For decoupled unit tests

main.swift

import UIKit

private func delegateClassName() -> String? {
    guard NSClassFromString("TestBundleName.TestingAppDelegate") == nil else {
        // when running fast tests, use a nil AppDelegate
        return nil
    }
    return NSStringFromClass(AppDelegate.self)
}

_ = UIApplicationMain(CommandLine.argc, CommandLine.unsafeArgv, nil, delegateClassName())