Is there a way to navigate to another page while on func viewDidLoad() using Swift?

Question

Upon opening my app, I want to be able to check if the UserDefaults.standard has a value on two different variables. If these variables are blank, the rest of the class will continue, but if these two variables have values, I want to navigate to another page. Is it possible to do this through viewDidLoad()? I was thinking of using viewDidLoad() because I had wanted the page change to happen before the user ends up viewing this.

This is what I have so far

    override func viewDidLoad() {
        super.viewDidLoad()

        let preferences = UserDefaults.standard

        if preferences.string(forKey: "useremail") != "" || preferences.string(forKey: "usernumber") != "" {
            transitionToMain()
        }
    }

    func transitionToMain() {

        let mainPage =
        storyboard?.instantiateViewController(withIdentifier: Constants.Storyboard.mainPage) as? MainPage

        view.window?.rootViewController = mainPage
        view.window?.makeKeyAndVisible()
    }

However, when I have tried running this, the page remains the same and does not change. The code I have for transitionToMain is what I have upon clicking buttons, and it works in those cases, but it does not for this. Is there a better way to go about this? What could be improved on so that this can properly work?

Thank you very much in advance.

Answer 1

if you want to call this in the viewDidLoad function then you must use the window reference from appDelgate since windows is nil because is not added to a window yet,

func transitionToMain() {

    let mainPage =
    storyboard?.instantiateViewController(withIdentifier: Constants.Storyboard.mainPage) as? MainPage
    let appDelgate = UIApplication.shared.delegate as! AppDelegate
    appDelegate.window?.rootViewController = mainPage
    appDelegate.window?.makeKeyAndVisible()
}