Change Window Root View Controller

ChangeWindowRootViewController.swift

import UIKit

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {

    var window: UIWindow?
    
    // MARK: - Handling Application Background State
    
    func applicationDidEnterBackground(_ application: UIApplication) {
        let blockViewController = BlockViewController()
        self.lastViewController = window?.rootViewController
        window?.rootViewController = blockViewController
    }

    func applicationWillEnterForeground(_ application: UIApplication) {
        window?.rootViewController = lastViewController
    }
}