Coordinator pattern in iOS Swift

Introduction

The Coordinator pattern helps organize navigation in an app. It keeps screens and navigation logic separate for easier management.

When your app has many screens and complex navigation.

When you want to avoid putting navigation code inside view controllers.

When you want to reuse navigation flows in different parts of your app.

When you want to make your app easier to test and maintain.

Syntax

iOS Swift

protocol Coordinator {
  var childCoordinators: [Coordinator] { get set }
  var navigationController: UINavigationController { get set }
  func start()
}

The Coordinator protocol defines a basic structure for coordinators.

The start() method begins the navigation flow.

Examples

This example shows a simple coordinator that starts by pushing a view controller.

iOS Swift

class MainCoordinator: Coordinator {
  var childCoordinators = [Coordinator]()
  var navigationController: UINavigationController

  init(navigationController: UINavigationController) {
    self.navigationController = navigationController
  }

  func start() {
    let vc = ViewController()
    vc.coordinator = self
    navigationController.pushViewController(vc, animated: true)
  }
}

This method shows how to start a child coordinator for a detail screen.

iOS Swift

func showDetail() {
  let detailCoordinator = DetailCoordinator(navigationController: navigationController)
  childCoordinators.append(detailCoordinator)
  detailCoordinator.start()
}

Sample App

This app uses the Coordinator pattern to manage navigation. The MainCoordinator starts with a home screen that has a button. When tapped, it navigates to a detail screen.

iOS Swift

import UIKit

protocol Coordinator {
  var childCoordinators: [Coordinator] { get set }
  var navigationController: UINavigationController { get set }
  func start()
}

class MainCoordinator: Coordinator {
  var childCoordinators = [Coordinator]()
  var navigationController: UINavigationController

  init(navigationController: UINavigationController) {
    self.navigationController = navigationController
  }

  func start() {
    let vc = ViewController()
    vc.coordinator = self
    navigationController.pushViewController(vc, animated: true)
  }

  func showDetail() {
    let detailVC = DetailViewController()
    navigationController.pushViewController(detailVC, animated: true)
  }
}

class ViewController: UIViewController {
  weak var coordinator: MainCoordinator?

  override func viewDidLoad() {
    super.viewDidLoad()
    view.backgroundColor = .white
    title = "Home"

    let button = UIButton(type: .system)
    button.setTitle("Go to Detail", for: .normal)
    button.addTarget(self, action: #selector(goToDetail), for: .touchUpInside)
    button.frame = CGRect(x: 100, y: 200, width: 150, height: 50)
    view.addSubview(button)
  }

  @objc func goToDetail() {
    coordinator?.showDetail()
  }
}

class DetailViewController: UIViewController {
  override func viewDidLoad() {
    super.viewDidLoad()
    view.backgroundColor = .lightGray
    title = "Detail"
  }
}

@main
class AppDelegate: UIResponder, UIApplicationDelegate {
  var window: UIWindow?
  var coordinator: MainCoordinator?

  func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
    let navController = UINavigationController()
    coordinator = MainCoordinator(navigationController: navController)
    coordinator?.start()

    window = UIWindow(frame: UIScreen.main.bounds)
    window?.rootViewController = navController
    window?.makeKeyAndVisible()

    return true
  }
}

Important Notes

Keep coordinators focused on navigation only, not UI details.

Use child coordinators to handle complex flows inside your app.

Make sure to keep references to child coordinators to avoid them being deallocated too soon.

Summary

The Coordinator pattern separates navigation from view controllers.

It makes your app easier to manage and test.

Use coordinators to start and control screen flows.

Practice

1. What is the main purpose of the Coordinator pattern in iOS development?

easy

A. To separate navigation logic from view controllers

B. To manage data persistence

C. To handle network requests

D. To style UI components

Solution

1. Step 1: Understand the role of Coordinator

   The Coordinator pattern is designed to manage navigation and screen flow in an app.

2. Step 2: Compare with other options

   Data persistence, network requests, and UI styling are handled by other parts of the app, not the Coordinator.

3. Final Answer:

   To separate navigation logic from view controllers -> Option A

4. Quick Check:

   Coordinator = navigation separation [OK]

Hint: Coordinator handles navigation, not data or UI [OK]

Common Mistakes:

• Confusing Coordinator with data managers
• Thinking Coordinator styles UI
• Assuming Coordinator handles network calls

2. Which of the following is the correct way to declare a basic Coordinator protocol in Swift?

easy

A. protocol Coordinator { func start() }

B. class Coordinator { func start() {} }

C. struct Coordinator { var start: () -> Void }

D. enum Coordinator { case start }

Solution

1. Step 1: Identify protocol syntax

   In Swift, protocols are declared with the protocol keyword and define method signatures without implementation.

2. Step 2: Check each option

   protocol Coordinator { func start() } correctly declares a protocol with a start() method signature. class Coordinator { func start() {} } is a class, not a protocol. struct Coordinator { var start: () -> Void } is a struct with a closure property, not a protocol. enum Coordinator { case start } is an enum, which is not used for protocols.

3. Final Answer:

   protocol Coordinator { func start() } -> Option A

4. Quick Check:

   Protocol syntax = protocol Coordinator { func start() } [OK]

Hint: Protocols use 'protocol' keyword and method signatures only [OK]

Common Mistakes:

• Using class instead of protocol
• Adding method implementation in protocol
• Confusing struct or enum with protocol

3. Given this Coordinator code snippet, what will be printed when appCoordinator.start() is called?

protocol Coordinator {
  func start()
}

class AppCoordinator: Coordinator {
  func start() {
    print("App started")
  }
}

let appCoordinator = AppCoordinator()
appCoordinator.start()

medium

A. start() method not found

B. No output

C. Compilation error

D. App started

Solution

1. Step 1: Analyze the class and method

   The AppCoordinator class implements the start() method which prints "App started".

2. Step 2: Check method call

   Calling appCoordinator.start() executes the print statement inside start().

3. Final Answer:

   App started -> Option D

4. Quick Check:

   Method call prints message [OK]

Hint: Calling start() runs its print statement [OK]

Common Mistakes:

• Assuming start() is missing
• Thinking code won't compile
• Expecting no output

4. Identify the error in this Coordinator implementation:

protocol Coordinator {
  func start()
}

class MainCoordinator: Coordinator {
  func start() {
    print("Starting main flow")
  }

  func startFlow() {
    print("Flow started")
  }
}

medium

A. startFlow() method is not part of the protocol

B. No error, code is correct

C. Missing start() method implementation

D. Protocol should be a class, not protocol

Solution

1. Step 1: Check protocol conformance

   The MainCoordinator class implements the required start() method from the Coordinator protocol.

2. Step 2: Verify additional methods

   Having extra methods like startFlow() is allowed and does not cause errors.

3. Final Answer:

   No error, code is correct -> Option B

4. Quick Check:

   Extra methods allowed, required methods implemented [OK]

Hint: Extra methods don't cause errors if required ones exist [OK]

Common Mistakes:

• Thinking extra methods must be in protocol
• Assuming missing methods cause errors
• Confusing protocol with class requirements

5. You want to start a login flow from your MainCoordinator and then show the home screen after login. Which approach best uses the Coordinator pattern to handle this navigation?

hard

A. LoginCoordinator handles login and home screens without notifying MainCoordinator

B. MainCoordinator directly pushes login and home view controllers without coordinators

C. MainCoordinator creates and starts a LoginCoordinator, then listens for login completion to start HomeCoordinator

D. Use a single view controller to manage login and home screens

Solution

1. Step 1: Understand Coordinator responsibilities

   MainCoordinator should manage the overall flow and delegate specific flows to child coordinators like LoginCoordinator.

2. Step 2: Evaluate navigation flow

   MainCoordinator creates LoginCoordinator to handle login, then listens for completion to start HomeCoordinator, keeping navigation logic centralized.

3. Step 3: Compare other options

   The other options break separation of concerns or do not coordinate flows properly.

4. Final Answer:

   MainCoordinator creates and starts a LoginCoordinator, then listens for login completion to start HomeCoordinator -> Option C

5. Quick Check:

   Parent coordinator manages child flows [OK]

Hint: Parent coordinator controls child coordinators and flow [OK]

Common Mistakes:

• Handling navigation only in view controllers
• Child coordinators acting independently
• Mixing multiple screens in one view controller