subclassing UIWindow while using storyboards

Solution 1:

UIWindow in a Storyboard project can be subclassed as explained in Apple's UIApplicationDelegate reference:

window
When a storyboard is being used, the application must present the storyboard by adding it to a window and putting that window on-screen. The application queries this property for the window. The retained reference to the window by this property is necessary to keep the window from being released. If the value of the property is nil (the default), the application creates a generic instance of UIWindow and assign it to this property for the delegate to reference. You may implement the getter method of this protocol to provide the application with a different window.

In other words in your AppDelegate implementation simply add the following getter

Objective-C

- (MyCustomWindow *)window
{    
    static MyCustomWindow *customWindow = nil;
    if (!customWindow) customWindow = [[MyCustomWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
    return customWindow;
}

Swift

var customWindow: MyCustomWindow?    
var window: UIWindow? {
    get {
        customWindow = customWindow ?? MyCustomWindow(frame: UIScreen.mainScreen().bounds)
        return customWindow
    }
    set { }
}

Solution 2:

Its not so hard you're going to first subclass UIWindow

class WinCustom : UIWindow{ 
....
}

then in AppDelegate:

func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {

    self.window = WinCustom(frame: UIScreen.main.bounds)

    self.window?.rootViewController = UIStoryboard(name: "Main", bundle: nil).instantiateInitialViewController()

    return true
}