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AdaptiveNavigationMethod.md

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Changing navigation method

So far, we have learned how to present UIViewControllers modally by starting ModalChild and NavigationChild instances. These classes allow you to define the context in which the view controller will be presented. It's guaranteed to be presented in the exact way that you expect.

However, there is a way to create a coordinator that adapts to the presentation context of the parent coordinator. You use AdaptiveCoordinator to achieve this goal:

class PaymentMethodCoordinator: AdaptiveCoordinator<PaymentMethodScreenResult> {
    override var viewController: UIViewController {
        let viewController = PaymentMethodViewController()
        
        return viewController
    }
}

Starting an AdaptiveCoordinator

ModalCoordinator and NavigationCoordinator both have self.navigation. For both classes, this object provides a very specialized interface to start new instances of both ModalCoordinator and NavigationCoordinator.

Additionally, self.navigation always conforms to AdaptiveNavigation protocol. This interface enables you to start AdaptiveCoordinators.

Here is the start method available on AdaptiveNavigation:

public struct AdaptiveContext {
    public static var current: AdaptiveContext

    public static var modal: AdaptiveContext

    public static func navigation(new navigationController: UINavigationController) -> AdaptiveContext

    public static func navigation(fallback navigationController: @autoclosure @escaping () -> UINavigationController) -> AdaptiveContext
}

public protocol AdaptiveNavigation {
    func start<NewResult>(_ child: AdaptiveCoordinator<NewResult>,
                          in context: AdaptiveContext,
                          animated: Bool,
                          _ completion: @escaping (NewResult?) -> Void)
}

Notice the context parameter. This parameter allows to control the way the child will be presented on screen.

AdaptiveContext.current

Starting with this context means that the child will be presented just like its parent:

Parent is presented Child will be presented
Via present Via present
Via push Via push

"Current" stands for "current presentation context"

AdaptiveContext.modal

This overrides the current context and enforces modal presentation:

Parent is presented Child will be presented
Via present Via present
Via push Via present

AdaptiveContext.navigation(new:)

This overrides the current context and starts a new UINavigationController

Parent is presented Child will be presented
Via present as a root of the given UINavigationController. The navigation controller will be presented modally on top of modally presented parent.
Via push as a root of the given UINavigationController. The navigation controller will be presented modally on top of the existing UINavigationController.

AdaptiveContext.navigation(fallback:)

This kind of context allows you to push if there is already a UINavigationController and to start a new UINavigationController if the parent didn't provide any.

This method accepts an @autoclosure which is not always evaluated.

Current Presentation Context Outcome
Already a UINavigationController It pushes the child unto the existing UINavigationController. fallback-autoclosure parameter never gets evaluated.
Modal presentation It creates the UINavigationController from the @autoclosure that you pass as fallback parameter. Then, it starts child as the rootViewController of the UINavigationController.

⚠️ Be careful not to create the UINavigationController ahead of passing it into autoclosure

Do this:

self.navigation.start(coordinator, in: .navigation(fallback: UINavigationController()), animated: true) { _ in }

Not this:

let navigationController = UINavigationController() // Already created

self.navigation.start(coordinator, in: .navigation(fallback: navigationController), animated: true) { _ in }

Starting proper coordinators from AdaptiveCoordinator

self.navigation also provides 2 methods that allow you to return to the realm of ModalCoordinator and NavigationCoordinator.

public protocol AdaptiveNavigation {
    func start<NewResult>(_ child: ModalCoordinator<NewResult>,
                          animated: Bool,
                          _ completion: @escaping (NewResult?) -> Void)
    
    func start<NewResult>(_ navigationController: UINavigationController,
                          _ child: NavigationCoordinator<NewResult>,
                          animated: Bool,
                          _ completion: @escaping (NewResult?) -> Void)
}

Of course, these instances of ModalCoordinator and NavigationCoordinator will see their proper navigation interfaces in self.navigation - the ModalNavigation and NavigationControllerNavigation.