Как использовать PayPal от настольного приложения? [закрытый]

В контроллере B вида реализуйте либо viewWillAppear, либо viewDidAppear. Там, используйте isMovingToParent и isBeingPresented, чтобы увидеть, при каких условиях оно появляется:

override func viewWillAppear(_ animated: Bool) {
    super.viewWillAppear(animated)

    if !isBeingPresented && !isMovingToParent {
        // this view controller is becoming visible because something that was covering it has been dismissed or popped
    }
}

Ниже приведено более общее использование этих свойств, которые могут оказаться полезными для людей:

override func viewWillAppear(_ animated: Bool) {
    super.viewWillAppear(animated)

    if isMovingToParent {
        // this view controller is becoming visible because it was just push onto a navigation controller or some other container view controller
    } else if isBeingPresented {
        // this view controller is becoming visible because it is being presented from another view controller
    } else if view.window == nil {
        // this view controller is becoming visible for the first time as the window's root view controller
    } else {
        // this view controller is becoming visible because something that was covering it has been dismissed or popped
    }
}