DispatchQueue.main.async {} in viewDidLoad

Question

I was wondering what happens if I call something asynchronously in main queue from viewDidLoad method. A little experiment showed me these results:

This code:

override func viewDidLoad() {
    super.viewDidLoad()

    firstSelector()
    DispatchQueue.main.async {
        self.secondSelector()
    }
    for i in 1...10 {
        print(i)
    }
    thirdSelector()

}

func firstSelector() {
    print("First selector fired")
}

func secondSelector() {
    print("Second selector fired")
}

func thirdSelector() {
    print("Third selector fired")
}

Gives these prints:

First selector fired
1
2
3
4
5
6
7
8
9
10
Third selector fired
Second selector fired

So the last one method that was called is secondSelector. I think this is because main queue is serial and when I call asynchronously some method (secondSelector in this case) it returns immediately and waits until all other methods will be completed. After queue is free of tasks it completes method that I called asynchronously. Am I right in my thoughts?

Answer 1

I used to ask a similar question. Let me quote the important part of the answer I got:

"Because the default runloop on the main thread has the special behaviour that, when run, it also processes for the main dispatch queue..."

When you do dispatch_async to a main thread, your block { self.secondSelector() } gets scheduled to a main run loop. Since viewDidLoad method is already being executed by the main run loop, your dispatched block with be processed after viewDidLoad and all other (possible) blocks or methods that were scheduled before your block will be executed.

Keep in mind that your question is about behaviour of dispatch_async when you dispatch to the main queue and main run loop from the main thread. viewDidLoad has nothing to do with it - the only thing where it is relevant here, is that UIViewController's lifecycle methods like viewDidLoad, viewWillAppear etc are all run on the main thread (processed by a main run loop). You will see the same behaviour with any method other than viewDidLoad given this method is run on a main thread. If you call dispatch_async from other thread you might be surprised by different results because in that case you will have two threads working at the same time (your other thread and a main thread to which you dispatched).