How to Set a New Root View Controller

How can i set a new Root viewController in Swift 4?

Try this out,

let storyBoard : UIStoryboard = UIStoryboard(name: "Main", bundle:nil)
let newViewController = storyBoard.instantiateViewController(withIdentifier: "YourViewController") as! YourViewController
let navigationController = UINavigationController(rootViewController: newViewController)            
let appdelegate = UIApplication.shared.delegate as! AppDelegate
appdelegate.window!.rootViewController = navigationController

How to change rootViewController in swift?

First you need to get the Window from SceneDelegate and use that window for rootViewController.

Suggestion: Whenever we are using rootViewController then no need to do present and push thing it will automatically set as a main home viewcontroller(Intial viewController)

 let window = (UIApplication.shared.connectedScenes.first!.delegate as! SceneDelegate).window
            let storyboard = UIStoryboard(name: "Main", bundle:nil)
        if UserDefaults.standard.bool(forKey: "LoggedIn") == false {
            let vc =storyboard.instantiateViewController(withIdentifier: "EnterPin") as! UINavigationController
            window?.rootViewController = vc
            window?.makeKeyAndVisible()
        } else if UserDefaults.standard.bool(forKey: "LoggedIn") == true {
            let vc =storyboard.instantiateViewController(withIdentifier: "MainController") as! UINavigationController
            window?.rootViewController = vc
            window?.makeKeyAndVisible()
        }

I suggest you do all this things in SceneDelegate file

func scene(_ scene: UIScene, willConnectTo session: UISceneSession, options connectionOptions: UIScene.ConnectionOptions) {
        guard let scene = (scene as? UIWindowScene) else { return }
        self.window = UIWindow(windowScene: scene)
        let storyboard = UIStoryboard(name: "Main", bundle: nil)
        if UserDefaults.standard.bool(forKey: "LoggedIn") == false {
            let vc = storyboard.instantiateViewController(withIdentifier: "EnterPin") as! UINavigationController
            self.window?.rootViewController = vc
            self.window?.makeKeyAndVisible()
        } else if UserDefaults.standard.bool(forKey: "LoggedIn") == true {
            let vc = storyboard.instantiateViewController(withIdentifier: "MainController") as! UINavigationController
            self.window?.rootViewController = vc
            self.window?.makeKeyAndVisible()
        }
    }

I Hope it is helpful for you!

UIViewController setting new root view when click button

In your appDelegate, add property:

final var window: UIWindow?

Call it like this

@IBAction func btnSignInPressed(_ sender: UIButton) {
    // if true take to home page
    DispatchQueue.main.async {
        let home = self.storyboard?.instantiateViewController(identifier: "HomePageViewController") as!
            HomePageViewController

        let window = UIApplication.shared.delegate!.window!!
        window.rootViewController = nil
        window.rootViewController = home
        
        UIView.transition(with: window, duration: 0.4, options: [.transitionCrossDissolve], animations: nil, completion: nil)
    }
}

Set rootViewController iOS 13

This is because AppDelegate doesn't have window property anymore.
Now you must use SceneDelegate's scene(_:willConnectTo:options:) method to change root view controller.
Like shown in this example:

    func scene(_ scene: UIScene, willConnectTo session: UISceneSession, options connectionOptions: UIScene.ConnectionOptions) {
        guard let scene = (scene as? UIWindowScene) else { return }

        // Instantiate UIWindow with scene
        let window = UIWindow(windowScene: scene)
        // Assign window to SceneDelegate window property
        self.window = window
        // Set initial view controller from Main storyboard as root view controller of UIWindow
        self.window?.rootViewController = UIStoryboard(name: "Main", bundle: nil).instantiateInitialViewController()
        // Present window to screen
        self.window?.makeKeyAndVisible()
    }

Programmatically change rootViewController of storyBoard

Objective-C:

UIStoryboard *storyboard = [UIStoryboard storyboardWithName:@"MainStoryboard" bundle:nil];
UITabBarController *rootViewController = [storyboard instantiateViewControllerWithIdentifier:@"tabBarcontroller"];
[[UIApplication sharedApplication].keyWindow setRootViewController:rootViewController];

Swift :

 let mainStoryboard: UIStoryboard = UIStoryboard(name: "Main", bundle: nil)
 let viewController = mainStoryboard.instantiateViewControllerWithIdentifier("tabBarcontroller") as UITabBarController  
   UIApplication.sharedApplication().keyWindow?.rootViewController = viewController;

Swift 3:

let mainStoryboard: UIStoryboard = UIStoryboard(name: "Main", bundle: nil)
let viewController = mainStoryboard.instantiateViewController(withIdentifier: "tabBarcontroller") as! UITabBarController
UIApplication.shared.keyWindow?.rootViewController = viewController

Swift 5:

let viewController = mainStoryboard.instantiateViewController(withIdentifier: "tabBarcontroller") as! UITabBarController
UIApplication.shared.windows.first?.rootViewController = viewController
UIApplication.shared.windows.first?.makeKeyAndVisible()

Or simply like this:

let viewController = mainStoryboard.instantiateViewController(withIdentifier: "tabBarcontroller") as! UITabBarController
self.view.window?.rootViewController = viewController
self.view.window?.makeKeyAndVisible()

Both works fine!