AppDelegate and SceneDelegate when supporting iOS 12 and 13

You do need to duplicate the code but you need to make sure it runs only on the correct system. In iOS 13 you don’t want that application delegate didFinishLaunching body code to run, so use an availability check to prevent it. In the same way, use availability to hide the window scene stuff from iOS 12.

Here's the basic sketch of a solution that runs correctly on both iOS 12 and iOS 13:

AppDelegate.Swift

import UIKit
@UIApplicationMain
class AppDelegate : UIResponder, UIApplicationDelegate {
    var window : UIWindow?
    func application(_ application: UIApplication,
        didFinishLaunchingWithOptions 
        launchOptions: [UIApplication.LaunchOptionsKey : Any]?)
        -> Bool {
            if #available(iOS 13, *) {
                // do only pure app launch stuff, not interface stuff
            } else {
                self.window = UIWindow()
                let vc = ViewController()
                self.window!.rootViewController = vc
                self.window!.makeKeyAndVisible()
                self.window!.backgroundColor = .red
            }
            return true
    }
}

SceneDelegate.swift

import UIKit
@available(iOS 13.0, *)
class SceneDelegate: UIResponder, UIWindowSceneDelegate {
    var window : UIWindow?
    func scene(_ scene: UIScene,
        willConnectTo session: UISceneSession,
        options connectionOptions: UIScene.ConnectionOptions) {
            if let windowScene = scene as? UIWindowScene {
                self.window = UIWindow(windowScene: windowScene) 
                let vc = ViewController()                      
                self.window!.rootViewController = vc             
                self.window!.makeKeyAndVisible()                 
                self.window!.backgroundColor = .red
            }
    }
}

ViewController.swift

import UIKit
class ViewController : UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()
        print("view did load")
        self.view.backgroundColor = .green
    }
}

Note that dealing with other duplicates, such as the application activating, is much simpler because if you support window scenes the application delegate method won't be called on iOS 12. So the problem is confined to this one situation, namely where you have window / root view controller manipulations to perform at launch (e.g. no storyboard).

This is work on me.

@available out the SceneDelegate.swift

As the SceneDelegate class is only available on iOS 13 and above, we have to tell the compiler to only include the class for iOS 13 and above. To do this, we will add this line "@available(iOS 13.0, *)" right above the SceneDelegate class declaration like this :

import UIKit

@available(iOS 13.0, *)
class SceneDelegate: UIResponder, UIWindowSceneDelegate {
//...
}

@available out some methods in AppDelegate.swift

Next, there are two new methods added in AppDelegate.swift, which only supports iOS 13 and above. We will add the same @available(iOS 13.0, *) on top of them as well :

// AppDelegate.swift

@available(iOS 13.0, *)
func application(_ application: UIApplication, configurationForConnecting connectingSceneSession: UISceneSession, options: UIScene.ConnectionOptions) -> UISceneConfiguration {
    // Called when a new scene session is being created.
    // Use this method to select a configuration to create the new scene with.
    return UISceneConfiguration(name: "Default Configuration", sessionRole: connectingSceneSession.role)
}

@available(iOS 13.0, *)
func application(_ application: UIApplication, didDiscardSceneSessions sceneSessions: Set<UISceneSession>) {
    // Called when the user discards a scene session.
    // If any sessions were discarded while the application was not running, this will be called shortly after application:didFinishLaunchingWithOptions.
    // Use this method to release any resources that were specific to the discarded scenes, as they will not return.
}

Add back the window to AppDelegate

If you build and run your app now, you will get a dark black screen, because there's no UIWindow initialized.

In iOS 12 and older, there's always a var window: UIWindow? variable located at the top of AppDelegate.swft. iOS 13 has moved this variable to SceneDelegate.swift, and now we are going to add back this variable to AppDelegate.

import UIKit

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {
     
    var window: UIWindow?
  
    // ...
}

Now Build and run your app on an iOS 12 devices, and it works!

I guess Apple really wants iOS developers to adopt and focus on iOS 13, to the extent that they don't mind breaking support for iOS 12 and older with default settings in Xcode.

If you are lazy to do these step manually every time, you can also download Xcode 10.3 in the Apple's developer download portal (require sign in with your Apple ID), create a new Xcode project using it, and then edit it using Xcode 11.

Xcode 11.* and Swift 5.*

Follow the steps given below after that your code will work fine for both iOS 12 and iOS 13 -

1. Remove the scene manifest from info.plist file
2. Remove scene delegate
3. Add window property inside the AppDelegate
4. Remove all the methods(2 methods mostly) related to the scene from AppDelegate

Hope this will work for someone. Happy Coding 🤓