观察者模式

require 'observer'

class User
  include Observable
  attr_accessor :username
  attr_accessor :password

  def initialize(username, password)
    @username = username
    @password = password
  end

  def change_username
    changed
    @username = "789"
    notify_observers
  end

end

class Foo
  def update
    puts "attribute has been updated"
  end
end

  user = User.new("123", "456")
  user.add_observer(Foo.new)
  user.change_username