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, $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_instance = $self->associated_class->get_meta_instance;
138 my $attr_name = $self->name;
140 $meta_instance->set_slot_value($_[0], $attr_name, $_[1]) if scalar(@_) == 2;
141 $meta_instance->get_slot_value($_[0], $attr_name);
145 sub generate_reader_method {
147 my $meta_instance = $self->associated_class->get_meta_instance;
148 my $attr_name = $self->name;
150 confess "Cannot assign a value to a read-only accessor" if @_ > 1;
151 $meta_instance->get_slot_value($_[0], $attr_name);
155 sub generate_writer_method {
157 my $meta_instance = $self->associated_class->get_meta_instance;
158 my $attr_name = $self->name;
160 $meta_instance->set_slot_value($_[0], $attr_name, $_[1]);
164 sub generate_predicate_method {
166 my $meta_instance = $self->associated_class->get_meta_instance;
167 my $attr_name = $self->name;
169 defined $meta_instance->get_slot_value($_[0], $attr_name) ? 1 : 0;
173 sub process_accessors {
174 my ($self, $type, $accessor) = @_;
175 if (reftype($accessor)) {
176 (reftype($accessor) eq 'HASH')
177 || confess "bad accessor/reader/writer/predicate format, must be a HASH ref";
178 my ($name, $method) = each %{$accessor};
179 return ($name, Class::MOP::Attribute::Accessor->wrap($method));
182 my $generator = $self->can('generate_' . $type . '_method');
184 || confess "There is no method generator for the type='$type'";
185 if (my $method = $self->$generator($self->name)) {
186 return ($accessor => Class::MOP::Attribute::Accessor->wrap($method));
188 confess "Could not create the '$type' method for " . $self->name . " because : $@";
192 sub install_accessors {
194 my $class = $self->associated_class;
197 $self->process_accessors('accessor' => $self->accessor())
198 ) if $self->has_accessor();
201 $self->process_accessors('reader' => $self->reader())
202 ) if $self->has_reader();
205 $self->process_accessors('writer' => $self->writer())
206 ) if $self->has_writer();
209 $self->process_accessors('predicate' => $self->predicate())
210 ) if $self->has_predicate();
215 my $_remove_accessor = sub {
216 my ($accessor, $class) = @_;
217 if (reftype($accessor) && reftype($accessor) eq 'HASH') {
218 ($accessor) = keys %{$accessor};
220 my $method = $class->get_method($accessor);
221 $class->remove_method($accessor)
222 if (blessed($method) && $method->isa('Class::MOP::Attribute::Accessor'));
225 sub remove_accessors {
227 $_remove_accessor->($self->accessor(), $self->associated_class()) if $self->has_accessor();
228 $_remove_accessor->($self->reader(), $self->associated_class()) if $self->has_reader();
229 $_remove_accessor->($self->writer(), $self->associated_class()) if $self->has_writer();
230 $_remove_accessor->($self->predicate(), $self->associated_class()) if $self->has_predicate();
236 package Class::MOP::Attribute::Accessor;
241 use Class::MOP::Method;
243 our $VERSION = '0.01';
245 our @ISA = ('Class::MOP::Method');
255 Class::MOP::Attribute - Attribute Meta Object
259 Class::MOP::Attribute->new('$foo' => (
260 accessor => 'foo', # dual purpose get/set accessor
261 predicate => 'has_foo' # predicate check for defined-ness
262 init_arg => '-foo', # class->new will look for a -foo key
263 default => 'BAR IS BAZ!' # if no -foo key is provided, use this
266 Class::MOP::Attribute->new('$.bar' => (
267 reader => 'bar', # getter
268 writer => 'set_bar', # setter
269 predicate => 'has_bar' # predicate check for defined-ness
270 init_arg => ':bar', # class->new will look for a :bar key
271 # no default value means it is undef
276 The Attribute Protocol is almost entirely an invention of this module,
277 and is completely optional to this MOP. This is because Perl 5 does not
278 have consistent notion of what is an attribute of a class. There are
279 so many ways in which this is done, and very few (if any) are
280 easily discoverable by this module.
282 So, all that said, this module attempts to inject some order into this
283 chaos, by introducing a consistent API which can be used to create
292 =item B<new ($name, ?%options)>
294 An attribute must (at the very least), have a C<$name>. All other
295 C<%options> are contained added as key-value pairs. Acceptable keys
302 This should be a string value representing the expected key in
303 an initialization hash. For instance, if we have an I<init_arg>
304 value of C<-foo>, then the following code will Just Work.
306 MyClass->meta->construct_instance(-foo => "Hello There");
308 In an init_arg is not assigned, it will automatically use the
313 The value of this key is the default value which
314 C<Class::MOP::Class::construct_instance> will initialize the
318 If the value is a simple scalar (string or number), then it can
319 be just passed as is. However, if you wish to initialize it with
320 a HASH or ARRAY ref, then you need to wrap that inside a CODE
323 Class::MOP::Attribute->new('@foo' => (
324 default => sub { [] },
329 Class::MOP::Attribute->new('%foo' => (
330 default => sub { {} },
333 If you wish to initialize an attribute with a CODE reference
334 itself, then you need to wrap that in a subroutine as well, like
337 Class::MOP::Attribute->new('&foo' => (
338 default => sub { sub { print "Hello World" } },
341 And lastly, if the value of your attribute is dependent upon
342 some other aspect of the instance structure, then you can take
343 advantage of the fact that when the I<default> value is a CODE
344 reference, it is passed the raw (unblessed) instance structure
345 as it's only argument. So you can do things like this:
347 Class::MOP::Attribute->new('$object_identity' => (
348 default => sub { Scalar::Util::refaddr($_[0]) },
351 This last feature is fairly limited as there is no gurantee of
352 the order of attribute initializations, so you cannot perform
353 any kind of dependent initializations. However, if this is
354 something you need, you could subclass B<Class::MOP::Class> and
355 this class to acheive it. However, this is currently left as
356 an exercise to the reader :).
360 The I<accessor>, I<reader>, I<writer> and I<predicate> keys can
361 contain either; the name of the method and an appropriate default
362 one will be generated for you, B<or> a HASH ref containing exactly one
363 key (which will be used as the name of the method) and one value,
364 which should contain a CODE reference which will be installed as
371 The I<accessor> is a standard perl-style read/write accessor. It will
372 return the value of the attribute, and if a value is passed as an argument,
373 it will assign that value to the attribute.
376 This method will properly handle the following code, by assigning an
377 C<undef> value to the attribute.
379 $object->set_something(undef);
383 This is a basic read-only accessor, it will just return the value of
388 This is a basic write accessor, it accepts a single argument, and
389 assigns that value to the attribute. This method does not intentially
390 return a value, however perl will return the result of the last
391 expression in the subroutine, which returns in this returning the
392 same value that it was passed.
395 This method will properly handle the following code, by assigning an
396 C<undef> value to the attribute.
398 $object->set_something();
402 This is a basic test to see if the value of the attribute is not
403 C<undef>. It will return true (C<1>) if the attribute's value is
404 defined, and false (C<0>) otherwise.
408 =item B<clone (%options)>
410 =item B<initialize_instance_slot ($instance, $params)>
416 These are all basic read-only value accessors for the values
417 passed into C<new>. I think they are pretty much self-explanitory.
433 =item B<default (?$instance)>
435 As noted in the documentation for C<new> above, if the I<default>
436 value is a CODE reference, this accessor will pass a single additional
437 argument C<$instance> into it and return the value.
441 Returns a list of slots required by the attribute. This is usually
442 just one, which is the name of the attribute.
446 =head2 Informational predicates
448 These are all basic predicate methods for the values passed into C<new>.
452 =item B<has_accessor>
458 =item B<has_predicate>
460 =item B<has_init_arg>
466 =head2 Class association
470 =item B<associated_class>
472 =item B<attach_to_class ($class)>
474 =item B<detach_from_class>
478 =item B<allocate_slots>
480 =item B<deallocate_slots>
484 =head2 Attribute Accessor generation
488 =item B<install_accessors>
490 This allows the attribute to generate and install code for it's own
491 I<accessor/reader/writer/predicate> methods. This is called by
492 C<Class::MOP::Class::add_attribute>.
494 This method will call C<process_accessors> for each of the possible
495 method types (accessor, reader, writer & predicate).
497 =item B<process_accessors ($type, $value)>
499 This takes a C<$type> (accessor, reader, writer or predicate), and
500 a C<$value> (the value passed into the constructor for each of the
501 different types). It will then either generate the method itself
502 (using the C<generate_*_method> methods listed below) or it will
503 use the custom method passed through the constructor.
507 =item B<generate_accessor_method>
509 =item B<generate_predicate_method>
511 =item B<generate_reader_method>
513 =item B<generate_writer_method>
517 =item B<remove_accessors>
519 This allows the attribute to remove the method for it's own
520 I<accessor/reader/writer/predicate>. This is called by
521 C<Class::MOP::Class::remove_attribute>.
531 This will return a B<Class::MOP::Class> instance which is related
534 It should also be noted that B<Class::MOP> will actually bootstrap
535 this module by installing a number of attribute meta-objects into
536 it's metaclass. This will allow this class to reap all the benifits
537 of the MOP when subclassing it.
543 Stevan Little E<lt>stevan@iinteractive.comE<gt>
545 =head1 COPYRIGHT AND LICENSE
547 Copyright 2006 by Infinity Interactive, Inc.
549 L<http://www.iinteractive.com>
551 This library is free software; you can redistribute it and/or modify
552 it under the same terms as Perl itself.