在 iOS 上，FCM 通知在后台收到，但不在前台收到

Question

我在后台收到推送到 iOS 设备的 FCM 消息，但在前台却收不到。我发现很多文章中其他人也有同样的问题，并通过添加 willPresentNotification 函数解决了它。但这并不能解决我的问题。我的 FCM 代码如下：

class FCMManager: NSObject, MessagingDelegate, UNUserNotificationCenterDelegate {

static let shared = FCMManager()
let database = Database.database().reference()

public func registerForPushNotifications() {
    UNUserNotificationCenter.current().delegate = self
    let authOptions: UNAuthorizationOptions = [.alert, .sound, .announcement, .badge, .carPlay, .providesAppNotificationSettings]
    UNUserNotificationCenter.current()
        .requestAuthorization( options: authOptions,
                               completionHandler: { granted, _ in
            guard granted else {
                print("Apple push notifications are not registered")
                return
            }
            print("Apple push notifications are registered")
        })
    Messaging.messaging().delegate = self
    DispatchQueue.main.async {
        UIApplication.shared.registerForRemoteNotifications()
    }
    updatePushTokenIfNeeded()
 }

 func messaging(_ messaging: Messaging, didReceiveRegistrationToken fcmToken: String?) {
     updatePushTokenIfNeeded()
  }
}

我的通知有效负载设置为：

 ["to" : "fcm_token",
  "notification" : ["title" : "Message Title",
                    "body" : "Message Body",
                    "sound" : "default",
                    "badge" : 1]
 ]

然后我的应用程序委托如下：

class AppDelegate: UIResponder, UIApplicationDelegate, MessagingDelegate, UNUserNotificationCenterDelegate {

  var window: UIWindow?
  let notificationCenter = UNUserNotificationCenter.current()

  func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
      FirebaseApp.configure()
      let fcmManager = FCMManager()
      fcmManager.registerForPushNotifications()
      return true
  }

  func userNotificationCenter(_ center: UNUserNotificationCenter, willPresent notification: UNNotification, withCompletionHandler completionHandler: @escaping (UNNotificationPresentationOptions) -> Void) {
      if #available(iOS 14, *) {
          completionHandler([.banner])
      } else {
          completionHandler([.alert])
      }
  }
}