em-dbus.rb

require 'dbus'
require 'eventmachine'

module EM
  module Avahi
    AVAHI         = "org.freedesktop.Avahi"
    AVAHI_SERVER  = "org.freedesktop.Avahi.Server"
    AVAHI_BROWSER = "org.freedesktop.Avahi.ServiceBrowser"

    class ServiceBrowser
      def initialize
        @bus        = DBus::SystemBus.instance
        @server     = @bus.introspect(AVAHI, '/')[AVAHI_SERVER]
        @connection = EM.watch(@bus.socket, DBusHandler) do |handler|
          handler.bus = @bus
        end
        @connection.notify_readable = true
      end

      def browse(service, &blk)
        browser = @server.ServiceBrowserNew(-1, -1, service, "", 0)
        matcher = DBus::MatchRule.new.tap do |mr|
          mr.type      = "signal"
          mr.interface = AVAHI_BROWSER
          mr.path      = browser.first
        end

        @bus.add_match(matcher) do |msg, _|
          blk.call(msg)
        end
      end
    end

    module DBusHandler
      attr_accessor :bus

      def notify_readable
        bus.update_buffer
        while msg = bus.pop_message
          bus.process(msg)
        end
      end
    end
  end
end

EM.run do
  trap("INT") { EM.stop }

  avahi = EM::Avahi::ServiceBrowser.new
  avahi.browse('_presence._tcp') do |msg|
    puts msg.inspect
  end
end