SwiftUI & Pre IOS 13 Support

This is a proof of concept sample project to support SwiftUI and pre IOS 13 code to coexist and provide support till IOS 9.

Requirements

The requirement came from an existing project which supports upto IOS 9 and will continue to support it for the foreseeable future. At the same time, there is a requirement to build a new version of the app using SwiftUI. The project does not try to provide a way for SwiftUI to work before IOS 13.

• Surface new SwiftUI version of the app for IOS 13 and above.
• Switch to current app for IOS 12 and below.

How it works

• In AppDelegate, mark new UISceneSession APIs as IOS 13 only. Conditionally build pre IOS 13 UI in didFinishLaunchingWithOptions callback.

    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
        guard #available(iOS 13.0, *) else {
            // Code for earlier OS
            let window = UIWindow()

            let rootViewController = ...
            window.rootViewController = rootViewController
            self.window = window
            window.makeKeyAndVisible()

            return true
        }

        return true
    }

    @available(iOS 13.0, *)
    func application(_ application: UIApplication, configurationForConnecting connectingSceneSession: UISceneSession, options: UIScene.ConnectionOptions) -> UISceneConfiguration {
    }

    @available(iOS 13.0, *)
    func application(_ application: UIApplication, didDiscardSceneSessions sceneSessions: Set<UISceneSession>) {
    }

• Mark SceneDelgate as IOS 13 only

@available(iOS 13.0, *)
class SceneDelegate: UIResponder, UIWindowSceneDelegate {
    var window: UIWindow?

    func scene(_ scene: UIScene, willConnectTo session: UISceneSession, options connectionOptions: UIScene.ConnectionOptions) {
    }

    func sceneDidDisconnect(_ scene: UIScene) {
    }

    func sceneDidBecomeActive(_ scene: UIScene) {
    }

    func sceneWillResignActive(_ scene: UIScene) {
    }

    func sceneWillEnterForeground(_ scene: UIScene) {
    }

    func sceneDidEnterBackground(_ scene: UIScene) {
    }
}

• Mark all SwiftUI app code as IOS 13 only.

@available(iOS 13.0.0, *)
struct ContentView: View {
    var body: some View {
    }
}

@available(iOS 13.0.0, *)
struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}