Ruby meta - instance_eval inside a block

gistfile1.rb

# Demonstrates a basic module mixin and a slightly advanced meta technique:
# the use_socket (line 13) wrapper method connects to a server and issues
# various SocketIO commands. I use instance_eval (line 16) with a recipient
# SocketIO object, not an instance of the caller (NodeMessenger).
# instance_eval runs the passed block with receiver of self, which is a
# SocketIO object.

# The general purpose of this code is to talk to a Socket.io server
require 'SocketIO'
module NodeSocket
  class NodeCantConnect < StandardError; end

  def use_socket(&block)
    SocketIO.connect(@chat.location) do
      after_start do
        instance_eval &block
        disconnect
      end
    end
    rescue Errno::ECONNREFUSED
      raise NodeCantConnect
  end

  private :use_socket
end

# node_messenger
require 'node_socket'
# def: send messages and events to chat sessions
class NodeMessenger
  include NodeSocket

  def initialize(chat)
    @chat = chat
  end

  # expects hash with keys :name and :data
  # users will see this
  def chat(msg)
    use_socket do
      send_event 'systemMessages', msg
    end
  end

  def startup
    use_socket do
      send_event 'startup', nil
    end
  end

  def shutdown
    use_socket do
      send_event 'shutdown', nil
    end
  end
end