Inheritable class attributes

gistfile1.rb

module InheritableAttributes
  def self.included base
    base.extend ClassMethods
  end

  module ClassMethods
    def inheritable_attr *attrs
      attrs.each do |attr|
        class_eval <<-CODE
          def self.#{attr} #{attr}=nil
            @#{attr} = #{attr} unless #{attr}.nil?
            @#{attr}
          end
        CODE
      end

      @inheritable_attrs ||= [:inheritable_attrs]
      @inheritable_attrs += attrs
    end

    def inherited subclass
      @inheritable_attrs.each do |attr|
        subclass.instance_variable_set("@#{attr}", instance_variable_get("@#{attr}"))
      end
    end
  end
end


module Foo
  class Bar
    include InheritableAttributes
    inheritable_attr :platform
    platform "desktop"
  end

  class Baz < Bar
  end

  class Tar < Baz
    platform "mobile"
  end

  class Cat < Baz
  end
end

puts Foo::Bar.platform
puts Foo::Baz.platform
puts Foo::Tar.platform
puts Foo::Cat.platform