Detecting sheet was dismissed on iOS 13

Another option to get back viewWillAppear and viewDidAppear is set

let vc = UIViewController()
vc.modalPresentationStyle = .fullScreen

this option cover full screen and after dismiss, calls above methods

Here's a code example of a parent view-controller which is notified when the child view-controller it presents as a sheet (i.e., in the default iOS 13 manner) is dismissed:

public final class Parent: UIViewController, UIAdaptivePresentationControllerDelegate
{
  // This is assuming that the segue is a storyboard segue; 
  // if you're manually presenting, just set the delegate there.
  public override func prepare(for segue: UIStoryboardSegue, sender: Any?)
  {
    if segue.identifier == "mySegue" {
      segue.destination.presentationController?.delegate = self;
    }
  }

  public func presentationControllerDidDismiss(
    _ presentationController: UIPresentationController)
  {
    // Only called when the sheet is dismissed by DRAGGING.
    // You'll need something extra if you call .dismiss() on the child.
    // (I found that overriding dismiss in the child and calling
    // presentationController.delegate?.presentationControllerDidDismiss
    // works well).
  }
}

Jerland2's answer is confused, since (a) the original questioner wanted to get a function call when the sheet is dismissed (whereas he implemented presentationControllerDidAttemptToDismiss, which is called when the user tries and fails to dismiss the sheet), and (b) setting isModalInPresentation is entirely orthogonal and in fact will make the presented sheet undismissable (which is the opposite of what OP wants).

Is there a way to detect that the presented view controller sheet was dismissed?

Yes.

Some other function I can override in the parent view controller rather than using some sort of delegate?

No. "Some sort of delegate" is how you do it. Make yourself the presentation controller's delegate and override presentationControllerDidDismiss(_:).

https://developer.apple.com/documentation/uikit/uiadaptivepresentationcontrollerdelegate/3229889-presentationcontrollerdiddismiss

---

The lack of a general runtime-generated event informing you that a presented view controller, whether fullscreen or not, has been dismissed, is indeed troublesome; but it's not a new issue, because there have always been non-fullscreen presented view controllers. It's just that now (in iOS 13) there are more of them! I devote a separate question-and-answer to this topic elsewhere: Unified UIViewController "became frontmost" detection?.