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

问题描述

我希望子类从其父级继承类级实例变量，但我似乎无法弄明白。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

然后我想要这样输出：

> 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，这段代码不起作用。基本上我想为父类继承的类级实例变量定义一个默认值。对于子子类，子类中的更改将是默认值。我希望这一切都能在不改变影响其父母或兄弟姐妹的一个类的价值的情况下发生。 Class_inheritable_accessor给出了我想要的行为......但对于一个类变量。

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.

我觉得我可能会问得太多。有什么想法？

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

将这个模块包含在一个类中，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获得子类时，它都会为每个声明的类级可继承实例变量（@inheritable_attributes）设置类级实例变量。

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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