How to convert Objective-C App Delegate to Swift?

Question

I am working on old projects converting all classes to Swift.

How can I convert the AppDelegate class to Swift?

What changes must me made in the Project settings or main.m?

Answer 1

A good starting point is to create a new Swift-Project to get the Template for the AppDelegate, or just copy the following code in your AppDelegate.swift class:

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {

    var window: UIWindow?
    
    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 throttle down OpenGL ES frame rates. 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.
    }
    
    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.
    }
}

In Swift the main.m and AppDelegate class have been merged by using the @UIApplicationMain annotation. Therefore main.m is not required any longer and should be deleted from your project.

It's also not required to change your Project setting, hence the @UIApplicationMain will do the work for you. Just be sure to set the correct Target Membership of your AppDelegate class if you have more build targets with different AppDelegates.