Firebase通知-打开应用程序时无通知（Swift）

Question

打开应用程序后，为什么通知不起作用？

UNUserNotificationCenter.current().delegate = self
let authOptions: UNAuthorizationOptions = [.alert, .badge, .sound]
UNUserNotificationCenter.current().requestAuthorization(
                options: authOptions,
                completionHandler: {_, _ in })
Messaging.messaging().delegate = self
application.registerForRemoteNotifications()
FirebaseApp.configure()