Ruby でファイルをモニタリングするとこんな感じなのかなぁ？

monitor.rb

require 'fssm'
require 'thread'

PATH = "/Users/monzou/sample.txt"

class Monitor

  def start(&block)
    puts "start monitoring @ #{Thread.current}"
    monitor = FSSM::Monitor.new
    monitor.path(File.dirname(PATH), File.basename(PATH)) do
      update(&block)
    end
    thread = Thread.new { monitor.run }
    thread.join
  end

end

class Model

  attr_accessor :value

  def initialize
    @value = ""
  end

end

puts "boot @ #{Thread.current}"

model = Model.new
monitor = Monitor.new
mutex = Mutex.new

monitor.start do
  puts "update @ #{Thread.current}"
  # is it thread-safe ?
  mutex.synchronize do
    model.value << "+"
    puts model.value
  end
end