2 package Class::MOP::Attribute;
7 use Class::MOP::Method::Accessor;
10 use Scalar::Util 'blessed', 'reftype', 'weaken';
12 our $VERSION = '0.23';
13 our $AUTHORITY = 'cpan:STEVAN';
15 use base 'Class::MOP::Object';
18 require Class::MOP::Class;
19 Class::MOP::Class->initialize(blessed($_[0]) || $_[0]);
22 # NOTE: (meta-circularity)
23 # This method will be replaced in the
24 # boostrap section of Class::MOP, by
25 # a new version which uses the
26 # &Class::MOP::Class::construct_instance
27 # method to build an attribute meta-object
28 # which itself is described with attribute
30 # - Ain't meta-circularity grand? :)
36 (defined $name && $name)
37 || confess "You must provide a name for the attribute";
39 $options{init_arg} = $name
40 if not exists $options{init_arg};
41 if(exists $options{builder}){
42 confess("builder must be a defined scalar value which is a method name")
43 if ref $options{builder} || !(defined $options{builder});
44 confess("Setting both default and builder is not allowed.")
45 if exists $options{default};
47 (is_default_a_coderef(\%options))
48 || confess("References are not allowed as default values, you must ".
49 "wrap then in a CODE reference (ex: sub { [] } and not [])")
50 if exists $options{default} && ref $options{default};
52 if( $options{required} and not( defined($options{builder}) || defined($options{init_arg}) || exists $options{default} ) ) {
53 confess("A required attribute must have either 'init_arg', 'builder', or 'default'");
57 '$!accessor' => $options{accessor},
58 '$!reader' => $options{reader},
59 '$!writer' => $options{writer},
60 '$!predicate' => $options{predicate},
61 '$!clearer' => $options{clearer},
62 '$!builder' => $options{builder},
63 '$!init_arg' => $options{init_arg},
64 '$!default' => $options{default},
65 '$!initializer' => $options{initializer},
66 # keep a weakened link to the
67 # class we are associated with
68 '$!associated_class' => undef,
69 # and a list of the methods
70 # associated with this attr
71 '@!associated_methods' => [],
73 # protect this from silliness
79 # this is a primative (and kludgy) clone operation
80 # for now, it will be replaced in the Class::MOP
81 # bootstrap with a proper one, however we know
82 # that this one will work fine for now.
87 || confess "Can only clone an instance";
88 return bless { %{$self}, %options } => blessed($self);
91 sub initialize_instance_slot {
92 my ($self, $meta_instance, $instance, $params) = @_;
93 my $init_arg = $self->{'$!init_arg'};
94 # try to fetch the init arg from the %params ...
96 # if nothing was in the %params, we can use the
97 # attribute's default value (if it has one)
98 if(defined $init_arg and exists $params->{$init_arg}){
99 $self->_set_initial_slot_value(
102 $params->{$init_arg},
105 elsif (defined $self->{'$!default'}) {
106 $self->_set_initial_slot_value(
109 $self->default($instance),
112 elsif (defined( my $builder = $self->{'$!builder'})) {
113 if ($builder = $instance->can($builder)) {
114 $self->_set_initial_slot_value(
121 confess(blessed($instance)." does not support builder method '". $self->{'$!builder'} ."' for attribute '" . $self->name . "'");
126 sub _set_initial_slot_value {
127 my ($self, $meta_instance, $instance, $value) = @_;
129 my $slot_name = $self->name;
131 return $meta_instance->set_slot_value($instance, $slot_name, $value)
132 unless $self->has_initializer;
135 $meta_instance->set_slot_value($instance, $slot_name, $_[0]);
138 my $initializer = $self->initializer;
140 # most things will just want to set a value, so make it first arg
141 $instance->$initializer($value, $callback, $self);
145 # the next bunch of methods will get bootstrapped
146 # away in the Class::MOP bootstrapping section
148 sub name { $_[0]->{'$!name'} }
150 sub associated_class { $_[0]->{'$!associated_class'} }
151 sub associated_methods { $_[0]->{'@!associated_methods'} }
153 sub has_accessor { defined($_[0]->{'$!accessor'}) ? 1 : 0 }
154 sub has_reader { defined($_[0]->{'$!reader'}) ? 1 : 0 }
155 sub has_writer { defined($_[0]->{'$!writer'}) ? 1 : 0 }
156 sub has_predicate { defined($_[0]->{'$!predicate'}) ? 1 : 0 }
157 sub has_clearer { defined($_[0]->{'$!clearer'}) ? 1 : 0 }
158 sub has_builder { defined($_[0]->{'$!builder'}) ? 1 : 0 }
159 sub has_init_arg { defined($_[0]->{'$!init_arg'}) ? 1 : 0 }
160 sub has_default { defined($_[0]->{'$!default'}) ? 1 : 0 }
161 sub has_initializer { defined($_[0]->{'$!initializer'}) ? 1 : 0 }
163 sub accessor { $_[0]->{'$!accessor'} }
164 sub reader { $_[0]->{'$!reader'} }
165 sub writer { $_[0]->{'$!writer'} }
166 sub predicate { $_[0]->{'$!predicate'} }
167 sub clearer { $_[0]->{'$!clearer'} }
168 sub builder { $_[0]->{'$!builder'} }
169 sub init_arg { $_[0]->{'$!init_arg'} }
170 sub initializer { $_[0]->{'$!initializer'} }
172 # end bootstrapped away method section.
173 # (all methods below here are kept intact)
175 sub get_read_method {
177 my $reader = $self->reader || $self->accessor;
179 return $reader unless ref $reader;
181 my ($name) = %$reader;
185 sub get_write_method {
187 my $writer = $self->writer || $self->accessor;
189 return $writer unless ref $writer;
191 my ($name) = %$writer;
195 sub get_read_method_ref {
197 if ((my $reader = $self->get_read_method) && $self->associated_class) {
198 return $self->associated_class->get_method($reader);
201 return sub { $self->get_value(@_) };
205 sub get_write_method_ref {
207 if ((my $writer = $self->get_write_method) && $self->associated_class) {
208 return $self->associated_class->get_method($writer);
211 return sub { $self->set_value(@_) };
215 sub is_default_a_coderef {
216 ('CODE' eq (reftype($_[0]->{'$!default'} || $_[0]->{default}) || ''))
220 my ($self, $instance) = @_;
221 if (defined $instance && $self->is_default_a_coderef) {
222 # if the default is a CODE ref, then
223 # we pass in the instance and default
224 # can return a value based on that
225 # instance. Somewhat crude, but works.
226 return $self->{'$!default'}->($instance);
228 $self->{'$!default'};
233 sub slots { (shift)->name }
237 sub attach_to_class {
238 my ($self, $class) = @_;
239 (blessed($class) && $class->isa('Class::MOP::Class'))
240 || confess "You must pass a Class::MOP::Class instance (or a subclass)";
241 weaken($self->{'$!associated_class'} = $class);
244 sub detach_from_class {
246 $self->{'$!associated_class'} = undef;
251 sub associate_method {
252 my ($self, $method) = @_;
253 push @{$self->{'@!associated_methods'}} => $method;
258 sub set_initial_value {
259 my ($self, $instance, $value) = @_;
260 $self->_set_initial_slot_value(
261 Class::MOP::Class->initialize(blessed($instance))->get_meta_instance,
268 my ($self, $instance, $value) = @_;
270 Class::MOP::Class->initialize(blessed($instance))
272 ->set_slot_value($instance, $self->name, $value);
276 my ($self, $instance) = @_;
278 Class::MOP::Class->initialize(blessed($instance))
280 ->get_slot_value($instance, $self->name);
284 my ($self, $instance) = @_;
286 Class::MOP::Class->initialize(blessed($instance))
288 ->is_slot_initialized($instance, $self->name);
292 my ($self, $instance) = @_;
294 Class::MOP::Class->initialize(blessed($instance))
296 ->deinitialize_slot($instance, $self->name);
301 sub accessor_metaclass { 'Class::MOP::Method::Accessor' }
303 sub process_accessors {
304 my ($self, $type, $accessor, $generate_as_inline_methods) = @_;
305 if (reftype($accessor)) {
306 (reftype($accessor) eq 'HASH')
307 || confess "bad accessor/reader/writer/predicate/clearer format, must be a HASH ref";
308 my ($name, $method) = %{$accessor};
309 $method = $self->accessor_metaclass->wrap($method);
310 $self->associate_method($method);
311 return ($name, $method);
314 my $inline_me = ($generate_as_inline_methods && $self->associated_class->instance_metaclass->is_inlinable);
317 $method = $self->accessor_metaclass->new(
319 is_inline => $inline_me,
320 accessor_type => $type,
323 confess "Could not create the '$type' method for " . $self->name . " because : $@" if $@;
324 $self->associate_method($method);
325 return ($accessor, $method);
329 sub install_accessors {
332 my $class = $self->associated_class;
335 $self->process_accessors('accessor' => $self->accessor(), $inline)
336 ) if $self->has_accessor();
339 $self->process_accessors('reader' => $self->reader(), $inline)
340 ) if $self->has_reader();
343 $self->process_accessors('writer' => $self->writer(), $inline)
344 ) if $self->has_writer();
347 $self->process_accessors('predicate' => $self->predicate(), $inline)
348 ) if $self->has_predicate();
351 $self->process_accessors('clearer' => $self->clearer(), $inline)
352 ) if $self->has_clearer();
358 my $_remove_accessor = sub {
359 my ($accessor, $class) = @_;
360 if (reftype($accessor) && reftype($accessor) eq 'HASH') {
361 ($accessor) = keys %{$accessor};
363 my $method = $class->get_method($accessor);
364 $class->remove_method($accessor)
365 if (blessed($method) && $method->isa('Class::MOP::Method::Accessor'));
368 sub remove_accessors {
371 # we really need to make sure to remove from the
372 # associates methods here as well. But this is
373 # such a slimly used method, I am not worried
374 # about it right now.
375 $_remove_accessor->($self->accessor(), $self->associated_class()) if $self->has_accessor();
376 $_remove_accessor->($self->reader(), $self->associated_class()) if $self->has_reader();
377 $_remove_accessor->($self->writer(), $self->associated_class()) if $self->has_writer();
378 $_remove_accessor->($self->predicate(), $self->associated_class()) if $self->has_predicate();
379 $_remove_accessor->($self->clearer(), $self->associated_class()) if $self->has_clearer();
393 Class::MOP::Attribute - Attribute Meta Object
397 Class::MOP::Attribute->new('$foo' => (
398 accessor => 'foo', # dual purpose get/set accessor
399 predicate => 'has_foo' # predicate check for defined-ness
400 init_arg => '-foo', # class->new will look for a -foo key
401 default => 'BAR IS BAZ!' # if no -foo key is provided, use this
404 Class::MOP::Attribute->new('$.bar' => (
405 reader => 'bar', # getter
406 writer => 'set_bar', # setter
407 predicate => 'has_bar' # predicate check for defined-ness
408 init_arg => ':bar', # class->new will look for a :bar key
409 # no default value means it is undef
414 The Attribute Protocol is almost entirely an invention of this module,
415 and is completely optional to this MOP. This is because Perl 5 does not
416 have consistent notion of what is an attribute of a class. There are
417 so many ways in which this is done, and very few (if any) are
418 easily discoverable by this module.
420 So, all that said, this module attempts to inject some order into this
421 chaos, by introducing a consistent API which can be used to create
430 =item B<new ($name, ?%options)>
432 An attribute must (at the very least), have a C<$name>. All other
433 C<%options> are contained added as key-value pairs. Acceptable keys
440 This should be a string value representing the expected key in
441 an initialization hash. For instance, if we have an I<init_arg>
442 value of C<-foo>, then the following code will Just Work.
444 MyClass->meta->construct_instance(-foo => "Hello There");
446 In an init_arg is not assigned, it will automatically use the
447 value of C<$name>. If an explicit C<undef> is given for an init_arg,
448 an attribute value can't be specified during initialization.
452 The value of this key is the name of the method that will be
453 called to obtain the value used to initialize the attribute.
454 This should be a method in the class associated with the attribute,
455 not a method in the attribute class itself.
459 The value of this key is the default value which
460 C<Class::MOP::Class::construct_instance> will initialize the
464 If the value is a simple scalar (string or number), then it can
465 be just passed as is. However, if you wish to initialize it with
466 a HASH or ARRAY ref, then you need to wrap that inside a CODE
469 Class::MOP::Attribute->new('@foo' => (
470 default => sub { [] },
475 Class::MOP::Attribute->new('%foo' => (
476 default => sub { {} },
479 If you wish to initialize an attribute with a CODE reference
480 itself, then you need to wrap that in a subroutine as well, like
483 Class::MOP::Attribute->new('&foo' => (
484 default => sub { sub { print "Hello World" } },
487 And lastly, if the value of your attribute is dependent upon
488 some other aspect of the instance structure, then you can take
489 advantage of the fact that when the I<default> value is a CODE
490 reference, it is passed the (as yet unfinished) instance structure
491 as it's only argument. So you can do things like this:
493 Class::MOP::Attribute->new('$object_identity' => (
494 default => sub { Scalar::Util::refaddr($_[0]) },
497 This last feature is fairly limited as there is no gurantee of
498 the order of attribute initializations, so you cannot perform
499 any kind of dependent initializations. However, if this is
500 something you need, you could subclass B<Class::MOP::Class> and
501 this class to acheive it. However, this is currently left as
502 an exercise to the reader :).
506 This may be a method name (referring to a method on the class with this
507 attribute) or a CODE ref. The initializer is used to set the attribute value
508 on an instance when the attribute is set during instance initialization. When
509 called, it is passed the instance (as the invocant), the value to set, a
510 slot-setting CODE ref, and the attribute meta-instance. The slot-setting code
511 is provided to make it easy to set the (possibly altered) value on the instance
512 without going through several more method calls.
514 This contrived example shows an initializer that sets the attribute to twice
517 Class::MOP::Attribute->new('$doubled' => (
519 my ($instance, $value, $set) = @_;
524 As method names can be given as initializers, one can easily make
525 attribute initialization use the writer:
527 Class::MOP::Attribute->new('$some_attr' => (
528 writer => 'some_attr',
529 initializer => 'some_attr',
532 Your writer will simply need to examine it's C<@_> and determine under
533 which context it is being called.
537 The I<accessor>, I<reader>, I<writer>, I<predicate> and I<clearer> keys can
538 contain either; the name of the method and an appropriate default one will be
539 generated for you, B<or> a HASH ref containing exactly one key (which will be
540 used as the name of the method) and one value, which should contain a CODE
541 reference which will be installed as the method itself.
547 The I<accessor> is a standard perl-style read/write accessor. It will
548 return the value of the attribute, and if a value is passed as an argument,
549 it will assign that value to the attribute.
552 This method will properly handle the following code, by assigning an
553 C<undef> value to the attribute.
555 $object->set_something(undef);
559 This is a basic read-only accessor, it will just return the value of
564 This is a basic write accessor, it accepts a single argument, and
565 assigns that value to the attribute. This method does not intentially
566 return a value, however perl will return the result of the last
567 expression in the subroutine, which returns in this returning the
568 same value that it was passed.
571 This method will properly handle the following code, by assigning an
572 C<undef> value to the attribute.
574 $object->set_something();
578 This is a basic test to see if any value has been set for the
579 attribute. It will return true (C<1>) if the attribute has been set
580 to any value (even C<undef>), and false (C<0>) otherwise.
583 The predicate will return true even when you set an attribute's
584 value to C<undef>. This behaviour has changed as of version 0.43. In
585 older versions, the predicate (erroneously) checked for attribute
586 value definedness, instead of presence as it is now.
588 If you really want to get rid of the value, you have to define and
589 use a I<clearer> (see below).
593 This is the a method that will uninitialize the attr, reverting lazy values
594 back to their "unfulfilled" state.
598 =item B<clone (%options)>
600 This will return a clone of the attribute instance, allowing the overriding
601 of various attributes through the C<%options> supplied.
603 =item B<initialize_instance_slot ($instance, $params)>
605 This method is used internally to initialize the approriate slot for this
606 attribute in a given C<$instance>, the C<$params> passed are those that were
607 passed to the constructor.
611 =head2 Value management
613 These methods are basically "backdoors" to the instance, which can be used
614 to bypass the regular accessors, but still stay within the context of the MOP.
616 These methods are not for general use, and should only be used if you really
617 know what you are doing.
621 =item B<set_value ($instance, $value)>
623 Set the value without going through the accessor. Note that this may be done to
624 even attributes with just read only accessors.
626 =item B<set_initial_value ($instance, $value)>
628 This method sets the value without going through the accessor -- but it is only
629 called when the instance data is first initialized.
631 =item B<get_value ($instance)>
633 Return the value without going through the accessor. Note that this may be done
634 even to attributes with just write only accessors.
636 =item B<has_value ($instance)>
638 Return a boolean indicating if the item in the C<$instance> has a value in it.
639 This is basically what the default C<predicate> method calls.
641 =item B<clear_value ($instance)>
643 This will clear the value in the C<$instance>. This is basically what the default
644 C<clearer> would call. Note that this may be done even if the attirbute does not
645 have any associated read, write or clear methods.
651 These are all basic read-only value accessors for the values
652 passed into C<new>. I think they are pretty much self-explanitory.
672 =item B<is_default_a_coderef>
674 =item B<default (?$instance)>
676 Return the default value for the attribute.
678 If you pass in an C<$instance> argument to this accessor and the
679 I<default> is a CODE reference, then the CODE reference will be
680 executed with the C<$instance> as its argument.
684 Return a list of slots required by the attribute. This is usually
685 just one, which is the name of the attribute.
687 =item B<get_read_method>
689 =item B<get_write_method>
691 Return the name of a method name suitable for reading / writing the value
692 of the attribute in the associated class. Suitable for use whether
693 C<reader> and C<writer> or C<accessor> was used.
695 =item B<get_read_method_ref>
697 =item B<get_write_method_ref>
699 Return the CODE reference of a method suitable for reading / writing the
700 value of the attribute in the associated class. Suitable for use whether
701 C<reader> and C<writer> or C<accessor> was specified or not.
703 NOTE: If no reader/writer/accessor was specified, this will use the
704 attribute get_value/set_value methods, which can be very inefficient.
708 =head2 Informational predicates
710 These are all basic predicate methods for the values passed into C<new>.
714 =item B<has_accessor>
720 =item B<has_predicate>
724 =item B<has_initializer>
726 =item B<has_init_arg>
734 =head2 Class association
736 These methods allow you to manage the attributes association with
737 the class that contains it. These methods should not be used
738 lightly, nor are they very magical, they are mostly used internally
739 and by metaclass instances.
743 =item B<associated_class>
745 This returns the metaclass this attribute is associated with.
747 =item B<attach_to_class ($class)>
749 This will store a weaken reference to C<$class> internally. You should
750 note that just changing the class assocation will not remove the attribute
751 from it's old class, and initialize it (and it's accessors) in the new
752 C<$class>. It is up to you to do this manually.
754 =item B<detach_from_class>
756 This will remove the weakened reference to the class. It does B<not>
757 remove the attribute itself from the class (or remove it's accessors),
758 you must do that yourself if you want too. Actually if that is what
759 you want to do, you should probably be looking at
760 L<Class::MOP::Class::remove_attribute> instead.
764 =head2 Attribute Accessor generation
768 =item B<accessor_metaclass>
770 Accessors are generated by an accessor metaclass, which is usually
771 a subclass of C<Class::MOP::Method::Accessor>. This method returns
772 the name of the accessor metaclass that this attribute uses.
774 =item B<associate_method ($method)>
776 This will associate a C<$method> with the given attribute which is
777 used internally by the accessor generator.
779 =item B<associated_methods>
781 This will return the list of methods which have been associated with
782 the C<associate_method> methods. This is a good way of seeing what
783 methods are used to manage a given attribute.
785 =item B<install_accessors>
787 This allows the attribute to generate and install code for it's own
788 I<accessor/reader/writer/predicate> methods. This is called by
789 C<Class::MOP::Class::add_attribute>.
791 This method will call C<process_accessors> for each of the possible
792 method types (accessor, reader, writer & predicate).
794 =item B<process_accessors ($type, $value)>
796 This takes a C<$type> (accessor, reader, writer or predicate), and
797 a C<$value> (the value passed into the constructor for each of the
798 different types). It will then either generate the method itself
799 (using the C<generate_*_method> methods listed below) or it will
800 use the custom method passed through the constructor.
802 =item B<remove_accessors>
804 This allows the attribute to remove the method for it's own
805 I<accessor/reader/writer/predicate/clearer>. This is called by
806 C<Class::MOP::Class::remove_attribute>.
808 NOTE: This does not currently remove methods from the list returned
809 by C<associated_methods>, that is on the TODO list.
819 This will return a B<Class::MOP::Class> instance which is related
822 It should also be noted that B<Class::MOP> will actually bootstrap
823 this module by installing a number of attribute meta-objects into
824 it's metaclass. This will allow this class to reap all the benefits
825 of the MOP when subclassing it.
831 Stevan Little E<lt>stevan@iinteractive.comE<gt>
833 =head1 COPYRIGHT AND LICENSE
835 Copyright 2006-2008 by Infinity Interactive, Inc.
837 L<http://www.iinteractive.com>
839 This library is free software; you can redistribute it and/or modify
840 it under the same terms as Perl itself.