How to check app going to background in SwiftUI

Issue #746

From iOS 13, the default is to support multiple scene, so the the old UIApplicationDelegate lifecycle does not work. Double check your Info.plist for UIApplicationSceneManifest key

<key>UIApplicationSceneManifest</key>
<dict>
    <key>UIApplicationSupportsMultipleScenes</key>
    <true/>
</dict>

One way to be notified about application life cycle is to use UIApplicationDelegateAdaptor and via NotificationCenter

import SwiftUI
import UIKit
import FontAwesomeSwiftUI

final class AppDelegate: NSObject, UIApplicationDelegate {
    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey : Any]? = nil) -> Bool {
        FontAwesome.register()
        PreferenceManager.shared.load()
        return true
    }
}

@main
struct MyAwesomeApp: App {
    @UIApplicationDelegateAdaptor(AppDelegate.self)
    var appDelegate

    var body: some Scene {
        WindowGroup {
            MainView(store: Store.shared)
                .onReceive(
                    NotificationCenter.default.publisher(
                        for: UIApplication.didEnterBackgroundNotification)) { _ in
                    PreferenceManager.shared.save()
                }
        }
    }
}