ball.rb

# A ping pong ball, which pings and pongs properly
class Ball

  SOUNDS = %w(ping pong)

  def initialize
    @hit_count = 0
  end

  def hit
    @hit_count += 1
    puts SOUNDS[@hit_count.remainder(2)] + " hit #{@hit_count} times"
  end

end

client_player.rb

#client player, connects to the server and starts playing
#as soon as it receives the ball from the server
require 'ball'
require 'player'

EM.run {
  EM.connect("127.0.0.1", 9797, Player)
}

player.rb

#ping pong player common behaviour, hit the ball
#sending it to the other end
require 'rubygems'
require 'eventmachine'

module Player
    
  include EM::P::ObjectProtocol

  def receive_object(ball)
    ball.hit
    send_object(ball)
  end
  
  def unbind
    puts "game stopped"
  end

end

server_player.rb

#server player, serves the ball to the client as
#soon as a client connects.
#It can play with many clients at the same time
require 'ball'
require 'player'

module ServerPlayer

  include Player

  def post_init
    puts "client connected starting serving ball"
    send_object(Ball.new)
  end

end

EM.run {
  EM.start_server("127.0.0.1", 9797, ServerPlayer)
}