Bypassing AppDelegate and Scene delegate for Unit testing in iOS

AppDelegate.swift

Remove @main or @UIApplicationMain

main.swift

// Add this file to the main target

import UIKit

let appDelegateClass: AnyClass = NSClassFromString("TestsAppDelegate") ?? AppDelegate.self

UIApplicationMain(CommandLine.argc, CommandLine.unsafeArgv, nil, NSStringFromClass(appDelegateClass))

TestsAppDelegate.swift

// Add this file to the unit test target

import UIKit

@objc(TestsAppDelegate)
class TestsAppDelegate: UIResponder, UIApplicationDelegate {

    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
        print(">>> Launching with Tests app delegate.")
        // Remove any cached scene configurations to ensure that TestingAppDelegate.application(_:configurationForConnecting:options:) is called and TestingSceneDelegate will be used when running unit tests. NOTE: THIS IS PRIVATE API AND MAY BREAK IN THE FUTURE!
        for sceneSession in application.openSessions {
            application.perform(Selector(("_removeSessionFromSessionSet:")), with: sceneSession)
        }
        return true
    }
    
    // MARK: UISceneSession Lifecycle

    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.
        let config = UISceneConfiguration(name: nil, sessionRole: connectingSceneSession.role)
        config.delegateClass = TestsSceneDelegate.self
        return config
    }
}

TestsSceneDelegate.swift

// Add this file to the unit test target

import UIKit

class TestsSceneDelegate: UIResponder, UIWindowSceneDelegate {

    var window: UIWindow?

    func scene(_ scene: UIScene, willConnectTo session: UISceneSession, options connectionOptions: UIScene.ConnectionOptions) {
        // Use this method to optionally configure and attach the UIWindow `window` to the provided UIWindowScene `scene`.
        // If using a storyboard, the `window` property will automatically be initialized and attached to the scene.
        // This delegate does not imply the connecting scene or session are new (see `application:configurationForConnectingSceneSession` instead).
        guard let windowScene = (scene as? UIWindowScene) else { return }
        window = UIWindow(windowScene: windowScene)
        window?.rootViewController = UIViewController()
        window?.makeKeyAndVisible()
    }
}