> sugar function. With |
171 | L<Moose::Exporter>, if your exporting class has an C<init_meta> |
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172 | method, L<Moose::Exporter> makes sure that this C<init_meta> method |
173 | gets called when your class is imported. |
174 | |
175 | Then in your C<init_meta> you can arrange for the caller to use your |
176 | subclasses: |
177 | |
178 | package MooseX::Embiggen; |
179 | |
180 | use Moose (); |
181 | use Moose::Exporter; |
182 | |
183 | use MooseX::Embiggen::Meta::Class; |
184 | use MooseX::Embiggen::Object; |
185 | |
186 | Moose::Exporter->setup_import_methods( also => 'Moose' ); |
187 | |
188 | sub init_meta { |
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189 | shift; # just your package name |
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190 | my %options = @_; |
191 | |
192 | return Moose->init_meta( |
193 | for_class => $options{for_class}, |
194 | metaclass => 'MooseX::Embiggen::Meta::Class', |
195 | base_class => 'MooseX::Embiggen::Object', |
196 | ); |
197 | } |
198 | |
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199 | NOTE: Make sure that your C<init_meta> returns the metaclass object, just as |
200 | C<< Moose->init_meta >> does. |
201 | |
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202 | =head2 Extensions as Metaclass (and Base Object) Roles |
203 | |
204 | Implementing your extensions as metaclass roles makes your extensions |
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205 | easy to apply, and cooperative with other role-based extensions for |
206 | metaclasses. |
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207 | |
208 | Just as with a subclass, you will probably want to package your |
209 | extensions for consumption with a single module that uses |
210 | L<Moose::Exporter>. However, in this case, you will use |
211 | L<Moose::Util::MetaRole> to apply all of your roles. The advantage of |
212 | using this module is that I<it preserves any subclassing or roles |
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213 | already applied to the user's metaclasses>. This means that your |
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214 | extension is cooperative I<by default>, and consumers of your |
215 | extension can easily use it with other role-based extensions. |
216 | |
217 | package MooseX::Embiggen; |
218 | |
219 | use Moose (); |
220 | use Moose::Exporter; |
221 | use Moose::Util::MetaRole; |
222 | |
223 | use MooseX::Embiggen::Role::Meta::Class; |
224 | use MooseX::Embiggen::Role::Meta::Attribute; |
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225 | use MooseX::Embiggen::Role::Meta::Method::Constructor; |
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226 | use MooseX::Embiggen::Role::Object; |
227 | |
228 | Moose::Exporter->setup_import_methods( also => 'Moose' ); |
229 | |
230 | sub init_meta { |
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231 | shift; # just your package name |
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232 | my %options = @_; |
233 | |
234 | Moose->init_meta(%options); |
235 | |
236 | my $meta = Moose::Util::MetaRole::apply_metaclass_roles( |
237 | for_class => $options{for_class}, |
238 | metaclass_roles => ['MooseX::Embiggen::Role::Meta::Class'], |
239 | attribute_metaclass_roles => |
240 | ['MooseX::Embiggen::Role::Meta::Attribute'], |
241 | constructor_class_roles => |
242 | ['MooseX::Embiggen::Role::Meta::Method::Constructor'], |
243 | ); |
244 | |
245 | Moose::Util::MetaRole::apply_base_class_roles( |
246 | for_class => $options{for_class}, |
247 | roles => ['MooseX::Embiggen::Role::Object'], |
248 | ); |
249 | |
250 | return $meta; |
251 | } |
252 | |
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253 | As you can see from this example, you can use L<Moose::Util::MetaRole> |
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254 | to apply roles to any metaclass, as well as the base object class. If |
255 | some other extension has already applied its own roles, they will be |
256 | preserved when your extension applies its roles, and vice versa. |
257 | |
258 | =head2 Providing Sugar |
259 | |
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260 | With L<Moose::Exporter>, you can also export your own sugar functions, |
261 | as well as those from other modules: |
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262 | |
263 | package MooseX::Embiggen; |
264 | |
265 | use Moose (); |
266 | use Moose::Exporter; |
267 | |
268 | Moose::Exporter->setup_import_methods( |
269 | with_caller => ['embiggen'], |
270 | also => 'Moose', |
271 | ); |
272 | |
273 | sub init_meta { ... } |
274 | |
275 | sub embiggen { |
276 | my $caller = shift; |
277 | $caller->meta()->embiggen(@_); |
278 | } |
279 | |
280 | And then the consumer of your extension can use your C<embiggen> sub: |
281 | |
282 | package Consumer; |
283 | |
284 | use MooseX::Embiggen; |
285 | |
286 | extends 'Thing'; |
287 | |
288 | embiggen ...; |
289 | |
290 | This can be combined with metaclass and base class roles quite easily. |
291 | |
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292 | =head1 LEGACY EXTENSION MECHANISMS |
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293 | |
294 | Before the existence of L<Moose::Exporter> and |
295 | L<Moose::Util::MetaRole>, there were a number of other ways to extend |
296 | Moose. In general, these methods were less cooperative, and only |
297 | worked well with a single extension. |
298 | |
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299 | These methods include L<metaclass.pm|metaclass>, L<Moose::Policy> |
300 | (which uses L<metaclass.pm|metaclass> under the hood), and various |
301 | hacks to do what L<Moose::Exporter> does. Please do not use these for |
302 | your own extensions. |
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303 | |
304 | Note that if you write a cooperative extension, it should cooperate |
305 | with older extensions, though older extensions generally do not |
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306 | cooperate with each other. |
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307 | |
308 | =head1 CONCLUSION |
309 | |
310 | If you can write your extension as one or more metaclass and base |
311 | object roles, please consider doing so. Make sure to read the docs for |
312 | L<Moose::Exporter> and L<Moose::Util::MetaRole> as well. |
313 | |
314 | =head1 AUTHOR |
315 | |
316 | Dave Rolsky E<lt>autarch@urth.orgE<gt> |
317 | |
318 | =head1 COPYRIGHT AND LICENSE |
319 | |
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320 | Copyright 2009 by Infinity Interactive, Inc. |
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321 | |
322 | L<http://www.iinteractive.com> |
323 | |
324 | This library is free software; you can redistribute it and/or modify |
325 | it under the same terms as Perl itself. |
326 | |
327 | =cut |