Minimal iOS 13 App - Life Cycle Events

For any iOS 13 app, you need an AppDelegate and a SceneDelegate and then you have to call UIApplicationMain function. You need an initial UIViewController object which you have to set as rootViewController property of the window object of the AppDelegate. In a typical iOS app, most of this work done through configurations buried inside Info.plist file leaving beginners in deep trouble in understanding the insights of the app ie. how it launches, what are the lifecycle events, etc.

In this app, I removed all the configurations and manually created everything needed to make beginners understand the whole app launching process.

1. App’s entry point is UIApplicationMain. In 3rd parameter you can pass ‘nil’ instead of NSStringFromClass(UIApplication.self), passing ‘nil’ will do the same thing. You can also pass a subclass of UIApplication, it is rarely done.

UIApplicationMain(
    CommandLine.argc,
    CommandLine.unsafeArgv,
    NSStringFromClass(UIApplication.self),
    NSStringFromClass(AppDelegate.self))

1. Your app must has an Application Delegate class (our case AppDelegate) conforming UIApplicationDelegate protocol. A valid UISceneConfiguration object must be retured from the lifecycle function: application(_ application: UIApplication, configurationForConnecting connectingSceneSession: UISceneSession, options: UIScene.ConnectionOptions) -> UISceneConfiguration. You have to set delegateClass property with your Scene Delegate class (SceneDelegate) conforming UIWindowSceneDelegate protocol.

func application(_ application: UIApplication, configurationForConnecting connectingSceneSession: UISceneSession, options: UIScene.ConnectionOptions) -> UISceneConfiguration {

    // MARK: Create UISceneConfiguration manually
    let sceneConfiguration = UISceneConfiguration(name: "Default", sessionRole: .windowApplication)
    sceneConfiguration.delegateClass = SceneDelegate.self

    return sceneConfiguration
}

1. Create a UIWindow object and set it to SceneDelegate’s window property in scene(_ scene: UIScene, willConnectTo session: UISceneSession, options connectionOptions: UIScene.ConnectionOptions) function. Set the rootViewController property of the window object with an object a UIViewController object.

func scene(_ scene: UIScene, willConnectTo session: UISceneSession, options connectionOptions: UIScene.ConnectionOptions) {

    guard let windowScene = (scene as? UIWindowScene) else { return }

    // MARK: Create UIWindow and rootViewController manually
    let window = UIWindow(windowScene: windowScene)
    window.frame = windowScene.coordinateSpace.bounds

    let viewController = ViewController()
    window.rootViewController = viewController

    self.window = window
    self.window?.makeKeyAndVisible()
}

In addition to new application life cycle, the app also demonstrated ViewController life cycle events. Logged the class name and the method name using below print statement:

print(String(describing: type(of: self)) + "::" + #function)

Checkout the Xcode 11 project @GitHub/mkhrussell/MinimalApp.