exposable.rb

require 'set'

module Lotus
  module Action
    module Exposable
      def self.included(base)
        base.extend(ClassMethods)
      end

      module ClassMethods
        def attr_reader(*names)
          self.attributes = names
          super
        end

        def attributes=(names)
          attributes
          @attributes += names
        end

        def attributes
          @attributes ||= Set.new
        end
      end

      def attributes
        self.class.attributes.each_with_object({}) do |attribute, result|
          result[attribute.to_sym] = public_send(attribute)
        end
      end
    end
  end
end

show.rb

class Show
  include Lotus::Action::Exposable
  attr_reader :color
  def call
    @color = '#c0ffee'
  end
end

__END__

> s = Show.new
=> #<Show:0x007f86b8aa0938>
> Show.attributes
=> #<Set: {:color}>
> s.call
=> "#c0ffee"
> s.attributes
=> {:color=>"#c0ffee"}