open-closed

event_subscriber.rb

class EventSubscriber
  
  def self.inherited subscriber
    @subscribers ||= []
    @subscribers << subscriber
  end
  
  def self.all
    @subscribers ||= []
  end
  
  def self.subscribers_for event
    all.select { |s| s.subscribe? event }
       .map    { |s| s.new }
  end
    
  def self.publish event
    subscribers_for(event).each { |s| s.execute event }
  end
  
  def execute event
    # do something, or initiate a new job
  end
  
end

send_an_email_for_new_accounts.rb

class SendAnEmailForNewAccounts < EventSubscriber
  
  def self.subscribe? event
    event.type == 'new account'
  end
  
  def execute event
    account = Account.find event.account_id
    EmailSender.send_email_for account
  end
  
end

z_destroy_robots.rb

class DestroyRobots
  
  def self.subscribe? event
    event.type == 'robot appeared'
  end
  
  def execute event
    robot = Robot.find event.robot_id
    RobotSmasher.smash robot
  end
  
end