rootViewController を別の ViewController に入れ替える [Swift3]

UIWindow+Ex.swift

import UIKit

extension UIWindow {
    func changeRootViewController(new viewController: UIViewController) {
        UIView.transition(
            with: UIApplication.shared.keyWindow!,
            duration: 0.5,
            options: .transitionCrossDissolve,
            animations: {
                UIApplication.shared.keyWindow?.rootViewController = viewController
            },
            completion: nil
        )
    }
}

UIWindow().changeRootViewController(new: UIViewController())