AppDelegate is the delegate of our project. The UIApplicationDelegate protocol defines methods that are called by the singleton UIApplication object in response to important events in the lifetime of your app.
The app delegate works alongside the app to ensure your app interacts properly with the system and with other apps. Specifically, the methods of the app will give you a chance to respond to important changes. For example, you use the methods of the app to delegate to respond to state transitions, such as when your app moves from foreground to background execution, and to respond to incoming notifications. In many cases, the methods of the append are the only way to receive these important notifications.
Swift tutorial programming
Tutorial language Swift