use @main in Xcode 12

Question

I want to run this code on iOS 13 and above how should I fix this error? I want to make this code could run on iOS 13 too.

@available(iOS 14.0, *)
@main

struct WeatherProApp: App {
  @Environment(\.scenePhase) private var scenePhase
  @UIApplicationDelegateAdaptor(AppDelegate.self) var appDelegate

  
  var body: some Scene {
    WindowGroup{
      let fetcher = WeatherFetcher()
      let viewModel = WeeklyWeatherViewModel(weatherFethcer: fetcher)
      WeeklyWeatherView(viewModel: viewModel)
    }
    .onChange(of: scenePhase) { (newScenePhase) in
      switch newScenePhase {
      case .active:
        print("scene is now active!")
      case .inactive:
        print("scene is now inactive!")
      case .background:
        print("scene is now in the background!")
      @unknown default:
        print("Apple must have added something new!")
      }
    }
  }
}

but it shows me this error

Answer 1

Following @the.blaggy answer, here is how I managed to run my project on iOS 13:

1. Create a SceneDelegate if you do not have one

SceneDelegate.swift

class SceneDelegate: UIResponder, UIWindowSceneDelegate {

    var window: UIWindow?

    func scene(_ scene: UIScene, willConnectTo session: UISceneSession, options connectionOptions: UIScene.ConnectionOptions) {
        let contentView = ContentView()

        // Use a UIHostingController as window root view controller.
        if let windowScene = scene as? UIWindowScene {
            let window = UIWindow(windowScene: windowScene)
            window.rootViewController = UIHostingController(rootView: contentView)
            self.window = window
            window.makeKeyAndVisible()
        }
    }
}

2. Open your info.plist as Source Code and add those lines :

Info.plist

   <key>UIApplicationSceneManifest</key>
       <dict>
           <key>UIApplicationSupportsMultipleScenes</key>
           <false/>
           <key>UISceneConfigurations</key>
           <dict>
           <key>UIWindowSceneSessionRoleApplication</key>
           <array>
               <dict>
                   <key>UISceneConfigurationName</key>
                   <string>Default Configuration</string>
                   <key>UISceneDelegateClassName</key>
                   <string>$(PRODUCT_MODULE_NAME).SceneDelegate</string>
               </dict>
           </array>
       </dict>
   </dict>

3. Add this in your WeatherProApp.swift

WeatherProApp.swift

    @main
    struct WeatherProAppWrapper {
        static func main() {
            if #available(iOS 14.0, *) {
                WeatherProApp.main()
            }
            else {
                UIApplicationMain(CommandLine.argc, CommandLine.unsafeArgv, nil, NSStringFromClass(SceneDelegate.self))
            }
        }
    }

Answer 2

Actually you can use the @main attribute in pre iOS 14 but you need an alternative AppDelegate and SceneDelegate (you can copy these two delegate classes from iOS 13 Xcode projects) and you have to do some extra wrapping.

First you have to apply the @main attribute in the following way to a struct with a main function which decides depending on the iOS version whether to use the WeatherProApp struct or the AppDelegate class to launch:

@main
struct WeatherProAppWrapper {
    static func main() {
        if #available(iOS 14.0, *) {
            WeatherProApp.main()
        }
        else {
            UIApplicationMain(CommandLine.argc, CommandLine.unsafeArgv, nil, NSStringFromClass(AppDelegate.self))
        }
    }
}

Afterwards you can use the shown implementation from your question, just remove the @main attribute, only use @available(iOS 14.0, *). E.g.:

@available(iOS 14.0, *)
struct WeatherProApp: App {
    var body: some Scene {
        WindowGroup{
            ContentView()
        }
    }
}

I'm not sure how familiar you're with UIKit but you have to do the same setup you did in your WindowGroup in the SceneDelegate class too.