Navigator

public class Navigator

Main class that is handling the navigation in a Tempura app.

Overview

When it comes to creating a complex app, the way you handle the navigation between different screens is an important factor on the final result.

We believe that relying on the native iOS navigation system, even in a Redux-like environment like Katana, is the right choice for our stack, because:

• there is no navigation code to write and maintain just to mimic the way native navigation works

• native navigation gestures will come for free and will stay up to date with new iOS releases

• the app will feel more “native”

For these reasons we found a way to reconcile the redux-like world of Katana with the imperative world of the iOS navigation.

The Routable protocol

If a Screen (read ViewController) takes an active part on the navigation (i.e. needs to present another screen) it must conform to the RoutableWithConfiguration protocol.

   protocol RoutableWithConfiguration: Routable {

     var routeIdentifier: RouteElementIdentifier { get }

     var navigationConfiguration: [NavigationRequest: NavigationInstruction] { get }
   }

   typealias RouteElementIdentifier = String

Each RoutableWithConfiguration can be asked by the Navigator navigation to perform a specific navigation task (like present another ViewController) based on the navigation action (Show, Hide) you dispatch.

The route

A Route is an array that represents a navigation path to a specific screen.

   typealias Route = [RouteElementIdentifier]

How the navigation works

Suppose we have a current Route like “screenA”, “screenB” the topmost ViewController/RoutableWithConfiguration in the visible hierarchy.

Tempura will expose two main navigation actions:

Show

   Show("screenC", animated: true)

When this action is dispatched, the Navigator will ask “screenB” (the topmost RoutableWithConfiguration to handle that action, looking at its navigationConfiguration).

In order to allow “screenB” to present “screenC”, we need to add a NavigationRequest inside the navigationConfiguration of “screenB” that will match the Show("screenC") action with a .show("screenC") NavigationRequest.

   extension ScreenB: RoutableWithConfiguration {

     // needed by the `Routable` protocol
     // to identify this ViewController in the hierarchy
     var routeIdentifier: RouteElementIdentifier {
       return "screenB"
     }

     // the `NavigationRequest`s that this ViewController is handling
     // with the `NavigationInstruction` to execute
     var navigationConfiguration: [NavigationRequest: NavigationInstruction] {
       return [
         .show("screenC"): .presentModally({ [unowned self] _ in
           let vc = ScreenC(store: self.store)
           return vc
         })
       ]
   }

Inside the .presentModally NavigationInstruction, we create and return the ViewController that is implementing the “screenC”. This will be used by the Navigator to call the appropriate UIKit navigation action ( a UIViewController.present(:animated:completion:) in this case).

If the Navigator will not find a matching NavigationRequest in the navigationConfiguration of “screenB”, it will ask to the next RoutableWithConfiguration in the visible hierarchy, in this case “screenA”. If nobody is matching that NavigationRequest, a fatalError is thrown.

Hide

Same will happen when a Hide function is dispatched.

   Hide("screenC", animated: true)

Looking at the current Route [“screenA”, “screenB”, “screenC”], the Navigator will ask the topmost RoutableWithConfiguration (“screenC” in this case) to match for a .hide("screenC") NavigationRequest.

Again, if we want “screenC” to be responsible for dismissing itself, we just need to implement the matching NavigationRequest in the navigationConfiguration:

   extension ScreenC: RoutableWithConfiguration {

     // needed by the `Routable` protocol
     // to identify this ViewController in the hierarchy
     var routeIdentifier: RouteElementIdentifier {
       return "screenC"
     }

     // the `NavigationRequest`s that this ViewController is handling
     // with the `NavigationInstruction` to execute
     var navigationConfiguration: [NavigationRequest: NavigationInstruction] {
       return [
         .hide("screenC"): .dismissModally(behaviour: .hard)
       ]
   }

If the Navigator will not find a matching NavigationRequest in the navigationConfiguration of “screenC”, it will ask to the next RoutableWithConfiguration in the visible hierarchy, in this case “screenB”. If nobody is matching that NavigationRequest, a fatalError is thrown.

Initializing the navigation

In order to use the navigation system you need to start the Navigator (typically in your AppDelegate) using the start(using:in:at:) method. In this method you need to specify a RootInstaller and a starting screen. The RootInstaller (typically your AppDelegate) will be responsible to handle the installation of the screen you specified before.

Doing so, the Navigator will call the RooInstaller.installRoot(identifier:context:completion:) method that will handle the setup of the screen to be shown.

   class AppDelegate: UIResponder, UIApplicationDelegate, RootInstaller {

     func application(_ application: UIApplication, didFinishLaunchingWithOptions ...) -> Bool {
       ...
       // setup the root of the navigation
       // this is done by invoking this method (and not in the init of the navigator)
       // because the navigator is instantiated by the Store.
       // this in turn will invoke the `installRootMethod` of the rootInstaller (self)
       navigator.start(using: self, in: self.window, at: "screenA")
       return true
     }

     // install the root of the app
     // this method is called by the navigator when needed
     // you must call the `completion` callback when the navigation has been completed
     func installRoot(identifier: RouteElementIdentifier, context: Any?, completion: () -> ()) -> Bool {
       let vc = ScreenAViewController(store: self.store)
       self.window.rootViewController = vc
       completion()
       return true
     }
   }