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 properties of the
-attribute. There are a many options, but in the simplest form you just
+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).
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 override the default naming
+extension system that lets you override the default naming
conventions. See L<Moose::Manual::MooseX> for more details.
=head2 Predicate and clearer methods
predicate methods for an attribute.
A predicate method tells you whether or not a given attribute is
-currently set. Note an attribute can be explicitly set to C<undef> or
-some other false value, but 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
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.
-Basically, all it says is that this attribute (C<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
for the C<default> option:
has 'size' => (
- is => 'rw',
+ is => 'ro',
default => 'medium',
predicate => 'has_size',
);
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 attribute
-C<lazy>. Laziness 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',
);
We strongly recommend that you use a C<builder> instead of a
C<default> for anything beyond the most trivial default.
+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
has 'size' => (
is => 'ro',
lazy => 1,
- builder => '_build_animal',
+ builder => '_build_size',
);
package Lilliputian;
C<lazy>:
has 'size' => (
- is => 'rw',
+ is => 'ro',
lazy => 1,
builder => '_build_size',
);
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',
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',
);
Both of these goals can be accomplished with the C<init_arg> option:
has 'bigness' => (
- is => 'rw',
+ is => 'ro',
init_arg => 'size',
);
the constructor. This is particularly handy for private attributes:
has '_genetic_code' => (
- is => 'rw',
+ is => 'ro',
lazy_build => 1,
init_arg => undef,
);
);
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 is called. Second, it is also called if the
-attribute is set via a lazy default or builder.
+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.
+
+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',
);
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' },
);
use MooseX::MetaDescription;
has 'size' => (
- is => 'rw',
+ is => 'ro',
traits => ['MooseX::MetaDescription::Meta::Trait'],
description => {
html_widget => 'text_input',
=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
projects / gitmo/Moose.git / blobdiff