UI setup class for test in Swift 4.0

TopLevelUIUtilities.swift

import XCTest
import UIKit

class TopLevelUIUtilities<T: UIViewController> {

    private var rootWindow: UIWindow!

    func setupTopLevelUI(withViewController viewController: T, withNavigationController navController: Bool = false) -> T {
        rootWindow = UIWindow(frame: UIScreen.main.bounds)
        rootWindow.isHidden = false
        if navController {
            rootWindow.rootViewController = UINavigationController(rootViewController: viewController).topViewController as! T
        } else {
            rootWindow.rootViewController = viewController
        }
        return rootWindow.rootViewController as! T
    }

    func loadTopLevelUI(withViewController viewController: T) {
        _ = viewController.view
        viewController.viewWillAppear(false)
        viewController.viewDidAppear(false)
    }

    func tearDownTopLevelUI() {
        guard let rootWindow = rootWindow,
            let rootViewController = rootWindow.rootViewController as? T else {
                XCTFail("tearDownTopLevelUI() was called without setupTopLevelUI() being called first")
                return
        }
        rootViewController.viewWillDisappear(false)
        rootViewController.viewDidDisappear(false)
        rootWindow.rootViewController = nil
        rootWindow.isHidden = true
        self.rootWindow = nil
    }
}