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When you start a new project or feature, you should spend some time thinking the architectural pattern to use. With a good analysis, you may avoid spending days on refactoring because of a messy codebase.

“How does an app transition from one view controller to another?”. This question is common and puzzling regarding iOS development. There are many answers, as every architecture has different implementation variations. Some do it from within the implementation of a view controller, while some use a router/coordinator, an object connecting view models.

Using the coordinator pattern in iOS apps lets us remove the job of app navigation from our view controllers, helping make them more manageable and more reusable, while also letting us adjust our app's flow whenever we need. Let's try hands on Implementing MVVM-C, Model-View-ViewModel-Coordinator

The best way to introduce Coordinator is probably to introduce the problem it aims to solve. As an iOS developer, Let's take a look at the snippet of code which uses the standard navigation

extension ListViewController: UITableViewDelegate,UITableViewDataSource { func tableView(_ tableView: UITableView, didSelectRowAt indexPath: IndexPath) {   ...   self.navigationController.pushViewController(detailsViewController, animated: true)  } }

We all have been doing this from the first day, and this seems simple enough. Here, The Navigation Controller is a parent of ListViewController, and yet its child commands it. UIViewController has a reference to UINavigationController in this example violates the parent-child hierarchy. So ListViewController has to responsible for managing navigation to DetailViewController, and it creates extra overhead, breaks the SOLID principles.

Another problem is that the data displayed in ViewControllers isn’t centralized. Imagine a situation where you have the following stack:

Do my 2nd UIViewController need to hold all data for 3rd or 4th UIViewController just to pass on? In that case, the VC2 receives and forwards data it doesn’t even need.

And what if you needed to add another VC between the VC2 and VC3 sometime in the future? The line grows longer and more complicated to debug, and for no good reason.

Coordinators

The Coordinator Pattern was first introduced to the iOS community by Soroush Khanlou (@khanlou) in his blog and during his presentation at the NSSpain conference

Coordinators are a great tool because they free our ViewController’s from a responsibility that they should not have. This helps us adhere to the  single responsibility principle, which makes our ViewController’s much leaner and easier to re-use.

Above Chaos, flow, can be easily fixed with coordinators. In short, they are objects that control one or more ViewControllers, and thus the flow of the app.

The  whole idea with Coordinator pattern is to keep a clear separation between layers. So ideally we want to avoid exposing a UIViewController to another, like following.

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Boosting Workflow with Swift Package Manager Sphinx Solutions Pvt. Ltd. 8 months ago Android and Jenkins: What Is the Limit? Part Two. Bruno Verachten 7 months ago The Ultimate Guide to Android Basics, Architecture… Abdullah Abdelhakeem 7 months ago class LoginViewController: UIViewController @IBAction func navigateToDashBoard(sender: Any) {     let destinationController = DashBoardViewController()     destinationController.user = self.loggedinUser     self.navigationController.pushViewController(destinationController, animation: true)   } }

Using Coordinator Pattern, we can make sure both UIViewControllers are separated.

// handles the responsibility if LoginViewController final class LoginCoordinator: BaseCoordinator {   private let navigationcontroller: UINavigationController   public weak var delegate: LoginCoordinatorDelegate?   init(navigationcontroller:UINavigationController) {    self.navigationcontroller = navigationcontroller   }      override func start() {     if let controller = self.loginController {       self.navigationcontroller.setViewControllers([controller], animated: false)     }   }      // init login-controller   lazy var loginController: LoginViewController? = {     let viewModel = LoginViewModel()     viewModel.coordinatorDelegate = self     let controller = UIStoryboard(name: "Login", bundle: nil).instantiateViewController(withIdentifier: "LoginViewController") as? LoginViewController     controller?.viewModel = viewModel     return controller   }() }

And handle navigation in coordinator.

extension LoginCoordinator: LoginViewModelCoordinatorDelegate {  func loginDidSuccess(with user: User) {   self.delegate?.didFinishLoginCordinator(coordinator: self, with: user)  }  func loginFailed(error: NSError) {   self.loginController?.displayAlertMessage(error: error)  } }

And For Dashboard.

// handles the responsibility if Dashboard flow class DashboardCoordinator: BaseCoordinator {   private let navigationcontroller: UINavigationController   public weak var delegate: DashboardCoordinatorDelegate?   private let user: User   init(navigationcontroller:UINavigationController, with user: User) {     self.navigationcontroller = navigationcontroller     self.user = user   }      override func start() {     if let controller = self.dashboardController {       self.navigationcontroller.setViewControllers([controller], animated: false)     }   }      // init dashboard-controller with viewmodel dependency injection   lazy var dashboardController: DashboardViewController? = {     let controller = UIStoryboard(name: "Dashboard", bundle: nil).instantiateViewController(withIdentifier: "DashboardViewController") as? DashboardViewController     let viewModel = DashboardViewModel(appUser: self.user)     controller?.viewModel = viewModel     controller?.viewModel?.coordinatorDelegate = self     return controller   }() }

Handling Action from Dashboard, 'Logout' in this example.

extension DashboardCoordinator: DashboardViewModelDelegate {   func logout() {     // logout     // show login   } }

Find the GitHub Repo:

References

• https://medium.com/sudo-by-icalia-labs/ios-architecture-mvvm-c-introduction-1-6-815204248518
• https://khanlou.com/2015/01/the-coordinator/

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