grocer.rb

module Fruit
  @fruits = []
  @mutex = Mutex.new
  def self.register_class(klass)
    @mutex.synchronize do
      @fruits << klass
    end
  end

  def self.fruit_classes
    @fruits
  end

  attr_reader :fruit_color
  def set_fruit_color(color)
    Fruit.register_class(self)
    @fruit_color = color
  end
end

class Apple
  extend Fruit
  set_fruit_color :green
end

class Orange
  extend Fruit
  set_fruit_color :orange
end

class Brocolli
end

Fruit.fruit_classes.each do |klass|
  puts klass.fruit_color
end