Grand Central Dispatch is the name of Apple's collection of task concurrency functions in libdispatch. GCD (as it's often called) is packed full of interesting features involving concurrency, queuing, timers, and more. Let's dive in and take a look at some simple and common use cases:

Common Usage

One of the most common ways to use GCD is to hop to a background queue to do some work, then hop back to the main queue to update the UI:

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)) {
  let resizedAvatarImage = self.resizeImage(avatarImage)

  dispatch_async(dispatch_get_main_queue()) {
    self.avatarImageView.image = resizedAvatarImage

Dispatch Once

If we wanted to run some code in viewDidAppear: but only wanted it to run the first time, we could store some flag in a Bool property. Or just use GCD:

struct Tokens {
  static var onceToken: dispatch_once_t = 0;

dispatch_once(&Tokens.onceToken) {

Dispatch After

We can also easily wait a specified amount of time, then run some code. Here we'll use this technique to pop back to the previous view controller a quick "beat" after the user selects an an option in a table view controller. (As seen in many apps, including

let delay = 0.5
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, Int64(delay * Double(NSEC_PER_SEC))), dispatch_get_main_queue()) {