gistfile1.rb

#!/usr/bin/env ruby -wKU

require "osx/cocoa"
include OSX

require_framework "ScriptingBridge"

class NextPreviewPlayer < NSObject

  def initialize
    @itunes         = SBApplication.applicationWithBundleIdentifier 'com.apple.iTunes'
    @default_center = NSDistributedNotificationCenter.defaultCenter
    @played         = 0
    
    add_observer
  end
  
  def player_info(notification)
    return unless (info = notification.userInfo).has_key?('Store URL')
    
    state = info['Player State']
    track = info['Track Number']
    
    case state
    when 'Stopped'
      play_next
    when 'Playing'
      track == @played ? stop : @played = track
    end
  end
  
  private
  
  def play_next
    dont_observe { @itunes.nextTrack }
    @itunes.playpause
  end
  
  def stop
    dont_observe { @itunes.stop }
    @played = 0
  end
  
  
  def add_observer
    @default_center.objc_send(:addObserver, self,
                                 :selector, :player_info,
                                     :name, 'com.apple.iTunes.playerInfo',
                                   :object, nil)
  end
  
  def remove_observer
    @default_center.removeObserver(self)
  end
  
  def dont_observe
    remove_observer
    yield
    add_observer
  end

  
end

NextPreviewPlayer.new
NSApplication.sharedApplication.run