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",
41 my $meta = $self->meta;
45 for my $attribute ( map { $meta->get_attribute($_) }
46 sort $meta->get_attribute_list ) {
48 if ( $attribute->isa('MyApp::Meta::Attribute::Labeled')
49 && $attribute->has_label ) {
50 $dump .= $attribute->label;
53 $dump .= $attribute->name;
56 my $reader = $attribute->get_read_method;
57 $dump .= ": " . $self->$reader . "\n";
65 my $app = MyApp::Website->new( url => "http://google.com", name => "Google" );
69 In this recipe, we begin to delve into the wonder of meta-programming.
70 Some readers may scoff and claim that this is the arena of only the
71 most twisted Moose developers. Absolutely not! Any sufficiently
72 twisted developer can benefit greatly from going more meta.
74 Our goal is to allow each attribute to have a human-readable "label"
75 attached to it. Such labels would be used when showing data to an end
76 user. In this recipe we label the C<url> attribute with "The site's
77 URL" and create a simple method showing how to use that label.
79 =head1 META-ATTRIBUTE OBJECTS
81 All the attributes of a Moose-based object are actually objects
82 themselves. These objects have methods and attributes. Let's look at
85 has 'x' => ( isa => 'Int', is => 'ro' );
86 has 'y' => ( isa => 'Int', is => 'rw' );
88 Internally, the metaclass for C<Point> has two
89 L<Moose::Meta::Attribute>. There are several methods for getting
90 meta-attributes out of a metaclass, one of which is
91 C<get_attribute_list>. This method is called on the metaclass object.
93 The C<get_attribute_list> method returns a list of attribute names. You can
94 then use C<get_attribute> to get the L<Moose::Meta::Attribute> object itself.
96 Once you this meta-attribute object, you can call methods on it like this:
98 print $point->meta->get_attribute('x')->type_constraint;
101 To add a label to our attributes there are two steps. First, we need a
102 new attribute metaclass that can store a label for an
103 attribute. Second, we need to create attributes that use that
108 We start by creating a new attribute metaclass.
110 package MyApp::Meta::Attribute::Labeled;
112 extends 'Moose::Meta::Attribute';
114 We can subclass a Moose metaclass in the same way that we subclass
120 predicate => 'has_label',
123 Again, this is standard Moose code.
125 Then we need to register our metaclass with Moose:
127 package Moose::Meta::Attribute::Custom::Labeled;
128 sub register_implementation { 'MyApp::Meta::Attribute::Labeled' }
130 This is a bit of magic that lets us use a short name, "Labeled", when
131 referring to our new metaclass.
133 That was the whole attribute metaclass.
135 Now we start using it.
137 package MyApp::Website;
139 use MyApp::Meta::Attribute::Labeled;
141 We have to load the metaclass to use it, just like any Perl class.
143 Finally, we use it for an attribute:
146 metaclass => 'Labeled',
149 label => "The site's URL",
152 This looks like a normal attribute declaration, except for two things,
153 the C<metaclass> and C<label> parameters. The C<metaclass> parameter
154 tells Moose we want to use a custom metaclass for this (one)
155 attribute. The C<label> parameter will be stored in the meta-attribute
158 The reason that we can pass the name C<Labeled>, instead of
159 C<MyApp::Meta::Attribute::Labeled>, is because of the
160 C<register_implementation> code we touched on previously.
162 When you pass a metaclass to C<has>, it will take the name you provide
163 and prefix it with C<Moose::Meta::Attribute::Custom::>. Then it calls
164 C<register_implementation> in the package. In this case, that means
165 Moose ends up calling
166 C<Moose::Meta::Attribute::Custom::Labeled::register_implementation>.
168 If this function exists, it should return the I<real> metaclass
169 package name. This is exactly what our code does, returning
170 C<MyApp::Meta::Attribute::Labeled>. This is a little convoluted, and
171 if you don't like it, you can always use the fully-qualified name.
173 We can access this meta-attribute and its label like this:
175 $website->meta->get_attribute('url')->label()
177 MyApp::Website->meta->get_attribute('url')->label()
179 We also have a regular attribute, C<name>:
186 This is a regular Moose attribute, because we have not specified a new
189 Finally, we have a C<dump> method, which creates a human-readable
190 representation of a C<MyApp::Website> object. It will use an
191 attribute's label if it has one.
196 my $meta = $self->meta;
200 for my $attribute ( map { $meta->get_attribute($_) }
201 sort $meta->get_attribute_list ) {
203 if ( $attribute->isa('MyApp::Meta::Attribute::Labeled')
204 && $attribute->has_label ) {
205 $dump .= $attribute->label;
208 This is a bit of defensive code. We cannot depend on every
209 meta-attribute having a label. Even if we define one for every
210 attribute in our class, a subclass may neglect to do so. Or a
211 superclass could add an attribute without a label.
213 We also check that the attribute has a label using the predicate we
214 defined. We could instead make the label C<required>. If we have a
215 label, we use it, otherwise we use the attribute name:
218 $dump .= $attribute->name;
221 my $reader = $attribute->get_read_method;
222 $dump .= ": " . $self->$reader . "\n";
228 The C<get_read_method> is part of the L<Moose::Meta::Attribute>
229 API. It returns the name of a method that can read the attribute's
230 value, I<when called on the real object> (don't call this on the
235 You might wonder why you'd bother with all this. You could just
236 hardcode "The Site's URL" in the C<dump> method. But we want to avoid
237 repetition. If you need the label once, you may need it elsewhere,
238 maybe in the C<as_form> method you write next.
240 Associating a label with an attribute just makes sense! The label is a
241 piece of information I<about> the attribute.
243 It's also important to realize that this was a trivial example. You
244 can make much more powerful metaclasses that I<do> things, as opposed
245 to just storing some more information. For example, you could
246 implement a metaclass that expires attributes after a certain amount
250 metaclass => 'TimedExpiry',
251 expires_after => { hours => 1 },
252 refresh_with => sub { get( $_[0]->url ) },
261 Shawn M Moore E<lt>sartak@gmail.comE<gt>
263 Dave Rolsky E<lt>autarch@urth.org<gt>
265 =head1 COPYRIGHT AND LICENSE
267 Copyright 2006-2009 by Infinity Interactive, Inc.
269 L<http://www.iinteractive.com>
271 This library is free software; you can redistribute it and/or modify
272 it under the same terms as Perl itself.
276 my $app = MyApp::Website->new( url => "http://google.com", name => "Google" );
278 $app->dump, q{name: Google
279 The site's URL: http://google.com
280 }, '... got the expected dump value'