6 Moose::Cookbook::Meta::Recipe3 - Labels implemented via attribute traits
10 package MyApp::Meta::Attribute::Trait::Labeled;
16 predicate => 'has_label',
19 package Moose::Meta::Attribute::Custom::Trait::Labeled;
20 sub register_implementation { 'MyApp::Meta::Attribute::Trait::Labeled' }
22 package MyApp::Website;
24 use MyApp::Meta::Attribute::Trait::Labeled;
27 traits => [qw/Labeled/],
30 label => "The site's URL",
41 # iterate over all the attributes in $self
42 my %attributes = %{ $self->meta->get_attribute_map };
43 while (my ($name, $attribute) = each %attributes) {
45 # print the label if available
46 if ($attribute->does('MyApp::Meta::Attribute::Trait::Labeled')
47 && $attribute->has_label) {
48 print $attribute->label;
50 # otherwise print the name
55 # print the attribute's value
56 my $reader = $attribute->get_read_method;
57 print ": " . $self->$reader . "\n";
62 my $app = MyApp::Website->new(url => "http://google.com", name => "Google");
67 This recipe is a continuation of
68 L<Moose::Cookbook::Meta::Recipe2>. Please read that recipe first.
72 In L<Moose::Cookbook::Meta::Recipe2>, we created an attribute
73 metaclass that gives attributes a "label" that can be set in
74 L<Moose/has>. That works well until you want a second meta-attribute,
75 or until you want to adjust the behavior of the attribute. You could
76 define a specialized attribute metaclass to use in every attribute.
77 However, you may want different attributes to have different
78 behaviors. You might end up with a unique attribute metaclass for
79 B<every single attribute>, with a lot of code copying and pasting!
81 Or, if you've been drinking deeply of the Moose kool-aid, you'll have a role
82 for each of the behaviors. One role would give a label meta-attribute. Another
83 role would signify that this attribute is not directly modifiable via the
84 REST interface. Another role would write to a logfile when this attribute
87 Unfortunately, you'd still be left with a bunch of attribute metaclasses that
88 do nothing but compose a bunch of roles. If only there were some way to specify
89 in L<Moose/has> a list of roles to apply to the attribute metaclass...
93 Roles that apply to metaclasses have a special name: traits. Don't let the
94 change in nomenclature fool you, B<traits are just roles>.
96 L<Moose/has> provides a C<traits> option. It takes a list of trait names to
97 compose into an anonymous metaclass. That means you do still have a bunch of
98 attribute metaclasses that do nothing but compose a bunch of roles, but they're
99 managed automatically by Moose. You don't need to declare them in advance, or
100 worry whether changing one will affect some other attribute.
102 What can traits do? Anything roles can do. They can add or refine attributes,
103 wrap methods, provide more methods, define an interface, etc. The only
104 difference is that you're now changing the attribute metaclass instead of a
109 A side-by-side look of the code examples in this recipe and recipe 2 should
110 indicate that defining and using a trait is very similar to defining and using
111 a new attribute metaclass.
113 package MyApp::Meta::Attribute::Trait::Labeled;
119 predicate => 'has_label',
122 Instead of subclassing L<Moose::Meta::Attribute>, we define a role. Traits
123 don't need any special methods or attributes. You just focus on whatever it is
124 you actually need to get done. Here we're adding a new meta-attribute for use
127 package Moose::Meta::Attribute::Custom::Trait::Labeled;
128 sub register_implementation { 'MyApp::Meta::Attribute::Trait::Labeled' }
130 Much like when we define a new attribute metaclass, we can provide a shorthand
131 name for the trait. Moose looks at the C<register_implementation> method in
132 C<Moose::Meta::Attribute::Custom::Trait::$TRAIT_NAME> to find the full
135 Now we begin writing our application logic. I'll only cover what has changed
139 traits => [qw/Labeled/],
142 label => "The site's URL",
145 L<Moose/has> provides a C<traits> option. Just pass the list of trait names and
146 it will compose them together to form the (anonymous) attribute metaclass used
147 by the attribute. We provide a label for the attribute in the same way.
149 # print the label if available
150 if ($attribute->does('MyApp::Meta::Attribute::Trait::Labeled')
151 && $attribute->has_label) {
152 print $attribute->label;
155 Previously, this code asked the question "Does this attribute use our attribute
156 metaclass?" Since we're now using a trait, we ask "Does this attribute's
157 metaclass do the C<Labeled> role?" If not, the attribute metaclass won't have
158 the C<has_label> method, and so it would be an error to blindly call
159 C<< $attribute->has_label >>.
161 That's all. Everything else is the same!
163 =head1 METACLASS + TRAIT
165 "But wait!" you protest. "I've already written all of my extensions as
166 attribute metaclasses. I don't want to break all that code out there."
168 All is not lost. If you rewrite your extension as a trait, then you can
169 easily get a regular metaclass extension out of it. You just compose the trait
170 in the attribute metaclass, as normal.
172 package MyApp::Meta::Attribute::Labeled;
174 extends 'Moose::Meta::Attribute';
175 with 'MyApp::Meta::Attribute::Trait::Labeled';
177 package Moose::Meta::Attribute::Custom::Labeled;
178 sub register_implementation { 'MyApp::Meta::Attribute::Labeled' }
180 Unfortunately, going the other way (providing a trait created from a metaclass)
181 is more tricky. Thus, defining your extensions as traits is just plain better
182 than defining them as subclassed metaclasses.
186 If you're extending your attributes, it's easier and more flexible to provide
187 composable bits of behavior than to subclass L<Moose::Meta::Attribute>.
188 Using traits (which are just roles applied to a metaclass!) let you choose
189 exactly which behaviors each attribute will have. Moose makes it easy to create
190 attribute metaclasses on the fly by providing a list of trait names to
195 Shawn M Moore E<lt>sartak@gmail.comE<gt>
197 =head1 COPYRIGHT AND LICENSE
199 Copyright 2006-2008 by Infinity Interactive, Inc.
201 L<http://www.iinteractive.com>
203 This library is free software; you can redistribute it and/or modify
204 it under the same terms as Perl itself.