在 Ruby 中继承类级实例变量? [英] Inherit class-level instance variables in Ruby?

问题描述

我希望子类从其父类继承类级实例变量，但我似乎无法弄清楚.基本上我正在寻找这样的功能:

I want a child class to inherit a class-level instance variable from its parent, but I can't seem to figure it out. Basically I'm looking for functionality like this:

class Alpha
  class_instance_inheritable_accessor :foo #
  @foo = [1, 2, 3]
end

class Beta < Alpha
  @foo << 4
  def self.bar
    @foo
  end
end

class Delta < Alpha
  @foo << 5
  def self.bar
    @foo
  end
end

class Gamma < Beta
  @foo << 'a'
  def self.bar
    @foo
  end
end

然后我希望它像这样输出:

And then I want this to output like this:

> Alpha.bar
# [1, 2, 3]

> Beta.bar
# [1, 2, 3, 4]

> Delta.bar
# [1, 2, 3, 5]

> Gamma.bar
# [1, 2, 3, 4, 'a']

显然，此代码不起作用.基本上我想为父类中的类级实例变量定义一个默认值，它的子类继承.子类中的更改将成为子子类的默认值.我希望这一切都发生，而不会影响一个类的值而影响其父级或兄弟级.Class_inheritable_accessor 给出了我想要的行为......但对于类变量.

Obviously, this code doesn't work. Basically I want to define a default value for a class-level instance variables in the parent class, which its subclasses inherit. A change in a subclass will be the default value then for a sub-subclass. I want this all to happen without a change in one class's value affecting its parent or siblings. Class_inheritable_accessor gives exactly the behavior I want... but for a class variable.

我觉得我可能要求太多了.有什么想法吗?

I feel like I might be asking too much. Any ideas?

推荐答案

使用 mixin:

module ClassLevelInheritableAttributes
  def self.included(base)
    base.extend(ClassMethods)    
  end

  module ClassMethods
    def inheritable_attributes(*args)
      @inheritable_attributes ||= [:inheritable_attributes]
      @inheritable_attributes += args
      args.each do |arg|
        class_eval %(
          class << self; attr_accessor :#{arg} end
        )
      end
      @inheritable_attributes
    end

    def inherited(subclass)
      @inheritable_attributes.each do |inheritable_attribute|
        instance_var = "@#{inheritable_attribute}"
        subclass.instance_variable_set(instance_var, instance_variable_get(instance_var))
      end
    end
  end
end

在一个类中包含这个模块，给它两个类方法:inheritable_attributes和inherited.
继承的类方法与所示模块中的 self.included 方法的工作方式相同.每当包含此模块的类被子类化时，它会为每个声明的类级可继承实例变量 (@inheritable_attributes) 设置一个类级实例变量.

Including this module in a class, gives it two class methods: inheritable_attributes and inherited.
The inherited class method works the same as the self.included method in the module shown. Whenever a class that includes this module gets subclassed, it sets a class level instance variable for each of declared class level inheritable instance variables (@inheritable_attributes).

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