ios 15 prewarming causing appwillLaunch method when prewarm is done

There is poor documentation about Prewarming:

Prewarming executes an app’s launch sequence up until, but not including, when main() calls UIApplicationMain(::::). https://developer.apple.com/documentation/uikit/app_and_environment/responding_to_the_launch_of_your_app/about_the_app_launch_sequence#3894431

But prewarming can call both willFinishLaunchingWithOptions and didFinishLaunchingWithOptions from my tests on my iPhone 13 with iOS 15.0.

I’ve created a test app which logs all calls to a string array:

main.swift

var logMessages = [String]()

func addLogMessage(_ text: String) {
    let date = Date().formatted(date: .omitted, time: .standard)
    logMessages.append("\(date): \(text)")
}

autoreleasepool {
    addLogMessage("main.swift")
    UIApplicationMain(CommandLine.argc, CommandLine.unsafeArgv, nil, NSStringFromClass(AppDelegate.self))
}

AppDelegate.swift

class AppDelegate: UIResponder, UIApplicationDelegate {
    
    override init() {
        super.init()
        addLogMessage("AppDelegate.init")
    }
    
    func application(_ application: UIApplication, willFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey : Any]? = nil) -> Bool {
        addLogMessage("willFinishLaunchingWithOptions")
        return true
    }

    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
        addLogMessage("didFinishLaunchingWithOptions")
        return true
    }

    func applicationProtectedDataDidBecomeAvailable(_ application: UIApplication) {
        addLogMessage("applicationProtectedDataDidBecomeAvailable")
    }

    func application(_ application: UIApplication, configurationForConnecting connectingSceneSession: UISceneSession, options: UIScene.ConnectionOptions) -> UISceneConfiguration {
        addLogMessage("configurationForConnecting")
        return UISceneConfiguration(name: "Default Configuration", sessionRole: connectingSceneSession.role)
    }
    …
}

SceneDelegate.swift

class SceneDelegate: UIResponder, UIWindowSceneDelegate {

    var window: UIWindow? {
        didSet {
            addLogMessage("SceneDelegate.didSet window")
        }
    }

    func scene(_ scene: UIScene, willConnectTo session: UISceneSession, options connectionOptions: UIScene.ConnectionOptions) {
        addLogMessage("SceneDelegate.willConnectTo")
        guard let _ = (scene as? UIWindowScene) else { return }
    }
    …
}

Then did the following:

1. Launch the app
2. Use it briefly
3. Force quit the app
4. Lock my device and leave it for ~30 minutes
5. Unlock the device
6. Launch the app again

My logs:

12:28:36 AM: main.swift
12:28:36 AM: AppDelegate.init
12:28:36 AM: willFinishLaunchingWithOptions
12:28:36 AM: didFinishLaunchingWithOptions

12:58:15 AM: applicationProtectedDataDidBecomeAvailable
12:58:15 AM: configurationForConnecting
12:58:15 AM: SceneDelegate.didSet window
12:58:15 AM: SceneDelegate.willConnectTo
12:58:15 AM: viewDidLoad

As you can see the system launched my app (prewarm) at 12:28:36 and held in background until my manual start at 12:58:15 and after gave access to protected data and loaded UI.

Apple's documentation is incorrect, here is the behaviour observed on iOS 15:

UIApplicationMain() always runs, including during prewarming.

What happens after this depends on whether your app uses the UIScene life-cycle.

• For apps that do support scenes:

  • application:didFinishLaunchingWithOptions: may be called (doesn't always happen)
  • scene:willConnectToSession:options: is not called - in fact the SceneDelegate is not created until the app is opened.

• For apps that do not support scenes:

  • application:didFinishLaunchingWithOptions: is not called.