Unit testing a view controller

ControllerLifecycle.swift

// From https://albertodebortoli.com/2018/03/12/easy-view-controller-unit-testing/

import XCTest
import UIKit

class ControllerLifecycle<T: UIViewController>
{
    private lazy var this = type(of: self).self
    private var rootWindow: UIWindow?
    var rootController: T? {
        return rootWindow?.rootViewController as? T
    }
    
    func setupWindow(storyboardName: String, controllerIdentifier: String)
    {
        let bundle = Bundle(for: T.self)
        guard let storyboard = this.createStoryboard(name: storyboardName, bundle: bundle),
              let controller = this.createController(storyboard: storyboard, identifier: controllerIdentifier)
        else {
            return
        }
        setupWindow(withViewController: controller)
    }
    
    func setupWindow(withViewController viewController: T)
    {
        rootWindow = UIWindow(frame: UIScreen.main.bounds)
        guard let rootWindow = rootWindow else { return }
        rootWindow.isHidden = false
        rootWindow.rootViewController = viewController
        _ = viewController.view
        viewController.viewWillAppear(false)
        viewController.viewDidAppear(false)
    }
    
    func tearDownWindow() {
        guard let rootWindow = rootWindow, let rootViewController = rootWindow.rootViewController as? T else {
            XCTFail("Expected to find a window with a root view controller. However: rootWindow = \(String(describing: self.rootWindow)), rootViewController = \(String(describing: self.rootWindow?.rootViewController)).")
            return
        }
        rootViewController.viewWillDisappear(false)
        rootViewController.viewDidDisappear(false)
        rootWindow.rootViewController = nil
        rootWindow.isHidden = true
        self.rootWindow = nil
    }

    /// Return a storyboard for the given name and bundle. Log a console error if not found.
    private static func createController(storyboard: UIStoryboard, identifier: String) -> T? {
        var controller: T? = nil
        do {
            try ObjC.catchException {
                controller = storyboard.instantiateViewController(withIdentifier: identifier) as? T
            }
        } catch {
            print("🚨 Storyboard \(storyboard) doesn't contain a view controller with identifier ‘\(identifier)’.")
        }
        return controller
    }
    
    /// Return a storyboard for the given name and bundle. Log a console error if not found.
    private static func createStoryboard(name: String, bundle: Bundle) -> UIStoryboard? {
        var storyboard: UIStoryboard? = nil
        do {
            try ObjC.catchException {
                storyboard = UIStoryboard(name: name, bundle: bundle)
            }
        } catch {
            print("🚨 Failed to create storyboard \(name): \(error)")
        }
        return storyboard
    }
}

ViewControllerTests.swift

import XCTest
import UIKit
@testable import Foobar

class ViewControllerTests: XCTestCase
{
    private typealias Controller = ViewController
    
    private lazy var this = type(of: self).self
    private var controllerLifecycle: ControllerLifecycle<Controller>?

    override func setUp() {
        super.setUp()
        controllerLifecycle = ControllerLifecycle<Controller>()
        controllerLifecycle?.setupWindow(storyboardName: "Main", controllerIdentifier: "ViewController")
        if let controller = controllerLifecycle?.rootController {
            print("Stub \(controller)")
        }
    }
    
    func testFoo(){
        print("this is here to trigger setUp() and tearDown()")
    }
    
    override func tearDown() {
        controllerLifecycle?.tearDownWindow()
        controllerLifecycle = nil
        super.tearDown()
    }
}