IOS app lifecycle

An iOS application runs into several states, which are called the state of the Application life cycle. Every iOS developer must be aware of the app life cycle, which helps to understand the application’s behavior. Every iOS application passes through the following states as it runs.

Not Running: the app is considered to be in a Not Running state when it is not yet launched or terminated by the system or user.
Inactive: the app is in an inactive state when it is in the foreground but receiving events. In other words, we can say that it acts like a bridge state in which the app remains briefly when it transitions to a different state.
Active: it is a normal mode for the app when it is in the foreground state and receiving all the user events.
Background: the app transitions into the background state when the user taps on the home screen while using the application, or it requires some extra execution time. When the app is about to be suspended, then also transitions into this state for a small amount of time. In this state, the app remains in the background and executes the code.
Suspended: in this state, the app remains in the background and doesn’t execute the code. The app is automatically moved to this state. In this state, the app remains in memory. However, the foreground apps are always given priority over suspended apps and can be purged any time without notice.

We must notice that when we build and run an iOS application in XCode, the main entry point of the application is UIApplicationDelegate, which is a protocol that the application must implement to get notified of several user events like app launch, the app goes into the background, the app goes to the foreground, push notifications, etc.

The UIApplicationDelegate contains certain app lifecycle methods that are notified when the app starts running. The UIApplicationDelegate methods are given below.

application: didFinishLaunchingWithOptions:-> Bool: when the application is launched initially, this method is called. We can do any initial setup for the application in this method like firebase configuration, user navigation, etc. The storyboard is loaded at this point, but we can maintain the state restoration.
applicationWillEnterForeground: this method is called after didFinishLaunchingWithOptions. It is also called when the application comes from background to foreground.
applicationDidBecomeActive: this method is called after applicationWillEnterForeground. If we need to perform any particular task when the app comes into the foreground, like font update, etc., then we can place the code in this method.
applicationWillResignActive: this method is notified when the application is about to become inactive. For example, the user receives a phone call; the user presses the home button, etc.).
applicationDidEnterBackground: this method is notified when the application goes into a background state after become inactive.
applicationWillTerminate: this method is called when the application is about to be finally terminated from memory. If we need to perform any final cleanups, then we can place the code in this method.

We can place the code which is to be run on app launch in didFinishLaunchingWithOptions method. For example, in an application in which user login is required to use the app, we can check if the user is already logged in to the application via checking UserDefaults. If the user is logged in, then we can navigate the user to the home screen; otherwise, we can navigate it to the login screen.

The AppDelegate class, which is created in the first place when we create any iOS project, is given below.

  @UIApplicationMain  class AppDelegate: UIResponder, UIApplicationDelegate {           func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {          // Override point for customization after application launch.                 return true             }        func applicationWillResignActive(_ application: UIApplication) {          // Sent when the application is about to move from active to inactive state. This can occur for certain types of temporary interruptions (such as an incoming phone call or SMS message) or when the user quits the application, and it begins the transition to the background state.          // Use this method to pause ongoing tasks, disable timers, and invalidate graphics rendering callbacks. Games should use this method to pause the game.       }            func applicationDidEnterBackground(_ application: UIApplication) {          // Use this method to release shared resources, save user data, invalidate timers, and store enough application state information to restore your application to its current state in case it is terminated later.          // If your application supports background execution, this method is called instead of applicationWillTerminate: when the user quits.      }            func applicationWillEnterForeground(_ application: UIApplication) {          // Called as part of the transition from the background to the active state; here you can undo many of the changes made on entering the background.          Utile.getCurrentViewController()?.viewDidLayoutSubviews()      }            func applicationDidBecomeActive(_ application: UIApplication) {                           // Restart any tasks that were paused (or not yet started) while the application was inactive. If the application was previously in the background, optionally refresh the user interface.                }            func applicationWillTerminate(_ application: UIApplication) {          // Called when the application is about to terminate. Save data if appropriate. See also applicationDidEnterBackground:.      }  

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