Ordinary roles can require that its consumers have a particular list of method names. Since parameterized roles have direct access to its consumer,you can inspect it and throw errors if the consumer does not meet your needs. 07001

角色专业化的细节与增强的类保持一致;它甚至不需要传递任何参数,它需要知道的是传递给角色的参数(要包装的字段列表).唯一的关键是必须在类上定义相关属性后使用该角色.

因此,消费类和角色的定义如下：

package My::Foo;
use Moose;

my @fields = qw(attr1 attr2);

has \@fields => (
    is => 'rw',default => sub { \@fields },);

with 'My::Role::X' => {};

1;

package My::Role::X;
use MooseX::Role::Parameterized;

role {
    my $p = shift;

    my %args = @_;

    # this should be a Moose::Meta::Class object
    my $target_Meta = $args{consumer};

    # get Class::MOP::Attribute object out of the Metaclass
    my $fields_attr = $target_Meta->find_attribute_by_name('fields');

    # extract the value of this attribute - should be a coderef
    my $fields_to_modify = $fields_attr->default;

    # evaluate the coderef to get the arrayref
    $fields_to_modify = &$fields_to_modify if ref $fields_to_modify eq 'CODE';

    around $_ => sub {
        # ...
    } for @$fields_to_modify;
};

1;

附录：我发现如果参数化角色使用另一个参数化角色,那么嵌套角色中的$target_Meta实际上将是父角色的元类(isa MooseX :: Role :: Parameterized :: Meta :: Role ::参数化),而不是消费类的元类(isa Moose :: Meta :: Class).为了派生适当的元类,您需要将其显式传递为参数.我已将此作为“最佳实践”模板添加到我的所有参数化角色中：

package MyApp::Role::SomeRole;

use MooseX::Role::Parameterized;

# because we are used by an earlier role,Meta is not actually the Meta of the
# consumer,but of the higher-level parameterized role.
parameter Metaclass => (
    is => 'ro',isa => 'Moose::Meta::Class',required => 1,);

# ... other parameters here...

role {
    my $params = shift;
    my %args = @_;

    # isa a Moose::Meta::Class
    my $Meta = $params->Metaclass;

    # class name of what is consuming us,om nom nom
    my $consumer = $Meta->name;

    # ... code here...

}; # end role
no Moose::Role;
1;

附录2：我进一步发现,如果将角色应用于对象实例而不是类,则角色中的$target_Meta实际上将是执行消耗的对象的类：

package main;
use My::Foo;
use Moose::Util;

my $foo = My::Foo->new;
Moose::Util::apply_all_roles($foo,MyApp::Role::SomeRole,{ parameter => 'value' });

package MyApp::Role::SomeRole;
use MooseX::Role::Parameterized;
# ... use same code as above (in addendum 1):

role {
    my $Meta = $args{consumer};
    my $consumer = $Meta->name;     # fail! My::Foo does not implement the 'name' method

因此,在参数化角色的开头提取元类时,此代码是必需的：

role {
    my $params = shift;
    my %args = @_;

    # could be a Moose::Meta::Class,or the object consuming us
    my $Meta = $args{consumer};
    $Meta = $Meta->Meta if not $Meta->isa('Moose::Meta::Class');   # <-- important!