NSNotificationCenter example https://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/Notifications/Articles/Registering.html#//apple_ref/doc/uid/20000723-CEGCCFID

viewcontroller.m

// here i register a MPMusicPlayerController object to the NSNotificationCenter
MPMusicPlayerController *musicPlayer = 
[MPMusicPlayerController applicationMusicPlayer];
[musicPlayer setQueueWithItemCollection: userMediaItemCollection];
[musicPlayer beginGeneratingPlaybackNotifications];

NSNotificationCenter *notificationCenter = [NSNotificationCenter defaultCenter];

[notificationCenter 
  addObserver:self
  selector:@selector(handleNowPlayingItemChanged:)
  name:@"MPMusicPlayerControllerNowPlayingItemDidChangeNotification"
  object:musicPlayer];

[notificationCenter
  addObserver:self
  selector:@selector(handlePlaybackStateChanged:)
  name:@"MPMusicPlayerCOntrollerPlaybackStateDidChangeNotification"
  object:musicPlayer];

[musicPlayer play];