app has black screen on device. works fine on simulator. no error messages.

Probably you have a sceneDelegate and you initiate window in AppDelegate.

As you hinted, you need to do it in SceneDelegate.

Your SceneDelegate should look like this:

import UIKit

class SceneDelegate: UIResponder, UIWindowSceneDelegate {

    var window: UIWindow?

    @available(iOS 13.0, *)
    func scene(_ scene: UIScene, willConnectTo session: UISceneSession, options connectionOptions: UIScene.ConnectionOptions) {
        // Use this method to optionally configure and attach the UIWindow `window` to the provided UIWindowScene `scene`.
        // If using a storyboard, the `window` property will automatically be initialized and attached to the scene.
        // This delegate does not imply the connecting scene or session are new (see `application:configurationForConnectingSceneSession` instead).
        guard let _ = (scene as? UIWindowScene) else { return }
    }

    @available(iOS 13.0, *)
    func sceneDidDisconnect(_ scene: UIScene) {
        // Called as the scene is being released by the system.
        // This occurs shortly after the scene enters the background, or when its session is discarded.
        // Release any resources associated with this scene that can be re-created the next time the scene connects.
        // The scene may re-connect later, as its session was not neccessarily discarded (see `application:didDiscardSceneSessions` instead).
    }

    @available(iOS 13.0, *)
    func sceneDidBecomeActive(_ scene: UIScene) {
        // Called when the scene has moved from an inactive state to an active state.
        // Use this method to restart any tasks that were paused (or not yet started) when the scene was inactive.
    }

    @available(iOS 13.0, *)
    func sceneWillResignActive(_ scene: UIScene) {
        // Called when the scene will move from an active state to an inactive state.
        // This may occur due to temporary interruptions (ex. an incoming phone call).
    }

    @available(iOS 13.0, *)
    func sceneWillEnterForeground(_ scene: UIScene) {
        // Called as the scene transitions from the background to the foreground.
        // Use this method to undo the changes made on entering the background.
        // Do it here, not in app delegate (iOS 13)
        NotificationCenter.default.post(name: .kRefreshPrefs, object: self)
    }

    @available(iOS 13.0, *)
    func sceneDidEnterBackground(_ scene: UIScene) {
        // Called as the scene transitions from the foreground to the background.
        // Use this method to save data, release shared resources, and store enough scene-specific state information
        // to restore the scene back to its current state.
    }

}

And AppDelegate like this:

class AppDelegate: UIResponder, UIApplicationDelegate {

    var window: UIWindow?   // For iOS 12

    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {

        // Override point for customization after application launch.
       
        return true
    }

    func applicationWillEnterForeground(_ application: UIApplication) {

    }

    // MARK: - UISceneSession Lifecycle

    @available(iOS 13.0, *)
    func application(_ application: UIApplication, configurationForConnecting connectingSceneSession: UISceneSession, options: UIScene.ConnectionOptions) -> UISceneConfiguration {
        // Called when a new scene session is being created.
        // Use this method to select a configuration to create the new scene with.
        return UISceneConfiguration(name: "Default Configuration", sessionRole: connectingSceneSession.role)
    }

    @available(iOS 13.0, *)
    func application(_ application: UIApplication, didDiscardSceneSessions sceneSessions: Set) {
        // Called when the user discards a scene session.
        // If any sessions were discarded while the application was not running, this will be called shortly after application:didFinishLaunchingWithOptions.
        // Use this method to release any resources that were specific to the discarded scenes, as they will not return.
    }

}

And don't forget to define the key in 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>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>

<key>UISceneStoryboardFile</key>

<string>Main</string>

</dict>

</array>

</dict>

</dict>

</plist>

Yes thats how the scene delegate looks like.

What should I be doing in both the files? I didn't get you.

Note:please give me objc code. Thanks.

Neerav

@ace.neerav

Check below link, if you are still facing same issue.

https://stackoverflow.com/questions/29441682/the-app-delegate-must-implement-the-window-property-if-it-wants-to-use-a-main-st