Xcode 12 项目与 iOS 13 SwiftUI 支持：未调用 SceneDelegate 函数

Question

I had created a project in Xcode 12 with iOS 14 as deployment target (bigger project). After some time I realized that I had to also be able to support iOS 13 devices. My project uses SwiftUI.

It is apparently not very straight-forward to simply change the deployment target to iOS 13 as some of the code generated by Xcode 12 is not backwards compatible with iOS 13.

So I essentially need to convert my Xcode 12-based project (currently with deployment target iOS 14.0) to also support iOS 13 .

Here's what I did to prepare for my project's downgrade:

• Install Xcode 11.7, create new project with SwiftUI support and ensure the deployment target is iOS 13.0. Thereby I know what Xcode believes the "iOS 13"-way should be, ie, using AppDelegate and SceneDelegate.
• Run that app - works fine on my iOS 14 device.
• Open Xcode 12, create new project with SwiftUI support, lower the deployment target to iOS 13.0 (because it's default set to 14.x), build it, and be presented with a bunch of compilation issues (as expected).

Changes then made to the Xcode 12 project:

• Comment out everything in the <project>App.swift file.
• Add AppDelegate.swift file with the same contents from the Xcode 11.7-generated project mentioned above.
• Add SceneDelegate.swift file with the same contents from the Xcode 11.7-generated project mentioned above.
• Add a LaunchScreen.storyboard file (because Xcode warned about its non-existence).
• Run the app.

The app runs, but the contents of ContentView is not shown. Just a black screen (black is probably because my device is in dark mode). Also, as you'll notice in the code below, I've inserted debug logs but only "didFinishLaunchingWithOptions" is printed in the console - that is, the scene delegate methods are not invoked, it seems.

I've (of course) Googled a lot, tried various things with Info.plist, but nothing seems to help.

Code:

AppDelegate.swift

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {
    
    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
        print("didFinishLaunchingWithOptions")
        return true
    }
    
    // MARK: UISceneSession Lifecycle
    
    func application(_ application: UIApplication, configurationForConnecting connectingSceneSession: UISceneSession, options: UIScene.ConnectionOptions) -> UISceneConfiguration {
        print("configurationForConnecting")      
    }
    
    func application(_ application: UIApplication, didDiscardSceneSessions sceneSessions: Set<UISceneSession>) {
        print("didDiscardSceneSessions")
    }
}

SceneDelegate.swift

class SceneDelegate: UIResponder, UIWindowSceneDelegate {
    
    var window: UIWindow?
    
    func scene(_ scene: UIScene, willConnectTo session: UISceneSession, options connectionOptions: UIScene.ConnectionOptions) {
        print("scene willConnectTo")
        guard let windowScene = scene as? UIWindowScene else { return }
        let window = UIWindow(windowScene: windowScene)
        window.rootViewController = UIHostingController(rootView: ContentView())
        self.window = window
        window.makeKeyAndVisible()
    }
    
    func sceneDidDisconnect(_ scene: UIScene) {
        print("sceneDidDisconnect")
    }
    
    func sceneDidBecomeActive(_ scene: UIScene) {
        print("sceneDidBecomeActive")
    }
    
    func sceneWillResignActive(_ scene: UIScene) {
        print("sceneWillResignActive")
    }
    
    func sceneWillEnterForeground(_ scene: UIScene) {
        print("sceneWillEnterForeground")
    }
    
    func sceneDidEnterBackground(_ scene: UIScene) {
        print("sceneDidEnterBackground")
    }
}

ContentView.swift

struct ContentView: View {
    
    var body: some View {
        Text("Hello, world!")
            .foregroundColor(.blue)
            .padding()
            .onAppear {
                print("ContentView")
            }
    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}

Info.plist

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
    <key>CFBundleDisplayName</key>
    <string></string>
    <key>LSApplicationCategoryType</key>
    <string></string>
    <key>CFBundleDevelopmentRegion</key>
    <string>$(DEVELOPMENT_LANGUAGE)</string>
    <key>CFBundleExecutable</key>
    <string>$(EXECUTABLE_NAME)</string>
    <key>CFBundleIdentifier</key>
    <string>$(PRODUCT_BUNDLE_IDENTIFIER)</string>
    <key>CFBundleInfoDictionaryVersion</key>
    <string>6.0</string>
    <key>CFBundleName</key>
    <string>$(PRODUCT_NAME)</string>
    <key>CFBundlePackageType</key>
    <string>$(PRODUCT_BUNDLE_PACKAGE_TYPE)</string>
    <key>CFBundleShortVersionString</key>
    <string>1.0</string>
    <key>CFBundleVersion</key>
    <string>1</string>
    <key>LSRequiresIPhoneOS</key>
    <true/>
    <key>UIApplicationSceneManifest</key>
    <dict>
        <key>UIApplicationSupportsMultipleScenes</key>
        <false/>
        <key>UISceneConfigurations</key>
        <dict>
            <key>UIWindowSceneSessionRoleApplication</key>
            <array>
                <dict>
                    <key>UISceneConfigurationName</key>
                    <string>Default Configuration</string>
                    <key>UISceneDelegateClassName</key>
                    <string>$(PRODUCT_MODULE_NAME).SceneDelegate</string>
                </dict>
            </array>
        </dict>
    </dict>
    <key>UIApplicationSupportsIndirectInputEvents</key>
    <true/>
    <key>UILaunchScreen</key>
    <dict/>
    <key>UILaunchStoryboardName</key>
    <string>LaunchScreen</string>
    <key>UIRequiredDeviceCapabilities</key>
    <array>
        <string>armv7</string>
    </array>
    <key>UISupportedInterfaceOrientations</key>
    <array>
        <string>UIInterfaceOrientationPortrait</string>
        <string>UIInterfaceOrientationLandscapeLeft</string>
        <string>UIInterfaceOrientationLandscapeRight</string>
    </array>
    <key>UISupportedInterfaceOrientations~ipad</key>
    <array>
        <string>UIInterfaceOrientationPortrait</string>
        <string>UIInterfaceOrientationPortraitUpsideDown</string>
        <string>UIInterfaceOrientationLandscapeLeft</string>
        <string>UIInterfaceOrientationLandscapeRight</string>
    </array>
</dict>
</plist>

Answer 1

I'm going to guess that you forgot to add the scene delegate file to the app target. When you build and run, look in the console and see if there's a complaint about not finding the scene delegate. That would explain the phenomenon.

Answer 2

If you've already built app using SwiftApp before implementation SceneDelegate , just uninstall and rebuild it. And then, you can resolve this problem.

persistentIdentifier will be generated by the system for creating new Scene when an app is launched at the first time. After that, the identifier will be same one every app open. So the app doesn't need to create new Scene.

That's why configurationForConnecting wasn't invoke but set up no rootview so it happens that your app shows black window.

The following WWDC19 session video(around 31:40) says about the identifier.
https://developer.apple.com/videos/play/wwdc2019/212/?time=1900 .

Hope it will help your problem.
Thank you.