func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {
    // initialize a new window which will cover the whole screen.
    window = UIWindow(frame: UIScreen.main.bounds)
    // create a variable which holds your view controller.
    let homeViewController = UIViewController()
    // set the background color of your viewController.
    homeViewController.view.backgroundColor = UIColor.purple
    // set the rootViewController of your window. This is the ViewController that will first show up when the app launches.
    window!.rootViewController = homeViewController
    // make the window visible.
    window!.makeKeyAndVisible()
    return true
}