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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 |
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 | |
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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 | |
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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' => ( |
70 | is => 'rw', |
71 | writer => '_set_weight', |
72 | ); |
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73 | |
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 |
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 override the default naming |
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 an attribute can be explicitly set to C<undef> or |
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106 | some other false value, but the predicate will return true. |
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107 | |
108 | The clearer method unsets the attribute. This is I<not> the |
109 | same as setting the value to C<undef>, but you can only distinguish |
110 | between them if you define a predicate method! |
111 | |
112 | Here's some code to illustrate the relationship between an accessor, |
113 | predicate, and clearer method. |
114 | |
115 | package Person; |
116 | |
117 | use Moose; |
118 | |
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119 | has 'ssn' => ( |
120 | is => 'rw', |
121 | clearer => 'clear_ssn', |
122 | predicate => 'has_ssn', |
123 | ); |
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124 | |
125 | ... |
126 | |
127 | my $person = Person->new(); |
128 | $person->has_ssn; # false |
129 | |
130 | $person->ssn(undef); |
131 | $person->ssn; # returns undef |
132 | $person->has_ssn; # true |
133 | |
134 | $person->clear_ssn; |
135 | $person->ssn; # returns undef |
136 | $person->has_ssn; # false |
137 | |
138 | $person->ssn('123-45-6789'); |
139 | $person->ssn; # returns '123-45-6789' |
140 | $person->has_ssn; # true |
141 | |
142 | my $person2 = Person->new( ssn => '111-22-3333'); |
143 | $person2->has_ssn; # true |
144 | |
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145 | By default, Moose does not make a predicate or clearer for you. You |
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146 | must explicitly provide names for them. |
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147 | |
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148 | =head2 Required or not? |
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149 | |
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150 | By default, all attributes are optional, and do not need to be |
151 | provided at object construction time. If you want to make an attribute |
152 | required, simply set the C<required> option to true: |
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153 | |
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154 | has 'name' => ( |
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155 | is => 'ro', |
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156 | required => 1, |
157 | ); |
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158 | |
159 | There are a couple caveats worth mentioning in regards to what |
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160 | "required" actually means. |
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161 | |
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162 | Basically, all it says is that this attribute (C<name>) must be provided |
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163 | to the constructor. It does not say anything about its value, so it |
164 | could be C<undef>. |
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165 | |
166 | If you define a clearer method on a required attribute, the clearer |
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167 | I<will> work, so even a required attribute can be unset after object |
168 | construction. |
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169 | |
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170 | This means that if you do make an attribute required, providing a |
171 | clearer doesn't make much sense. In some cases, it might be handy to |
172 | have a I<private> C<clearer> and C<predicate> for a required |
173 | attribute. |
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174 | |
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175 | =head2 Default and builder methods |
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176 | |
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177 | Attributes can have default values, and Moose provides two ways to |
178 | specify that default. |
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179 | |
180 | In the simplest form, you simply provide a non-reference scalar value |
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181 | for the C<default> option: |
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182 | |
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183 | has 'size' => ( |
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184 | is => 'ro', |
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185 | default => 'medium', |
186 | predicate => 'has_size', |
187 | ); |
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188 | |
189 | If the size attribute is not provided to the constructor, then it ends |
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190 | up being set to C<medium>: |
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191 | |
192 | my $person = Person->new(); |
193 | $person->size; # medium |
194 | $person->has_size; # true |
195 | |
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196 | You can also provide a subroutine reference for C<default>. This |
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197 | reference will be called as a method on the object. |
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198 | |
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199 | has 'size' => ( |
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200 | is => 'ro', |
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201 | default => |
202 | sub { ( 'small', 'medium', 'large' )[ int( rand 3 ) ] }, |
203 | predicate => 'has_size', |
204 | ); |
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205 | |
206 | This is dumb example, but it illustrates the point that the subroutine |
207 | will be called for every new object created. |
208 | |
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209 | When you provide a C<default> subroutine reference, it is called as a |
210 | method on the object, with no additional parameters: |
211 | |
212 | has 'size' => ( |
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213 | is => 'ro', |
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214 | default => sub { |
215 | my $self = shift; |
216 | |
217 | return $self->height > 200 ? 'big' : 'average'; |
218 | }, |
219 | ); |
220 | |
221 | When the C<default> is called during object construction, it may be |
222 | called before other attributes have been set. If your default is |
223 | dependent on other parts of the object's state, you can make the |
224 | attribute C<lazy>. Laziness is covered in the next section. |
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225 | |
226 | If you want to use a reference of any sort as the default value, you |
227 | must return it from a subroutine. This is necessary because otherwise |
228 | Perl would instantiate the reference exactly once, and it would be |
229 | shared by all objects: |
230 | |
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231 | has 'mapping' => ( |
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232 | is => 'ro', |
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233 | default => {}, # wrong! |
234 | ); |
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235 | |
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236 | Moose will throw an error if you pass a bare non-subroutine reference |
237 | as the default. |
238 | |
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239 | If Moose allowed this then the default mapping attribute could easily |
240 | end up shared across many objects. Instead, wrap it in a subroutine |
241 | reference: |
242 | |
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243 | has 'mapping' => ( |
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244 | is => 'ro', |
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245 | default => sub { {} }, # right! |
246 | ); |
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247 | |
248 | This is a bit awkward, but it's just the way Perl works. |
249 | |
250 | As an alternative to using a subroutine reference, you can instead |
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251 | supply a C<builder> method for your attribute: |
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252 | |
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253 | has 'size' => ( |
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254 | is => 'ro', |
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255 | builder => '_build_size', |
256 | predicate => 'has_size', |
257 | ); |
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258 | |
259 | sub _build_size { |
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260 | return ( 'small', 'medium', 'large' )[ int( rand 3 ) ]; |
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261 | } |
262 | |
263 | This has several advantages. First, it moves a chunk of code to its |
264 | own named method, which improves readability and code |
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265 | organization. |
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266 | |
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267 | We strongly recommend that you use a C<builder> instead of a |
268 | C<default> for anything beyond the most trivial default. |
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269 | |
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270 | A C<builder>, just like a C<default>, is called as a method on the |
271 | object with no additional parameters. |
272 | |
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273 | =head3 Builders allow subclassing |
274 | |
275 | Because the C<builder> is called I<by name>, it goes through Perl's |
276 | method resolution. This means that builder methods are both |
277 | inheritable and overridable. |
278 | |
279 | If we subclass our C<Person> class, we can override C<_build_size>: |
280 | |
281 | package Lilliputian; |
282 | |
283 | use Moose; |
284 | extends 'Person'; |
285 | |
286 | sub _build_size { return 'small' } |
287 | |
288 | =head3 Builders can be composed from roles |
289 | |
290 | Because builders are called by name, they work well with roles. For |
291 | example, a role could provide an attribute but require that the |
292 | consuming class provide the C<builder>: |
293 | |
294 | package HasSize; |
295 | use Moose::Role; |
296 | |
297 | requires '_build_size'; |
298 | |
299 | has 'size' => ( |
300 | is => 'ro', |
301 | lazy => 1, |
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302 | builder => '_build_size', |
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303 | ); |
304 | |
305 | package Lilliputian; |
306 | use Moose; |
307 | |
308 | with 'HasSize'; |
309 | |
310 | sub _build_size { return 'small' } |
311 | |
312 | Roles are covered in L<Moose::Manual::Roles>. |
313 | |
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314 | =head2 Laziness and C<lazy_build> |
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315 | |
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316 | Moose lets you defer attribute population by making an attribute |
317 | C<lazy>: |
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318 | |
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319 | has 'size' => ( |
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320 | is => 'ro', |
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321 | lazy => 1, |
322 | builder => '_build_size', |
323 | ); |
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324 | |
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325 | When C<lazy> is true, the default is not generated until the reader |
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326 | method is called, rather than at object construction time. There are |
327 | several reasons you might choose to do this. |
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328 | |
329 | First, if the default value for this attribute depends on some other |
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330 | attributes, then the attribute I<must> be C<lazy>. During object |
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331 | construction, defaults are not generated in a predictable order, so |
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332 | you cannot count on some other attribute being populated when |
333 | generating a default. |
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334 | |
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335 | Second, there's often no reason to calculate a default before it's |
336 | needed. Making an attribute C<lazy> lets you defer the cost until the |
337 | attribute is needed. If the attribute is I<never> needed, you save |
338 | some CPU time. |
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339 | |
340 | We recommend that you make any attribute with a builder or non-trivial |
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341 | default C<lazy> as a matter of course. |
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342 | |
343 | To facilitate this, you can simply specify the C<lazy_build> attribute |
344 | option. This bundles up a number of options together: |
345 | |
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346 | has 'size' => ( |
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347 | is => 'ro', |
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348 | lazy_build => 1, |
349 | ); |
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350 | |
351 | This is the same as specifying all of these options: |
352 | |
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353 | has 'size' => ( |
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354 | is => 'ro', |
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355 | lazy => 1, |
356 | builder => '_build_size', |
357 | clearer => 'clear_size', |
358 | predicate => 'has_size', |
359 | ); |
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360 | |
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361 | If your attribute name starts with an underscore (C<_>), then the clearer |
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362 | and predicate will as well: |
363 | |
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364 | has '_size' => ( |
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365 | is => 'ro', |
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366 | lazy_build => 1, |
367 | ); |
8d38e631 |
368 | |
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369 | becomes: |
8d38e631 |
370 | |
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371 | has '_size' => ( |
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372 | is => 'ro', |
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373 | lazy => 1, |
374 | builder => '_build__size', |
375 | clearer => '_clear_size', |
376 | predicate => '_has_size', |
377 | ); |
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378 | |
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379 | Note the doubled underscore in the builder name. Internally, Moose |
380 | simply prepends the attribute name with "_build_" to come up with the |
381 | builder name. |
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382 | |
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383 | If you don't like the names that C<lazy_build> generates, you can |
384 | always provide your own: |
385 | |
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386 | has 'size' => ( |
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387 | is => 'ro', |
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388 | lazy_build => 1, |
389 | clearer => '_clear_size', |
390 | ); |
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391 | |
392 | Options that you explicitly provide are always used in favor of |
393 | Moose's internal defaults. |
394 | |
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395 | =head2 Constructor parameters (C<init_arg>) |
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396 | |
397 | By default, each attribute can be passed by name to the class's |
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398 | constructor. On occasion, you may want to use a different name for |
d4b96efa |
399 | the constructor parameter. You may also want to make an attribute |
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400 | unsettable via the constructor. |
d4b96efa |
401 | |
0c38f882 |
402 | Both of these goals can be accomplished with the C<init_arg> option: |
d4b96efa |
403 | |
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404 | has 'bigness' => ( |
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405 | is => 'ro', |
d983b81e |
406 | init_arg => 'size', |
407 | ); |
d4b96efa |
408 | |
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409 | Now we have an attribute named "bigness", but we pass C<size> to the |
0c38f882 |
410 | constructor. |
d4b96efa |
411 | |
0c38f882 |
412 | Even more useful is the ability to disable setting an attribute via |
413 | the constructor. This is particularly handy for private attributes: |
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414 | |
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415 | has '_genetic_code' => ( |
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416 | is => 'ro', |
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417 | lazy_build => 1, |
418 | init_arg => undef, |
d983b81e |
419 | ); |
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420 | |
421 | By setting the C<init_arg> to C<undef>, we make it impossible to set |
422 | this attribute when creating a new object. |
423 | |
d67ce58f |
424 | =head2 Weak references |
0b9488c8 |
425 | |
426 | Moose has built-in support for weak references. If you set the |
d4b96efa |
427 | C<weak_ref> option to a true value, then it will call |
0b9488c8 |
428 | C<Scalar::Util::weaken> whenever the attribute is set: |
429 | |
d983b81e |
430 | has 'parent' => ( |
431 | is => 'rw', |
432 | weak_ref => 1, |
433 | ); |
0b9488c8 |
434 | |
435 | $node->parent($parent_node); |
436 | |
437 | This is very useful when you're building objects that may contain |
438 | circular references. |
439 | |
440 | =head2 Triggers |
441 | |
d4b96efa |
442 | A C<trigger> is a subroutine that is called whenever the attribute is |
443 | set: |
0b9488c8 |
444 | |
d983b81e |
445 | has 'size' => ( |
446 | is => 'rw', |
447 | trigger => \&_size_set, |
448 | ); |
0b9488c8 |
449 | |
450 | sub _size_set { |
451 | my ( $self, $size, $meta_attr ) = @_; |
452 | |
d4b96efa |
453 | warn $self->name, " size is now $size\n"; |
0b9488c8 |
454 | } |
455 | |
456 | The trigger is called as a method, and receives the new value as well |
d4b96efa |
457 | as the L<Moose::Meta::Attribute> object for the attribute. The trigger |
458 | 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 |
463 | writing). Second, it is also called if the when an attribute's value |
464 | is passed to the constructor. |
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 | |
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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 | |
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493 | Attributes can define methods which simply delegate to their values: |
0b9488c8 |
494 | |
d983b81e |
495 | has 'hair_color' => ( |
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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 | |
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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 |
0c39debe |
525 | L<MooseX::AttributeHelpers::Collection::Hash>. |
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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>) |
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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 |
603 | aspects of their aspects. We recommend that you read the |
604 | L<Moose::Manual::Delegation> and L<Moose::Manual::Types> documents to |
605 | get a more complete understanding of attribute features. |
606 | |
607 | =head1 A FEW MORE OPTIONS |
608 | |
609 | Moose has lots of attribute options. The ones listed below are |
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610 | superseded by some more modern features, but are covered for the sake |
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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 |
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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 |
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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. |
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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 |