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🎌 Powerful navigation library for iOS based on the coordinator pattern
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README.md

Build Status CocoaPods Compatible Carthage Compatible Documentation Platform License

“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.

To better answer the question, we are building XCoordinator, a navigation framework based on the Coordinator pattern. It's especially useful for implementing MVVM-C, Model-View-ViewModel-Coordinator:

🏃‍♂️Getting started

Create an enum with all of the navigation paths for a particular flow, i.e. a group of closely connected scenes. (It is up to you when to create a Route/Coordinator. As our rule of thumb, create a new Route/Coordinator whenever a new root view controller, e.g. a new navigation controller or a tab bar controller, is needed.).

Whereas the Route describes which routes can be triggered in a flow, the Coordinator is responsible for the preparation of transitions based on routes being triggered. We could, therefore, prepare multiple coordinators for the same route, which differ in which transitions are executed for each route.

In the following example, we create the UserListRoute enum to define triggers of a flow of our application. UserListRoute offers routes to open the home screen, display a list of users, to open a specific user and to log out. The UserListCoordinator is implemented to prepare transitions for the triggered routes. When a UserListCoordinator is shown, it triggers the .home route to display a HomeViewController.

enum UserListRoute: Route {
    case home
    case users
    case user(String)
    case registerUsersPeek(from: Container)
    case logout
}

class UserListCoordinator: NavigationCoordinator<UserListRoute> {
    init() {
        super.init(initialRoute: .home)
    }

    override func prepareTransition(for route: UserListRoute) -> NavigationTransition {
        switch route {
        case .home:
            let viewController = HomeViewController.instantiateFromNib()
            let viewModel = HomeViewModelImpl(router: anyRouter)
            viewController.bind(to: viewModel)
            return .push(viewController)
        case .users:
            let viewController = UsersViewController.instantiateFromNib()
            let viewModel = UsersViewModelImpl(router: anyRouter)
            viewController.bind(to: viewModel)
            return .push(viewController, animation: .interactiveFade)
        case .user(let username):
            let coordinator = UserCoordinator(user: username)
            return .present(coordinator, animation: .default)
        case .registerUsersPeek(let source):
            return registerPeek(for: source, route: .users)
        case .logout:
            return .dismiss()
        }
    }
}

Routes are triggered from within Coordinators or ViewModels. In the following, we describe how to trigger routes from within a ViewModel. The router of the current flow is injected into the ViewModel.

class HomeViewModel {
    let router: AnyRouter<HomeRoute>

    init(router: AnyRouter<HomeRoute>) {
        self.router = router
    }

    /* ... */

    func usersButtonPressed() {
        router.trigger(.users)
    }
}

🏗 Organizing an app's structure with XCoordinator

In general, an app's structure is defined by nesting coordinators and view controllers. You can transition (i.e. push, present, pop, dismiss) to a different coordinator whenever your app changes to a different flow. Within a flow, we transition between viewControllers.

Example: In UserListCoordinator.prepareTransition(for:) we change from the UserListRoute to the UserRoute whenever the UserListRoute.user route is triggered. By dismissing a viewController in UserListRoute.logout, we additionally switch back to the previous flow - in this case the HomeRoute.

To achieve this behavior, every Coordinator has its own rootViewController. This would be a UINavigationController in the case of a NavigationCoordinator, a UITabBarController in the case of a TabBarCoordinator, etc. When transitioning to a Coordinator/Router, this rootViewController is used as the destination view controller.

🏁 Using XCoordinator from App Launch

To use coordinators from the launch of the app, make sure to create the app's window programmatically in AppDelegate.swift (Don't forget to remove Main Storyboard file base name from Info.plist). Then, set the coordinator as the root of the window's view hierarchy in the AppDelegate.didFinishLaunching.

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {
    let window: UIWindow! = UIWindow()
    let router = AppCoordinator().anyRouter

    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {
        router.setRoot(for: window)
        return true
    }
}

🤸‍♂️ Extras

For more advanced use, XCoordinator offers many more customization options. We introduce custom animated transitions and deep linking. Furthermore, extensions for use in reactive programming with RxSwift and options to split up huge routes are described.

🌗 Custom Transitions

Custom animated transitions define presentation and dismissal animations. You can specify Animation objects in prepareTransition(for:) in your coordinator for several common transitions, such as present, dismiss, push and pop. Specifying no animation (nil) results in not overriding previously set animations. Use Animation.default to reset previously set animation to the default animations UIKit offers.

class UsersCoordinator: NavigationCoordinator<UserRoute> {

    /* ... */
    
    override func prepareTransition(for route: UserRoute) -> NavigationTransition {
        switch route {
        case .user(let name):
            let animation = Animation(
                presentationAnimation: YourAwesomePresentationTransitionAnimation(),
                dismissalAnimation: YourAwesomeDismissalTransitionAnimation()
            )
            let viewController = UserViewController.instantiateFromNib()
            let viewModel = UserViewModelImpl(coordinator: coordinator, name: name)
            viewController.bind(to: viewModel)
            return .push(viewController, animation: animation)
        /* ... */
        }
    }
}

🛤 Deep Linking

Deep Linking can be used to chain different routes together. In contrast to the .multiple transition, deep linking can identify routers based on previous transitions (e.g. when pushing or presenting a router), which enables chaining of routes of different types. Keep in mind, that you cannot access higher-level routers anymore once you trigger a route on a lower level of the router hierarchy.

class AppCoordinator: NavigationCoordinator<AppRoute> {

    /* ... */

    override func prepareTransition(for route: AppRoute) -> NavigationTransition {
        switch route {
        /* ... */
        case .deep:
            return deepLink(AppRoute.login, AppRoute.home, HomeRoute.news, HomeRoute.dismiss)
        }
    }
}

⚠️ XCoordinator does not check at compile-time, whether a deep link can be executed. Rather it uses assertionFailures to inform about incorrect chaining at runtime, when it cannot find an appriopriate router for a given route. Keep this in mind when changing the structure of your app.

🚏 Redirection

Let's assume, there is a route type called HugeRoute with more than 10 routes. To decrease coupling, HugeRoute needs to be split up into mutliple route types. As you will discover, many routes in HugeRoute use transitions dependent on a specific rootViewController, such as push, show, pop, etc. If splitting up routes by introducing a new router/coordinator is not an option, XCoordinator has two solutions for you to solve such a case: RedirectionRouter and RedirectionCoordinator.

A RedirectionRouter can be used to map a new route type onto a generalized SuperRoute. A RedirectionRouter is independent of the TransitionType of its superRouter. You can use RedirectionRouter.init(viewController:superRouter:map:) or subclassing by overriding mapToSuperRoute(_:) to create a RedirectionRouter.

A RedirectionCoordinator is not dependent on a specific SuperRoute, instead it uses its superTransitionPerformer to perform transitions. Due to constraints of UIKit, this is especially helpful when nesting routes dependent on a UINavigationController, since pushing navigation controllers on top of each other is not possible. Similar to the RedirectionRouter, you can use a RedirectionCoordinator by providing a prepareTransition-closure to map from a route to a transition or by subclassing including the overriding of prepareTransition(for:).

The following table describes how using a RedirectionRouter, RedirectionCoordinator and creating a new coordinator independent from a superCoordinator stack up:

RedirectionRouter RedirectionCoordinator New Coordinator
Dependencies on superCoordinator on SuperRoute on TransitionType none
Type constraint of superCoordinator Router TransitionPerformer none
Accessibility of superCoordinator map RouteType to SuperRoute map RouteType to TransitionType none
Transition definition in superCoordinator constrained by TransitionType of superCoordinator independent from superCoordinator

The following code example illustrates how a RedirectionCoordinator is initialized and used. The use of a RedirectionRouter is similar, but one would need different type-constraints in SubCoordinator.init and override mapToSuperRoute instead of prepareTransition.

class SuperCoordinator: NavigationCoordinator<SuperRoute> {
    /* ... */
    
    override func prepareTransition(for route: SuperRoute) -> NavigationTransition {
        switch route {
        /* ... */
        case .subCoordinator:
            let subCoordinator = SubCoordinator(superCoordinator: self)
            return .push(subCoordinator)
        }
    }
}

class SubCoordinator: RedirectionCoordinator<SubRoute, NavigationTransition> {
    init<T: TransitionPerformer>(superCoordinator: T) where T.TransitionType == NavigationTransition {
        let viewController = SubCoordinatorViewController()
        super.init(viewController: viewController, superTransitionPerformer: superCoordinator, prepareTransition: nil)
        let viewModel = SubCoordinatorViewModel(router: anyRouter)
        // this viewModel has to be initialized after `super.init`
        // since it needs `self` to create the `anyRouter`.
        viewController.bind(to: viewModel)
    }
    
    /* ... */
    
    override func prepareTransition(for route: SubRoute) -> NavigationTransition {
        // you can prepare routes here just like in any other NavigationCoordinator
    }
}

🚀 RxSwift extensions

Reactive programming can be very useful to keep the state of view and model consistent in a MVVM architecture. Instead of relying on the completion handler of the trigger method available in any Router, you can also use our RxSwift-extension. In the example application, we use Actions (from the Action framework) to trigger routes on certain UI events - e.g. to trigger LoginRoute.home in LoginViewModel, when the login button is tapped.

class LoginViewModelImpl: LoginViewModel, LoginViewModelInput, LoginViewModelOutput {

    private let router: AnyRouter<AppRoute>

    private lazy var loginAction = CocoaAction { [unowned self] in
        return self.router.rx.trigger(.home)
    }

    /* ... */
}

In addition to the above-mentioned approach, the reactive trigger extension can also be used to sequence different transitions by using the flatMap operator, as can be seen in the following:

let doneWithBothTransitions = 
    router.rx.trigger(.home)
        .flatMap { [unowned router] in router.rx.trigger(.news) }
        .map { true }
        .startWith(false)

📚 Documentation & Example app

To get more information about XCoordinator, check out the documentation. Additionally, this repository serves as an example project using XCoordinator.

👨‍✈️ Why coordinators

  • Separation of responsibilities with the coordinator being the only component knowing anything related to the flow of your application.

  • Reusable Views and ViewModels because they do not contain any navigation logic.

  • Less coupling between components

  • Changeable navigation: Each coordinator is only responsible for one component and does not need to make assumptions about its parent. It can therefore be placed wherever we want to.

The Coordinator by Soroush Khanlou

⁉️ Why XCoordinator

  • Actual navigation code is already written and abstracted away.
  • Clear separation of concerns:
    • Coordinator: Coordinates routing of a set of routes.
    • Route: Describes navigation path.
    • Transition: Describe transition type and animation to new view.
  • Reuse coordinators, routers and transitions in different combinations.
  • Full support for custom transitions/animations.
  • Support for embedding child views / container views.
  • Generic BasicCoordinator classes suitable for many use cases and therefore less need to write your own coordinators.
  • Full support for your own coordinator classes conforming to our Coordinator protocol
    • You can also start with one of the following types to get a head start: NavigationCoordinator, ViewCoordinator, TabBarCoordinator and more.
  • Generic AnyRouter type erasure class encapsulates all types of coordinators and routers supporting the same set of routes. Therefore you can easily replace coordinators.
  • Use of enum for routes gives you autocompletion and type safety to perform only transition to routes supported by the coordinator.

🔩 Components

🎢 Route

Describes possible navigation paths within a flow, a collection of closely related scenes.

👨‍✈️ Coordinator / Router

An object loading views and creating viewModels based on triggered routes. A Coordinator creates and performs transitions to these scenes based on the data transferred via the route. In contrast to the coordinator, a router can be seen as an abstraction from that concept limited to triggering routes. Often, a Router is used to abstract from a specific coordinator in ViewModels.

🌗 Transition

Transitions describe the navigation from one view to another. Transitions are available based on the type of the root view controller in use. Example: Whereas ViewTransition only supports basic transitions that every root view controller supports, NavigationTransition adds navigation controller specific transitions.

The available transition types include:

  • present presents a view controller on top of the view hierarchy - use presentOnRoot in case you want to present from the root view controller
  • embed embeds a view controller into a container view
  • dismiss dismisses the top most presented view controller - use dismissToRoot to call dismiss on the root view controller
  • none does nothing, may be used to ignore routes or for testing purposes
  • push pushes a view controller to the navigation stack (only in NavigationTransition)
  • pop pops the top view controller from the navigation stack (only in NavigationTransition)
  • popToRoot pops all the view controllers on the navigation stack except the root view controller (only in NavigationTransition)

XCoordinator additionally supports common transitions for UITabBarController, UISplitViewController and UIPageViewController root view controllers.

🛠 Installation

CocoaPods

To integrate XCoordinator into your Xcode project using CocoaPods, add this to your Podfile:

pod 'XCoordinator', '~> 1.0'

To use the RxSwift extensions, add this to your Podfile:

pod 'XCoordinator/RxSwift', '~> 1.0'

Carthage

To integrate XCoordinator into your Xcode project using Carthage, add this to your Cartfile:

github "quickbirdstudios/XCoordinator" ~> 1.0

Then run carthage update.

If this is your first time using Carthage in the project, you'll need to go through some additional steps as explained over at Carthage.

Manually

If you prefer not to use any of the dependency managers, you can integrate XCoordinator into your project manually, by downloading the source code and placing the files on your project directory.

👤 Author

This framework is created with ❤️ by QuickBird Studios.

To get more information on XCoordinator check out our blog post.

❤️ Contributing

Open an issue if you need help, if you found a bug, or if you want to discuss a feature request. If you feel like having a chat about XCoordinator with the developers and other users, join our Slack Workspace.

Open a PR if you want to make changes to XCoordinator.

📃 License

XCoordinator is released under an MIT license. See License.md for more information.

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