iOS: load main interface programmatically

AppDelegate.m

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
    UIStoryboard *mainStoryboard = nil;
    
    BOOL showLogin = NO;
    if(showLogin == NO)
    {
        // mainStoryboard method looks like:
        // return [UIStoryboard storyboardWithName:@"Main" bundle:nil];
        mainStoryboard = [UIStoryboard mainStoryboard]; 
    }
    else
    {
        mainStoryboard = [UIStoryboard loginStoryboard];
    }
    
    
    UIViewController *rootController = [mainStoryboard instantiateInitialViewController];
    NSAssert(rootController, @"no user interface, what is this? a web service??");
    self.window.rootViewController = rootController;
    [self.window makeKeyAndVisible];
    return YES;
}

AppDelegate.swift

    func application(application: UIApplication, didFinishLaunchingWithOptions launchOptions: [NSObject: AnyObject]?) -> Bool {
        self.window = UIWindow(frame: UIScreen.mainScreen().bounds)
        var mainStoryboard: UIStoryboard? = nil
        
        let showTutorial = true
        
        if(showTutorial) {
            mainStoryboard = UIStoryboard.tutorialStoryboard()
        }
        else {
            // mainStoryboard method looks like:
            // return UIStoryboard(name: "Main", bundle: nil)
            mainStoryboard = UIStoryboard.mainStoryboard()
        }
        
        let rootController = mainStoryboard!.instantiateInitialViewController()
        assert(rootController != nil, "no user interface, must be the D day")
        self.window?.rootViewController = rootController
        self.window?.makeKeyAndVisible()
        
        return true
    }

Note that as of Swift 4 that UIScreen.mainScreen().bounds (line 2) has been renamed to UIScreen.main.bounds