NAME¶
Moose::Cookbook::Meta::Labeled_AttributeTrait - Labels implemented via attribute
traits
VERSION¶
version 2.0603
SYNOPSIS¶
package MyApp::Meta::Attribute::Trait::Labeled;
use Moose::Role;
Moose::Util::meta_attribute_alias('Labeled');
has label => (
is => 'rw',
isa => 'Str',
predicate => 'has_label',
);
package MyApp::Website;
use Moose;
has url => (
traits => [qw/Labeled/],
is => 'rw',
isa => 'Str',
label => "The site's URL",
);
has name => (
is => 'rw',
isa => 'Str',
);
sub dump {
my $self = shift;
my $meta = $self->meta;
my $dump = '';
for my $attribute ( map { $meta->get_attribute($_) }
sort $meta->get_attribute_list ) {
if ( $attribute->does('MyApp::Meta::Attribute::Trait::Labeled')
&& $attribute->has_label ) {
$dump .= $attribute->label;
}
else {
$dump .= $attribute->name;
}
my $reader = $attribute->get_read_method;
$dump .= ": " . $self->$reader . "\n";
}
return $dump;
}
package main;
my $app = MyApp::Website->new( url => "http://google.com", name => "Google" );
SUMMARY¶
In this recipe, we begin to delve into the wonder of meta-programming. Some
readers may scoff and claim that this is the arena of only the most twisted
Moose developers. Absolutely not! Any sufficiently twisted developer can
benefit greatly from going more meta.
Our goal is to allow each attribute to have a human-readable "label"
attached to it. Such labels would be used when showing data to an end user. In
this recipe we label the "url" attribute with "The site's
URL" and create a simple method showing how to use that label.
All the attributes of a Moose-based object are actually objects themselves.
These objects have methods and attributes. Let's look at a concrete example.
has 'x' => ( isa => 'Int', is => 'ro' );
has 'y' => ( isa => 'Int', is => 'rw' );
Internally, the metaclass for "Point" has two Moose::Meta::Attribute
objects. There are several methods for getting meta-attributes out of a
metaclass, one of which is "get_attribute_list". This method is
called on the metaclass object.
The "get_attribute_list" method returns a list of attribute names. You
can then use "get_attribute" to get the Moose::Meta::Attribute
object itself.
Once you have this meta-attribute object, you can call methods on it like this:
print $point->meta->get_attribute('x')->type_constraint;
=> Int
To add a label to our attributes there are two steps. First, we need a new
attribute metaclass trait that can store a label for an attribute. Second, we
need to apply that trait to our attributes.
TRAITS¶
Roles that apply to metaclasses have a special name: traits. Don't let the
change in nomenclature fool you,
traits are just roles.
"has" in Moose allows you to pass a "traits" parameter for
an attribute. This parameter takes a list of trait names which are composed
into an anonymous metaclass, and that anonymous metaclass is used for the
attribute.
Yes, we still have lots of metaclasses in the background, but they're managed by
Moose for you.
Traits can do anything roles can do. They can add or refine attributes, wrap
methods, provide more methods, define an interface, etc. The only difference
is that you're now changing the attribute metaclass instead of a user-level
class.
DISSECTION¶
We start by creating a package for our trait.
package MyApp::Meta::Attribute::Trait::Labeled;
use Moose::Role;
has label => (
is => 'rw',
isa => 'Str',
predicate => 'has_label',
);
You can see that a trait is just a Moose::Role. In this case, our role contains
a single attribute, "label". Any attribute which does this trait
will now have a label.
We also register our trait with Moose:
Moose::Util::meta_attribute_alias('Labeled');
This allows Moose to find our trait by the short name "Labeled" when
passed to the "traits" attribute option, rather than requiring the
full package name to be specified.
Finally, we pass our trait when defining an attribute:
has url => (
traits => [qw/Labeled/],
is => 'rw',
isa => 'Str',
label => "The site's URL",
);
The "traits" parameter contains a list of trait names. Moose will
build an anonymous attribute metaclass from these traits and use it for this
attribute.
The reason that we can pass the name "Labeled", instead of
"MyApp::Meta::Attribute::Trait::Labeled", is because of the
"register_implementation" code we touched on previously.
When you pass a metaclass to "has", it will take the name you provide
and prefix it with "Moose::Meta::Attribute::Custom::Trait::". Then
it calls "register_implementation" in the package. In this case,
that means Moose ends up calling "register_implementation" from
"Moose::Meta::Attribute::Custom::Trait::Labeled".
If this function exists, it should return the
real trait's package name.
This is exactly what our code does, returning
"MyApp::Meta::Attribute::Trait::Labeled". This is a little
convoluted, and if you don't like it, you can always use the fully-qualified
name.
We can access this meta-attribute and its label like this:
$website->meta->get_attribute('url')->label()
MyApp::Website->meta->get_attribute('url')->label()
We also have a regular attribute, "name":
has name => (
is => 'rw',
isa => 'Str',
);
Finally, we have a "dump" method, which creates a human-readable
representation of a "MyApp::Website" object. It will use an
attribute's label if it has one.
sub dump {
my $self = shift;
my $meta = $self->meta;
my $dump = '';
for my $attribute ( map { $meta->get_attribute($_) }
sort $meta->get_attribute_list ) {
if ( $attribute->does('MyApp::Meta::Attribute::Trait::Labeled')
&& $attribute->has_label ) {
$dump .= $attribute->label;
}
This is a bit of defensive code. We cannot depend on every meta-attribute having
a label. Even if we define one for every attribute in our class, a subclass
may neglect to do so. Or a superclass could add an attribute without a label.
We also check that the attribute has a label using the predicate we defined. We
could instead make the label "required". If we have a label, we use
it, otherwise we use the attribute name:
else {
$dump .= $attribute->name;
}
my $reader = $attribute->get_read_method;
$dump .= ": " . $self->$reader . "\n";
}
return $dump;
}
The "get_read_method" is part of the Moose::Meta::Attribute API. It
returns the name of a method that can read the attribute's value,
when
called on the real object (don't call this on the meta-attribute).
CONCLUSION¶
You might wonder why you'd bother with all this. You could just hardcode
"The Site's URL" in the "dump" method. But we want to
avoid repetition. If you need the label once, you may need it elsewhere, maybe
in the "as_form" method you write next.
Associating a label with an attribute just makes sense! The label is a piece of
information
about the attribute.
It's also important to realize that this was a trivial example. You can make
much more powerful metaclasses that
do things, as opposed to just
storing some more information. For example, you could implement a metaclass
that expires attributes after a certain amount of time:
has site_cache => (
traits => ['TimedExpiry'],
expires_after => { hours => 1 },
refresh_with => sub { get( $_[0]->url ) },
isa => 'Str',
is => 'ro',
);
The sky's the limit!
AUTHOR¶
Moose is maintained by the Moose Cabal, along with the help of many
contributors. See "CABAL" in Moose and "CONTRIBUTORS" in
Moose for details.
COPYRIGHT AND LICENSE¶
This software is copyright (c) 2012 by Infinity Interactive, Inc..
This is free software; you can redistribute it and/or modify it under the same
terms as the Perl 5 programming language system itself.