2 package Class::MOP::Attribute;
8 use Scalar::Util 'blessed', 'reftype', 'weaken';
10 our $VERSION = '0.08';
13 require Class::MOP::Class;
14 Class::MOP::Class->initialize(blessed($_[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($self);
63 sub initialize_instance_slot {
64 my ($self, $meta_instance, $instance, $params) = @_;
65 my $init_arg = $self->{init_arg};
66 # try to fetch the init arg from the %params ...
68 $val = $params->{$init_arg} if exists $params->{$init_arg};
69 # if nothing was in the %params, we can use the
70 # attribute's default value (if it has one)
71 if (!defined $val && defined $self->{default}) {
72 $val = $self->default($instance);
74 $self->associated_class
76 ->set_slot_value($instance, $self->name, $val);
80 # the next bunch of methods will get bootstrapped
81 # away in the Class::MOP bootstrapping section
83 sub name { $_[0]->{name} }
85 sub associated_class { $_[0]->{associated_class} }
87 sub has_accessor { defined($_[0]->{accessor}) ? 1 : 0 }
88 sub has_reader { defined($_[0]->{reader}) ? 1 : 0 }
89 sub has_writer { defined($_[0]->{writer}) ? 1 : 0 }
90 sub has_predicate { defined($_[0]->{predicate}) ? 1 : 0 }
91 sub has_init_arg { defined($_[0]->{init_arg}) ? 1 : 0 }
92 sub has_default { defined($_[0]->{default}) ? 1 : 0 }
94 sub accessor { $_[0]->{accessor} }
95 sub reader { $_[0]->{reader} }
96 sub writer { $_[0]->{writer} }
97 sub predicate { $_[0]->{predicate} }
98 sub init_arg { $_[0]->{init_arg} }
100 # end bootstrapped away method section.
101 # (all methods below here are kept intact)
105 if (reftype($self->{default}) && reftype($self->{default}) eq 'CODE') {
106 # if the default is a CODE ref, then
107 # we pass in the instance and default
108 # can return a value based on that
109 # instance. Somewhat crude, but works.
110 return $self->{default}->(shift);
117 sub slots { (shift)->name }
121 sub attach_to_class {
122 my ($self, $class) = @_;
123 (blessed($class) && $class->isa('Class::MOP::Class'))
124 || confess "You must pass a Class::MOP::Class instance (or a subclass)";
125 weaken($self->{associated_class} = $class);
128 sub detach_from_class {
130 $self->{associated_class} = undef;
133 ## Method generation helpers
135 sub generate_accessor_method {
137 my $meta_class = $self->associated_class;
138 my $attr_name = $self->name;
140 my $meta_instance = $meta_class->get_meta_instance;
141 $meta_instance->set_slot_value($_[0], $attr_name, $_[1]) if scalar(@_) == 2;
142 $meta_instance->get_slot_value($_[0], $attr_name);
146 sub generate_reader_method {
148 my $meta_class = $self->associated_class;
149 my $attr_name = $self->name;
151 confess "Cannot assign a value to a read-only accessor" if @_ > 1;
152 $meta_class->get_meta_instance
153 ->get_slot_value($_[0], $attr_name);
157 sub generate_writer_method {
159 my $meta_class = $self->associated_class;
160 my $attr_name = $self->name;
162 $meta_class->get_meta_instance
163 ->set_slot_value($_[0], $attr_name, $_[1]);
167 sub generate_predicate_method {
169 my $meta_class = $self->associated_class;
170 my $attr_name = $self->name;
172 defined $meta_class->get_meta_instance
173 ->get_slot_value($_[0], $attr_name) ? 1 : 0;
177 sub process_accessors {
178 my ($self, $type, $accessor) = @_;
179 if (reftype($accessor)) {
180 (reftype($accessor) eq 'HASH')
181 || confess "bad accessor/reader/writer/predicate format, must be a HASH ref";
182 my ($name, $method) = each %{$accessor};
183 return ($name, Class::MOP::Attribute::Accessor->wrap($method));
186 my $generator = $self->can('generate_' . $type . '_method');
188 || confess "There is no method generator for the type='$type'";
189 if (my $method = $self->$generator($self->name)) {
190 return ($accessor => Class::MOP::Attribute::Accessor->wrap($method));
192 confess "Could not create the '$type' method for " . $self->name . " because : $@";
196 sub install_accessors {
198 my $class = $self->associated_class;
201 $self->process_accessors('accessor' => $self->accessor())
202 ) if $self->has_accessor();
205 $self->process_accessors('reader' => $self->reader())
206 ) if $self->has_reader();
209 $self->process_accessors('writer' => $self->writer())
210 ) if $self->has_writer();
213 $self->process_accessors('predicate' => $self->predicate())
214 ) if $self->has_predicate();
219 my $_remove_accessor = sub {
220 my ($accessor, $class) = @_;
221 if (reftype($accessor) && reftype($accessor) eq 'HASH') {
222 ($accessor) = keys %{$accessor};
224 my $method = $class->get_method($accessor);
225 $class->remove_method($accessor)
226 if (blessed($method) && $method->isa('Class::MOP::Attribute::Accessor'));
229 sub remove_accessors {
231 $_remove_accessor->($self->accessor(), $self->associated_class()) if $self->has_accessor();
232 $_remove_accessor->($self->reader(), $self->associated_class()) if $self->has_reader();
233 $_remove_accessor->($self->writer(), $self->associated_class()) if $self->has_writer();
234 $_remove_accessor->($self->predicate(), $self->associated_class()) if $self->has_predicate();
240 package Class::MOP::Attribute::Accessor;
245 use Class::MOP::Method;
247 our $VERSION = '0.01';
249 our @ISA = ('Class::MOP::Method');
259 Class::MOP::Attribute - Attribute Meta Object
263 Class::MOP::Attribute->new('$foo' => (
264 accessor => 'foo', # dual purpose get/set accessor
265 predicate => 'has_foo' # predicate check for defined-ness
266 init_arg => '-foo', # class->new will look for a -foo key
267 default => 'BAR IS BAZ!' # if no -foo key is provided, use this
270 Class::MOP::Attribute->new('$.bar' => (
271 reader => 'bar', # getter
272 writer => 'set_bar', # setter
273 predicate => 'has_bar' # predicate check for defined-ness
274 init_arg => ':bar', # class->new will look for a :bar key
275 # no default value means it is undef
280 The Attribute Protocol is almost entirely an invention of this module,
281 and is completely optional to this MOP. This is because Perl 5 does not
282 have consistent notion of what is an attribute of a class. There are
283 so many ways in which this is done, and very few (if any) are
284 easily discoverable by this module.
286 So, all that said, this module attempts to inject some order into this
287 chaos, by introducing a consistent API which can be used to create
296 =item B<new ($name, ?%options)>
298 An attribute must (at the very least), have a C<$name>. All other
299 C<%options> are contained added as key-value pairs. Acceptable keys
306 This should be a string value representing the expected key in
307 an initialization hash. For instance, if we have an I<init_arg>
308 value of C<-foo>, then the following code will Just Work.
310 MyClass->meta->construct_instance(-foo => "Hello There");
312 In an init_arg is not assigned, it will automatically use the
317 The value of this key is the default value which
318 C<Class::MOP::Class::construct_instance> will initialize the
322 If the value is a simple scalar (string or number), then it can
323 be just passed as is. However, if you wish to initialize it with
324 a HASH or ARRAY ref, then you need to wrap that inside a CODE
327 Class::MOP::Attribute->new('@foo' => (
328 default => sub { [] },
333 Class::MOP::Attribute->new('%foo' => (
334 default => sub { {} },
337 If you wish to initialize an attribute with a CODE reference
338 itself, then you need to wrap that in a subroutine as well, like
341 Class::MOP::Attribute->new('&foo' => (
342 default => sub { sub { print "Hello World" } },
345 And lastly, if the value of your attribute is dependent upon
346 some other aspect of the instance structure, then you can take
347 advantage of the fact that when the I<default> value is a CODE
348 reference, it is passed the raw (unblessed) instance structure
349 as it's only argument. So you can do things like this:
351 Class::MOP::Attribute->new('$object_identity' => (
352 default => sub { Scalar::Util::refaddr($_[0]) },
355 This last feature is fairly limited as there is no gurantee of
356 the order of attribute initializations, so you cannot perform
357 any kind of dependent initializations. However, if this is
358 something you need, you could subclass B<Class::MOP::Class> and
359 this class to acheive it. However, this is currently left as
360 an exercise to the reader :).
364 The I<accessor>, I<reader>, I<writer> and I<predicate> keys can
365 contain either; the name of the method and an appropriate default
366 one will be generated for you, B<or> a HASH ref containing exactly one
367 key (which will be used as the name of the method) and one value,
368 which should contain a CODE reference which will be installed as
375 The I<accessor> is a standard perl-style read/write accessor. It will
376 return the value of the attribute, and if a value is passed as an argument,
377 it will assign that value to the attribute.
380 This method will properly handle the following code, by assigning an
381 C<undef> value to the attribute.
383 $object->set_something(undef);
387 This is a basic read-only accessor, it will just return the value of
392 This is a basic write accessor, it accepts a single argument, and
393 assigns that value to the attribute. This method does not intentially
394 return a value, however perl will return the result of the last
395 expression in the subroutine, which returns in this returning the
396 same value that it was passed.
399 This method will properly handle the following code, by assigning an
400 C<undef> value to the attribute.
402 $object->set_something();
406 This is a basic test to see if the value of the attribute is not
407 C<undef>. It will return true (C<1>) if the attribute's value is
408 defined, and false (C<0>) otherwise.
412 =item B<clone (%options)>
414 =item B<initialize_instance_slot ($instance, $params)>
420 These are all basic read-only value accessors for the values
421 passed into C<new>. I think they are pretty much self-explanitory.
437 =item B<default (?$instance)>
439 As noted in the documentation for C<new> above, if the I<default>
440 value is a CODE reference, this accessor will pass a single additional
441 argument C<$instance> into it and return the value.
445 Returns a list of slots required by the attribute. This is usually
446 just one, which is the name of the attribute.
450 =head2 Informational predicates
452 These are all basic predicate methods for the values passed into C<new>.
456 =item B<has_accessor>
462 =item B<has_predicate>
464 =item B<has_init_arg>
470 =head2 Class association
474 =item B<associated_class>
476 =item B<attach_to_class ($class)>
478 =item B<detach_from_class>
482 =item B<allocate_slots>
484 =item B<deallocate_slots>
488 =head2 Attribute Accessor generation
492 =item B<install_accessors>
494 This allows the attribute to generate and install code for it's own
495 I<accessor/reader/writer/predicate> methods. This is called by
496 C<Class::MOP::Class::add_attribute>.
498 This method will call C<process_accessors> for each of the possible
499 method types (accessor, reader, writer & predicate).
501 =item B<process_accessors ($type, $value)>
503 This takes a C<$type> (accessor, reader, writer or predicate), and
504 a C<$value> (the value passed into the constructor for each of the
505 different types). It will then either generate the method itself
506 (using the C<generate_*_method> methods listed below) or it will
507 use the custom method passed through the constructor.
511 =item B<generate_accessor_method>
513 =item B<generate_predicate_method>
515 =item B<generate_reader_method>
517 =item B<generate_writer_method>
521 =item B<remove_accessors>
523 This allows the attribute to remove the method for it's own
524 I<accessor/reader/writer/predicate>. This is called by
525 C<Class::MOP::Class::remove_attribute>.
535 This will return a B<Class::MOP::Class> instance which is related
538 It should also be noted that B<Class::MOP> will actually bootstrap
539 this module by installing a number of attribute meta-objects into
540 it's metaclass. This will allow this class to reap all the benifits
541 of the MOP when subclassing it.
547 Stevan Little E<lt>stevan@iinteractive.comE<gt>
549 =head1 COPYRIGHT AND LICENSE
551 Copyright 2006 by Infinity Interactive, Inc.
553 L<http://www.iinteractive.com>
555 This library is free software; you can redistribute it and/or modify
556 it under the same terms as Perl itself.