Difference between DispatchQueue.main.async and Dispatch.main.asyncAfter(.now(),{})

Question

Is there a difference between DispatchQueue.main.async and Dispatch.main.asyncAfter(.now(),{})

My question is specifically about asyncAfter with .now() as the delay time.

The second question is if both methods allow the current UI thread to end before executing the closure in the next runloop?

I find that displaying an alert using UIAlertController in the closure of the former causes some erratic behavior. And with the latter, together with a delay of about 0.3 seconds, it works well.

Answer 1

There is no difference if you chooses to use asyncAfter with .now. Here is the proof. Both function should start executing immediately after getting called

Answer 2

There is no difference. I tested by invoking it 100 times, and the callbacks are executed in the same order they're enqueued.

I further tested by having 100 queues, to see how it behaves with parallelism, and the callbacks are still executed in the same order as they are enqueued. So, the conclusion is that asyncAfter(.now()) is same as async:

func testOneThread() {
  var lastCallbackRun = 0

  for i in 1...100 {
    DispatchQueue.main.asyncAfter(.now()) {
      assert(lastCallbackRun < i)
      lastCallbackRun = i
      if i == 100 {
        NSLog("done")
      }
    }
  }
}

func testAsync() {
  for _ in 0...100 {
    let q = DispatchQueue(label: "whatever")
    q.async(execute: testOneThread)
    Unmanaged<DispatchQueue>.passRetained(q)
  }
}

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {

  func application(_ application: UIApplication,
                   didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {
    testAsync()
    // The rest of it.
  }
}