Skip site navigation (1)Skip section navigation (2)

FreeBSD Manual Pages

  
 
  

home | help
Moose::Cookbook::Meta:UsereConMributedoPerloDocueta::Labeled_AttributeTrait(3)

NAME
       Moose::Cookbook::Meta::Labeled_AttributeTrait - Labels implemented via
       attribute traits

VERSION
       version 2.2013

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.

META-ATTRIBUTE OBJECTS
       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
       "Moose::Meta::Attribute::Custom::Trait::Labeled::register_implementation".

       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!

AUTHORS
       o   Stevan Little <stevan.little@iinteractive.com>

       o   Dave	Rolsky <autarch@urth.org>

       o   Jesse Luehrs	<doy@tozt.net>

       o   Shawn M Moore <code@sartak.org>

       o   xxxx	x<section>xx'xx	(Yuval Kogman) <nothingmuch@woobling.org>

       o   Karen Etheridge <ether@cpan.org>

       o   Florian Ragwitz <rafl@debian.org>

       o   Hans	Dieter Pearcey <hdp@weftsoar.net>

       o   Chris Prather <chris@prather.org>

       o   Matt	S Trout	<mst@shadowcat.co.uk>

COPYRIGHT AND LICENSE
       This software is	copyright (c) 2006 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.

perl v5.32.0		      Moose::Cookbook::Meta::Labeled_AttributeTrait(3)

NAME | VERSION | SYNOPSIS | SUMMARY | META-ATTRIBUTE OBJECTS | TRAITS | DISSECTION | CONCLUSION | AUTHORS | COPYRIGHT AND LICENSE

Want to link to this manual page? Use this URL:
<https://www.freebsd.org/cgi/man.cgi?query=Moose::Cookbook::Meta::Labeled_AttributeTrait&sektion=3&manpath=FreeBSD+12.2-RELEASE+and+Ports>

home | help