App operation without app delegate methods

Question

Can you help me to understand why app still work if I delete default app delegate methods such as applicationWillResignActive and applicationDidEnterBackground?

Answer 1

In part, the declaration of the UIApplicationDelegate protocol looks like this:

public protocol UIApplicationDelegate : NSObjectProtocol {

    optional public func applicationDidFinishLaunching(_ application: UIApplication)

    optional public func application(_ application: UIApplication, willFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey : Any]? = nil) -> Bool

    optional public func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey : Any]? = nil) -> Bool

    optional public func applicationDidBecomeActive(_ application: UIApplication)

    optional public func applicationWillResignActive(_ application: UIApplication)

   ...

Note that the functions are declared as optional. This means that code that conforms to the protocol does not have to implement those functions.

If your app doesn't need to do anything special when it enters the background, then you don't need to implement applicationDidEnterBackground. If your app did need to do something, then you would implement that function.