How do I replicate class_inheritable_accessor's behavior in Rails 3.1?

问题

Beginning with Rails 3.1, class_inheritable_accessor produces deprecation warnings, telling me to use class_attribute instead. But class_attribute behaves differently in an important way which I will demonstrate.

A typical use of class_inheritable_attribute would be a presenter class, like so:

module Presenter
  class Base
    class_inheritable_accessor :presented
    self.presented = {}

    def self.presents(*types)
      types_and_classes = types.extract_options!
      types.each {|t| types_and_classes[t] = t.to_s.tableize.classify.constantize }
      attr_accessor *types_and_classes.keys
      types_and_classes.keys.each do |t|
        presented[t] = types_and_classes[t]
      end
    end
  end
end

class PresenterTest < Presenter::Base
  presents :user, :person
end

Presenter::Base.presented => {}
PresenterTest.presented => {:user => User, :person => Person}

But using class_attribute, subclasses will pollute their parents:

Presenter::Base => {:user => User, :person => Person}

Which is not desired behavior at all. Is there another type of accessor that behaves the correct way, or do I need to switch to another pattern altogether? How should I replicate the same behavior without class_inheritable_accessor?

回答1:

class_attribute won't pollute its parent if it's used as intended. Make sure you're not changing the mutable items in-place.

types_and_classes.keys.each do |t|
  self.presented = presented.merge({t => types_and_classes[t]})
end

回答2:

Give this a try, a great way to have attributes set on the class, and not polluted by inheritance or instances.

class Class

  private

  # Sets Unique Variables on a resource
  def inheritable_attr_accessor(*attrs)
    attrs.each do |attr|

      # Class Setter, Defining Setter
      define_singleton_method "#{attr}=".to_sym do |value|
        define_singleton_method attr do
          value
        end
      end

      # Default to nil
      self.send("#{attr}=".to_sym, nil)

      # Instance Setter
      define_method "#{attr}=".to_sym do |value|
        eval("@_#{attr} = value")
      end

      # Instance Getter, gets class var if nil
      define_method attr do
        eval("defined?(@_#{attr})") ? eval("@_#{attr}") : self.class.send(attr)
      end

    end
  end

end

来源：https://stackoverflow.com/questions/6617769/how-do-i-replicate-class-inheritable-accessors-behavior-in-rails-3-1