Base protocol for adopting Coordinator pattern.

Coordinator.swift

import UIKit
import Foundation

protocol Coordinator: class {
    typealias ExitHandler = (Coordinator) -> Void
    var childCoordinators: [Coordinator] { get set }
    var navigationController: UINavigationController { get set }
    var initialViewController: UIViewController? { get }
    func willNavigate(from: UIViewController, to: UIViewController)
    func start()
    var exit: ExitHandler? { get set }
}

// MARK: - Default implementations
extension Coordinator {
    func willNavigate(from: UIViewController, to: UIViewController) {
        
    }
}

// MARK: - Navigation Helpers
extension Coordinator {
    func end() {
        exit?(self)
    }
    
    func push(_ viewController: UIViewController, animated: Bool) {
        viewController.onViewDidAppear {
            let vcType = String(describing: type(of: viewController))
            let presentingCoordinator = String(describing: Self.self)
            log.debug("[\(presentingCoordinator)] \(vcType) -> viewDidAppear", type: .coordinator, tag: .none)
        }
        
        navigationController.pushViewController(viewController, animated: animated)
    }
    
    func pop(animated: Bool, completion: (() -> Void)? = nil) {
        navigationController.popViewController(animated: animated, completion: completion)
    }
    
    func present(_ viewController: UIViewController, animated: Bool, completion: (() -> Void)?) {
        if !(viewController is UINavigationController) {
            viewController.onViewDidAppear {
                let vcType = String(describing: type(of: viewController))
                let presentingCoordinator = String(describing: Self.self)
                log.debug("[\(presentingCoordinator)] \(vcType) -> viewDidAppear", type: .coordinator, tag: .none)
            }
        }

        navigationController.topViewController?.present(viewController, animated: animated, completion: completion)
    }
    
    func startChild(_ coordinator: Coordinator, modal: Bool, animated: Bool, onExit: ExitHandler?) {
        // Root coordinator is the delegate for all navigationControllers
        coordinator.navigationController.delegate = navigationController.delegate
        coordinator.exit = onExit
        coordinator.start()
        if modal {
            present(coordinator.navigationController, animated: true, completion: nil)
        }
        childCoordinators.append(coordinator)
    }
    
    func dismiss(animated: Bool, completion: (() -> Void)?) {
        navigationController.visibleViewController?.dismiss(animated: animated, completion: completion)
    }
    
}

// MARK: - Child coordinator inspection
extension Coordinator {
    
    var isActive: Bool {
        return currentCoordinator === self
    }
    
    var currentCoordinator: Coordinator {
        return childCoordinators.last?.currentCoordinator ?? childCoordinators.last ?? self
    }
    
    func removeChildCoordinator<T>(ofType: T.Type) {
        for c in childCoordinators {
            if c is T {
                remove(coordinator: c)
                return
            }
            c.removeChildCoordinator(ofType: T.self)
        }
    }
    
    func remove(coordinator: Coordinator) {
        for (offset, c) in childCoordinators.enumerated() {
            if c === coordinator {
                childCoordinators.remove(at: offset)
                return
            }
            c.remove(coordinator: coordinator)
        }
    }
    
    var allChildCoordinators: [Coordinator] {
        var result = [Coordinator]()
        for c in childCoordinators {
           result += c.allChildCoordinators
        }
        return result + childCoordinators
    }
    
    func childCoordinator<T: Coordinator>(ofType: T.Type) -> T? {
        for c in childCoordinators {
            return (c as? T) ?? c.childCoordinator(ofType: T.self)
        }
        
        return nil
    }

}

internal extension Coordinator {
    static func createNavigationController() -> UINavigationController {
        let navigation = UINavigationController()
        navigation.isNavigationBarHidden = true
        navigation.delegate = (UIApplication.shared.delegate as? AppDelegate)?.navigationCoordinator?.navigationController.delegate
        return navigation
    }
}


extension UINavigationController {
    
    func pushViewController(_ viewController: UIViewController, animated: Bool = true, completion: (() -> Void)?) {
        CATransaction.begin()
        CATransaction.setCompletionBlock(completion)
        pushViewController(viewController, animated: animated)
        CATransaction.commit()
    }
    
    func popViewController(animated: Bool, completion: (() -> Void)?) {
        CATransaction.begin()
        CATransaction.setCompletionBlock(completion)
        popViewController(animated: animated)
        CATransaction.commit()
    }
    
    func popToRootViewController(animated: Bool, completion: (() -> Void)?) {
        CATransaction.begin()
        CATransaction.setCompletionBlock(completion)
        popToRootViewController(animated: animated)
        CATransaction.commit()
    }
}

Note this is relying on an additional extension for logging presented viewcontrollers lifecycle.

Also initialViewController could be omitted.