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1 | =pod |
2 | |
3 | =head1 NAME |
4 | |
5 | Moose::Manual::Attribute - Object attributes with Moose |
6 | |
7 | =head1 INTRODUCTION |
8 | |
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9 | Moose attributes have many properties, and attributes are probably the |
10 | single most powerful and flexible part of Moose. You can create a |
11 | powerful class simply by declaring attributes. In fact, it's possible |
12 | to have classes that consist solely of attribute declarations. |
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13 | |
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14 | An attribute is a property that every member of a class has. For |
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15 | example, we might say that "every Person object has a first name and |
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16 | last name". Attributes can be optional, so that we can say "some Person |
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17 | objects have a social security number (and some don't)". |
18 | |
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19 | At its simplest, an attribute can be thought of as a named value (as |
20 | in a hash) that can be read and set. However, attributes can also have |
21 | defaults, type constraints, delegation and much more. |
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22 | |
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23 | In other languages, attributes are also referred to as slots or |
24 | properties. |
25 | |
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26 | =head1 ATTRIBUTE OPTIONS |
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27 | |
28 | Use the C<has> function to declare an attribute: |
29 | |
30 | package Person; |
31 | |
32 | use Moose; |
33 | |
34 | has 'first_name' => ( is => 'rw' ); |
35 | |
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36 | This says that all person objects have an optional read-write |
37 | "first_name" attribute. |
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38 | |
39 | =head2 Read-write Vs Read-only |
40 | |
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41 | The options passed to C<has> define the properties of the |
42 | attribute. There are a many options, but in the simplest form you just |
43 | need to set C<is>, which can be either C<rw> (read-write) or C<ro> |
44 | (read-only). |
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45 | |
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46 | (In fact, you could even omit C<is>, but that gives you an attribute |
47 | that has no accessors, which is pointless unless you're doing some |
48 | deep, dark magic). |
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49 | |
50 | =head2 Accessor Methods |
51 | |
52 | Each attribute has one or more accessor methods. An accessor lets you |
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53 | read and write the value of that attribute for an object. |
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54 | |
55 | By default, the accessor method has the same name as the attribute. If |
56 | you declared your attribute as C<ro> then your accessor will be |
57 | read-only. If you declared it read-write, you get a read-write |
58 | accessor. Simple. |
59 | |
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60 | Given our Person example above, we now have a single C<first_name> |
61 | accessor that can read or write a person object's first name. |
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62 | |
63 | If you want, you can also explicitly specify the method names to be |
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64 | used for reading and writing an attribute's value. This is |
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65 | particularly handy when you'd like an attribute to be publically |
66 | readable, but only privately settable. For example: |
67 | |
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68 | has 'weight' => ( |
69 | is => 'rw', |
70 | writer => '_set_weight', |
71 | ); |
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72 | |
73 | This might be useful if weight is calculated based on other methods, |
74 | for example every time the C<eat> method is called, we might adjust |
75 | weight. This lets us hide the implementation details of weight |
76 | changes, but still provide the weight value to users of the class. |
77 | |
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78 | Some people might prefer to have distinct methods for reading and |
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79 | writing. In I<Perl Best Practices>, Damian Conway recommends that |
80 | reader methods start with "get_" and writer methods start with "set_". |
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81 | |
82 | We can do exactly that by providing names for both the C<reader> and |
83 | C<writer> methods: |
84 | |
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85 | has 'weight' => ( |
86 | is => 'rw', |
87 | reader => 'get_weight', |
88 | writer => 'set_weight', |
89 | ); |
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90 | |
91 | If you're thinking that doing this over and over would be insanely |
92 | tedious, you're right! Fortunately, Moose provides a powerful |
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93 | extension system that lets override the default naming |
94 | conventions. See L<Moose::Manual::MooseX> for more details. |
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95 | |
96 | =head2 Predicate and Clearer Methods |
97 | |
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98 | Moose allows you to explicitly distinguish between a false or |
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99 | undefined attribute value and an attribute which has not been set. If |
100 | you want to access this information, you must define clearer and |
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101 | predicate methods for an attribute. |
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102 | |
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103 | A predicate method tells you whether or not a given attribute is |
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104 | currently set. Note an attribute can be explicitly set to undef or |
105 | some other false value, but the predicate will return true. |
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106 | |
107 | The clearer method unsets the attribute. This is I<not> the |
108 | same as setting the value to C<undef>, but you can only distinguish |
109 | between them if you define a predicate method! |
110 | |
111 | Here's some code to illustrate the relationship between an accessor, |
112 | predicate, and clearer method. |
113 | |
114 | package Person; |
115 | |
116 | use Moose; |
117 | |
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118 | has 'ssn' => ( |
119 | is => 'rw', |
120 | clearer => 'clear_ssn', |
121 | predicate => 'has_ssn', |
122 | ); |
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123 | |
124 | ... |
125 | |
126 | my $person = Person->new(); |
127 | $person->has_ssn; # false |
128 | |
129 | $person->ssn(undef); |
130 | $person->ssn; # returns undef |
131 | $person->has_ssn; # true |
132 | |
133 | $person->clear_ssn; |
134 | $person->ssn; # returns undef |
135 | $person->has_ssn; # false |
136 | |
137 | $person->ssn('123-45-6789'); |
138 | $person->ssn; # returns '123-45-6789' |
139 | $person->has_ssn; # true |
140 | |
141 | my $person2 = Person->new( ssn => '111-22-3333'); |
142 | $person2->has_ssn; # true |
143 | |
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144 | By default, Moose does not make a predicate or clearer for you. You |
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145 | must explicitly provide names for them. |
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146 | |
147 | =head2 Required or Not? |
148 | |
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149 | By default, all attributes are optional, and do not need to be |
150 | provided at object construction time. If you want to make an attribute |
151 | required, simply set the C<required> option to true: |
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152 | |
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153 | has 'name' => ( |
154 | is => 'rw', |
155 | required => 1, |
156 | ); |
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157 | |
158 | There are a couple caveats worth mentioning in regards to what |
159 | required actually means. |
160 | |
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161 | Basically, all it says is that this attribute (name) must be provided |
162 | to the constructor. It does not say anything about its value, so it |
163 | could be C<undef>. |
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164 | |
165 | If you define a clearer method on a required attribute, the clearer |
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166 | I<will> work, so even a required attribute can be unset after object |
167 | construction. |
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168 | |
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169 | This means that if you do make an attribute required, providing a |
170 | clearer doesn't make much sense. In some cases, it might be handy to |
171 | have a I<private> C<clearer> and C<predicate> for a required |
172 | attribute. |
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173 | |
174 | =head2 Default and Builder Methods |
175 | |
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176 | Attributes can have default values, and Moose provides two ways to |
177 | specify that default. |
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178 | |
179 | In the simplest form, you simply provide a non-reference scalar value |
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180 | for the C<default> option: |
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181 | |
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182 | has 'size' => ( |
183 | is => 'rw', |
184 | default => 'medium', |
185 | predicate => 'has_size', |
186 | ); |
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187 | |
188 | If the size attribute is not provided to the constructor, then it ends |
189 | up being set to "medium": |
190 | |
191 | my $person = Person->new(); |
192 | $person->size; # medium |
193 | $person->has_size; # true |
194 | |
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195 | You can also provide a subroutine reference for C<default>. This |
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196 | reference will be called as a method on the object. |
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197 | |
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198 | has 'size' => ( |
199 | is => 'rw', |
200 | default => |
201 | sub { ( 'small', 'medium', 'large' )[ int( rand 3 ) ] }, |
202 | predicate => 'has_size', |
203 | ); |
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204 | |
205 | This is dumb example, but it illustrates the point that the subroutine |
206 | will be called for every new object created. |
207 | |
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208 | Of course, if it's called during object construction, it may be called |
209 | before other attributes have been set. If your default is dependent on |
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210 | other parts of the object's state, you can make the attribute |
211 | C<lazy>. Laziness is covered in the next section. |
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212 | |
213 | If you want to use a reference of any sort as the default value, you |
214 | must return it from a subroutine. This is necessary because otherwise |
215 | Perl would instantiate the reference exactly once, and it would be |
216 | shared by all objects: |
217 | |
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218 | has 'mapping' => ( |
219 | is => 'rw', |
220 | default => {}, # wrong! |
221 | ); |
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222 | |
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223 | Moose will throw an error if you pass a bare non-subroutine reference |
224 | as the default. |
225 | |
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226 | If Moose allowed this then the default mapping attribute could easily |
227 | end up shared across many objects. Instead, wrap it in a subroutine |
228 | reference: |
229 | |
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230 | has 'mapping' => ( |
231 | is => 'rw', |
232 | default => sub { {} }, # right! |
233 | ); |
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234 | |
235 | This is a bit awkward, but it's just the way Perl works. |
236 | |
237 | As an alternative to using a subroutine reference, you can instead |
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238 | supply a C<builder> method for your attribute: |
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239 | |
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240 | has 'size' => ( |
241 | is => 'rw', |
242 | builder => '_build_size', |
243 | predicate => 'has_size', |
244 | ); |
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245 | |
246 | sub _build_size { |
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247 | return ( 'small', 'medium', 'large' )[ int( rand 3 ) ]; |
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248 | } |
249 | |
250 | This has several advantages. First, it moves a chunk of code to its |
251 | own named method, which improves readability and code |
252 | organization. Second, the C<_build_size> method can be overridden in |
253 | subclasses. |
254 | |
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255 | We strongly recommend that you use a C<builder> instead of a |
256 | C<default> for anything beyond the most trivial default. |
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257 | |
258 | =head2 Laziness and lazy_build |
259 | |
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260 | Moose lets you defer attribute population by making an attribute |
261 | C<lazy>: |
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262 | |
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263 | has 'size' => ( |
264 | is => 'rw', |
265 | lazy => 1, |
266 | builder => '_build_size', |
267 | ); |
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268 | |
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269 | When C<lazy> is true, the default is not generated until the reader |
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270 | method is called, rather than at object construction time. There are |
271 | several reasons you might choose to do this. |
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272 | |
273 | First, if the default value for this attribute depends on some other |
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274 | attributes, then the attribute I<must> be C<lazy>. During object |
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275 | construction, defaults are not generated in a predictable order, so |
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276 | you cannot count on some other attribute being populated when |
277 | generating a default. |
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278 | |
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279 | Second, there's often no reason to calculate a default before it's |
280 | needed. Making an attribute C<lazy> lets you defer the cost until the |
281 | attribute is needed. If the attribute is I<never> needed, you save |
282 | some CPU time. |
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283 | |
284 | We recommend that you make any attribute with a builder or non-trivial |
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285 | default C<lazy> as a matter of course. |
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286 | |
287 | To facilitate this, you can simply specify the C<lazy_build> attribute |
288 | option. This bundles up a number of options together: |
289 | |
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290 | has 'size' => ( |
291 | is => 'rw', |
292 | lazy_build => 1, |
293 | ); |
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294 | |
295 | This is the same as specifying all of these options: |
296 | |
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297 | has 'size' => ( |
298 | is => 'rw', |
299 | lazy => 1, |
300 | builder => '_build_size', |
301 | clearer => 'clear_size', |
302 | predicate => 'has_size', |
303 | ); |
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304 | |
305 | If your attribute name starts with an underscore (_), then the clearer |
306 | and predicate will as well: |
307 | |
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308 | has '_size' => ( |
309 | is => 'rw', |
310 | lazy_build => 1, |
311 | ); |
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312 | |
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313 | becomes: |
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314 | |
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315 | has '_size' => ( |
316 | is => 'rw', |
317 | lazy => 1, |
318 | builder => '_build__size', |
319 | clearer => '_clear_size', |
320 | predicate => '_has_size', |
321 | ); |
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322 | |
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323 | Note the doubled underscore in the builder name. Internally, Moose |
324 | simply prepends the attribute name with "_build_" to come up with the |
325 | builder name. |
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326 | |
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327 | If you don't like the names that C<lazy_build> generates, you can |
328 | always provide your own: |
329 | |
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330 | has 'size' => ( |
331 | is => 'rw', |
332 | lazy_build => 1, |
333 | clearer => '_clear_size', |
334 | ); |
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335 | |
336 | Options that you explicitly provide are always used in favor of |
337 | Moose's internal defaults. |
338 | |
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339 | =head2 Constructor Parameters (init_arg) |
340 | |
341 | By default, each attribute can be passed by name to the class's |
342 | constructor. On occassion, you may want to use a different name for |
343 | the constructor parameter. You may also want to make an attribute |
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344 | unsettable via the constructor. |
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345 | |
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346 | Both of these goals can be accomplished with the C<init_arg> option: |
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347 | |
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348 | has 'bigness' => ( |
349 | is => 'rw', |
350 | init_arg => 'size', |
351 | ); |
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352 | |
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353 | Now we have an attribute named bigness, but we pass C<size> to the |
354 | constructor. |
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355 | |
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356 | Even more useful is the ability to disable setting an attribute via |
357 | the constructor. This is particularly handy for private attributes: |
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358 | |
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359 | has '_genetic_code' => ( |
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360 | is => 'rw', |
361 | lazy_build => 1, |
362 | init_arg => undef, |
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363 | ); |
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364 | |
365 | By setting the C<init_arg> to C<undef>, we make it impossible to set |
366 | this attribute when creating a new object. |
367 | |
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368 | =head2 Weak References |
369 | |
370 | Moose has built-in support for weak references. If you set the |
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371 | C<weak_ref> option to a true value, then it will call |
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372 | C<Scalar::Util::weaken> whenever the attribute is set: |
373 | |
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374 | has 'parent' => ( |
375 | is => 'rw', |
376 | weak_ref => 1, |
377 | ); |
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378 | |
379 | $node->parent($parent_node); |
380 | |
381 | This is very useful when you're building objects that may contain |
382 | circular references. |
383 | |
384 | =head2 Triggers |
385 | |
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386 | A C<trigger> is a subroutine that is called whenever the attribute is |
387 | set: |
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388 | |
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389 | has 'size' => ( |
390 | is => 'rw', |
391 | trigger => \&_size_set, |
392 | ); |
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393 | |
394 | sub _size_set { |
395 | my ( $self, $size, $meta_attr ) = @_; |
396 | |
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397 | warn $self->name, " size is now $size\n"; |
0b9488c8 |
398 | } |
399 | |
400 | The trigger is called as a method, and receives the new value as well |
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401 | as the L<Moose::Meta::Attribute> object for the attribute. The trigger |
402 | is called I<after> the value is set. |
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403 | |
404 | =head2 Attribute Types |
405 | |
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406 | Attributes can be restricted to only accept certain types: |
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407 | |
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408 | has 'first_name' => ( |
409 | is => 'rw', |
410 | isa => 'Str', |
411 | ); |
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412 | |
413 | This says that the first_name attribute must be a string. |
414 | |
415 | Moose also provides a shortcut for specifying that an attribute only |
416 | accepts objects that do a certain role: |
417 | |
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418 | has 'weapon' => ( |
419 | is => 'rw', |
420 | does => 'MyApp::Weapon', |
421 | ); |
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422 | |
423 | See the L<Moose::Manual::Types> documentation for a complete |
424 | discussion of Moose's type system. |
425 | |
426 | =head2 Delegation |
427 | |
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428 | Attributes can define methods which simple delegate to their values: |
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429 | |
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430 | has 'hair_color' => ( |
431 | is => 'rw', |
432 | isa => 'Graphics::Color::RGB', |
433 | handles => { hair_color_hex => 'as_hex_string' }, |
434 | ); |
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435 | |
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436 | This adds a new method, C<hair_color_hex>. When someone calls |
437 | C<hair_color_hex>, internally, the object just calls C<< |
438 | $self->hair_color->as_hex_string >>. |
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439 | |
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440 | See L<Moose::Manual::Delegation> for documentation on how to set up |
0b9488c8 |
441 | delegation methods. |
442 | |
443 | =head2 Metaclass and traits |
444 | |
445 | One of Moose's best features is that it can be extended in all sorts |
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446 | of ways through the use of custom metaclasses and metaclass traits. |
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447 | |
448 | When declaring an attribute, you can declare a metaclass or a set of |
449 | traits for the attribute: |
450 | |
451 | use MooseX::AttributeHelpers; |
452 | |
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453 | has 'mapping' => ( |
454 | metaclass => 'Collection::Hash', |
455 | is => 'ro', |
456 | default => sub { {} }, |
457 | ); |
0b9488c8 |
458 | |
459 | In this case, the metaclass C<Collection::Hash> really refers to |
460 | C<MooseX::AttributeHelpers::Collection::Hash>. |
461 | |
462 | You can also apply one or more traits to an attribute: |
463 | |
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464 | use MooseX::MetaDescription; |
465 | |
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466 | has 'size' => ( |
467 | is => 'rw', |
468 | traits => ['MooseX::MetaDescription::Meta::Trait'], |
469 | description => { |
470 | html_widget => 'text_input', |
471 | serialize_as => 'element', |
472 | }, |
473 | ); |
0b9488c8 |
474 | |
475 | The advantage of traits is that you can mix more than one of them |
476 | together easily (in fact, a trait is just a role under the hood). |
477 | |
478 | There are a number of MooseX modules on CPAN which provide useful |
479 | attribute metaclasses and traits. See L<Moose::Manual::MooseX> for |
480 | some examples. You can also write your own metaclasses and traits. See |
481 | the "Meta" and "Extending" recipes in L<Moose::Cookbook> for examples. |
482 | |
7261aa9d |
483 | =head1 ATTRIBUTE INHERITANCE |
0b9488c8 |
484 | |
485 | By default, a child inherits all of its parent class(es)' attributes |
d4b96efa |
486 | as-is. However, you can explicitly change some aspects of the |
487 | inherited attribute in the child class. |
0b9488c8 |
488 | |
489 | The options that can be overridden in a subclass are: |
490 | |
491 | =over 4 |
492 | |
493 | =item * default |
494 | |
495 | =item * coerce |
496 | |
497 | =item * required |
498 | |
499 | =item * documentation |
500 | |
501 | =item * lazy |
502 | |
503 | =item * isa |
504 | |
505 | =item * handles |
506 | |
507 | =item * builder |
508 | |
509 | =item * metaclass |
510 | |
511 | =item * traits |
512 | |
513 | =back |
514 | |
515 | To override an attribute, you simply prepend its name with a plus sign |
516 | (+): |
517 | |
518 | package LazyPerson; |
519 | |
520 | use Moose; |
521 | |
522 | extends 'Person'; |
523 | |
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524 | has '+first_name' => ( |
525 | lazy => 1, |
526 | default => 'Bill', |
527 | ); |
0b9488c8 |
528 | |
529 | Now the C<first_name> attribute in C<LazyPerson> is lazy, and defaults |
530 | to C<'Bill'>. |
531 | |
532 | We recommend that you exercise caution when changing the type (C<isa>) |
0c38f882 |
533 | of an inherited attribute. |
0b9488c8 |
534 | |
7261aa9d |
535 | =head1 MORE ON ATTRIBUTES |
536 | |
537 | Moose attributes are a big topic, and this document glosses over a few |
538 | aspects of their aspects. We recommend that you read the |
539 | L<Moose::Manual::Delegation> and L<Moose::Manual::Types> documents to |
540 | get a more complete understanding of attribute features. |
541 | |
542 | =head1 A FEW MORE OPTIONS |
543 | |
544 | Moose has lots of attribute options. The ones listed below are |
545 | superceded by some more modern features, but are covered for the sake |
546 | of completeness. |
547 | |
0b9488c8 |
548 | =head2 The C<documentation> option |
549 | |
550 | You can provide a piece of documentation as a string for an attribute: |
551 | |
d983b81e |
552 | has 'first_name' => ( |
553 | is => 'rw', |
554 | documentation => q{The person's first (personal) name}, |
555 | ); |
0b9488c8 |
556 | |
557 | Moose does absolutely nothing with this information other than store |
558 | it. |
559 | |
0b9488c8 |
560 | =head2 The C<auto_deref> Option |
561 | |
562 | If your attribute is an array reference or hash reference, the |
563 | C<auto_deref> option will make Moose de-reference the value when it is |
564 | returned from the reader method: |
565 | |
566 | my %map = $object->mapping; |
567 | |
568 | This option only works if your attribute is explicitly typed as an |
569 | ArrayRef or HashRef. |
570 | |
571 | However, we recommend that you use C<MooseX::AttributeHelpers> for |
572 | these types of attributes, which gives you much more control over how |
573 | they are accessed and manipulated. |
574 | |
575 | =head2 Initializer |
576 | |
577 | Moose provides an attribute option called C<initializer>. This is |
578 | similar to C<builder>, except that it is I<only> called during object |
579 | construction. |
580 | |
581 | This option is inherited from C<Class::MOP>, but we recommend that you |
582 | use a C<builder> (which is Moose-only) instead. |
583 | |
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584 | =head1 AUTHOR |
585 | |
586 | Dave Rolsky E<lt>autarch@urth.orgE<gt> |
587 | |
588 | =head1 COPYRIGHT AND LICENSE |
589 | |
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590 | Copyright 2009 by Infinity Interactive, Inc. |
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591 | |
592 | L<http://www.iinteractive.com> |
593 | |
594 | This library is free software; you can redistribute it and/or modify |
595 | it under the same terms as Perl itself. |
596 | |
597 | =cut |