为MooseX::ClassAttribute编写Moose::Role的构建器

Question

我想定义一个具有Class属性和构建器的角色。不管我试过什么都失败了。我找到了两个解决办法：

1. 不是使用class属性，而是使用普通属性。这在我的情况下是有问题的，因为我想修改我的实际代码中的属性(对于所有实例)。
2. 将构建器放入类中(而不是角色)。这也是有问题的，因为它意味着修改所有使用此角色的类。

下面是一个很小的例子：

package MyRole;
use Moose::Role;
use MooseX::ClassAttribute;

sub _build_value {
    return "in MyRole";
}

class_has 'value' => (
    is => 'ro',
    isa => 'Str',
    builder => '_build_value',
);

1;

package Appli;
use Moose;

with 'MyRole';

1;

package main;

my $e=Appli->new();

print $e->value, "\n";

其结果是：

$ perl ./test.pl 
Appli does not support builder method '_build_value' for attribute 'value' at /usr/share/perl5/MooseX/ClassAttribute/Trait/Attribute.pm line 81.
Class::MOP::Class:::around(CODE(0x13d6c78), Moose::Meta::Class::__ANON__::SERIAL::11=HASH(0x1982460), "Appli") called at /usr/lib/x86_64-linux-gnu/perl5/5.22/Class/MOP/Method/Wrapped.pm line 164
Moose::Meta::Class::__ANON__::SERIAL::11::_wrapped__call_builder(Moose::Meta::Class::__ANON__::SERIAL::11=HASH(0x1982460), "Appli") called at /usr/lib/x86_64-linux-gnu/perl5/5.22/Class/MOP/Method/Wrapped.pm line 95
Moose::Meta::Class::__ANON__::SERIAL::11::_call_builder(Moose::Meta::Class::__ANON__::SERIAL::11=HASH(0x1982460), "Appli") called at /usr/share/perl5/MooseX/ClassAttribute/Trait/Attribute.pm line 54
MooseX::ClassAttribute::Trait::Attribute::_initialize(Moose::Meta::Class::__ANON__::SERIAL::11=HASH(0x1982460), Moose::Meta::Class::__ANON__::SERIAL::10=HASH(0x19683d0)) called at /usr/share/perl5/MooseX/ClassAttribute/Trait/Attribute.pm line 32
Class::MOP::Class:::after(Moose::Meta::Class::__ANON__::SERIAL::11=HASH(0x1982460), Moose::Meta::Class::__ANON__::SERIAL::10=HASH(0x19683d0)) called at /usr/lib/x86_64-linux-gnu/perl5/5.22/Class/MOP/Method/Wrapped.pm line 57
Moose::Meta::Class::__ANON__::SERIAL::11::_wrapped_attach_to_class(Moose::Meta::Class::__ANON__::SERIAL::11=HASH(0x1982460), Moose::Meta::Class::__ANON__::SERIAL::10=HASH(0x19683d0)) called at /usr/lib/x86_64-linux-gnu/perl5/5.22/Class/MOP/Method/Wrapped.pm line 95
Moose::Meta::Class::__ANON__::SERIAL::11::attach_to_class(Moose::Meta::Class::__ANON__::SERIAL::11=HASH(0x1982460), Moose::Meta::Class::__ANON__::SERIAL::10=HASH(0x19683d0)) called at /usr/share/perl5/MooseX/ClassAttribute/Trait/Class.pm line 66
MooseX::ClassAttribute::Trait::Class::_attach_class_attribute(Moose::Meta::Class::__ANON__::SERIAL::10=HASH(0x19683d0), Moose::Meta::Class::__ANON__::SERIAL::11=HASH(0x1982460)) called at /usr/share/perl5/MooseX/ClassAttribute/Trait/Mixin/HasClassAttributes.pm line 40
MooseX::ClassAttribute::Trait::Mixin::HasClassAttributes::add_class_attribute(Moose::Meta::Class::__ANON__::SERIAL::10=HASH(0x19683d0), Moose::Meta::Class::__ANON__::SERIAL::11=HASH(0x1982460)) called at /usr/share/perl5/MooseX/ClassAttribute/Trait/Class.pm line 41
Class::MOP::Class:::around(CODE(0x17fae38), Moose::Meta::Class::__ANON__::SERIAL::10=HASH(0x19683d0), Moose::Meta::Class::__ANON__::SERIAL::11=HASH(0x1982460)) called at /usr/lib/x86_64-linux-gnu/perl5/5.22/Class/MOP/Method/Wrapped.pm line 164
Moose::Meta::Class::__ANON__::SERIAL::10::_wrapped_add_class_attribute(Moose::Meta::Class::__ANON__::SERIAL::10=HASH(0x19683d0), Moose::Meta::Class::__ANON__::SERIAL::11=HASH(0x1982460)) called at /usr/lib/x86_64-linux-gnu/perl5/5.22/Class/MOP/Method/Wrapped.pm line 95
Moose::Meta::Class::__ANON__::SERIAL::10::add_class_attribute(Moose::Meta::Class::__ANON__::SERIAL::10=HASH(0x19683d0), Moose::Meta::Class::__ANON__::SERIAL::11=HASH(0x1982460)) called at /usr/share/perl5/MooseX/ClassAttribute/Trait/Application/ToClass.pm line 44
MooseX::ClassAttribute::Trait::Application::ToClass::_apply_class_attributes(Moose::Meta::Class::__ANON__::SERIAL::8=HASH(0x1a29940), Moose::Meta::Class::__ANON__::SERIAL::1=HASH(0x1a27298), Moose::Meta::Class::__ANON__::SERIAL::10=HASH(0x19683d0)) called at /usr/share/perl5/MooseX/ClassAttribute/Trait/Application.pm line 13
Class::MOP::Class:::after(Moose::Meta::Class::__ANON__::SERIAL::8=HASH(0x1a29940), Moose::Meta::Class::__ANON__::SERIAL::1=HASH(0x1a27298), Moose::Meta::Class::__ANON__::SERIAL::10=HASH(0x19683d0)) called at /usr/lib/x86_64-linux-gnu/perl5/5.22/Class/MOP/Method/Wrapped.pm line 57
Moose::Meta::Class::__ANON__::SERIAL::8::_wrapped_apply_attributes(Moose::Meta::Class::__ANON__::SERIAL::8=HASH(0x1a29940), Moose::Meta::Class::__ANON__::SERIAL::1=HASH(0x1a27298), Moose::Meta::Class::__ANON__::SERIAL::10=HASH(0x19683d0)) called at /usr/lib/x86_64-linux-gnu/perl5/5.22/Class/MOP/Method/Wrapped.pm line 95
Moose::Meta::Class::__ANON__::SERIAL::8::apply_attributes(Moose::Meta::Class::__ANON__::SERIAL::8=HASH(0x1a29940), Moose::Meta::Class::__ANON__::SERIAL::1=HASH(0x1a27298), Moose::Meta::Class::__ANON__::SERIAL::10=HASH(0x19683d0)) called at /usr/lib/x86_64-linux-gnu/perl5/5.22/Moose/Meta/Role/Application.pm line 59
Moose::Meta::Role::Application::apply(Moose::Meta::Class::__ANON__::SERIAL::8=HASH(0x1a29940), Moose::Meta::Class::__ANON__::SERIAL::1=HASH(0x1a27298), Moose::Meta::Class::__ANON__::SERIAL::10=HASH(0x19683d0)) called at /usr/lib/x86_64-linux-gnu/perl5/5.22/Moose/Meta/Role/Application/ToClass.pm line 31
Moose::Meta::Role::Application::ToClass::apply(Moose::Meta::Class::__ANON__::SERIAL::8=HASH(0x1a29940), Moose::Meta::Class::__ANON__::SERIAL::1=HASH(0x1a27298), Moose::Meta::Class::__ANON__::SERIAL::10=HASH(0x19683d0)) called at /usr/share/perl5/MooseX/ClassAttribute/Trait/Application/ToClass.pm line 27
Class::MOP::Class:::around(CODE(0x14ae360), Moose::Meta::Class::__ANON__::SERIAL::8=HASH(0x1a29940), Moose::Meta::Class::__ANON__::SERIAL::1=HASH(0x1a27298), Moose::Meta::Class=HASH(0x175d658), HASH(0x1a22b28)) called at /usr/lib/x86_64-linux-gnu/perl5/5.22/Class/MOP/Method/Wrapped.pm line 164
Moose::Meta::Class::__ANON__::SERIAL::8::_wrapped_apply(Moose::Meta::Class::__ANON__::SERIAL::8=HASH(0x1a29940), Moose::Meta::Class::__ANON__::SERIAL::1=HASH(0x1a27298), Moose::Meta::Class=HASH(0x175d658), HASH(0x1a22b28)) called at /usr/lib/x86_64-linux-gnu/perl5/5.22/Class/MOP/Method/Wrapped.pm line 95
Moose::Meta::Class::__ANON__::SERIAL::8::apply(Moose::Meta::Class::__ANON__::SERIAL::8=HASH(0x1a29940), Moose::Meta::Class::__ANON__::SERIAL::1=HASH(0x1a27298), Moose::Meta::Class=HASH(0x175d658), HASH(0x1a22b28)) called at /usr/lib/x86_64-linux-gnu/perl5/5.22/Moose/Meta/Role.pm line 472
Moose::Meta::Role::apply(Moose::Meta::Class::__ANON__::SERIAL::1=HASH(0x1a27298), Moose::Meta::Class=HASH(0x175d658)) called at /usr/lib/x86_64-linux-gnu/perl5/5.22/Moose/Util.pm line 172
Moose::Util::_apply_all_roles(Moose::Meta::Class=HASH(0x175d658), undef, "MyRole") called at /usr/lib/x86_64-linux-gnu/perl5/5.22/Moose/Util.pm line 114
Moose::Util::apply_all_roles(Moose::Meta::Class=HASH(0x175d658), "MyRole") called at /usr/lib/x86_64-linux-gnu/perl5/5.22/Moose.pm line 59
Moose::with(Moose::Meta::Class=HASH(0x175d658), "MyRole") called at /usr/lib/x86_64-linux-gnu/perl5/5.22/Moose/Exporter.pm line 419
Moose::with("MyRole") called at ./test.pl line 20

更新：i通过添加“懒惰的=> 1”来解决这个问题。那它就完美地工作了。如果没有“懒惰”，那么在角色完全导入之前很可能会调用构建器。

Answer 1

在这个上下文中使用lazy与其说是一种解决办法，不如说是正确的解决方案。MooseX::ClassAttribute被实现为一个Moose::Role。在MyRole完全组合成Appli之前，Perl调用class_has，后者试图调用_build_value作为Appli类的一种方法。由于该方法还没有被组合成Appli，MooseX::ClassAttribute就会出现上述错误。使用lazy将对_build_value的评估推迟到MyRole完全组合成Appli之后。