add_instance_attributes_dynamically.rb

# add a method that allows an object to add dyn. new instance methods.
module A
  klass.instance_eval do
    send(:attr_accessor, :observers)
    instance_variable_set("@#{:observers}", [])
  end
end

# including this object prepends a class' initializer
module B
  module Initializer
    def initialize
      @observers = []
      super
    end
  end

  def self.included(klass)
    klass.send :prepend, Initializer
  end
end