2 package Class::MOP::Attribute;
8 use Scalar::Util 'blessed', 'reftype', 'weaken';
10 our $VERSION = '0.05';
13 require Class::MOP::Class;
14 Class::MOP::Class->initialize(blessed($_[0]) || $_[0]);
17 # NOTE: (meta-circularity)
18 # This method will be replaces in the
19 # boostrap section of Class::MOP, by
20 # a new version which uses the
21 # &Class::MOP::Class::construct_instance
22 # method to build an attribute meta-object
23 # which itself is described with attribute
25 # - Ain't meta-circularity grand? :)
31 (defined $name && $name)
32 || confess "You must provide a name for the attribute";
33 $options{init_arg} = $name
34 if not exists $options{init_arg};
38 accessor => $options{accessor},
39 reader => $options{reader},
40 writer => $options{writer},
41 predicate => $options{predicate},
42 init_arg => $options{init_arg},
43 default => $options{default},
44 # keep a weakened link to the
45 # class we are associated with
46 associated_class => undef,
51 # this is a primative (and kludgy) clone operation
52 # for now, it will be repleace in the Class::MOP
53 # bootstrap with a proper one, however we know
54 # that this one will work fine for now.
59 || confess "Can only clone an instance";
60 return bless { %{$self}, %options } => blessed($self);
64 # the next bunch of methods will get bootstrapped
65 # away in the Class::MOP bootstrapping section
67 sub name { $_[0]->{name} }
69 sub associated_class { $_[0]->{associated_class} }
71 sub has_accessor { defined($_[0]->{accessor}) ? 1 : 0 }
72 sub has_reader { defined($_[0]->{reader}) ? 1 : 0 }
73 sub has_writer { defined($_[0]->{writer}) ? 1 : 0 }
74 sub has_predicate { defined($_[0]->{predicate}) ? 1 : 0 }
75 sub has_init_arg { defined($_[0]->{init_arg}) ? 1 : 0 }
76 sub has_default { defined($_[0]->{default}) ? 1 : 0 }
78 sub accessor { $_[0]->{accessor} }
79 sub reader { $_[0]->{reader} }
80 sub writer { $_[0]->{writer} }
81 sub predicate { $_[0]->{predicate} }
82 sub init_arg { $_[0]->{init_arg} }
84 # end bootstrapped away method section.
85 # (all methods below here are kept intact)
89 if (reftype($self->{default}) && reftype($self->{default}) eq 'CODE') {
90 # if the default is a CODE ref, then
91 # we pass in the instance and default
92 # can return a value based on that
93 # instance. Somewhat crude, but works.
94 return $self->{default}->(shift);
101 sub attach_to_class {
102 my ($self, $class) = @_;
103 (blessed($class) && $class->isa('Class::MOP::Class'))
104 || confess "You must pass a Class::MOP::Class instance (or a subclass)";
105 weaken($self->{associated_class} = $class);
108 sub detach_from_class {
110 $self->{associated_class} = undef;
113 ## Method generation helpers
115 sub generate_accessor_method {
116 my ($self, $attr_name) = @_;
118 $_[0]->{$attr_name} = $_[1] if scalar(@_) == 2;
123 sub generate_reader_method {
124 my ($self, $attr_name) = @_;
126 confess "Cannot assign a value to a read-only accessor" if @_ > 1;
131 sub generate_writer_method {
132 my ($self, $attr_name) = @_;
133 sub { $_[0]->{$attr_name} = $_[1] };
136 sub generate_predicate_method {
137 my ($self, $attr_name) = @_;
138 sub { defined $_[0]->{$attr_name} ? 1 : 0 };
141 sub process_accessors {
142 my ($self, $type, $accessor) = @_;
143 if (reftype($accessor)) {
144 (reftype($accessor) eq 'HASH')
145 || confess "bad accessor/reader/writer/predicate format, must be a HASH ref";
146 my ($name, $method) = each %{$accessor};
147 return ($name, Class::MOP::Attribute::Accessor->wrap($method));
150 my $generator = $self->can('generate_' . $type . '_method');
152 || confess "There is no method generator for the type='$type'";
153 if (my $method = $self->$generator($self->name)) {
154 return ($accessor => Class::MOP::Attribute::Accessor->wrap($method));
156 confess "Could not create the '$type' method for " . $self->name . " because : $@";
160 sub install_accessors {
162 my $class = $self->associated_class;
165 $self->process_accessors('accessor' => $self->accessor())
166 ) if $self->has_accessor();
169 $self->process_accessors('reader' => $self->reader())
170 ) if $self->has_reader();
173 $self->process_accessors('writer' => $self->writer())
174 ) if $self->has_writer();
177 $self->process_accessors('predicate' => $self->predicate())
178 ) if $self->has_predicate();
183 my $_remove_accessor = sub {
184 my ($accessor, $class) = @_;
185 if (reftype($accessor) && reftype($accessor) eq 'HASH') {
186 ($accessor) = keys %{$accessor};
188 my $method = $class->get_method($accessor);
189 $class->remove_method($accessor)
190 if (blessed($method) && $method->isa('Class::MOP::Attribute::Accessor'));
193 sub remove_accessors {
195 $_remove_accessor->($self->accessor(), $self->associated_class()) if $self->has_accessor();
196 $_remove_accessor->($self->reader(), $self->associated_class()) if $self->has_reader();
197 $_remove_accessor->($self->writer(), $self->associated_class()) if $self->has_writer();
198 $_remove_accessor->($self->predicate(), $self->associated_class()) if $self->has_predicate();
204 package Class::MOP::Attribute::Accessor;
209 use Class::MOP::Method;
211 our $VERSION = '0.01';
213 our @ISA = ('Class::MOP::Method');
223 Class::MOP::Attribute - Attribute Meta Object
227 Class::MOP::Attribute->new('$foo' => (
228 accessor => 'foo', # dual purpose get/set accessor
229 predicate => 'has_foo' # predicate check for defined-ness
230 init_arg => '-foo', # class->new will look for a -foo key
231 default => 'BAR IS BAZ!' # if no -foo key is provided, use this
234 Class::MOP::Attribute->new('$.bar' => (
235 reader => 'bar', # getter
236 writer => 'set_bar', # setter
237 predicate => 'has_bar' # predicate check for defined-ness
238 init_arg => ':bar', # class->new will look for a :bar key
239 # no default value means it is undef
244 The Attribute Protocol is almost entirely an invention of this module,
245 and is completely optional to this MOP. This is because Perl 5 does not
246 have consistent notion of what is an attribute of a class. There are
247 so many ways in which this is done, and very few (if any) are
248 easily discoverable by this module.
250 So, all that said, this module attempts to inject some order into this
251 chaos, by introducing a consistent API which can be used to create
260 =item B<new ($name, ?%options)>
262 An attribute must (at the very least), have a C<$name>. All other
263 C<%options> are contained added as key-value pairs. Acceptable keys
266 =item B<clone (%options)>
272 This should be a string value representing the expected key in
273 an initialization hash. For instance, if we have an I<init_arg>
274 value of C<-foo>, then the following code will Just Work.
276 MyClass->meta->construct_instance(-foo => "Hello There");
278 In an init_arg is not assigned, it will automatically use the
283 The value of this key is the default value which
284 C<Class::MOP::Class::construct_instance> will initialize the
288 If the value is a simple scalar (string or number), then it can
289 be just passed as is. However, if you wish to initialize it with
290 a HASH or ARRAY ref, then you need to wrap that inside a CODE
293 Class::MOP::Attribute->new('@foo' => (
294 default => sub { [] },
299 Class::MOP::Attribute->new('%foo' => (
300 default => sub { {} },
303 If you wish to initialize an attribute with a CODE reference
304 itself, then you need to wrap that in a subroutine as well, like
307 Class::MOP::Attribute->new('&foo' => (
308 default => sub { sub { print "Hello World" } },
311 And lastly, if the value of your attribute is dependent upon
312 some other aspect of the instance structure, then you can take
313 advantage of the fact that when the I<default> value is a CODE
314 reference, it is passed the raw (unblessed) instance structure
315 as it's only argument. So you can do things like this:
317 Class::MOP::Attribute->new('$object_identity' => (
318 default => sub { Scalar::Util::refaddr($_[0]) },
321 This last feature is fairly limited as there is no gurantee of
322 the order of attribute initializations, so you cannot perform
323 any kind of dependent initializations. However, if this is
324 something you need, you could subclass B<Class::MOP::Class> and
325 this class to acheive it. However, this is currently left as
326 an exercise to the reader :).
330 The I<accessor>, I<reader>, I<writer> and I<predicate> keys can
331 contain either; the name of the method and an appropriate default
332 one will be generated for you, B<or> a HASH ref containing exactly one
333 key (which will be used as the name of the method) and one value,
334 which should contain a CODE reference which will be installed as
341 The I<accessor> is a standard perl-style read/write accessor. It will
342 return the value of the attribute, and if a value is passed as an argument,
343 it will assign that value to the attribute.
346 This method will properly handle the following code, by assigning an
347 C<undef> value to the attribute.
349 $object->set_something(undef);
353 This is a basic read-only accessor, it will just return the value of
358 This is a basic write accessor, it accepts a single argument, and
359 assigns that value to the attribute. This method does not intentially
360 return a value, however perl will return the result of the last
361 expression in the subroutine, which returns in this returning the
362 same value that it was passed.
365 This method will properly handle the following code, by assigning an
366 C<undef> value to the attribute.
368 $object->set_something();
372 This is a basic test to see if the value of the attribute is not
373 C<undef>. It will return true (C<1>) if the attribute's value is
374 defined, and false (C<0>) otherwise.
382 These are all basic read-only value accessors for the values
383 passed into C<new>. I think they are pretty much self-explanitory.
399 =item B<default (?$instance)>
401 As noted in the documentation for C<new> above, if the I<default>
402 value is a CODE reference, this accessor will pass a single additional
403 argument C<$instance> into it and return the value.
407 =head2 Informational predicates
409 These are all basic predicate methods for the values passed into C<new>.
413 =item B<has_accessor>
419 =item B<has_predicate>
421 =item B<has_init_arg>
427 =head2 Class association
431 =item B<associated_class>
433 =item B<attach_to_class ($class)>
435 =item B<detach_from_class>
439 =head2 Attribute Accessor generation
443 =item B<install_accessors>
445 This allows the attribute to generate and install code for it's own
446 I<accessor/reader/writer/predicate> methods. This is called by
447 C<Class::MOP::Class::add_attribute>.
449 This method will call C<process_accessors> for each of the possible
450 method types (accessor, reader, writer & predicate).
452 =item B<process_accessors ($type, $value)>
454 This takes a C<$type> (accessor, reader, writer or predicate), and
455 a C<$value> (the value passed into the constructor for each of the
456 different types). It will then either generate the method itself
457 (using the C<generate_*_method> methods listed below) or it will
458 use the custom method passed through the constructor.
462 =item B<generate_accessor_method ($attr_name)>
464 =item B<generate_predicate_method ($attr_name)>
466 =item B<generate_reader_method ($attr_name)>
468 =item B<generate_writer_method ($attr_name)>
472 =item B<remove_accessors>
474 This allows the attribute to remove the method for it's own
475 I<accessor/reader/writer/predicate>. This is called by
476 C<Class::MOP::Class::remove_attribute>.
486 This will return a B<Class::MOP::Class> instance which is related
489 It should also be noted that B<Class::MOP> will actually bootstrap
490 this module by installing a number of attribute meta-objects into
491 it's metaclass. This will allow this class to reap all the benifits
492 of the MOP when subclassing it.
498 Stevan Little E<lt>stevan@iinteractive.comE<gt>
500 =head1 COPYRIGHT AND LICENSE
502 Copyright 2006 by Infinity Interactive, Inc.
504 L<http://www.iinteractive.com>
506 This library is free software; you can redistribute it and/or modify
507 it under the same terms as Perl itself.