SwiftUI iOS集成Firebase推送报错：获取FCM Token前未设置APNS设备令牌

大约一年前推送通知功能正常，之后未再关注，不清楚是否因Swift/SwiftUI/Xcode/iOS更新引发当前报错：

APNS device token not set before retrieving FCM Token for Sender ID 'xxxxxxxxxxx'.Be sure to re-retrieve the FCM token once the APNS device token is set.

已尝试的解决方案 #

• 撤销旧APNS令牌并在Firebase项目设置中替换为新令牌
• 更新至最新版Firebase SDK
• 参考不同教程实现，尝试多种AppDelegate类
• 将FirebaseAppDelegateProxyEnabled设为false

问题困惑 #

有相关评论指出问题源于函数调用时机不当，但无法理解具体含义。使用该评论提供的代码片段后，应用启动数秒后token仍为nil。

当前使用的AppDelegate代码 #

class AppDelegate: NSObject, UIApplicationDelegate {
    
    let gcmMessageIDKey = "gcm.message_id"
    
    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey : Any]? = nil) -> Bool {
        
        FirebaseApp.configure()
        UNUserNotificationCenter.current().delegate = self
        
        UNUserNotificationCenter.current().requestAuthorization(options: [.alert, .badge, .sound]) { (granted, error) in
        }
        
        application.registerForRemoteNotifications()
        Messaging.messaging().delegate = self

        return true
    }
    
    func application(_ application: UIApplication, didReceiveRemoteNotification userInfo: [AnyHashable: Any], fetchCompletionHandler completionHandler: @escaping (UIBackgroundFetchResult) -> Void) {
        if let messageID = userInfo[gcmMessageIDKey] {
            print("Message ID: \(messageID)")
        }
        
        guard let aps = userInfo["aps"] as? [String: AnyObject] else {
            completionHandler(.failed)
            return
        }
        print("got something, aka the \(aps)")

        // Print full message.
        print(userInfo)

        completionHandler(UIBackgroundFetchResult.newData)
    }
    
    func application(_ application: UIApplication, didFailToRegisterForRemoteNotificationsWithError error: Error) {
        print("Unable to register for remote notifications: \(error.localizedDescription)")
    }

    func application(_ application: UIApplication, didRegisterForRemoteNotificationsWithDeviceToken deviceToken: Data) {
        Messaging.messaging().apnsToken = deviceToken
    }
      
}

extension AppDelegate: UNUserNotificationCenterDelegate {
  // Receive displayed notifications for iOS 10 devices.
  func userNotificationCenter(_ center: UNUserNotificationCenter,
                              willPresent notification: UNNotification,
                              withCompletionHandler completionHandler: @escaping (UNNotificationPresentationOptions)
                                -> Void) {
    let userInfo = notification.request.content.userInfo

    // With swizzling disabled you must let Messaging know about the message, for Analytics
    // Messaging.messaging().appDidReceiveMessage(userInfo)

    // ...

    // Print full message.
    print(userInfo)

    // Change this to your preferred presentation option
      completionHandler([.banner, .badge, .sound])
  }

  func userNotificationCenter(_ center: UNUserNotificationCenter,
                              didReceive response: UNNotificationResponse,
                              withCompletionHandler completionHandler: @escaping () -> Void) {
    let userInfo = response.notification.request.content.userInfo

    print(userInfo)

    completionHandler()
  }
}

extension AppDelegate: MessagingDelegate {
  // [START refresh_token]
  func messaging(_ messaging: Messaging, didReceiveRegistrationToken fcmToken: String?) {
    print("Firebase registration token: \(String(describing: fcmToken))")

    let dataDict: [String: String] = ["token": fcmToken ?? ""]
    NotificationCenter.default.post(
      name: Notification.Name("FCMToken"),
      object: nil,
      userInfo: dataDict
    )

  }

}

希望有人能帮我找出问题所在。

内容的提问来源于stack exchange，提问作者TheGreatCornholio