2 package Class::MOP::Attribute;
7 use Class::MOP::Method::Accessor;
10 use Scalar::Util 'blessed', 'weaken';
12 our $VERSION = '0.65';
13 our $AUTHORITY = 'cpan:STEVAN';
15 use base 'Class::MOP::Object';
17 # NOTE: (meta-circularity)
18 # This method will be replaced 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? :)
27 my ( $class, @args ) = @_;
29 unshift @args, "name" if @args % 2 == 1;
32 my $name = $options{name};
34 (defined $name && $name)
35 || confess "You must provide a name for the attribute";
37 $options{init_arg} = $name
38 if not exists $options{init_arg};
39 if(exists $options{builder}){
40 confess("builder must be a defined scalar value which is a method name")
41 if ref $options{builder} || !(defined $options{builder});
42 confess("Setting both default and builder is not allowed.")
43 if exists $options{default};
45 (is_default_a_coderef(\%options))
46 || confess("References are not allowed as default values, you must ".
47 "wrap the default of '$name' in a CODE reference (ex: sub { [] } and not [])")
48 if exists $options{default} && ref $options{default};
50 if( $options{required} and not( defined($options{builder}) || defined($options{init_arg}) || exists $options{default} ) ) {
51 confess("A required attribute must have either 'init_arg', 'builder', or 'default'");
54 $class->_new(%options);
58 my ( $class, %options ) = @_;
61 'name' => $options{name},
62 'accessor' => $options{accessor},
63 'reader' => $options{reader},
64 'writer' => $options{writer},
65 'predicate' => $options{predicate},
66 'clearer' => $options{clearer},
67 'builder' => $options{builder},
68 'init_arg' => $options{init_arg},
69 'default' => $options{default},
70 'initializer' => $options{initializer},
71 # keep a weakened link to the
72 # class we are associated with
73 'associated_class' => undef,
74 # and a list of the methods
75 # associated with this attr
76 'associated_methods' => [],
81 # this is a primative (and kludgy) clone operation
82 # for now, it will be replaced in the Class::MOP
83 # bootstrap with a proper one, however we know
84 # that this one will work fine for now.
89 || confess "Can only clone an instance";
90 return bless { %{$self}, %options } => ref($self);
93 sub initialize_instance_slot {
94 my ($self, $meta_instance, $instance, $params) = @_;
95 my $init_arg = $self->{'init_arg'};
97 # try to fetch the init arg from the %params ...
99 # if nothing was in the %params, we can use the
100 # attribute's default value (if it has one)
101 if(defined $init_arg and exists $params->{$init_arg}){
102 $self->_set_initial_slot_value(
105 $params->{$init_arg},
108 elsif (defined $self->{'default'}) {
109 $self->_set_initial_slot_value(
112 $self->default($instance),
115 elsif (defined( my $builder = $self->{'builder'})) {
116 if ($builder = $instance->can($builder)) {
117 $self->_set_initial_slot_value(
124 confess(ref($instance)." does not support builder method '". $self->{'builder'} ."' for attribute '" . $self->name . "'");
129 sub _set_initial_slot_value {
130 my ($self, $meta_instance, $instance, $value) = @_;
132 my $slot_name = $self->name;
134 return $meta_instance->set_slot_value($instance, $slot_name, $value)
135 unless $self->has_initializer;
138 $meta_instance->set_slot_value($instance, $slot_name, $_[0]);
141 my $initializer = $self->initializer;
143 # most things will just want to set a value, so make it first arg
144 $instance->$initializer($value, $callback, $self);
148 # the next bunch of methods will get bootstrapped
149 # away in the Class::MOP bootstrapping section
151 sub name { $_[0]->{'name'} }
153 sub associated_class { $_[0]->{'associated_class'} }
154 sub associated_methods { $_[0]->{'associated_methods'} }
156 sub has_accessor { defined($_[0]->{'accessor'}) }
157 sub has_reader { defined($_[0]->{'reader'}) }
158 sub has_writer { defined($_[0]->{'writer'}) }
159 sub has_predicate { defined($_[0]->{'predicate'}) }
160 sub has_clearer { defined($_[0]->{'clearer'}) }
161 sub has_builder { defined($_[0]->{'builder'}) }
162 sub has_init_arg { defined($_[0]->{'init_arg'}) }
163 sub has_default { defined($_[0]->{'default'}) }
164 sub has_initializer { defined($_[0]->{'initializer'}) }
166 sub accessor { $_[0]->{'accessor'} }
167 sub reader { $_[0]->{'reader'} }
168 sub writer { $_[0]->{'writer'} }
169 sub predicate { $_[0]->{'predicate'} }
170 sub clearer { $_[0]->{'clearer'} }
171 sub builder { $_[0]->{'builder'} }
172 sub init_arg { $_[0]->{'init_arg'} }
173 sub initializer { $_[0]->{'initializer'} }
175 # end bootstrapped away method section.
176 # (all methods below here are kept intact)
178 sub has_read_method { $_[0]->has_reader || $_[0]->has_accessor }
179 sub has_write_method { $_[0]->has_writer || $_[0]->has_accessor }
181 sub get_read_method {
183 my $reader = $self->reader || $self->accessor;
185 return $reader unless ref $reader;
187 my ($name) = %$reader;
191 sub get_write_method {
193 my $writer = $self->writer || $self->accessor;
195 return $writer unless ref $writer;
197 my ($name) = %$writer;
201 sub get_read_method_ref {
203 if ((my $reader = $self->get_read_method) && $self->associated_class) {
204 return $self->associated_class->get_method($reader);
207 my $code = sub { $self->get_value(@_) };
208 if (my $class = $self->associated_class) {
209 return $class->method_metaclass->wrap(
211 package_name => $class->name,
221 sub get_write_method_ref {
223 if ((my $writer = $self->get_write_method) && $self->associated_class) {
224 return $self->associated_class->get_method($writer);
227 my $code = sub { $self->set_value(@_) };
228 if (my $class = $self->associated_class) {
229 return $class->method_metaclass->wrap(
231 package_name => $class->name,
241 sub is_default_a_coderef {
242 ('CODE' eq ref($_[0]->{'default'} || $_[0]->{default}))
246 my ($self, $instance) = @_;
247 if (defined $instance && $self->is_default_a_coderef) {
248 # if the default is a CODE ref, then
249 # we pass in the instance and default
250 # can return a value based on that
251 # instance. Somewhat crude, but works.
252 return $self->{'default'}->($instance);
259 sub slots { (shift)->name }
263 sub attach_to_class {
264 my ($self, $class) = @_;
265 (blessed($class) && $class->isa('Class::MOP::Class'))
266 || confess "You must pass a Class::MOP::Class instance (or a subclass)";
267 weaken($self->{'associated_class'} = $class);
270 sub detach_from_class {
272 $self->{'associated_class'} = undef;
277 sub associate_method {
278 my ($self, $method) = @_;
279 push @{$self->{'associated_methods'}} => $method;
284 sub set_initial_value {
285 my ($self, $instance, $value) = @_;
286 $self->_set_initial_slot_value(
287 Class::MOP::Class->initialize(ref($instance))->get_meta_instance,
294 my ($self, $instance, $value) = @_;
296 Class::MOP::Class->initialize(ref($instance))
298 ->set_slot_value($instance, $self->name, $value);
302 my ($self, $instance) = @_;
304 Class::MOP::Class->initialize(ref($instance))
306 ->get_slot_value($instance, $self->name);
310 my ($self, $instance) = @_;
312 Class::MOP::Class->initialize(ref($instance))
314 ->is_slot_initialized($instance, $self->name);
318 my ($self, $instance) = @_;
320 Class::MOP::Class->initialize(ref($instance))
322 ->deinitialize_slot($instance, $self->name);
327 sub accessor_metaclass { 'Class::MOP::Method::Accessor' }
329 sub process_accessors {
330 my ($self, $type, $accessor, $generate_as_inline_methods) = @_;
331 if (ref($accessor)) {
332 (ref($accessor) eq 'HASH')
333 || confess "bad accessor/reader/writer/predicate/clearer format, must be a HASH ref";
334 my ($name, $method) = %{$accessor};
335 $method = $self->accessor_metaclass->wrap(
337 package_name => $self->associated_class->name,
340 $self->associate_method($method);
341 return ($name, $method);
344 my $inline_me = ($generate_as_inline_methods && $self->associated_class->instance_metaclass->is_inlinable);
347 $method = $self->accessor_metaclass->new(
349 is_inline => $inline_me,
350 accessor_type => $type,
351 package_name => $self->associated_class->name,
355 confess "Could not create the '$type' method for " . $self->name . " because : $@" if $@;
356 $self->associate_method($method);
357 return ($accessor, $method);
361 sub install_accessors {
364 my $class = $self->associated_class;
367 $self->process_accessors('accessor' => $self->accessor(), $inline)
368 ) if $self->has_accessor();
371 $self->process_accessors('reader' => $self->reader(), $inline)
372 ) if $self->has_reader();
375 $self->process_accessors('writer' => $self->writer(), $inline)
376 ) if $self->has_writer();
379 $self->process_accessors('predicate' => $self->predicate(), $inline)
380 ) if $self->has_predicate();
383 $self->process_accessors('clearer' => $self->clearer(), $inline)
384 ) if $self->has_clearer();
390 my $_remove_accessor = sub {
391 my ($accessor, $class) = @_;
392 if (ref($accessor) && ref($accessor) eq 'HASH') {
393 ($accessor) = keys %{$accessor};
395 my $method = $class->get_method($accessor);
396 $class->remove_method($accessor)
397 if (ref($method) && $method->isa('Class::MOP::Method::Accessor'));
400 sub remove_accessors {
403 # we really need to make sure to remove from the
404 # associates methods here as well. But this is
405 # such a slimly used method, I am not worried
406 # about it right now.
407 $_remove_accessor->($self->accessor(), $self->associated_class()) if $self->has_accessor();
408 $_remove_accessor->($self->reader(), $self->associated_class()) if $self->has_reader();
409 $_remove_accessor->($self->writer(), $self->associated_class()) if $self->has_writer();
410 $_remove_accessor->($self->predicate(), $self->associated_class()) if $self->has_predicate();
411 $_remove_accessor->($self->clearer(), $self->associated_class()) if $self->has_clearer();
425 Class::MOP::Attribute - Attribute Meta Object
429 Class::MOP::Attribute->new( foo => (
430 accessor => 'foo', # dual purpose get/set accessor
431 predicate => 'has_foo' # predicate check for defined-ness
432 init_arg => '-foo', # class->new will look for a -foo key
433 default => 'BAR IS BAZ!' # if no -foo key is provided, use this
436 Class::MOP::Attribute->new( bar => (
437 reader => 'bar', # getter
438 writer => 'set_bar', # setter
439 predicate => 'has_bar' # predicate check for defined-ness
440 init_arg => ':bar', # class->new will look for a :bar key
441 # no default value means it is undef
446 The Attribute Protocol is almost entirely an invention of this module,
447 and is completely optional to this MOP. This is because Perl 5 does not
448 have consistent notion of what is an attribute of a class. There are
449 so many ways in which this is done, and very few (if any) are
450 easily discoverable by this module.
452 So, all that said, this module attempts to inject some order into this
453 chaos, by introducing a consistent API which can be used to create
462 =item B<new ($name, ?%options)>
464 An attribute must (at the very least), have a C<$name>. All other
465 C<%options> are contained added as key-value pairs. Acceptable keys
472 This should be a string value representing the expected key in
473 an initialization hash. For instance, if we have an I<init_arg>
474 value of C<-foo>, then the following code will Just Work.
476 MyClass->meta->construct_instance(-foo => "Hello There");
478 In an init_arg is not assigned, it will automatically use the
479 value of C<$name>. If an explicit C<undef> is given for an init_arg,
480 an attribute value can't be specified during initialization.
484 The value of this key is the name of the method that will be
485 called to obtain the value used to initialize the attribute.
486 This should be a method in the class associated with the attribute,
487 not a method in the attribute class itself.
491 The value of this key is the default value which
492 C<Class::MOP::Class::construct_instance> will initialize the
496 If the value is a simple scalar (string or number), then it can
497 be just passed as is. However, if you wish to initialize it with
498 a HASH or ARRAY ref, then you need to wrap that inside a CODE
501 Class::MOP::Attribute->new('@foo' => (
502 default => sub { [] },
507 Class::MOP::Attribute->new('%foo' => (
508 default => sub { {} },
511 If you wish to initialize an attribute with a CODE reference
512 itself, then you need to wrap that in a subroutine as well, like
515 Class::MOP::Attribute->new('&foo' => (
516 default => sub { sub { print "Hello World" } },
519 And lastly, if the value of your attribute is dependent upon
520 some other aspect of the instance structure, then you can take
521 advantage of the fact that when the I<default> value is a CODE
522 reference, it is passed the (as yet unfinished) instance structure
523 as it's only argument. So you can do things like this:
525 Class::MOP::Attribute->new('$object_identity' => (
526 default => sub { Scalar::Util::refaddr($_[0]) },
529 This last feature is fairly limited as there is no gurantee of
530 the order of attribute initializations, so you cannot perform
531 any kind of dependent initializations. However, if this is
532 something you need, you could subclass B<Class::MOP::Class> and
533 this class to acheive it. However, this is currently left as
534 an exercise to the reader :).
538 This may be a method name (referring to a method on the class with this
539 attribute) or a CODE ref. The initializer is used to set the attribute value
540 on an instance when the attribute is set during instance initialization. When
541 called, it is passed the instance (as the invocant), the value to set, a
542 slot-setting CODE ref, and the attribute meta-instance. The slot-setting code
543 is provided to make it easy to set the (possibly altered) value on the instance
544 without going through several more method calls.
546 This contrived example shows an initializer that sets the attribute to twice
549 Class::MOP::Attribute->new('$doubled' => (
551 my ($instance, $value, $set) = @_;
556 As method names can be given as initializers, one can easily make
557 attribute initialization use the writer:
559 Class::MOP::Attribute->new('$some_attr' => (
560 writer => 'some_attr',
561 initializer => 'some_attr',
564 Your writer will simply need to examine it's C<@_> and determine under
565 which context it is being called.
569 The I<accessor>, I<reader>, I<writer>, I<predicate> and I<clearer> keys can
570 contain either; the name of the method and an appropriate default one will be
571 generated for you, B<or> a HASH ref containing exactly one key (which will be
572 used as the name of the method) and one value, which should contain a CODE
573 reference which will be installed as the method itself.
579 The I<accessor> is a standard perl-style read/write accessor. It will
580 return the value of the attribute, and if a value is passed as an argument,
581 it will assign that value to the attribute.
584 This method will properly handle the following code, by assigning an
585 C<undef> value to the attribute.
587 $object->set_something(undef);
591 This is a basic read-only accessor, it will just return the value of
596 This is a basic write accessor, it accepts a single argument, and
597 assigns that value to the attribute. This method does not intentially
598 return a value, however perl will return the result of the last
599 expression in the subroutine, which returns in this returning the
600 same value that it was passed.
603 This method will properly handle the following code, by assigning an
604 C<undef> value to the attribute.
606 $object->set_something();
610 This is a basic test to see if any value has been set for the
611 attribute. It will return true (C<1>) if the attribute has been set
612 to any value (even C<undef>), and false (C<0>) otherwise.
615 The predicate will return true even when you set an attribute's
616 value to C<undef>. This behaviour has changed as of version 0.43. In
617 older versions, the predicate (erroneously) checked for attribute
618 value definedness, instead of presence as it is now.
620 If you really want to get rid of the value, you have to define and
621 use a I<clearer> (see below).
625 This is the a method that will uninitialize the attr, reverting lazy values
626 back to their "unfulfilled" state.
630 =item B<clone (%options)>
632 This will return a clone of the attribute instance, allowing the overriding
633 of various attributes through the C<%options> supplied.
635 =item B<initialize_instance_slot ($instance, $params)>
637 This method is used internally to initialize the approriate slot for this
638 attribute in a given C<$instance>, the C<$params> passed are those that were
639 passed to the constructor.
643 =head2 Value management
645 These methods are basically "backdoors" to the instance, which can be used
646 to bypass the regular accessors, but still stay within the context of the MOP.
648 These methods are not for general use, and should only be used if you really
649 know what you are doing.
653 =item B<set_value ($instance, $value)>
655 Set the value without going through the accessor. Note that this may be done to
656 even attributes with just read only accessors.
658 =item B<set_initial_value ($instance, $value)>
660 This method sets the value without going through the accessor -- but it is only
661 called when the instance data is first initialized.
663 =item B<get_value ($instance)>
665 Return the value without going through the accessor. Note that this may be done
666 even to attributes with just write only accessors.
668 =item B<has_value ($instance)>
670 Return a boolean indicating if the item in the C<$instance> has a value in it.
671 This is basically what the default C<predicate> method calls.
673 =item B<clear_value ($instance)>
675 This will clear the value in the C<$instance>. This is basically what the default
676 C<clearer> would call. Note that this may be done even if the attirbute does not
677 have any associated read, write or clear methods.
683 These are all basic read-only value accessors for the values
684 passed into C<new>. I think they are pretty much self-explanitory.
704 =item B<is_default_a_coderef>
706 =item B<default (?$instance)>
708 Return the default value for the attribute.
710 If you pass in an C<$instance> argument to this accessor and the
711 I<default> is a CODE reference, then the CODE reference will be
712 executed with the C<$instance> as its argument.
716 Return a list of slots required by the attribute. This is usually
717 just one, which is the name of the attribute.
719 =item B<get_read_method>
721 =item B<get_write_method>
723 Return the name of a method name suitable for reading / writing the value
724 of the attribute in the associated class. Suitable for use whether
725 C<reader> and C<writer> or C<accessor> was used.
727 =item B<get_read_method_ref>
729 =item B<get_write_method_ref>
731 Return the CODE reference of a method suitable for reading / writing the
732 value of the attribute in the associated class. Suitable for use whether
733 C<reader> and C<writer> or C<accessor> was specified or not.
735 NOTE: If no reader/writer/accessor was specified, this will use the
736 attribute get_value/set_value methods, which can be very inefficient.
738 =item B<has_read_method>
740 =item B<has_write_method>
742 Return whether a method exists suitable for reading / writing the value
743 of the attribute in the associated class. Suitable for use whether
744 C<reader> and C<writer> or C<accessor> was used.
748 =head2 Informational predicates
750 These are all basic predicate methods for the values passed into C<new>.
754 =item B<has_accessor>
760 =item B<has_predicate>
764 =item B<has_initializer>
766 =item B<has_init_arg>
774 =head2 Class association
776 These methods allow you to manage the attributes association with
777 the class that contains it. These methods should not be used
778 lightly, nor are they very magical, they are mostly used internally
779 and by metaclass instances.
783 =item B<associated_class>
785 This returns the metaclass this attribute is associated with.
787 =item B<attach_to_class ($class)>
789 This will store a weaken reference to C<$class> internally. You should
790 note that just changing the class assocation will not remove the attribute
791 from it's old class, and initialize it (and it's accessors) in the new
792 C<$class>. It is up to you to do this manually.
794 =item B<detach_from_class>
796 This will remove the weakened reference to the class. It does B<not>
797 remove the attribute itself from the class (or remove it's accessors),
798 you must do that yourself if you want too. Actually if that is what
799 you want to do, you should probably be looking at
800 L<Class::MOP::Class::remove_attribute> instead.
804 =head2 Attribute Accessor generation
808 =item B<accessor_metaclass>
810 Accessors are generated by an accessor metaclass, which is usually
811 a subclass of C<Class::MOP::Method::Accessor>. This method returns
812 the name of the accessor metaclass that this attribute uses.
814 =item B<associate_method ($method)>
816 This will associate a C<$method> with the given attribute which is
817 used internally by the accessor generator.
819 =item B<associated_methods>
821 This will return the list of methods which have been associated with
822 the C<associate_method> methods. This is a good way of seeing what
823 methods are used to manage a given attribute.
825 =item B<install_accessors>
827 This allows the attribute to generate and install code for it's own
828 I<accessor/reader/writer/predicate> methods. This is called by
829 C<Class::MOP::Class::add_attribute>.
831 This method will call C<process_accessors> for each of the possible
832 method types (accessor, reader, writer & predicate).
834 =item B<process_accessors ($type, $value)>
836 This takes a C<$type> (accessor, reader, writer or predicate), and
837 a C<$value> (the value passed into the constructor for each of the
838 different types). It will then either generate the method itself
839 (using the C<generate_*_method> methods listed below) or it will
840 use the custom method passed through the constructor.
842 =item B<remove_accessors>
844 This allows the attribute to remove the method for it's own
845 I<accessor/reader/writer/predicate/clearer>. This is called by
846 C<Class::MOP::Class::remove_attribute>.
848 NOTE: This does not currently remove methods from the list returned
849 by C<associated_methods>, that is on the TODO list.
859 This will return a B<Class::MOP::Class> instance which is related
862 It should also be noted that B<Class::MOP> will actually bootstrap
863 this module by installing a number of attribute meta-objects into
864 it's metaclass. This will allow this class to reap all the benefits
865 of the MOP when subclassing it.
871 Stevan Little E<lt>stevan@iinteractive.comE<gt>
873 =head1 COPYRIGHT AND LICENSE
875 Copyright 2006-2008 by Infinity Interactive, Inc.
877 L<http://www.iinteractive.com>
879 This library is free software; you can redistribute it and/or modify
880 it under the same terms as Perl itself.