ObserverPattern | project demostrates how correctly implement Observer pattern | Pub Sub library

You're using a UITabBarController to host the views. The system initially only loads the view controller it needs to display (ViewController in this case). Then, once you click on the tab for the SecondVC, it loads that view controller.

You can verify this by putting a print statement in viewDidLoad of SecondVC.

You can also verify that if you navigate to SecondVC before going back to ViewController and pressing Notify, both view controllers get notified in this scenario.

So, it's not a bug -- it's just an implementation detail of when the views are loaded.

If you want to find a way to make sure that SecondVC has access to that information when it's loaded, you have two options:

1. Rely on a different system of propagating the state
2. Put your notification listener in required init?(coder: NSCoder) instead of viewDidLoad (this does get called during setup). This has a caveat though: the UILabel won't be loaded yet, so you'd have to store that state for loading later. Trying to access mySecondLabel before viewDidLoad will result in a crash.

Update Updated code that stores the Notification in the event that you want to use the init method: