sleeper.rb

#!/usr/bin/env ruby -wKU

require "osx/cocoa"
include OSX

class Sleeper < NSObject

  def initialize
    NSWorkspace.sharedWorkspace.notificationCenter.objc_send(
      :addObserver, self,
      :selector,    :on_sleep,
      :name,        NSWorkspaceWillSleepNotification,
      :object,      nil
    ) 
  end
  
  def on_sleep(notification)
    p :notification => notification
  end
  
end

Sleeper.new
NSApplication.sharedApplication.run