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1 | =pod |
2 | |
3 | =head1 NAME |
4 | |
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5 | Moose::Manual::Attributes - Object attributes with Moose |
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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 C<Person> object has a first name and |
16 | last name". Attributes can be optional, so that we can say "some C<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 C<Person> objects have an optional read-write |
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37 | "first_name" attribute. |
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38 | |
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39 | =head2 Read-write vs. read-only |
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40 | |
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41 | The options passed to C<has> define the properties of the |
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42 | attribute. There are many options, but in the simplest form you just |
0c38f882 |
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 | |
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50 | =head2 Accessor methods |
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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 | |
c5934de1 |
60 | Given our C<Person> example above, we now have a single C<first_name> |
61 | accessor that can read or write a C<Person> object's C<first_name> |
62 | attribute's value. |
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63 | |
64 | If you want, you can also explicitly specify the method names to be |
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65 | used for reading and writing an attribute's value. This is |
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66 | particularly handy when you'd like an attribute to be publicly |
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67 | readable, but only privately settable. For example: |
68 | |
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69 | has 'weight' => ( |
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70 | is => 'ro', |
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71 | writer => '_set_weight', |
72 | ); |
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73 | |
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74 | This might be useful if weight is calculated based on other methods. |
75 | For example, every time the C<eat> method is called, we might adjust |
8d38e631 |
76 | weight. This lets us hide the implementation details of weight |
77 | changes, but still provide the weight value to users of the class. |
78 | |
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79 | Some people might prefer to have distinct methods for reading and |
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80 | writing. In I<Perl Best Practices>, Damian Conway recommends that |
81 | reader methods start with "get_" and writer methods start with "set_". |
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82 | |
83 | We can do exactly that by providing names for both the C<reader> and |
84 | C<writer> methods: |
85 | |
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86 | has 'weight' => ( |
87 | is => 'rw', |
88 | reader => 'get_weight', |
89 | writer => 'set_weight', |
90 | ); |
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91 | |
92 | If you're thinking that doing this over and over would be insanely |
93 | tedious, you're right! Fortunately, Moose provides a powerful |
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94 | extension system that lets you override the default naming |
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95 | conventions. See L<Moose::Manual::MooseX> for more details. |
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96 | |
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97 | =head2 Predicate and clearer methods |
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98 | |
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99 | Moose allows you to explicitly distinguish between a false or |
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100 | undefined attribute value and an attribute which has not been set. If |
101 | you want to access this information, you must define clearer and |
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102 | predicate methods for an attribute. |
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103 | |
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104 | A predicate method tells you whether or not a given attribute is |
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105 | currently set. Note that an attribute can be explicitly set to |
106 | C<undef> or some other false value, but the predicate will return |
107 | true. |
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108 | |
109 | The clearer method unsets the attribute. This is I<not> the |
110 | same as setting the value to C<undef>, but you can only distinguish |
111 | between them if you define a predicate method! |
112 | |
113 | Here's some code to illustrate the relationship between an accessor, |
114 | predicate, and clearer method. |
115 | |
116 | package Person; |
117 | |
118 | use Moose; |
119 | |
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120 | has 'ssn' => ( |
121 | is => 'rw', |
122 | clearer => 'clear_ssn', |
123 | predicate => 'has_ssn', |
124 | ); |
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125 | |
126 | ... |
127 | |
128 | my $person = Person->new(); |
129 | $person->has_ssn; # false |
130 | |
131 | $person->ssn(undef); |
132 | $person->ssn; # returns undef |
133 | $person->has_ssn; # true |
134 | |
135 | $person->clear_ssn; |
136 | $person->ssn; # returns undef |
137 | $person->has_ssn; # false |
138 | |
139 | $person->ssn('123-45-6789'); |
140 | $person->ssn; # returns '123-45-6789' |
141 | $person->has_ssn; # true |
142 | |
143 | my $person2 = Person->new( ssn => '111-22-3333'); |
144 | $person2->has_ssn; # true |
145 | |
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146 | By default, Moose does not make a predicate or clearer for you. You |
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147 | must explicitly provide names for them. |
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148 | |
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149 | =head2 Required or not? |
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150 | |
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151 | By default, all attributes are optional, and do not need to be |
152 | provided at object construction time. If you want to make an attribute |
153 | required, simply set the C<required> option to true: |
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154 | |
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155 | has 'name' => ( |
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156 | is => 'ro', |
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157 | required => 1, |
158 | ); |
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159 | |
160 | There are a couple caveats worth mentioning in regards to what |
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161 | "required" actually means. |
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162 | |
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163 | Basically, all it says is that this attribute (C<name>) must be provided to |
164 | the constructor, or be lazy with either a default or a builder. It does not |
165 | say anything about its value, so it could be C<undef>. |
8d38e631 |
166 | |
167 | If you define a clearer method on a required attribute, the clearer |
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168 | I<will> work, so even a required attribute can be unset after object |
169 | construction. |
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170 | |
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171 | This means that if you do make an attribute required, providing a |
172 | clearer doesn't make much sense. In some cases, it might be handy to |
173 | have a I<private> C<clearer> and C<predicate> for a required |
174 | attribute. |
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175 | |
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176 | =head2 Default and builder methods |
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177 | |
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178 | Attributes can have default values, and Moose provides two ways to |
179 | specify that default. |
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180 | |
181 | In the simplest form, you simply provide a non-reference scalar value |
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182 | for the C<default> option: |
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183 | |
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184 | has 'size' => ( |
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185 | is => 'ro', |
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186 | default => 'medium', |
187 | predicate => 'has_size', |
188 | ); |
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189 | |
190 | If the size attribute is not provided to the constructor, then it ends |
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191 | up being set to C<medium>: |
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192 | |
193 | my $person = Person->new(); |
194 | $person->size; # medium |
195 | $person->has_size; # true |
196 | |
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197 | You can also provide a subroutine reference for C<default>. This |
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198 | reference will be called as a method on the object. |
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199 | |
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200 | has 'size' => ( |
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201 | is => 'ro', |
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202 | default => |
203 | sub { ( 'small', 'medium', 'large' )[ int( rand 3 ) ] }, |
204 | predicate => 'has_size', |
205 | ); |
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206 | |
207 | This is dumb example, but it illustrates the point that the subroutine |
208 | will be called for every new object created. |
209 | |
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210 | When you provide a C<default> subroutine reference, it is called as a |
211 | method on the object, with no additional parameters: |
212 | |
213 | has 'size' => ( |
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214 | is => 'ro', |
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215 | default => sub { |
216 | my $self = shift; |
217 | |
218 | return $self->height > 200 ? 'big' : 'average'; |
219 | }, |
220 | ); |
221 | |
222 | When the C<default> is called during object construction, it may be |
223 | called before other attributes have been set. If your default is |
224 | dependent on other parts of the object's state, you can make the |
225 | attribute C<lazy>. Laziness is covered in the next section. |
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226 | |
227 | If you want to use a reference of any sort as the default value, you |
228 | must return it from a subroutine. This is necessary because otherwise |
229 | Perl would instantiate the reference exactly once, and it would be |
230 | shared by all objects: |
231 | |
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232 | has 'mapping' => ( |
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233 | is => 'ro', |
d983b81e |
234 | default => {}, # wrong! |
235 | ); |
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236 | |
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237 | Moose will throw an error if you pass a bare non-subroutine reference |
238 | as the default. |
239 | |
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240 | If Moose allowed this then the default mapping attribute could easily |
241 | end up shared across many objects. Instead, wrap it in a subroutine |
242 | reference: |
243 | |
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244 | has 'mapping' => ( |
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245 | is => 'ro', |
d983b81e |
246 | default => sub { {} }, # right! |
247 | ); |
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248 | |
249 | This is a bit awkward, but it's just the way Perl works. |
250 | |
251 | As an alternative to using a subroutine reference, you can instead |
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252 | supply a C<builder> method for your attribute: |
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253 | |
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254 | has 'size' => ( |
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255 | is => 'ro', |
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256 | builder => '_build_size', |
257 | predicate => 'has_size', |
258 | ); |
8d38e631 |
259 | |
260 | sub _build_size { |
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261 | return ( 'small', 'medium', 'large' )[ int( rand 3 ) ]; |
8d38e631 |
262 | } |
263 | |
264 | This has several advantages. First, it moves a chunk of code to its |
265 | own named method, which improves readability and code |
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266 | organization. |
8d38e631 |
267 | |
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268 | We strongly recommend that you use a C<builder> instead of a |
269 | C<default> for anything beyond the most trivial default. |
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270 | |
9e8cb6f7 |
271 | A C<builder>, just like a C<default>, is called as a method on the |
272 | object with no additional parameters. |
273 | |
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274 | =head3 Builders allow subclassing |
275 | |
276 | Because the C<builder> is called I<by name>, it goes through Perl's |
277 | method resolution. This means that builder methods are both |
278 | inheritable and overridable. |
279 | |
280 | If we subclass our C<Person> class, we can override C<_build_size>: |
281 | |
282 | package Lilliputian; |
283 | |
284 | use Moose; |
285 | extends 'Person'; |
286 | |
287 | sub _build_size { return 'small' } |
288 | |
289 | =head3 Builders can be composed from roles |
290 | |
291 | Because builders are called by name, they work well with roles. For |
292 | example, a role could provide an attribute but require that the |
293 | consuming class provide the C<builder>: |
294 | |
295 | package HasSize; |
296 | use Moose::Role; |
297 | |
298 | requires '_build_size'; |
299 | |
300 | has 'size' => ( |
301 | is => 'ro', |
302 | lazy => 1, |
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303 | builder => '_build_size', |
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304 | ); |
305 | |
306 | package Lilliputian; |
307 | use Moose; |
308 | |
309 | with 'HasSize'; |
310 | |
311 | sub _build_size { return 'small' } |
312 | |
313 | Roles are covered in L<Moose::Manual::Roles>. |
314 | |
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315 | =head2 Laziness and C<lazy_build> |
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316 | |
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317 | Moose lets you defer attribute population by making an attribute |
318 | C<lazy>: |
8d38e631 |
319 | |
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320 | has 'size' => ( |
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321 | is => 'ro', |
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322 | lazy => 1, |
323 | builder => '_build_size', |
324 | ); |
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325 | |
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326 | When C<lazy> is true, the default is not generated until the reader |
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327 | method is called, rather than at object construction time. There are |
328 | several reasons you might choose to do this. |
8d38e631 |
329 | |
330 | First, if the default value for this attribute depends on some other |
0b9488c8 |
331 | attributes, then the attribute I<must> be C<lazy>. During object |
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332 | construction, defaults are not generated in a predictable order, so |
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333 | you cannot count on some other attribute being populated when |
334 | generating a default. |
8d38e631 |
335 | |
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336 | Second, there's often no reason to calculate a default before it's |
337 | needed. Making an attribute C<lazy> lets you defer the cost until the |
338 | attribute is needed. If the attribute is I<never> needed, you save |
339 | some CPU time. |
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340 | |
341 | We recommend that you make any attribute with a builder or non-trivial |
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342 | default C<lazy> as a matter of course. |
8d38e631 |
343 | |
344 | To facilitate this, you can simply specify the C<lazy_build> attribute |
345 | option. This bundles up a number of options together: |
346 | |
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347 | has 'size' => ( |
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348 | is => 'ro', |
d983b81e |
349 | lazy_build => 1, |
350 | ); |
8d38e631 |
351 | |
352 | This is the same as specifying all of these options: |
353 | |
d983b81e |
354 | has 'size' => ( |
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355 | is => 'ro', |
d983b81e |
356 | lazy => 1, |
357 | builder => '_build_size', |
358 | clearer => 'clear_size', |
359 | predicate => 'has_size', |
360 | ); |
8d38e631 |
361 | |
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362 | If your attribute name starts with an underscore (C<_>), then the clearer |
8d38e631 |
363 | and predicate will as well: |
364 | |
d983b81e |
365 | has '_size' => ( |
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366 | is => 'ro', |
d983b81e |
367 | lazy_build => 1, |
368 | ); |
8d38e631 |
369 | |
0b9488c8 |
370 | becomes: |
8d38e631 |
371 | |
d983b81e |
372 | has '_size' => ( |
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373 | is => 'ro', |
d983b81e |
374 | lazy => 1, |
375 | builder => '_build__size', |
376 | clearer => '_clear_size', |
377 | predicate => '_has_size', |
378 | ); |
8d38e631 |
379 | |
0b9488c8 |
380 | Note the doubled underscore in the builder name. Internally, Moose |
381 | simply prepends the attribute name with "_build_" to come up with the |
382 | builder name. |
8d38e631 |
383 | |
0b9488c8 |
384 | If you don't like the names that C<lazy_build> generates, you can |
385 | always provide your own: |
386 | |
d983b81e |
387 | has 'size' => ( |
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388 | is => 'ro', |
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389 | lazy_build => 1, |
390 | clearer => '_clear_size', |
391 | ); |
0b9488c8 |
392 | |
393 | Options that you explicitly provide are always used in favor of |
394 | Moose's internal defaults. |
395 | |
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396 | =head2 Constructor parameters (C<init_arg>) |
d4b96efa |
397 | |
398 | By default, each attribute can be passed by name to the class's |
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399 | constructor. On occasion, you may want to use a different name for |
d4b96efa |
400 | the constructor parameter. You may also want to make an attribute |
0c38f882 |
401 | unsettable via the constructor. |
d4b96efa |
402 | |
0c38f882 |
403 | Both of these goals can be accomplished with the C<init_arg> option: |
d4b96efa |
404 | |
d983b81e |
405 | has 'bigness' => ( |
f977e776 |
406 | is => 'ro', |
d983b81e |
407 | init_arg => 'size', |
408 | ); |
d4b96efa |
409 | |
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410 | Now we have an attribute named "bigness", but we pass C<size> to the |
0c38f882 |
411 | constructor. |
d4b96efa |
412 | |
0c38f882 |
413 | Even more useful is the ability to disable setting an attribute via |
414 | the constructor. This is particularly handy for private attributes: |
d4b96efa |
415 | |
d983b81e |
416 | has '_genetic_code' => ( |
f977e776 |
417 | is => 'ro', |
0c38f882 |
418 | lazy_build => 1, |
419 | init_arg => undef, |
d983b81e |
420 | ); |
d4b96efa |
421 | |
422 | By setting the C<init_arg> to C<undef>, we make it impossible to set |
423 | this attribute when creating a new object. |
424 | |
d67ce58f |
425 | =head2 Weak references |
0b9488c8 |
426 | |
427 | Moose has built-in support for weak references. If you set the |
d4b96efa |
428 | C<weak_ref> option to a true value, then it will call |
0b9488c8 |
429 | C<Scalar::Util::weaken> whenever the attribute is set: |
430 | |
d983b81e |
431 | has 'parent' => ( |
432 | is => 'rw', |
433 | weak_ref => 1, |
434 | ); |
0b9488c8 |
435 | |
436 | $node->parent($parent_node); |
437 | |
438 | This is very useful when you're building objects that may contain |
439 | circular references. |
440 | |
441 | =head2 Triggers |
442 | |
d4b96efa |
443 | A C<trigger> is a subroutine that is called whenever the attribute is |
444 | set: |
0b9488c8 |
445 | |
d983b81e |
446 | has 'size' => ( |
447 | is => 'rw', |
448 | trigger => \&_size_set, |
449 | ); |
0b9488c8 |
450 | |
451 | sub _size_set { |
c2685d20 |
452 | my ( $self, $size ) = @_; |
0b9488c8 |
453 | |
d4b96efa |
454 | warn $self->name, " size is now $size\n"; |
0b9488c8 |
455 | } |
456 | |
c2685d20 |
457 | The trigger is called as a method, and receives the new value as its argument. |
458 | The trigger is called I<after> the value is set. |
0b9488c8 |
459 | |
9c9484bf |
460 | This differs from an after method modifier in two ways. First, a |
461 | trigger is only called when the attribute is set, as opposed to |
efe388d9 |
462 | whenever the accessor method is called (for reading or |
dab94063 |
463 | writing). Second, it is also called when an attribute's value is |
464 | passed to the constructor. |
efe388d9 |
465 | |
466 | However, triggers are I<not> called when an attribute is populated |
467 | from a C<default> or C<builder> |
9c9484bf |
468 | |
d67ce58f |
469 | =head2 Attribute types |
0b9488c8 |
470 | |
d4b96efa |
471 | Attributes can be restricted to only accept certain types: |
0b9488c8 |
472 | |
d983b81e |
473 | has 'first_name' => ( |
f977e776 |
474 | is => 'ro', |
d983b81e |
475 | isa => 'Str', |
476 | ); |
0b9488c8 |
477 | |
0c39debe |
478 | This says that the C<first_name> attribute must be a string. |
0b9488c8 |
479 | |
480 | Moose also provides a shortcut for specifying that an attribute only |
481 | accepts objects that do a certain role: |
482 | |
d983b81e |
483 | has 'weapon' => ( |
484 | is => 'rw', |
485 | does => 'MyApp::Weapon', |
486 | ); |
0b9488c8 |
487 | |
488 | See the L<Moose::Manual::Types> documentation for a complete |
489 | discussion of Moose's type system. |
490 | |
491 | =head2 Delegation |
492 | |
0c39debe |
493 | Attributes can define methods which simply delegate to their values: |
0b9488c8 |
494 | |
d983b81e |
495 | has 'hair_color' => ( |
f977e776 |
496 | is => 'ro', |
d983b81e |
497 | isa => 'Graphics::Color::RGB', |
498 | handles => { hair_color_hex => 'as_hex_string' }, |
499 | ); |
0b9488c8 |
500 | |
d4b96efa |
501 | This adds a new method, C<hair_color_hex>. When someone calls |
502 | C<hair_color_hex>, internally, the object just calls C<< |
503 | $self->hair_color->as_hex_string >>. |
0b9488c8 |
504 | |
0c38f882 |
505 | See L<Moose::Manual::Delegation> for documentation on how to set up |
0b9488c8 |
506 | delegation methods. |
507 | |
508 | =head2 Metaclass and traits |
509 | |
510 | One of Moose's best features is that it can be extended in all sorts |
d4b96efa |
511 | of ways through the use of custom metaclasses and metaclass traits. |
0b9488c8 |
512 | |
513 | When declaring an attribute, you can declare a metaclass or a set of |
514 | traits for the attribute: |
515 | |
516 | use MooseX::AttributeHelpers; |
517 | |
d983b81e |
518 | has 'mapping' => ( |
519 | metaclass => 'Collection::Hash', |
520 | is => 'ro', |
521 | default => sub { {} }, |
522 | ); |
0b9488c8 |
523 | |
524 | In this case, the metaclass C<Collection::Hash> really refers to |
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525 | L<MooseX::AttributeHelpers::Collection::Hash>. |
0b9488c8 |
526 | |
527 | You can also apply one or more traits to an attribute: |
528 | |
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529 | use MooseX::MetaDescription; |
530 | |
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531 | has 'size' => ( |
f977e776 |
532 | is => 'ro', |
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533 | traits => ['MooseX::MetaDescription::Meta::Trait'], |
534 | description => { |
535 | html_widget => 'text_input', |
536 | serialize_as => 'element', |
537 | }, |
538 | ); |
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539 | |
540 | The advantage of traits is that you can mix more than one of them |
541 | together easily (in fact, a trait is just a role under the hood). |
542 | |
543 | There are a number of MooseX modules on CPAN which provide useful |
544 | attribute metaclasses and traits. See L<Moose::Manual::MooseX> for |
545 | some examples. You can also write your own metaclasses and traits. See |
546 | the "Meta" and "Extending" recipes in L<Moose::Cookbook> for examples. |
547 | |
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548 | =head1 ATTRIBUTE INHERITANCE |
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549 | |
550 | By default, a child inherits all of its parent class(es)' attributes |
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551 | as-is. However, you can explicitly change some aspects of the |
552 | inherited attribute in the child class. |
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553 | |
554 | The options that can be overridden in a subclass are: |
555 | |
556 | =over 4 |
557 | |
558 | =item * default |
559 | |
560 | =item * coerce |
561 | |
562 | =item * required |
563 | |
564 | =item * documentation |
565 | |
566 | =item * lazy |
567 | |
568 | =item * isa |
569 | |
570 | =item * handles |
571 | |
572 | =item * builder |
573 | |
574 | =item * metaclass |
575 | |
576 | =item * traits |
577 | |
578 | =back |
579 | |
580 | To override an attribute, you simply prepend its name with a plus sign |
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581 | (C<+>): |
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582 | |
583 | package LazyPerson; |
584 | |
585 | use Moose; |
586 | |
587 | extends 'Person'; |
588 | |
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589 | has '+first_name' => ( |
590 | lazy => 1, |
591 | default => 'Bill', |
592 | ); |
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593 | |
594 | Now the C<first_name> attribute in C<LazyPerson> is lazy, and defaults |
595 | to C<'Bill'>. |
596 | |
597 | We recommend that you exercise caution when changing the type (C<isa>) |
0c38f882 |
598 | of an inherited attribute. |
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599 | |
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600 | =head1 MORE ON ATTRIBUTES |
601 | |
602 | Moose attributes are a big topic, and this document glosses over a few |
dab94063 |
603 | aspects. We recommend that you read the L<Moose::Manual::Delegation> |
604 | and L<Moose::Manual::Types> documents to get a more complete |
605 | understanding of attribute features. |
7261aa9d |
606 | |
607 | =head1 A FEW MORE OPTIONS |
608 | |
609 | Moose has lots of attribute options. The ones listed below are |
636f25f3 |
610 | superseded by some more modern features, but are covered for the sake |
7261aa9d |
611 | of completeness. |
612 | |
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613 | =head2 The C<documentation> option |
614 | |
615 | You can provide a piece of documentation as a string for an attribute: |
616 | |
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617 | has 'first_name' => ( |
618 | is => 'rw', |
619 | documentation => q{The person's first (personal) name}, |
620 | ); |
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621 | |
622 | Moose does absolutely nothing with this information other than store |
623 | it. |
624 | |
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625 | =head2 The C<auto_deref> option |
0b9488c8 |
626 | |
627 | If your attribute is an array reference or hash reference, the |
636f25f3 |
628 | C<auto_deref> option will make Moose dereference the value when it is |
0b9488c8 |
629 | returned from the reader method: |
630 | |
631 | my %map = $object->mapping; |
632 | |
633 | This option only works if your attribute is explicitly typed as an |
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634 | C<ArrayRef> or C<HashRef>. |
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635 | |
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636 | However, we recommend that you use L<MooseX::AttributeHelpers> for |
0b9488c8 |
637 | these types of attributes, which gives you much more control over how |
638 | they are accessed and manipulated. |
639 | |
640 | =head2 Initializer |
641 | |
642 | Moose provides an attribute option called C<initializer>. This is |
643 | similar to C<builder>, except that it is I<only> called during object |
644 | construction. |
645 | |
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646 | This option is inherited from L<Class::MOP>, but we recommend that you |
0b9488c8 |
647 | use a C<builder> (which is Moose-only) instead. |
648 | |
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649 | =head1 AUTHOR |
650 | |
651 | Dave Rolsky E<lt>autarch@urth.orgE<gt> |
652 | |
653 | =head1 COPYRIGHT AND LICENSE |
654 | |
2840a3b2 |
655 | Copyright 2009 by Infinity Interactive, Inc. |
d4b96efa |
656 | |
657 | L<http://www.iinteractive.com> |
658 | |
659 | This library is free software; you can redistribute it and/or modify |
660 | it under the same terms as Perl itself. |
661 | |
662 | =cut |