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

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.

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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.