x.rb

class EventHandler
  def initialize
    @callbacks = Hash.new { |h,k| h[k] = [] }
  end

  def on(event, callback)
    @callbacks[event] << callback
  end

  def trigger(event, *args)
    @callbacks[event].each { |callback| callback.call(*args) }
  end
end

class Mailer
  def self.call(message)
    new(message).deliver
  end
    
  def initialize(message)
    @message = message
  end

  def deliver
    puts @message
  end
end

class RedisStats
  def self.call(message)
    new(message).analyze
  end

  def initialize(message)
    @message = message
  end

  def analyze
    puts @message.length
  end
end

event_handler = EventHandler.new

event_handler.on :auto_posted, Mailer
event_handler.on :auto_posted, RedisStats

event_handler.trigger(:auto_posted, "Hello World")