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.