ios - Unified UIViewController "became frontmost" detection?

Question

In my mind, these situations are all parallel:

• My view controller presented another view controller fullscreen, which has now been dismissed

• My view controller presented another view controller not fullscreen, which has now been dismissed

• My view controller presented a popover, which has now been dismissed

• My view controller pushed another view controller, which has now been popped

In every case, my view controller ceased to be the "frontmost" view controller, and then became "frontmost" again. I find it curious that iOS has no single blanket "became frontmost" event sent to my view controller that covers all these situations.

I think I can cover each of those cases individually, and I think those are all the cases I need to cover, but the resulting code is confusing and scattered:

• viewDidAppear detects popping of a pushed view controller and dismissal of a fullscreen presented view controller

• popover delegate message detect dismissal of a popover

• not sure what detects dismissal of a nonfullscreen presented view controller

How do people handle this coherently and elegantly?

Answer 1

What the cases have in common is not the appearance of the original view controller but the disappearance of the presented/pushed view controller. Therefore, one simple and clear solution seems to be a protocol-and-delegate architecture. Declare a pair of protocols, as follows:

protocol Home : class {
    func comingHome()
}
protocol Away : class {
    var home : Home? {get set}
}
extension Away where Self : UIViewController {
    func notifyComingHome() {
        if self.isBeingDismissed || self.isMovingFromParent {
            self.home?.comingHome()
        }
    }
}

• The home view controller must adopt Home, and must set each view controller's home to self when it presents or pushes it.

• The presented or pushed view controllers must adopt Away, and must implement viewWillDisappear as follows:

  override func viewWillDisappear(_ animated: Bool) {
      super.viewWillDisappear(animated)
      self.notifyComingHome()
  }
  

This works for the four cases listed in the question. It's a pity, though, that Cocoa Touch doesn't do this for you automatically.

---

EDIT This approach has become even more important in my apps now that iOS 13 has forced nonfullscreen presented view controllers upon us. Also, I have subclassed UIAlertController so that it conforms to Away.