2 package Class::MOP::Attribute;
8 use Scalar::Util 'blessed', 'reftype', 'weaken';
10 our $VERSION = '0.06';
13 require Class::MOP::Class;
14 Class::MOP::Class->initialize(blessed($_[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 && $self->has_default) {
72 $val = $self->default($instance);
74 $instance->{$self->name} = $val;
78 # the next bunch of methods will get bootstrapped
79 # away in the Class::MOP bootstrapping section
81 sub name { $_[0]->{name} }
83 sub associated_class { $_[0]->{associated_class} }
85 sub has_accessor { defined($_[0]->{accessor}) ? 1 : 0 }
86 sub has_reader { defined($_[0]->{reader}) ? 1 : 0 }
87 sub has_writer { defined($_[0]->{writer}) ? 1 : 0 }
88 sub has_predicate { defined($_[0]->{predicate}) ? 1 : 0 }
89 sub has_init_arg { defined($_[0]->{init_arg}) ? 1 : 0 }
90 sub has_default { defined($_[0]->{default}) ? 1 : 0 }
92 sub accessor { $_[0]->{accessor} }
93 sub reader { $_[0]->{reader} }
94 sub writer { $_[0]->{writer} }
95 sub predicate { $_[0]->{predicate} }
96 sub init_arg { $_[0]->{init_arg} }
98 # end bootstrapped away method section.
99 # (all methods below here are kept intact)
103 if (reftype($self->{default}) && reftype($self->{default}) eq 'CODE') {
104 # if the default is a CODE ref, then
105 # we pass in the instance and default
106 # can return a value based on that
107 # instance. Somewhat crude, but works.
108 return $self->{default}->(shift);
115 sub attach_to_class {
116 my ($self, $class) = @_;
117 (blessed($class) && $class->isa('Class::MOP::Class'))
118 || confess "You must pass a Class::MOP::Class instance (or a subclass)";
119 weaken($self->{associated_class} = $class);
122 sub detach_from_class {
124 $self->{associated_class} = undef;
127 ## Method generation helpers
129 sub generate_accessor_method {
130 my ($self, $attr_name) = @_;
132 $_[0]->{$attr_name} = $_[1] if scalar(@_) == 2;
137 sub generate_reader_method {
138 my ($self, $attr_name) = @_;
140 confess "Cannot assign a value to a read-only accessor" if @_ > 1;
145 sub generate_writer_method {
146 my ($self, $attr_name) = @_;
147 sub { $_[0]->{$attr_name} = $_[1] };
150 sub generate_predicate_method {
151 my ($self, $attr_name) = @_;
152 sub { defined $_[0]->{$attr_name} ? 1 : 0 };
155 sub process_accessors {
156 my ($self, $type, $accessor) = @_;
157 if (reftype($accessor)) {
158 (reftype($accessor) eq 'HASH')
159 || confess "bad accessor/reader/writer/predicate format, must be a HASH ref";
160 my ($name, $method) = each %{$accessor};
161 return ($name, Class::MOP::Attribute::Accessor->wrap($method));
164 my $generator = $self->can('generate_' . $type . '_method');
166 || confess "There is no method generator for the type='$type'";
167 if (my $method = $self->$generator($self->name)) {
168 return ($accessor => Class::MOP::Attribute::Accessor->wrap($method));
170 confess "Could not create the '$type' method for " . $self->name . " because : $@";
174 sub install_accessors {
176 my $class = $self->associated_class;
179 $self->process_accessors('accessor' => $self->accessor())
180 ) if $self->has_accessor();
183 $self->process_accessors('reader' => $self->reader())
184 ) if $self->has_reader();
187 $self->process_accessors('writer' => $self->writer())
188 ) if $self->has_writer();
191 $self->process_accessors('predicate' => $self->predicate())
192 ) if $self->has_predicate();
197 my $_remove_accessor = sub {
198 my ($accessor, $class) = @_;
199 if (reftype($accessor) && reftype($accessor) eq 'HASH') {
200 ($accessor) = keys %{$accessor};
202 my $method = $class->get_method($accessor);
203 $class->remove_method($accessor)
204 if (blessed($method) && $method->isa('Class::MOP::Attribute::Accessor'));
207 sub remove_accessors {
209 $_remove_accessor->($self->accessor(), $self->associated_class()) if $self->has_accessor();
210 $_remove_accessor->($self->reader(), $self->associated_class()) if $self->has_reader();
211 $_remove_accessor->($self->writer(), $self->associated_class()) if $self->has_writer();
212 $_remove_accessor->($self->predicate(), $self->associated_class()) if $self->has_predicate();
218 package Class::MOP::Attribute::Accessor;
223 use Class::MOP::Method;
225 our $VERSION = '0.01';
227 our @ISA = ('Class::MOP::Method');
237 Class::MOP::Attribute - Attribute Meta Object
241 Class::MOP::Attribute->new('$foo' => (
242 accessor => 'foo', # dual purpose get/set accessor
243 predicate => 'has_foo' # predicate check for defined-ness
244 init_arg => '-foo', # class->new will look for a -foo key
245 default => 'BAR IS BAZ!' # if no -foo key is provided, use this
248 Class::MOP::Attribute->new('$.bar' => (
249 reader => 'bar', # getter
250 writer => 'set_bar', # setter
251 predicate => 'has_bar' # predicate check for defined-ness
252 init_arg => ':bar', # class->new will look for a :bar key
253 # no default value means it is undef
258 The Attribute Protocol is almost entirely an invention of this module,
259 and is completely optional to this MOP. This is because Perl 5 does not
260 have consistent notion of what is an attribute of a class. There are
261 so many ways in which this is done, and very few (if any) are
262 easily discoverable by this module.
264 So, all that said, this module attempts to inject some order into this
265 chaos, by introducing a consistent API which can be used to create
274 =item B<new ($name, ?%options)>
276 An attribute must (at the very least), have a C<$name>. All other
277 C<%options> are contained added as key-value pairs. Acceptable keys
284 This should be a string value representing the expected key in
285 an initialization hash. For instance, if we have an I<init_arg>
286 value of C<-foo>, then the following code will Just Work.
288 MyClass->meta->construct_instance(-foo => "Hello There");
290 In an init_arg is not assigned, it will automatically use the
295 The value of this key is the default value which
296 C<Class::MOP::Class::construct_instance> will initialize the
300 If the value is a simple scalar (string or number), then it can
301 be just passed as is. However, if you wish to initialize it with
302 a HASH or ARRAY ref, then you need to wrap that inside a CODE
305 Class::MOP::Attribute->new('@foo' => (
306 default => sub { [] },
311 Class::MOP::Attribute->new('%foo' => (
312 default => sub { {} },
315 If you wish to initialize an attribute with a CODE reference
316 itself, then you need to wrap that in a subroutine as well, like
319 Class::MOP::Attribute->new('&foo' => (
320 default => sub { sub { print "Hello World" } },
323 And lastly, if the value of your attribute is dependent upon
324 some other aspect of the instance structure, then you can take
325 advantage of the fact that when the I<default> value is a CODE
326 reference, it is passed the raw (unblessed) instance structure
327 as it's only argument. So you can do things like this:
329 Class::MOP::Attribute->new('$object_identity' => (
330 default => sub { Scalar::Util::refaddr($_[0]) },
333 This last feature is fairly limited as there is no gurantee of
334 the order of attribute initializations, so you cannot perform
335 any kind of dependent initializations. However, if this is
336 something you need, you could subclass B<Class::MOP::Class> and
337 this class to acheive it. However, this is currently left as
338 an exercise to the reader :).
342 The I<accessor>, I<reader>, I<writer> and I<predicate> keys can
343 contain either; the name of the method and an appropriate default
344 one will be generated for you, B<or> a HASH ref containing exactly one
345 key (which will be used as the name of the method) and one value,
346 which should contain a CODE reference which will be installed as
353 The I<accessor> is a standard perl-style read/write accessor. It will
354 return the value of the attribute, and if a value is passed as an argument,
355 it will assign that value to the attribute.
358 This method will properly handle the following code, by assigning an
359 C<undef> value to the attribute.
361 $object->set_something(undef);
365 This is a basic read-only accessor, it will just return the value of
370 This is a basic write accessor, it accepts a single argument, and
371 assigns that value to the attribute. This method does not intentially
372 return a value, however perl will return the result of the last
373 expression in the subroutine, which returns in this returning the
374 same value that it was passed.
377 This method will properly handle the following code, by assigning an
378 C<undef> value to the attribute.
380 $object->set_something();
384 This is a basic test to see if the value of the attribute is not
385 C<undef>. It will return true (C<1>) if the attribute's value is
386 defined, and false (C<0>) otherwise.
390 =item B<clone (%options)>
392 =item B<initialize_instance_slot ($instance, $params)>
398 These are all basic read-only value accessors for the values
399 passed into C<new>. I think they are pretty much self-explanitory.
415 =item B<default (?$instance)>
417 As noted in the documentation for C<new> above, if the I<default>
418 value is a CODE reference, this accessor will pass a single additional
419 argument C<$instance> into it and return the value.
423 =head2 Informational predicates
425 These are all basic predicate methods for the values passed into C<new>.
429 =item B<has_accessor>
435 =item B<has_predicate>
437 =item B<has_init_arg>
443 =head2 Class association
447 =item B<associated_class>
449 =item B<attach_to_class ($class)>
451 =item B<detach_from_class>
455 =head2 Attribute Accessor generation
459 =item B<install_accessors>
461 This allows the attribute to generate and install code for it's own
462 I<accessor/reader/writer/predicate> methods. This is called by
463 C<Class::MOP::Class::add_attribute>.
465 This method will call C<process_accessors> for each of the possible
466 method types (accessor, reader, writer & predicate).
468 =item B<process_accessors ($type, $value)>
470 This takes a C<$type> (accessor, reader, writer or predicate), and
471 a C<$value> (the value passed into the constructor for each of the
472 different types). It will then either generate the method itself
473 (using the C<generate_*_method> methods listed below) or it will
474 use the custom method passed through the constructor.
478 =item B<generate_accessor_method ($attr_name)>
480 =item B<generate_predicate_method ($attr_name)>
482 =item B<generate_reader_method ($attr_name)>
484 =item B<generate_writer_method ($attr_name)>
488 =item B<remove_accessors>
490 This allows the attribute to remove the method for it's own
491 I<accessor/reader/writer/predicate>. This is called by
492 C<Class::MOP::Class::remove_attribute>.
502 This will return a B<Class::MOP::Class> instance which is related
505 It should also be noted that B<Class::MOP> will actually bootstrap
506 this module by installing a number of attribute meta-objects into
507 it's metaclass. This will allow this class to reap all the benifits
508 of the MOP when subclassing it.
514 Stevan Little E<lt>stevan@iinteractive.comE<gt>
516 =head1 COPYRIGHT AND LICENSE
518 Copyright 2006 by Infinity Interactive, Inc.
520 L<http://www.iinteractive.com>
522 This library is free software; you can redistribute it and/or modify
523 it under the same terms as Perl itself.