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1 | package Moose::Cookbook::Meta::Recipe2; |
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2 | |
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3 | # ABSTRACT: A meta-attribute, attributes with labels |
4 | |
5 | __END__ |
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6 | |
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7 | |
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8 | =pod |
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9 | |
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10 | =head1 SYNOPSIS |
11 | |
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12 | package MyApp::Meta::Attribute::Labeled; |
13 | use Moose; |
14 | extends 'Moose::Meta::Attribute'; |
15 | |
16 | has label => ( |
17 | is => 'rw', |
18 | isa => 'Str', |
19 | predicate => 'has_label', |
20 | ); |
21 | |
22 | package Moose::Meta::Attribute::Custom::Labeled; |
23 | sub register_implementation {'MyApp::Meta::Attribute::Labeled'} |
24 | |
25 | package MyApp::Website; |
26 | use Moose; |
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27 | |
28 | has url => ( |
29 | metaclass => 'Labeled', |
30 | is => 'rw', |
31 | isa => 'Str', |
32 | label => "The site's URL", |
33 | ); |
34 | |
35 | has name => ( |
36 | is => 'rw', |
37 | isa => 'Str', |
38 | ); |
39 | |
40 | sub dump { |
41 | my $self = shift; |
42 | |
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43 | my $meta = $self->meta; |
44 | |
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45 | my $dump = ''; |
46 | |
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47 | for my $attribute ( map { $meta->get_attribute($_) } |
48 | sort $meta->get_attribute_list ) { |
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49 | |
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50 | if ( $attribute->isa('MyApp::Meta::Attribute::Labeled') |
51 | && $attribute->has_label ) { |
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52 | $dump .= $attribute->label; |
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53 | } |
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54 | else { |
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55 | $dump .= $attribute->name; |
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56 | } |
57 | |
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58 | my $reader = $attribute->get_read_method; |
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59 | $dump .= ": " . $self->$reader . "\n"; |
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60 | } |
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61 | |
62 | return $dump; |
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63 | } |
64 | |
65 | package main; |
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66 | |
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67 | my $app = MyApp::Website->new( url => "http://google.com", name => "Google" ); |
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68 | |
69 | =head1 SUMMARY |
70 | |
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71 | In this recipe, we begin to delve into the wonder of meta-programming. |
72 | Some readers may scoff and claim that this is the arena of only the |
73 | most twisted Moose developers. Absolutely not! Any sufficiently |
74 | twisted developer can benefit greatly from going more meta. |
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75 | |
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76 | Our goal is to allow each attribute to have a human-readable "label" |
77 | attached to it. Such labels would be used when showing data to an end |
78 | user. In this recipe we label the C<url> attribute with "The site's |
79 | URL" and create a simple method showing how to use that label. |
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80 | |
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81 | The proper, modern way to extend attributes (using a role instead of a |
82 | subclass) is described in L<Moose::Cookbook::Meta::Recipe3>, but that recipe |
83 | assumes you've read and at least tried to understand this one. |
84 | |
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85 | =head1 META-ATTRIBUTE OBJECTS |
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86 | |
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87 | All the attributes of a Moose-based object are actually objects |
88 | themselves. These objects have methods and attributes. Let's look at |
89 | a concrete example. |
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90 | |
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91 | has 'x' => ( isa => 'Int', is => 'ro' ); |
92 | has 'y' => ( isa => 'Int', is => 'rw' ); |
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93 | |
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94 | Internally, the metaclass for C<Point> has two |
95 | L<Moose::Meta::Attribute>. There are several methods for getting |
96 | meta-attributes out of a metaclass, one of which is |
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97 | C<get_attribute_list>. This method is called on the metaclass object. |
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98 | |
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99 | The C<get_attribute_list> method returns a list of attribute names. You can |
100 | then use C<get_attribute> to get the L<Moose::Meta::Attribute> object itself. |
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101 | |
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102 | Once you have this meta-attribute object, you can call methods on it like this: |
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103 | |
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104 | print $point->meta->get_attribute('x')->type_constraint; |
105 | => Int |
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106 | |
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107 | To add a label to our attributes there are two steps. First, we need a |
108 | new attribute metaclass that can store a label for an |
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109 | attribute. Second, we need to create attributes that use that |
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110 | attribute metaclass. |
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111 | |
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112 | =head1 RECIPE REVIEW |
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113 | |
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114 | We start by creating a new attribute metaclass. |
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115 | |
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116 | package MyApp::Meta::Attribute::Labeled; |
117 | use Moose; |
118 | extends 'Moose::Meta::Attribute'; |
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119 | |
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120 | We can subclass a Moose metaclass in the same way that we subclass |
121 | anything else. |
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122 | |
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123 | has label => ( |
124 | is => 'rw', |
125 | isa => 'Str', |
126 | predicate => 'has_label', |
127 | ); |
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128 | |
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129 | Again, this is standard Moose code. |
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130 | |
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131 | Then we need to register our metaclass with Moose: |
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132 | |
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133 | package Moose::Meta::Attribute::Custom::Labeled; |
134 | sub register_implementation { 'MyApp::Meta::Attribute::Labeled' } |
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135 | |
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136 | This is a bit of magic that lets us use a short name, "Labeled", when |
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137 | referring to our new metaclass. |
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138 | |
139 | That was the whole attribute metaclass. |
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140 | |
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141 | Now we start using it. |
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142 | |
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143 | package MyApp::Website; |
144 | use Moose; |
145 | use MyApp::Meta::Attribute::Labeled; |
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146 | |
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147 | We have to load the metaclass to use it, just like any Perl class. |
148 | |
149 | Finally, we use it for an attribute: |
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150 | |
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151 | has url => ( |
152 | metaclass => 'Labeled', |
153 | is => 'rw', |
154 | isa => 'Str', |
155 | label => "The site's URL", |
156 | ); |
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157 | |
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158 | This looks like a normal attribute declaration, except for two things, |
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159 | the C<metaclass> and C<label> parameters. The C<metaclass> parameter |
160 | tells Moose we want to use a custom metaclass for this (one) |
161 | attribute. The C<label> parameter will be stored in the meta-attribute |
162 | object. |
163 | |
164 | The reason that we can pass the name C<Labeled>, instead of |
165 | C<MyApp::Meta::Attribute::Labeled>, is because of the |
166 | C<register_implementation> code we touched on previously. |
167 | |
168 | When you pass a metaclass to C<has>, it will take the name you provide |
169 | and prefix it with C<Moose::Meta::Attribute::Custom::>. Then it calls |
170 | C<register_implementation> in the package. In this case, that means |
171 | Moose ends up calling |
172 | C<Moose::Meta::Attribute::Custom::Labeled::register_implementation>. |
173 | |
174 | If this function exists, it should return the I<real> metaclass |
175 | package name. This is exactly what our code does, returning |
176 | C<MyApp::Meta::Attribute::Labeled>. This is a little convoluted, and |
177 | if you don't like it, you can always use the fully-qualified name. |
178 | |
179 | We can access this meta-attribute and its label like this: |
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180 | |
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181 | $website->meta->get_attribute('url')->label() |
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182 | |
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183 | MyApp::Website->meta->get_attribute('url')->label() |
184 | |
185 | We also have a regular attribute, C<name>: |
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186 | |
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187 | has name => ( |
188 | is => 'rw', |
189 | isa => 'Str', |
190 | ); |
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191 | |
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192 | This is a regular Moose attribute, because we have not specified a new |
193 | metaclass. |
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194 | |
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195 | Finally, we have a C<dump> method, which creates a human-readable |
196 | representation of a C<MyApp::Website> object. It will use an |
197 | attribute's label if it has one. |
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198 | |
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199 | sub dump { |
200 | my $self = shift; |
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201 | |
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202 | my $meta = $self->meta; |
203 | |
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204 | my $dump = ''; |
205 | |
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206 | for my $attribute ( map { $meta->get_attribute($_) } |
207 | sort $meta->get_attribute_list ) { |
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208 | |
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209 | if ( $attribute->isa('MyApp::Meta::Attribute::Labeled') |
210 | && $attribute->has_label ) { |
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211 | $dump .= $attribute->label; |
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212 | } |
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213 | |
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214 | This is a bit of defensive code. We cannot depend on every |
215 | meta-attribute having a label. Even if we define one for every |
216 | attribute in our class, a subclass may neglect to do so. Or a |
217 | superclass could add an attribute without a label. |
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218 | |
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219 | We also check that the attribute has a label using the predicate we |
220 | defined. We could instead make the label C<required>. If we have a |
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221 | label, we use it, otherwise we use the attribute name: |
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222 | |
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223 | else { |
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224 | $dump .= $attribute->name; |
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225 | } |
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226 | |
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227 | my $reader = $attribute->get_read_method; |
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228 | $dump .= ": " . $self->$reader . "\n"; |
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229 | } |
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230 | |
231 | return $dump; |
6a7e3999 |
232 | } |
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233 | |
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234 | The C<get_read_method> is part of the L<Moose::Meta::Attribute> |
235 | API. It returns the name of a method that can read the attribute's |
236 | value, I<when called on the real object> (don't call this on the |
237 | meta-attribute). |
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238 | |
239 | =head1 CONCLUSION |
240 | |
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241 | You might wonder why you'd bother with all this. You could just |
242 | hardcode "The Site's URL" in the C<dump> method. But we want to avoid |
243 | repetition. If you need the label once, you may need it elsewhere, |
244 | maybe in the C<as_form> method you write next. |
245 | |
246 | Associating a label with an attribute just makes sense! The label is a |
247 | piece of information I<about> the attribute. |
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248 | |
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249 | It's also important to realize that this was a trivial example. You |
250 | can make much more powerful metaclasses that I<do> things, as opposed |
251 | to just storing some more information. For example, you could |
252 | implement a metaclass that expires attributes after a certain amount |
253 | of time: |
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254 | |
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255 | has site_cache => ( |
256 | metaclass => 'TimedExpiry', |
257 | expires_after => { hours => 1 }, |
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258 | refresh_with => sub { get( $_[0]->url ) }, |
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259 | isa => 'Str', |
260 | is => 'ro', |
261 | ); |
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262 | |
263 | The sky's the limit! |
264 | |
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265 | =begin testing |
266 | |
267 | my $app = MyApp::Website->new( url => "http://google.com", name => "Google" ); |
268 | is( |
269 | $app->dump, q{name: Google |
270 | The site's URL: http://google.com |
271 | }, '... got the expected dump value' |
272 | ); |
273 | |
274 | =end testing |
275 | |
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276 | =cut |
277 | |