2 package Class::MOP::Attribute;
7 use Class::MOP::Method::Accessor;
10 use Scalar::Util 'blessed', 'weaken';
12 our $VERSION = '0.72';
13 $VERSION = eval $VERSION;
14 our $AUTHORITY = 'cpan:STEVAN';
16 use base 'Class::MOP::Object';
18 # NOTE: (meta-circularity)
19 # This method will be replaced in the
20 # boostrap section of Class::MOP, by
21 # a new version which uses the
22 # &Class::MOP::Class::construct_instance
23 # method to build an attribute meta-object
24 # which itself is described with attribute
26 # - Ain't meta-circularity grand? :)
28 my ( $class, @args ) = @_;
30 unshift @args, "name" if @args % 2 == 1;
33 my $name = $options{name};
35 (defined $name && $name)
36 || confess "You must provide a name for the attribute";
38 $options{init_arg} = $name
39 if not exists $options{init_arg};
40 if(exists $options{builder}){
41 confess("builder must be a defined scalar value which is a method name")
42 if ref $options{builder} || !(defined $options{builder});
43 confess("Setting both default and builder is not allowed.")
44 if exists $options{default};
46 (is_default_a_coderef(\%options))
47 || confess("References are not allowed as default values, you must ".
48 "wrap the default of '$name' in a CODE reference (ex: sub { [] } and not [])")
49 if exists $options{default} && ref $options{default};
51 if( $options{required} and not( defined($options{builder}) || defined($options{init_arg}) || exists $options{default} ) ) {
52 confess("A required attribute must have either 'init_arg', 'builder', or 'default'");
55 $class->_new(\%options);
60 my $options = @_ == 1 ? $_[0] : {@_};
63 'name' => $options->{name},
64 'accessor' => $options->{accessor},
65 'reader' => $options->{reader},
66 'writer' => $options->{writer},
67 'predicate' => $options->{predicate},
68 'clearer' => $options->{clearer},
69 'builder' => $options->{builder},
70 'init_arg' => $options->{init_arg},
71 'default' => $options->{default},
72 'initializer' => $options->{initializer},
73 # keep a weakened link to the
74 # class we are associated with
75 'associated_class' => undef,
76 # and a list of the methods
77 # associated with this attr
78 'associated_methods' => [],
83 # this is a primative (and kludgy) clone operation
84 # for now, it will be replaced in the Class::MOP
85 # bootstrap with a proper one, however we know
86 # that this one will work fine for now.
91 || confess "Can only clone an instance";
92 return bless { %{$self}, %options } => ref($self);
95 sub initialize_instance_slot {
96 my ($self, $meta_instance, $instance, $params) = @_;
97 my $init_arg = $self->{'init_arg'};
99 # try to fetch the init arg from the %params ...
101 # if nothing was in the %params, we can use the
102 # attribute's default value (if it has one)
103 if(defined $init_arg and exists $params->{$init_arg}){
104 $self->_set_initial_slot_value(
107 $params->{$init_arg},
110 elsif (defined $self->{'default'}) {
111 $self->_set_initial_slot_value(
114 $self->default($instance),
117 elsif (defined( my $builder = $self->{'builder'})) {
118 if ($builder = $instance->can($builder)) {
119 $self->_set_initial_slot_value(
126 confess(ref($instance)." does not support builder method '". $self->{'builder'} ."' for attribute '" . $self->name . "'");
131 sub _set_initial_slot_value {
132 my ($self, $meta_instance, $instance, $value) = @_;
134 my $slot_name = $self->name;
136 return $meta_instance->set_slot_value($instance, $slot_name, $value)
137 unless $self->has_initializer;
140 $meta_instance->set_slot_value($instance, $slot_name, $_[0]);
143 my $initializer = $self->initializer;
145 # most things will just want to set a value, so make it first arg
146 $instance->$initializer($value, $callback, $self);
150 # the next bunch of methods will get bootstrapped
151 # away in the Class::MOP bootstrapping section
153 sub name { $_[0]->{'name'} }
155 sub associated_class { $_[0]->{'associated_class'} }
156 sub associated_methods { $_[0]->{'associated_methods'} }
158 sub has_accessor { defined($_[0]->{'accessor'}) }
159 sub has_reader { defined($_[0]->{'reader'}) }
160 sub has_writer { defined($_[0]->{'writer'}) }
161 sub has_predicate { defined($_[0]->{'predicate'}) }
162 sub has_clearer { defined($_[0]->{'clearer'}) }
163 sub has_builder { defined($_[0]->{'builder'}) }
164 sub has_init_arg { defined($_[0]->{'init_arg'}) }
165 sub has_default { defined($_[0]->{'default'}) }
166 sub has_initializer { defined($_[0]->{'initializer'}) }
168 sub accessor { $_[0]->{'accessor'} }
169 sub reader { $_[0]->{'reader'} }
170 sub writer { $_[0]->{'writer'} }
171 sub predicate { $_[0]->{'predicate'} }
172 sub clearer { $_[0]->{'clearer'} }
173 sub builder { $_[0]->{'builder'} }
174 sub init_arg { $_[0]->{'init_arg'} }
175 sub initializer { $_[0]->{'initializer'} }
177 # end bootstrapped away method section.
178 # (all methods below here are kept intact)
180 sub has_read_method { $_[0]->has_reader || $_[0]->has_accessor }
181 sub has_write_method { $_[0]->has_writer || $_[0]->has_accessor }
183 sub get_read_method {
185 my $reader = $self->reader || $self->accessor;
187 return $reader unless ref $reader;
189 my ($name) = %$reader;
193 sub get_write_method {
195 my $writer = $self->writer || $self->accessor;
197 return $writer unless ref $writer;
199 my ($name) = %$writer;
203 sub get_read_method_ref {
205 if ((my $reader = $self->get_read_method) && $self->associated_class) {
206 return $self->associated_class->get_method($reader);
209 my $code = sub { $self->get_value(@_) };
210 if (my $class = $self->associated_class) {
211 return $class->method_metaclass->wrap(
213 package_name => $class->name,
223 sub get_write_method_ref {
225 if ((my $writer = $self->get_write_method) && $self->associated_class) {
226 return $self->associated_class->get_method($writer);
229 my $code = sub { $self->set_value(@_) };
230 if (my $class = $self->associated_class) {
231 return $class->method_metaclass->wrap(
233 package_name => $class->name,
243 sub is_default_a_coderef {
244 my ($value) = $_[0]->{'default'};
245 return unless ref($value);
246 return ref($value) eq 'CODE' || (blessed($value) && $value->can('(&{}'));
250 my ($self, $instance) = @_;
251 if (defined $instance && $self->is_default_a_coderef) {
252 # if the default is a CODE ref, then
253 # we pass in the instance and default
254 # can return a value based on that
255 # instance. Somewhat crude, but works.
256 return $self->{'default'}->($instance);
263 sub slots { (shift)->name }
267 sub attach_to_class {
268 my ($self, $class) = @_;
269 (blessed($class) && $class->isa('Class::MOP::Class'))
270 || confess "You must pass a Class::MOP::Class instance (or a subclass)";
271 weaken($self->{'associated_class'} = $class);
274 sub detach_from_class {
276 $self->{'associated_class'} = undef;
281 sub associate_method {
282 my ($self, $method) = @_;
283 push @{$self->{'associated_methods'}} => $method;
288 sub set_initial_value {
289 my ($self, $instance, $value) = @_;
290 $self->_set_initial_slot_value(
291 Class::MOP::Class->initialize(ref($instance))->get_meta_instance,
298 my ($self, $instance, $value) = @_;
300 Class::MOP::Class->initialize(ref($instance))
302 ->set_slot_value($instance, $self->name, $value);
306 my ($self, $instance) = @_;
308 Class::MOP::Class->initialize(ref($instance))
310 ->get_slot_value($instance, $self->name);
314 my ($self, $instance) = @_;
316 Class::MOP::Class->initialize(ref($instance))
318 ->is_slot_initialized($instance, $self->name);
322 my ($self, $instance) = @_;
324 Class::MOP::Class->initialize(ref($instance))
326 ->deinitialize_slot($instance, $self->name);
331 sub accessor_metaclass { 'Class::MOP::Method::Accessor' }
333 sub process_accessors {
334 my ($self, $type, $accessor, $generate_as_inline_methods) = @_;
335 if (ref($accessor)) {
336 (ref($accessor) eq 'HASH')
337 || confess "bad accessor/reader/writer/predicate/clearer format, must be a HASH ref";
338 my ($name, $method) = %{$accessor};
339 $method = $self->accessor_metaclass->wrap(
341 package_name => $self->associated_class->name,
344 $self->associate_method($method);
345 return ($name, $method);
348 my $inline_me = ($generate_as_inline_methods && $self->associated_class->instance_metaclass->is_inlinable);
351 $method = $self->accessor_metaclass->new(
353 is_inline => $inline_me,
354 accessor_type => $type,
355 package_name => $self->associated_class->name,
359 confess "Could not create the '$type' method for " . $self->name . " because : $@" if $@;
360 $self->associate_method($method);
361 return ($accessor, $method);
365 sub install_accessors {
368 my $class = $self->associated_class;
371 $self->process_accessors('accessor' => $self->accessor(), $inline)
372 ) if $self->has_accessor();
375 $self->process_accessors('reader' => $self->reader(), $inline)
376 ) if $self->has_reader();
379 $self->process_accessors('writer' => $self->writer(), $inline)
380 ) if $self->has_writer();
383 $self->process_accessors('predicate' => $self->predicate(), $inline)
384 ) if $self->has_predicate();
387 $self->process_accessors('clearer' => $self->clearer(), $inline)
388 ) if $self->has_clearer();
394 my $_remove_accessor = sub {
395 my ($accessor, $class) = @_;
396 if (ref($accessor) && ref($accessor) eq 'HASH') {
397 ($accessor) = keys %{$accessor};
399 my $method = $class->get_method($accessor);
400 $class->remove_method($accessor)
401 if (ref($method) && $method->isa('Class::MOP::Method::Accessor'));
404 sub remove_accessors {
407 # we really need to make sure to remove from the
408 # associates methods here as well. But this is
409 # such a slimly used method, I am not worried
410 # about it right now.
411 $_remove_accessor->($self->accessor(), $self->associated_class()) if $self->has_accessor();
412 $_remove_accessor->($self->reader(), $self->associated_class()) if $self->has_reader();
413 $_remove_accessor->($self->writer(), $self->associated_class()) if $self->has_writer();
414 $_remove_accessor->($self->predicate(), $self->associated_class()) if $self->has_predicate();
415 $_remove_accessor->($self->clearer(), $self->associated_class()) if $self->has_clearer();
429 Class::MOP::Attribute - Attribute Meta Object
433 Class::MOP::Attribute->new( foo => (
434 accessor => 'foo', # dual purpose get/set accessor
435 predicate => 'has_foo' # predicate check for defined-ness
436 init_arg => '-foo', # class->new will look for a -foo key
437 default => 'BAR IS BAZ!' # if no -foo key is provided, use this
440 Class::MOP::Attribute->new( bar => (
441 reader => 'bar', # getter
442 writer => 'set_bar', # setter
443 predicate => 'has_bar' # predicate check for defined-ness
444 init_arg => ':bar', # class->new will look for a :bar key
445 # no default value means it is undef
450 The Attribute Protocol is almost entirely an invention of this module,
451 and is completely optional to this MOP. This is because Perl 5 does not
452 have consistent notion of what is an attribute of a class. There are
453 so many ways in which this is done, and very few (if any) are
454 easily discoverable by this module.
456 So, all that said, this module attempts to inject some order into this
457 chaos, by introducing a consistent API which can be used to create
466 =item B<new ($name, ?%options)>
468 An attribute must (at the very least), have a C<$name>. All other
469 C<%options> are contained added as key-value pairs. Acceptable keys
476 This should be a string value representing the expected key in
477 an initialization hash. For instance, if we have an I<init_arg>
478 value of C<-foo>, then the following code will Just Work.
480 MyClass->meta->construct_instance(-foo => "Hello There");
482 In an init_arg is not assigned, it will automatically use the
483 value of C<$name>. If an explicit C<undef> is given for an init_arg,
484 an attribute value can't be specified during initialization.
488 The value of this key is the name of the method that will be
489 called to obtain the value used to initialize the attribute.
490 This should be a method in the class associated with the attribute,
491 not a method in the attribute class itself.
495 The value of this key is the default value which
496 C<Class::MOP::Class::construct_instance> will initialize the
500 If the value is a simple scalar (string or number), then it can
501 be just passed as is. However, if you wish to initialize it with
502 a HASH or ARRAY ref, then you need to wrap that inside a CODE
505 Class::MOP::Attribute->new('@foo' => (
506 default => sub { [] },
511 Class::MOP::Attribute->new('%foo' => (
512 default => sub { {} },
515 If you wish to initialize an attribute with a CODE reference
516 itself, then you need to wrap that in a subroutine as well, like
519 Class::MOP::Attribute->new('&foo' => (
520 default => sub { sub { print "Hello World" } },
523 And lastly, if the value of your attribute is dependent upon
524 some other aspect of the instance structure, then you can take
525 advantage of the fact that when the I<default> value is a CODE
526 reference, it is passed the (as yet unfinished) instance structure
527 as it's only argument. So you can do things like this:
529 Class::MOP::Attribute->new('$object_identity' => (
530 default => sub { Scalar::Util::refaddr($_[0]) },
533 This last feature is fairly limited as there is no gurantee of
534 the order of attribute initializations, so you cannot perform
535 any kind of dependent initializations. However, if this is
536 something you need, you could subclass B<Class::MOP::Class> and
537 this class to acheive it. However, this is currently left as
538 an exercise to the reader :).
542 This may be a method name (referring to a method on the class with this
543 attribute) or a CODE ref. The initializer is used to set the attribute value
544 on an instance when the attribute is set during instance initialization. When
545 called, it is passed the instance (as the invocant), the value to set, a
546 slot-setting CODE ref, and the attribute meta-instance. The slot-setting code
547 is provided to make it easy to set the (possibly altered) value on the instance
548 without going through several more method calls.
550 This contrived example shows an initializer that sets the attribute to twice
553 Class::MOP::Attribute->new('$doubled' => (
555 my ($instance, $value, $set) = @_;
560 As method names can be given as initializers, one can easily make
561 attribute initialization use the writer:
563 Class::MOP::Attribute->new('$some_attr' => (
564 writer => 'some_attr',
565 initializer => 'some_attr',
568 Your writer will simply need to examine it's C<@_> and determine under
569 which context it is being called.
573 The I<accessor>, I<reader>, I<writer>, I<predicate> and I<clearer> keys can
574 contain either; the name of the method and an appropriate default one will be
575 generated for you, B<or> a HASH ref containing exactly one key (which will be
576 used as the name of the method) and one value, which should contain a CODE
577 reference which will be installed as the method itself.
583 The I<accessor> is a standard perl-style read/write accessor. It will
584 return the value of the attribute, and if a value is passed as an argument,
585 it will assign that value to the attribute.
588 This method will properly handle the following code, by assigning an
589 C<undef> value to the attribute.
591 $object->set_something(undef);
595 This is a basic read-only accessor, it will just return the value of
600 This is a basic write accessor, it accepts a single argument, and
601 assigns that value to the attribute. This method does not intentially
602 return a value, however perl will return the result of the last
603 expression in the subroutine, which returns in this returning the
604 same value that it was passed.
607 This method will properly handle the following code, by assigning an
608 C<undef> value to the attribute.
610 $object->set_something();
614 This is a basic test to see if any value has been set for the
615 attribute. It will return true (C<1>) if the attribute has been set
616 to any value (even C<undef>), and false (C<0>) otherwise.
619 The predicate will return true even when you set an attribute's
620 value to C<undef>. This behaviour has changed as of version 0.43. In
621 older versions, the predicate (erroneously) checked for attribute
622 value definedness, instead of presence as it is now.
624 If you really want to get rid of the value, you have to define and
625 use a I<clearer> (see below).
629 This is the a method that will uninitialize the attr, reverting lazy values
630 back to their "unfulfilled" state.
634 =item B<clone (%options)>
636 This will return a clone of the attribute instance, allowing the overriding
637 of various attributes through the C<%options> supplied.
639 =item B<initialize_instance_slot ($instance, $params)>
641 This method is used internally to initialize the approriate slot for this
642 attribute in a given C<$instance>, the C<$params> passed are those that were
643 passed to the constructor.
647 =head2 Value management
649 These methods are basically "backdoors" to the instance, which can be used
650 to bypass the regular accessors, but still stay within the context of the MOP.
652 These methods are not for general use, and should only be used if you really
653 know what you are doing.
657 =item B<set_value ($instance, $value)>
659 Set the value without going through the accessor. Note that this may be done to
660 even attributes with just read only accessors.
662 =item B<set_initial_value ($instance, $value)>
664 This method sets the value without going through the accessor -- but it is only
665 called when the instance data is first initialized.
667 =item B<get_value ($instance)>
669 Return the value without going through the accessor. Note that this may be done
670 even to attributes with just write only accessors.
672 =item B<has_value ($instance)>
674 Return a boolean indicating if the item in the C<$instance> has a value in it.
675 This is basically what the default C<predicate> method calls.
677 =item B<clear_value ($instance)>
679 This will clear the value in the C<$instance>. This is basically what the default
680 C<clearer> would call. Note that this may be done even if the attirbute does not
681 have any associated read, write or clear methods.
687 These are all basic read-only value accessors for the values
688 passed into C<new>. I think they are pretty much self-explanitory.
708 =item B<is_default_a_coderef>
710 =item B<default (?$instance)>
712 Return the default value for the attribute.
714 If you pass in an C<$instance> argument to this accessor and the
715 I<default> is a CODE reference, then the CODE reference will be
716 executed with the C<$instance> as its argument.
720 Return a list of slots required by the attribute. This is usually
721 just one, which is the name of the attribute.
723 =item B<get_read_method>
725 =item B<get_write_method>
727 Return the name of a method name suitable for reading / writing the value
728 of the attribute in the associated class. Suitable for use whether
729 C<reader> and C<writer> or C<accessor> was used.
731 =item B<get_read_method_ref>
733 =item B<get_write_method_ref>
735 Return the CODE reference of a method suitable for reading / writing the
736 value of the attribute in the associated class. Suitable for use whether
737 C<reader> and C<writer> or C<accessor> was specified or not.
739 NOTE: If no reader/writer/accessor was specified, this will use the
740 attribute get_value/set_value methods, which can be very inefficient.
742 =item B<has_read_method>
744 =item B<has_write_method>
746 Return whether a method exists suitable for reading / writing the value
747 of the attribute in the associated class. Suitable for use whether
748 C<reader> and C<writer> or C<accessor> was used.
752 =head2 Informational predicates
754 These are all basic predicate methods for the values passed into C<new>.
758 =item B<has_accessor>
764 =item B<has_predicate>
768 =item B<has_initializer>
770 =item B<has_init_arg>
778 =head2 Class association
780 These methods allow you to manage the attributes association with
781 the class that contains it. These methods should not be used
782 lightly, nor are they very magical, they are mostly used internally
783 and by metaclass instances.
787 =item B<associated_class>
789 This returns the metaclass this attribute is associated with.
791 =item B<attach_to_class ($class)>
793 This will store a weaken reference to C<$class> internally. You should
794 note that just changing the class assocation will not remove the attribute
795 from it's old class, and initialize it (and it's accessors) in the new
796 C<$class>. It is up to you to do this manually.
798 =item B<detach_from_class>
800 This will remove the weakened reference to the class. It does B<not>
801 remove the attribute itself from the class (or remove it's accessors),
802 you must do that yourself if you want too. Actually if that is what
803 you want to do, you should probably be looking at
804 L<Class::MOP::Class::remove_attribute> instead.
808 =head2 Attribute Accessor generation
812 =item B<accessor_metaclass>
814 Accessors are generated by an accessor metaclass, which is usually
815 a subclass of C<Class::MOP::Method::Accessor>. This method returns
816 the name of the accessor metaclass that this attribute uses.
818 =item B<associate_method ($method)>
820 This will associate a C<$method> with the given attribute which is
821 used internally by the accessor generator.
823 =item B<associated_methods>
825 This will return the list of methods which have been associated with
826 the C<associate_method> methods. This is a good way of seeing what
827 methods are used to manage a given attribute.
829 =item B<install_accessors>
831 This allows the attribute to generate and install code for it's own
832 I<accessor/reader/writer/predicate> methods. This is called by
833 C<Class::MOP::Class::add_attribute>.
835 This method will call C<process_accessors> for each of the possible
836 method types (accessor, reader, writer & predicate).
838 =item B<process_accessors ($type, $value)>
840 This takes a C<$type> (accessor, reader, writer or predicate), and
841 a C<$value> (the value passed into the constructor for each of the
842 different types). It will then either generate the method itself
843 (using the C<generate_*_method> methods listed below) or it will
844 use the custom method passed through the constructor.
846 =item B<remove_accessors>
848 This allows the attribute to remove the method for it's own
849 I<accessor/reader/writer/predicate/clearer>. This is called by
850 C<Class::MOP::Class::remove_attribute>.
852 NOTE: This does not currently remove methods from the list returned
853 by C<associated_methods>, that is on the TODO list.
863 This will return a B<Class::MOP::Class> instance which is related
866 It should also be noted that B<Class::MOP> will actually bootstrap
867 this module by installing a number of attribute meta-objects into
868 it's metaclass. This will allow this class to reap all the benefits
869 of the MOP when subclassing it.
875 Stevan Little E<lt>stevan@iinteractive.comE<gt>
877 =head1 COPYRIGHT AND LICENSE
879 Copyright 2006-2008 by Infinity Interactive, Inc.
881 L<http://www.iinteractive.com>
883 This library is free software; you can redistribute it and/or modify
884 it under the same terms as Perl itself.