6 Moose::Cookbook::Meta::Recipe2 - A meta-attribute, attributes with labels
10 package MyApp::Meta::Attribute::Labeled;
12 extends 'Moose::Meta::Attribute';
17 predicate => 'has_label',
20 package Moose::Meta::Attribute::Custom::Labeled;
21 sub register_implementation {'MyApp::Meta::Attribute::Labeled'}
23 package MyApp::Website;
27 metaclass => 'Labeled',
30 label => "The site's URL",
43 my %attributes = %{ $self->meta->get_attribute_map };
44 for my $name ( sort keys %attributes ) {
45 my $attribute = $attributes{$name};
47 if ( $attribute->isa('MyApp::Meta::Attribute::Labeled')
48 && $attribute->has_label ) {
49 $dump .= $attribute->label;
55 my $reader = $attribute->get_read_method;
56 $dump .= ": " . $self->$reader . "\n";
64 my $app = MyApp::Website->new( url => "http://google.com", name => "Google" );
68 In this recipe, we begin to delve into the wonder of meta-programming.
69 Some readers may scoff and claim that this is the arena of only the
70 most twisted Moose developers. Absolutely not! Any sufficiently
71 twisted developer can benefit greatly from going more meta.
73 Our goal is to allow each attribute to have a human-readable "label"
74 attached to it. Such labels would be used when showing data to an end
75 user. In this recipe we label the C<url> attribute with "The site's
76 URL" and create a simple method showing how to use that label.
78 =head1 META-ATTRIBUTE OBJECTS
80 All the attributes of a Moose-based object are actually objects
81 themselves. These objects have methods and attributes. Let's look at
84 has 'x' => ( isa => 'Int', is => 'ro' );
85 has 'y' => ( isa => 'Int', is => 'rw' );
87 Internally, the metaclass for C<Point> has two
88 L<Moose::Meta::Attribute>. There are several methods for getting
89 meta-attributes out of a metaclass, one of which is
90 C<get_attribute_map>. This method is called on the metaclass object.
92 The C<get_attribute_map> method returns a hash reference that maps
93 attribute names to their objects. In our case, C<get_attribute_map>
94 might return something that looks like the following:
97 x => $attr_object_for_x,
98 y => $attr_object_for_y,
101 You can also get a single L<Moose::Meta::Attribute> with
102 C<get_attribute('name')>. Once you have this meta-attribute object,
103 you can call methods on it like this:
105 print $point->meta->get_attribute('x')->type_constraint;
108 To add a label to our attributes there are two steps. First, we need a
109 new attribute metaclass that can store a label for an
110 attribute. Second, we need to create attributes that use that
115 We start by creating a new attribute metaclass.
117 package MyApp::Meta::Attribute::Labeled;
119 extends 'Moose::Meta::Attribute';
121 We can subclass a Moose metaclass in the same way that we subclass
127 predicate => 'has_label',
130 Again, this is standard Moose code.
132 Then we need to register our metaclass with Moose:
134 package Moose::Meta::Attribute::Custom::Labeled;
135 sub register_implementation { 'MyApp::Meta::Attribute::Labeled' }
137 This is a bit of magic that lets us use a short name, "Labeled", when
138 referring to our new metaclass.
140 That was the whole attribute metaclass.
142 Now we start using it.
144 package MyApp::Website;
146 use MyApp::Meta::Attribute::Labeled;
148 We have to load the metaclass to use it, just like any Perl class.
150 Finally, we use it for an attribute:
153 metaclass => 'Labeled',
156 label => "The site's URL",
159 This looks like a normal attribute declaration, except for two things,
160 the C<metaclass> and C<label> parameters. The C<metaclass> parameter
161 tells Moose we want to use a custom metaclass for this (one)
162 attribute. The C<label> parameter will be stored in the meta-attribute
165 The reason that we can pass the name C<Labeled>, instead of
166 C<MyApp::Meta::Attribute::Labeled>, is because of the
167 C<register_implementation> code we touched on previously.
169 When you pass a metaclass to C<has>, it will take the name you provide
170 and prefix it with C<Moose::Meta::Attribute::Custom::>. Then it calls
171 C<register_implementation> in the package. In this case, that means
172 Moose ends up calling
173 C<Moose::Meta::Attribute::Custom::Labeled::register_implementation>.
175 If this function exists, it should return the I<real> metaclass
176 package name. This is exactly what our code does, returning
177 C<MyApp::Meta::Attribute::Labeled>. This is a little convoluted, and
178 if you don't like it, you can always use the fully-qualified name.
180 We can access this meta-attribute and its label like this:
182 $website->meta->get_attribute('url')->label()
184 MyApp::Website->meta->get_attribute('url')->label()
186 We also have a regular attribute, C<name>:
193 This is a regular Moose attribute, because we have not specified a new
196 Finally, we have a C<dump> method, which creates a human-readable
197 representation of a C<MyApp::Website> object. It will use an
198 attribute's label if it has one.
205 my %attributes = %{ $self->meta->get_attribute_map };
206 for my $name ( sort keys %attributes ) {
207 my $attribute = $attributes{$name};
209 if ( $attribute->isa('MyApp::Meta::Attribute::Labeled')
210 && $attribute->has_label ) {
211 $dump .= $attribute->label;
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.
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
221 label, we use it, otherwise we use the attribute name:
227 my $reader = $attribute->get_read_method;
228 $dump .= ": " . $self->$reader . "\n";
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
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.
246 Associating a label with an attribute just makes sense! The label is a
247 piece of information I<about> the attribute.
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
256 metaclass => 'TimedExpiry',
257 expires_after => { hours => 1 },
258 refresh_with => sub { get( $_[0]->url ) },
267 Shawn M Moore E<lt>sartak@gmail.comE<gt>
269 Dave Rolsky E<lt>autarch@urth.org<gt>
271 =head1 COPYRIGHT AND LICENSE
273 Copyright 2006-2009 by Infinity Interactive, Inc.
275 L<http://www.iinteractive.com>
277 This library is free software; you can redistribute it and/or modify
278 it under the same terms as Perl itself.
282 my $app = MyApp::Website->new( url => "http://google.com", name => "Google" );
284 $app->dump, q{name: Google
285 The site's URL: http://google.com
286 }, '... got the expected dump value'