=head1 NAME
-Moose::Manual::Attribute - Object attributes with Moose
+Moose::Manual::Attributes - Object attributes with Moose
=head1 INTRODUCTION
-Moose has many attribute-related features, and attributes are probably
-the single most useful aspect of Moose. You can do a lot in a class
-just by declaring attributes. In fact, it's quite possible to have
-classes that consist solely of attribute declarations.
+Moose attributes have many properties, and attributes are probably the
+single most powerful and flexible part of Moose. You can create a
+powerful class simply by declaring attributes. In fact, it's possible
+to have classes that consist solely of attribute declarations.
-An Attribute is a property that every member of a class has. For
-example, we might say that "every Person object has a first name and
-last name". Attributes can be optional, so that we can say "some Person
+An attribute is a property that every member of a class has. For
+example, we might say that "every C<Person> object has a first name and
+last name". Attributes can be optional, so that we can say "some C<Person>
objects have a social security number (and some don't)".
At its simplest, an attribute can be thought of as a named value (as
in a hash) that can be read and set. However, attributes can also have
defaults, type constraints, delegation and much more.
+In other languages, attributes are also referred to as slots or
+properties.
+
=head1 ATTRIBUTE OPTIONS
Use the C<has> function to declare an attribute:
has 'first_name' => ( is => 'rw' );
-This says that all person objects have an optional read-write
+This says that all C<Person> objects have an optional read-write
"first_name" attribute.
-=head2 Read-write Vs Read-only
+=head2 Read-write vs. read-only
-The options passed to C<has> define the details of the
-attribute. There are a lot of options you can put here, but in the
-simplest form you just need to include C<is>, which can be either
-C<rw> (read-write) or C<ro> (read-only).
+The options passed to C<has> define the properties of the
+attribute. There are many options, but in the simplest form you just
+need to set C<is>, which can be either C<rw> (read-write) or C<ro>
+(read-only).
-(In fact, you could even omit C<is>, but that leaves you with an
-attribute that has no accessors, which is pointless unless you're
-doing some deep, dark magic).
+(In fact, you could even omit C<is>, but that gives you an attribute
+that has no accessors, which is pointless unless you're doing some
+deep, dark magic).
-=head2 Accessor Methods
+=head2 Accessor methods
Each attribute has one or more accessor methods. An accessor lets you
read and write the value of that attribute for an object.
read-only. If you declared it read-write, you get a read-write
accessor. Simple.
-Given our Person example above, we now have a single C<first_name>
-accessor that can read or write a person object's first name.
+Given our C<Person> example above, we now have a single C<first_name>
+accessor that can read or write a C<Person> object's C<first_name>
+attribute's value.
If you want, you can also explicitly specify the method names to be
used for reading and writing an attribute's value. This is
-particularly handy when you'd like an attribute to be publically
+particularly handy when you'd like an attribute to be publicly
readable, but only privately settable. For example:
has 'weight' => (
- is => 'rw',
+ is => 'ro',
writer => '_set_weight',
);
-This might be useful if weight is calculated based on other methods,
-for example every time the C<eat> method is called, we might adjust
+This might be useful if weight is calculated based on other methods.
+For example, every time the C<eat> method is called, we might adjust
weight. This lets us hide the implementation details of weight
changes, but still provide the weight value to users of the class.
If you're thinking that doing this over and over would be insanely
tedious, you're right! Fortunately, Moose provides a powerful
-extension system that lets you do things like override the default
-accessor method conventions. See L<Moose::Manual::MooseX> for more
-details.
+extension system that lets you override the default naming
+conventions. See L<Moose::Manual::MooseX> for more details.
-=head2 Predicate and Clearer Methods
+=head2 Predicate and clearer methods
Moose allows you to explicitly distinguish between a false or
-undefined attribute value and an attribute which is not set. If you
-want to be able access this information, you must define clearer and
+undefined attribute value and an attribute which has not been set. If
+you want to access this information, you must define clearer and
predicate methods for an attribute.
A predicate method tells you whether or not a given attribute is
-currently set. Note that even if the attribute was explicitly set to
-undef or some other false value, the predicate will return true.
+currently set. Note that an attribute can be explicitly set to
+C<undef> or some other false value, but the predicate will return
+true.
The clearer method unsets the attribute. This is I<not> the
same as setting the value to C<undef>, but you can only distinguish
$person2->has_ssn; # true
By default, Moose does not make a predicate or clearer for you. You
-must explicitly provide method names for these options if you want
-them.
+must explicitly provide names for them.
-=head2 Required or Not?
+=head2 Required or not?
By default, all attributes are optional, and do not need to be
provided at object construction time. If you want to make an attribute
required, simply set the C<required> option to true:
has 'name' => (
- is => 'rw',
+ is => 'ro',
required => 1,
);
There are a couple caveats worth mentioning in regards to what
-required actually means.
+"required" actually means.
-Basically, all it says is that this attribute (name) must be provided
-to the constructor. It does not say anything about its value, so it
-could be C<undef>.
+Basically, all it says is that this attribute (C<name>) must be provided to
+the constructor, or be lazy with either a default or a builder. It does not
+say anything about its value, so it could be C<undef>.
If you define a clearer method on a required attribute, the clearer
I<will> work, so even a required attribute can be unset after object
have a I<private> C<clearer> and C<predicate> for a required
attribute.
-=head2 Default and Builder Methods
+=head2 Default and builder methods
Attributes can have default values, and Moose provides two ways to
specify that default.
for the C<default> option:
has 'size' => (
- is => 'rw',
+ is => 'ro',
default => 'medium',
predicate => 'has_size',
);
If the size attribute is not provided to the constructor, then it ends
-up being set to "medium":
+up being set to C<medium>:
my $person = Person->new();
$person->size; # medium
$person->has_size; # true
You can also provide a subroutine reference for C<default>. This
-reference will be called a method on the object.
+reference will be called as a method on the object.
has 'size' => (
- is => 'rw',
+ is => 'ro',
default =>
sub { ( 'small', 'medium', 'large' )[ int( rand 3 ) ] },
predicate => 'has_size',
This is dumb example, but it illustrates the point that the subroutine
will be called for every new object created.
-Of course, if it's called during object construction, it may be called
-before other attributes have been set. If your default is dependent on
-other parts of the object's state, you can make the default c<lazy>,
-which is covered in the next section.
+When you provide a C<default> subroutine reference, it is called as a
+method on the object, with no additional parameters:
+
+ has 'size' => (
+ is => 'ro',
+ default => sub {
+ my $self = shift;
+
+ return $self->height > 200 ? 'big' : 'average';
+ },
+ );
+
+When the C<default> is called during object construction, it may be
+called before other attributes have been set. If your default is
+dependent on other parts of the object's state, you can make the
+attribute C<lazy>. Laziness is covered in the next section.
If you want to use a reference of any sort as the default value, you
must return it from a subroutine. This is necessary because otherwise
shared by all objects:
has 'mapping' => (
- is => 'rw',
+ is => 'ro',
default => {}, # wrong!
);
reference:
has 'mapping' => (
- is => 'rw',
+ is => 'ro',
default => sub { {} }, # right!
);
supply a C<builder> method for your attribute:
has 'size' => (
- is => 'rw',
+ is => 'ro',
builder => '_build_size',
predicate => 'has_size',
);
This has several advantages. First, it moves a chunk of code to its
own named method, which improves readability and code
-organization. Second, the C<_build_size> method can be overridden in
-subclasses.
+organization.
We strongly recommend that you use a C<builder> instead of a
C<default> for anything beyond the most trivial default.
-=head2 Laziness and lazy_build
+A C<builder>, just like a C<default>, is called as a method on the
+object with no additional parameters.
+
+=head3 Builders allow subclassing
+
+Because the C<builder> is called I<by name>, it goes through Perl's
+method resolution. This means that builder methods are both
+inheritable and overridable.
+
+If we subclass our C<Person> class, we can override C<_build_size>:
+
+ package Lilliputian;
+
+ use Moose;
+ extends 'Person';
+
+ sub _build_size { return 'small' }
+
+=head3 Builders can be composed from roles
+
+Because builders are called by name, they work well with roles. For
+example, a role could provide an attribute but require that the
+consuming class provide the C<builder>:
+
+ package HasSize;
+ use Moose::Role;
+
+ requires '_build_size';
+
+ has 'size' => (
+ is => 'ro',
+ lazy => 1,
+ builder => '_build_size',
+ );
+
+ package Lilliputian;
+ use Moose;
+
+ with 'HasSize';
+
+ sub _build_size { return 'small' }
+
+Roles are covered in L<Moose::Manual::Roles>.
+
+=head2 Laziness and C<lazy_build>
Moose lets you defer attribute population by making an attribute
C<lazy>:
has 'size' => (
- is => 'rw',
+ is => 'ro',
lazy => 1,
builder => '_build_size',
);
-When C<lazy> is true, the attribute is not populated until the reader
+When C<lazy> is true, the default is not generated until the reader
method is called, rather than at object construction time. There are
several reasons you might choose to do this.
First, if the default value for this attribute depends on some other
attributes, then the attribute I<must> be C<lazy>. During object
construction, defaults are not generated in a predictable order, so
-you cannot count on some other attribute being populated in a non-lazy
-default subroutine.
+you cannot count on some other attribute being populated when
+generating a default.
Second, there's often no reason to calculate a default before it's
needed. Making an attribute C<lazy> lets you defer the cost until the
option. This bundles up a number of options together:
has 'size' => (
- is => 'rw',
+ is => 'ro',
lazy_build => 1,
);
This is the same as specifying all of these options:
has 'size' => (
- is => 'rw',
+ is => 'ro',
lazy => 1,
builder => '_build_size',
clearer => 'clear_size',
predicate => 'has_size',
);
-If your attribute name starts with an underscore (_), then the clearer
+If your attribute name starts with an underscore (C<_>), then the clearer
and predicate will as well:
has '_size' => (
- is => 'rw',
+ is => 'ro',
lazy_build => 1,
);
becomes:
has '_size' => (
- is => 'rw',
+ is => 'ro',
lazy => 1,
builder => '_build__size',
clearer => '_clear_size',
always provide your own:
has 'size' => (
- is => 'rw',
+ is => 'ro',
lazy_build => 1,
clearer => '_clear_size',
);
Options that you explicitly provide are always used in favor of
Moose's internal defaults.
-=head2 Constructor Parameters (init_arg)
+=head2 Constructor parameters (C<init_arg>)
By default, each attribute can be passed by name to the class's
-constructor. On occassion, you may want to use a different name for
+constructor. On occasion, you may want to use a different name for
the constructor parameter. You may also want to make an attribute
-unsettable from the constructor.
+unsettable via the constructor.
-Both of these things can be done by providing a value for the
-C<init_arg> option:
+Both of these goals can be accomplished with the C<init_arg> option:
has 'bigness' => (
- is => 'rw',
+ is => 'ro',
init_arg => 'size',
);
-Now we have an attribute named bigness, but to set it during object
-construction we pass C<size> to the constructor.
+Now we have an attribute named "bigness", but we pass C<size> to the
+constructor.
-Even more useful is the ability to disable setting attribute. This is
-particularly handy for private attributes:
+Even more useful is the ability to disable setting an attribute via
+the constructor. This is particularly handy for private attributes:
has '_genetic_code' => (
- is => 'rw',
- init_arg => undef,
+ is => 'ro',
+ lazy_build => 1,
+ init_arg => undef,
);
By setting the C<init_arg> to C<undef>, we make it impossible to set
this attribute when creating a new object.
-=head2 Weak References
+=head2 Weak references
Moose has built-in support for weak references. If you set the
C<weak_ref> option to a true value, then it will call
);
sub _size_set {
- my ( $self, $size, $meta_attr ) = @_;
+ my ( $self, $size ) = @_;
warn $self->name, " size is now $size\n";
}
-The trigger is called as a method, and receives the new value as well
-as the L<Moose::Meta::Attribute> object for the attribute. The trigger
-is called I<after> the value is set.
+The trigger is called as a method, and receives the new value as its argument.
+The trigger is called I<after> the value is set.
+
+This differs from an after method modifier in two ways. First, a
+trigger is only called when the attribute is set, as opposed to
+whenever the accessor method is called (for reading or
+writing). Second, it is also called when an attribute's value is
+passed to the constructor.
-=head2 Attribute Types
+However, triggers are I<not> called when an attribute is populated
+from a C<default> or C<builder>
+
+=head2 Attribute types
Attributes can be restricted to only accept certain types:
has 'first_name' => (
- is => 'rw',
+ is => 'ro',
isa => 'Str',
);
-This says that the first_name attribute must be a string.
+This says that the C<first_name> attribute must be a string.
Moose also provides a shortcut for specifying that an attribute only
accepts objects that do a certain role:
=head2 Delegation
-Attributes can define methods which simple delegate to their values:
+Attributes can define methods which simply delegate to their values:
has 'hair_color' => (
- is => 'rw',
+ is => 'ro',
isa => 'Graphics::Color::RGB',
handles => { hair_color_hex => 'as_hex_string' },
);
C<hair_color_hex>, internally, the object just calls C<<
$self->hair_color->as_hex_string >>.
-See L<Moose::Manual::Delegation> for more details on how to set up
+See L<Moose::Manual::Delegation> for documentation on how to set up
delegation methods.
=head2 Metaclass and traits
);
In this case, the metaclass C<Collection::Hash> really refers to
-C<MooseX::AttributeHelpers::Collection::Hash>.
+L<MooseX::AttributeHelpers::Collection::Hash>.
You can also apply one or more traits to an attribute:
use MooseX::MetaDescription;
has 'size' => (
- is => 'rw',
+ is => 'ro',
traits => ['MooseX::MetaDescription::Meta::Trait'],
description => {
html_widget => 'text_input',
=back
To override an attribute, you simply prepend its name with a plus sign
-(+):
+(C<+>):
package LazyPerson;
to C<'Bill'>.
We recommend that you exercise caution when changing the type (C<isa>)
-of an inherited attribute. It's best to only make the new type a
-subtype of the one accepted by the parent.
+of an inherited attribute.
=head1 MORE ON ATTRIBUTES
Moose attributes are a big topic, and this document glosses over a few
-aspects of their aspects. We recommend that you read the
-L<Moose::Manual::Delegation> and L<Moose::Manual::Types> documents to
-get a more complete understanding of attribute features.
+aspects. We recommend that you read the L<Moose::Manual::Delegation>
+and L<Moose::Manual::Types> documents to get a more complete
+understanding of attribute features.
=head1 A FEW MORE OPTIONS
Moose has lots of attribute options. The ones listed below are
-superceded by some more modern features, but are covered for the sake
+superseded by some more modern features, but are covered for the sake
of completeness.
=head2 The C<documentation> option
Moose does absolutely nothing with this information other than store
it.
-As an alternative, you might want to look at the
-C<MooseX::MetaDescription> module, which lets you attach a
-"description" to each attribute. This description is a hashref that
-can include meta-information intended for use in other code, as well
-as documentation information.
-
-=head2 The C<auto_deref> Option
+=head2 The C<auto_deref> option
If your attribute is an array reference or hash reference, the
-C<auto_deref> option will make Moose de-reference the value when it is
+C<auto_deref> option will make Moose dereference the value when it is
returned from the reader method:
my %map = $object->mapping;
This option only works if your attribute is explicitly typed as an
-ArrayRef or HashRef.
+C<ArrayRef> or C<HashRef>.
-However, we recommend that you use C<MooseX::AttributeHelpers> for
+However, we recommend that you use L<MooseX::AttributeHelpers> for
these types of attributes, which gives you much more control over how
they are accessed and manipulated.
similar to C<builder>, except that it is I<only> called during object
construction.
-This option is inherited from C<Class::MOP>, but we recommend that you
+This option is inherited from L<Class::MOP>, but we recommend that you
use a C<builder> (which is Moose-only) instead.
=head1 AUTHOR
=head1 COPYRIGHT AND LICENSE
-Copyright 2008 by Infinity Interactive, Inc.
+Copyright 2009 by Infinity Interactive, Inc.
L<http://www.iinteractive.com>
projects / gitmo/Moose.git / blobdiff