使用 Moose，如何设置“ro”的值？属性 Trait，在运行时？

发布于 2024-10-03 07:06:35 字数 438 浏览 7 评论 0原文

我有一个属性特征，我想在其他类属性的基础上设置它。我真的想要一个属性特征的默认值，它获取类 $self 的副本，而不是属性的元。我想在我的班级中做这样的事情：

after 'BUILD' => sub {                                                     
  my $self = shift;                                                        
  $self->meta->get_attribute('id')->column_name( $self->_unique_key_name );
};

但是，我想保留我的属性特质RO？这可能吗。我知道 MOP 允许设置类属性的值，但我不知道如何在元属性上设置属性。

原文

I've got an Attribute Trait that I want to set on the basis of other class attributes. I realy want a default on an Attribute Trait that gets a copy of the class $self and not the meta for the attribute. I want to do something like this in my class:

after 'BUILD' => sub {                                                     
  my $self = shift;                                                        
  $self->meta->get_attribute('id')->column_name( $self->_unique_key_name );
};

But, I want to keep my attribute trait RO? Is this possible. I know the MOP allows one to set the value of a class-attribute, but I can't figure out how to set an attribute on the meta-attribute.

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时光无声 2024-10-10 07:06:35

这听起来像是一个非常奇怪的设计（为什么元类需要它所描述的类的实例？）——但是您可以通过使用元类的元类轻松地做到这一点（请记住，Moose 元类是使用 MOP 引导的本身）：

$self->meta->meta->get_attribute("foo")->default($some_value);

还请记住，如果默认值本身是引用，则需要将它们包装在 coderef 中： $some_value = sub { $instance };

实际上，这不起作用 - 'default' 是只读的。我建议您重新考虑您的设计，而不是在如此低的级别上摆弄 MOP - 例如，将您的“默认”存储在另一个属性中，并编写一个委托给它的默认子：

package MyApp::Meta::Attribute::Trait::Foo;

# set at runtime, when we have an instance to store here
has _default_of_foo => (
    is => 'rw', isa => 'Object',
);
has foo => (
    is => 'ro', isa => 'Object',
    lazy => 1,
    default => sub { shift->_default_of_foo },
);

This sounds like a really odd design (why would a metaclass need an instance of a class it is describing?) -- but you can do this easily enough by using the metaclass of the metaclass (remember that Moose meta classes are bootstrapped using the MOP itself):

$self->meta->meta->get_attribute("foo")->default($some_value);

Also remember that defaults need to be wrapped in a coderef if they are references themselves: $some_value = sub { $instance };

Actually, this won't work - 'default' is read only. Rather than fiddling with the MOP at such a low level, I would urge you to reconsider your design - e.g. store your 'default' in another attribute, and writing a default sub that delegated to it:

package MyApp::Meta::Attribute::Trait::Foo;

# set at runtime, when we have an instance to store here
has _default_of_foo => (
    is => 'rw', isa => 'Object',
);
has foo => (
    is => 'ro', isa => 'Object',
    lazy => 1,
    default => sub { shift->_default_of_foo },
);

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