GCD.swift

//
// Reference: https://www.raywenderlich.com/79150/grand-central-dispatch-tutorial-swift-part-2
//

var GlobalMainQueue: dispatch_queue_t {
  return dispatch_get_main_queue()
}
 
var GlobalUserInteractiveQueue: dispatch_queue_t {
  return dispatch_get_global_queue(Int(QOS_CLASS_USER_INTERACTIVE.value), 0)
}
 
var GlobalUserInitiatedQueue: dispatch_queue_t {
  return dispatch_get_global_queue(Int(QOS_CLASS_USER_INITIATED.value), 0)
}
 
var GlobalUtilityQueue: dispatch_queue_t {
  return dispatch_get_global_queue(Int(QOS_CLASS_UTILITY.value), 0)
}
 
var GlobalBackgroundQueue: dispatch_queue_t {
  return dispatch_get_global_queue(Int(QOS_CLASS_BACKGROUND.value), 0)
}

//
// Dispatch Group
//
func downloadPhotosWithCompletion(completion: BatchPhotoDownloadingCompletionClosure?) {
  var storedError: NSError!
  var downloadGroup = dispatch_group_create()
 
  for address in [OverlyAttachedGirlfriendURLString, SuccessKidURLString, LotsOfFacesURLString]
  {
    let url = NSURL(string: address)
    dispatch_group_enter(downloadGroup)
    let photo = DownloadPhoto(url: url!) {
      image, error in
      if let error = error {
        storedError = error
      }
      dispatch_group_leave(downloadGroup)
    }
    PhotoManager.sharedManager.addPhoto(photo)
  }
 
  dispatch_group_notify(downloadGroup, GlobalMainQueue) {
    if let completion = completion {
      completion(error: storedError)
    }
  }
}

//
// Dispatch Barrier
//
private let concurrentPhotoQueue = dispatch_queue_create("com.raywenderlich.GooglyPuff.photoQueue", DISPATCH_QUEUE_CONCURRENT)

func addPhoto(photo: Photo) {
  dispatch_barrier_async(concurrentPhotoQueue) { // 1
    self._photos.append(photo) // 2
    dispatch_async(GlobalMainQueue) { // 3
      self.postContentAddedNotification()
    }
  }
}

var photos: [Photo] {
  var photosCopy: [Photo]!
  dispatch_sync(concurrentPhotoQueue) { // 1
    photosCopy = self._photos // 2
  }
  return photosCopy
}

//
// Dispatch Source
//
#if DEBUG
private var signalSource: dispatch_source_t!
private var signalOnceToken = dispatch_once_t()
#endif
 
override func viewDidLoad() {
  super.viewDidLoad()
 
  #if DEBUG 
  dispatch_once(&signalOnceToken) { 
    let queue = dispatch_get_main_queue()
    self.signalSource = dispatch_source_create(DISPATCH_SOURCE_TYPE_SIGNAL, UInt(SIGSTOP), 0, queue) 
    if let source = self.signalSource { 
      dispatch_source_set_event_handler(source) { 
        NSLog("Hi, I am: \(self.description)")
      }
      dispatch_resume(source)
    }
  }
  #endif
}

(lldb) po self.navigationItem.prompt = "WOOT!"
  
(lldb) e let $vc = unsafeBitCast(0x7fd0b3e22bc0, GooglyPuff.PhotoCollectionViewController.self)
(lldb) po $vc.navigationItem.prompt = "WOOT!"