em.rb

require 'eventmachine'
require 'logger'

class LoggedConnection < EM::Connection
  attr_accessor :logger

  def initialize(*args)
    self.logger = args.pop[:logger]
    super
  end

  def post_init
    logger.info "post_init"
  end

  def receive_data(data)
    logger.info "receive_data: #{data}"
    EM.stop
  end
end

module Client
  def post_init
    send_data "post_init"
  end
end

EM.run do
  logger = Logger.new(STDOUT)

  EM.start_server 'localhost', 3333, LoggedConnection, :logger => logger # do |connection|
  # XXX: server block is called after post_init
  #
  #  connection.logger = logger
  # end

  EM.connect 'localhost', 3333, Client
end