App wide operations

Get the top most UIViewController

A common approach to get the top most UIViewController is to get the RootViewController of your active UIWindow. I wrote an extension for this:

extension UIApplication {

func topViewController(_ base: UIViewController? = UIApplication.shared.keyWindow?.rootViewController) -> UIViewController {
    
    if let nav = base as? UINavigationController {
        return topViewController(nav.visibleViewController)
    }
    
    if let tab = base as? UITabBarController {
        if let selected = tab.selectedViewController {
            return topViewController(selected)
        }
    }
    
    if let presented = base?.presentedViewController {
        return topViewController(presented)
    }
    
    return base!
}

Intercept System Events

Using the NotificationCenter of iOS, which can be very powerful, you are able to intercept certain app-wide events:

NotificationCenter.default.addObserver(
        self,
        selector: #selector(ViewController.do(_:)),
        name: NSNotification.Name.UIApplicationDidBecomeActive,
        object: nil)

You can register for a lot of more events, just take a look at https://developer.apple.com/reference/foundation/nsnotification.name.