ddd_example3_2.rb

class Post < ActiveRecord::Base
  has_many :comments
end

class TwitterNotification < ActiveRecord::Observer
  observe :post
 
  def after_create post
    TwitterService.send_tweet post.subject
  end
end
 
class TwitterService
  def self.send_tweet subject
    twitter = Twitter.login(username, password)
    twitter.send_tweet generate_tweet_from_subject(subject)
  end
  
  ...
end