SwiftUI app life cycle iOS14 where to put AppDelegate code?

You should not put that kind of codes in the app delegate at all or you will end up facing the Massive App Delegate. Instead, you should consider refactoring your code to more meaningful pieces and then put the right part in the right place. For this case, the only thing you need is to be sure that the code is executing those functions once the app is ready and only once. So the init method could be great:

@main
struct MyApp: App {
    init() {
        setupFirebase()
    }

    var body: some Scene {
        WindowGroup {
            ContentView()
        }
    }
}

private extension MyApp {
    func setupFirebase() {
        FirebaseConfiguration.shared.setLoggerLevel(.min)
        FirebaseApp.configure()
    }
}

---

AppDelegate ?

You can have your own custom class and assign it as the delegate. But note that it will not work for events that happen before assignment. For example:

class CustomDelegate: NSObject, UIApplicationDelegate {
    static let Shared = CustomDelegate()
}

And later:

UIApplication.shared.delegate = CustomDelegate.Shared

Observing For Notifications

Most of AppDelegate methods are actually observing on notifications that you can observe manually instead of defining a new class. For example:

NotificationCenter.default.addObserver(
    self,
    selector: #selector(<#T##@objc method#>),
    name: UIApplication.didBecomeActiveNotification,
    object: nil
)

---

Native AppDelegate Wrapper

You can directly inject app delegate into the @main struct:

@UIApplicationDelegateAdaptor(CustomDelegate.self) var appDelegate

Note: Using AppDelegate

Remember that adding AppDelegate means that you are killing default multiplatform support and you have to check for platform manually.

Overriding the initializer in your App also works:

import SwiftUI
import Firebase

@main
struct BookSpineApp: App {
  
  init() {
    FirebaseApp.configure()
  }
  
  var body: some Scene {
    WindowGroup {
      BooksListView()
    }
  }
}

Find a more detailed write-up here:

• The Ultimate Guide to the SwiftUI 2 Application Life Cycle
• Firebase and the new SwiftUI 2 Application Life Cycle

Here is a solution for SwiftUI life-cycle. Tested with Xcode 12b / iOS 14

import SwiftUI
import UIKit

// no changes in your AppDelegate class
class AppDelegate: NSObject, UIApplicationDelegate {
    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey : Any]? = nil) -> Bool {
        print(">> your code here !!")
        return true
    }
}

@main
struct Testing_SwiftUI2App: App {

    // inject into SwiftUI life-cycle via adaptor !!!
    @UIApplicationDelegateAdaptor(AppDelegate.self) var appDelegate

    var body: some Scene {
        WindowGroup {
            ContentView()
        }
    }
}