How to properly implement Navigator pattern - swift

Question

I am following John Sundell's post to implement a Navigator pattern (https://www.swiftbysundell.com/posts/navigation-in-swift). The basic idea is that, in contrast to Coordinator pattern, each view controller could simply call navigator.navigate(to: .someScreen) without having to know other view controllers.
My question is that, since in order to construct a view controller I need a navigator, to construct a navigator I need a navigation controller, but I want to make the view controller the root of the navigation controller, what's the best way to resolve this circular dependency in a way that respects the best practices of dependency injection?
Below is the idea of Navigator pattern as illustrated by Sundell
Navigator
protocol Navigator {
associatedtype Destination
func navigate(to destination: Destination)
}
class LoginNavigator: Navigator {
enum Destination {
case loginCompleted(user: User)
case signup
}
private weak var navigationController: UINavigationController?
private let viewControllerFactory: LoginViewControllerFactory
init(navigationController: UINavigationController,
viewControllerFactory: LoginViewControllerFactory) {
self.navigationController = navigationController
self.viewControllerFactory = viewControllerFactory
}
func navigate(to destination: Destination) {
let viewController = makeViewController(for: destination)
navigationController?.pushViewController(viewController, animated: true)
}
private func makeViewController(for destination: Destination) -> UIViewController {
switch destination {
case .loginCompleted(let user):
return viewControllerFactory.makeWelcomeViewController(forUser: user)
case .signup:
return viewControllerFactory.makeSignUpViewController()
}
}
}
View Controller
class LoginViewController: UIViewController {
private let navigator: LoginNavigator
init(navigator: LoginNavigator) {
self.navigator = navigator
super.init(nibName: nil, bundle: nil)
}
private func handleLoginButtonTap() {
navigator.navigate(to: .loginCompleted(user: user))
}
private func handleSignUpButtonTap() {
navigator.navigate(to: .signup)
}
}
Now in AppDelegate I want to do something like
let factory = LoginViewControllerFactory()
let loginViewController = factory.makeLoginViewController()
let rootNavigationController = UINavigationController(rootViewController: loginViewController)
window?.rootViewController = rootNavigationController
But I somehow have to pass the rootNavigationController into the factory in order for the loginViewController to be properly constructed right? Because it needs a navigator, which needs the navigation controller. How to do that?

Answer 1

I also was recently trying to implement Sundell's Navigator pattern and ran into this same circular dependency. I had to add some additional behavior to the initial Navigator to handle this odd bootstrap issue. I believe subsequent Navigators in your app can perfectly follow the blog's suggestion.
Here is the new initial Navigator code using JGuo's (the OP) example:
class LoginNavigator: Navigator {
enum Destination {
case loginCompleted(user: User)
case signup
}
private var navigationController: UINavigationController?
// This ^ doesn't need to be weak, as we will instantiate it here.
private let viewControllerFactory: LoginViewControllerFactory
// New:
private let appWindow: UIWindow?
private var isBootstrapped = false
// We will use this ^ to know whether or not to set the root VC
init(appWindow: UIWindow?, // Pass in your app's UIWindow from the AppDelegate
viewControllerFactory: LoginViewControllerFactory) {
self.appWindow = appWindow
self.viewControllerFactory = viewControllerFactory
}
func navigate(to destination: Destination) {
let viewController = makeViewController(for: destination)
// We'll either call bootstrap or push depending on
// if this is the first time we've launched the app, indicated by isBootstrapped
if self.isBootstrapped {
self.pushViewController(viewController)
} else {
bootstrap(rootViewController: viewController)
self.isBootstrapped = true
}
}
private func makeViewController(for destination: Destination) -> UIViewController {
switch destination {
case .loginCompleted(let user):
return viewControllerFactory.makeWelcomeViewController(forUser: user)
case .signup:
return viewControllerFactory.makeSignUpViewController()
}
}
// Add these two new helper functions below:
private func bootstrap(rootViewController: UIViewController) {
self.navigationController = UINavigationController(rootViewController: rootViewController)
self.appWindow?.rootViewController = self.navigationController
}
private func pushViewController(_ viewController: UIViewController) {
// Setup navigation look & feel appropriate to your app design...
navigationController?.setNavigationBarHidden(true, animated: false)
self.navigationController?.pushViewController(viewController, animated: true)
}
}
And inside the AppDelegate now:
class AppDelegate: UIResponder, UIApplicationDelegate {
var window: UIWindow?
func application(_ application: UIApplication,
didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
window = UIWindow(frame: UIScreen.main.bounds)
let factory = LoginViewControllerFactory()
let loginViewController = factory.makeLoginViewController()
loginViewController.navigate(to: .signup) // <- Ideally we wouldn't need to signup on app launch always, but this is the basic idea.
window?.makeKeyAndVisible()
return true
}
...
}

Answer 2

Does this solve it? in AppDelegate:
let factory = LoginViewControllerFactory()
let navController = UINavigationController()
let loginNavigator = LoginNavigator(navigationController: navController, viewControllerFactory: factory)
loginNavigator.navigate(to: .signup) // The example doesn't have a .login Destination, but it can easily be added to the factory, so using .signup instead
window?.rootViewController = navController

Answer 3

Instead of having the rootViewController as a property of the LoginViewControllerFactory, I would suggest to pass it as an argument when calling the 'make' functions:
return viewControllerFactory.makeWelcomeViewController(forUser: user, with: rootViewController)