6 Moose::Cookbook::Recipe11 - The meta-attribute example
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;
25 use MyApp::Meta::Attribute::Labeled;
28 metaclass => 'Labeled',
31 label => "The site's URL",
42 # iterate over all the attributes in $self
43 my %attributes = %{ $self->meta->get_attribute_map };
44 while (my ($name, $meta_attribute) = each %attributes) {
46 # print the label if available
47 if ($meta_attribute->isa('MyApp::Meta::Attribute::Labeled')
48 && $meta_attribute->has_label) {
49 print $meta_attribute->label;
51 # otherwise print the name
56 # print the attribute's value
57 my $reader = $meta_attribute->get_read_method;
58 print ": " . $self->$reader . "\n";
63 my $app = MyApp::Website->new(url => "http://google.com", name => "Google");
68 In this recipe, we begin to really delve into the wonder of meta-programming.
69 Some readers may scoff and claim that this is the arena only of the most
70 twisted Moose developers. Absolutely not! Any sufficiently twisted developer
71 can benefit greatly from going more meta.
73 The high-level goal of this recipe's code is to allow each attribute to have a
74 human-readable "label" attached to it. Such labels would be used when showing
75 data to an end user. In this recipe we label the "url" attribute with "The
76 site's URL" and create a simple method to demonstrate how to use that label.
78 =head1 REAL ATTRIBUTES 101
80 All the attributes of a Moose-based object are actually objects themselves.
81 These objects have methods and (surprisingly) attributes. Let's look at a
84 has 'x' => (isa => 'Int', is => 'ro');
85 has 'y' => (isa => 'Int', is => 'rw');
87 Ahh, the veritable x and y of the Point example. Internally, every Point has an
88 x object and a y object. They have methods (such as "get_value") and attributes
89 (such as "is_lazy"). What class are they instances of?
90 L<Moose::Meta::Attribute>. You don't normally see the objects lurking behind
91 the scenes, because you usually just use C<< $point->x >> and C<< $point->y >>
92 and forget that there's a lot of machinery lying in such methods.
94 So you have a C<$point> object, which has C<x> and C<y> methods. How can you
95 actually access the objects behind these attributes? Here's one way:
97 $point->meta->get_attribute_map()
99 C<get_attribute_map> returns a hash reference that maps attribute names to
100 their objects. In our case, C<get_attribute_map> might return something that
101 looks like the following:
104 x => Moose::Meta::Attribute=HASH(0x196c23c),
105 y => Moose::Meta::Attribute=HASH(0x18d1690),
108 Here's one thing you can do now that you can interact with the attribute's
111 print $point->meta->get_attribute_map->{x}->type_constraint;
114 (As an aside, it's not called C<< ->isa >> because C<< $obj->isa >> is already
117 So to actually beef up attributes, what we need to do is:
121 =item Create a new attribute metaclass
123 =item Create attributes using that new metaclass
127 Moose makes both of these easy!
129 Let's start dissecting the recipe's code.
133 We get the ball rolling by creating a new attribute metaclass. It starts off
134 somewhat ungloriously.
136 package MyApp::Meta::Attribute::Labeled;
138 extends 'Moose::Meta::Attribute';
140 You subclass metaclasses the same way you subclass regular classes. (Extra
141 credit: how in the actual hell can you use the MOP to extend itself?) Moving
147 predicate => 'has_label',
150 Now things get a little icky. We're adding a attribute to the attribute
151 metaclass. For clarity, I'm going to call this a meta-attribute.
153 This creates a new meta-attribute in the C<MyApp::Meta::Attribute::Labeled>
154 metaclass. The new meta-attribute's name is 'label'. The predicate just creates
155 a method that asks the question "Does this attribute have a value?"
157 Of course, if you step a foot back, you can see that this is really just adding
158 an attribute to a class. Don't be alarmed!
160 package Moose::Meta::Attribute::Custom::Labeled;
161 sub register_implementation { 'MyApp::Meta::Attribute::Labeled' }
163 This registers the new metaclass with Moose. That way attributes can actually
164 use it. More on what this is doing in a moment.
166 Note that we're done defining the new metaclass! Only nine lines of code, and
167 not particularly difficult lines, either. Now to start using the metaclass.
169 package MyApp::Website;
171 use MyApp::Meta::Attribute::Labeled;
173 Nothing new here. We do have to actually load our metaclass to be able to use
177 metaclass => 'Labeled',
180 label => "The site's URL",
183 Ah ha! Now we're using the metaclass. We're adding a new attribute, C<url>, to
184 C<MyApp::Website>. C<has> lets you set the metaclass of the attribute.
185 Ordinarily (as we've seen), the metaclass is C<Moose::Meta::Attribute>.
187 When C<has> sees that you're using a new metaclass, it will take the
188 metaclass's name, prepend C<Moose::Meta::Attribute::Custom::>, and call the
189 C<register_implementation> function in that package. So here Moose calls
190 C<Moose::Meta::Attribute::Custom::Labeled::register_implementation>. We defined
191 that function in the beginning -- it just returns our "real" metaclass'
192 package, C<MyApp::Meta::Attribute::Labeled>. So Moose uses that metaclass for
193 the attribute. It may seem a bit convoluted, but the alternative would be to
194 use C<< metaclass => 'MyApp::Meta::Attribute::Labeled' >> on every attribute.
195 As usual, Moose optimizes in favor of the end user, not the metaprogrammer. :)
197 Finally, we see that C<has> is setting our new meta-attribute, C<label>, to
205 You do not of course need to use the new metaclass for all new attributes.
207 Now we begin defining a method that will dump the C<MyApp::Website> instance
213 # iterate over all the attributes in $self
214 my %attributes = %{ $self->meta->get_attribute_map };
215 while (my ($name, $meta_attribute) = each %attributes) {
217 We covered the latter two lines of code earlier.
219 # print the label if available
220 if ($meta_attribute->isa('MyApp::Meta::Attribute::Labeled')
221 && $meta_attribute->has_label) {
222 print $meta_attribute->label;
225 Note that we have two checks here. The first is "is this attribute an instance
226 of C<MyApp::Meta::Attribute::Labeled>?". It's good to code defensively, even if
227 all of your attributes have this metaclass. You never know when someone is
228 going to subclass your work of art, poorly. The second check is "does this
229 attribute have a label?". This method was defined in the new metaclass as the
230 "predicate". If we pass both checks, we print the attribute's label. The
231 C<< ->label >> method was defined in the new metaclass as the "reader".
233 # otherwise print the name
238 Another good, defensive coding practice: Provide reasonable defaults.
240 # print the attribute's value
241 my $reader = $meta_attribute->get_read_method;
242 print ": " . $self->$reader . "\n";
246 Here's another example of using the attribute metaclass. C<<
247 $meta_attribute->get_read_method >> returns the name of the method that can
248 invoked on the original object to read the attribute's value. C<<
249 $self->$reader >> is an example of "reflection". Instead of using the name of
250 the method, we're using a variable with the name of the method in it. Perl
251 doesn't mind. Another way to write this would be
252 C<< $self->can($reader)->() >>.
255 my $app = MyApp::Website->new(url => "http://google.com", name => "Google");
258 And finish off the example with a script to show off our newfound magic.
262 Why oh why would you want to go through all of these contortions when you can
263 just print "The site's URL" directly in the C<dump> method? For one, the DRY
264 (Don't Repeat Yourself) principle. If you have it in the C<dump> method, you'll
265 probably also have it in the C<as_form> method, and C<to_file>, and so on. So
266 why not have a method that maps attribute names to labels? That could work, but
267 why not include the label where it belongs, in the attribute's definition?
268 That way you're also less likely to forget to add the label.
270 More importantly, this was a very simple example. Your metaclasses aren't
271 limited to just adding new meta-attributes. For example, you could implement
272 a metaclass that expires attributes after a certain amount of time.
275 metaclass => 'Expiry',
276 expires_after => '1 hour',
277 refresh_with => sub { my $self = shift; get($self->url) },
285 Shawn M Moore E<lt>sartak@gmail.comE<gt>
287 =head1 COPYRIGHT AND LICENSE
289 Copyright 2006, 2007 by Infinity Interactive, Inc.
291 L<http://www.iinteractive.com>
293 This library is free software; you can redistribute it and/or modify
294 it under the same terms as Perl itself.