我在iOS 10.3.3设备上进行测试，直到我将UNUserNotificationCenter的委托设置为UIApplication.shared.delegate as? UNUserNotificationCenterDelegate，才将AppDelegate中的实现委托设置为空委托。但在当前视图控制器中实现空委托并不起作用。
AppDelegate.swift:

extension AppDelegate: UNUserNotificationCenterDelegate { }

下面是我的视图控制器中的方法：

private func registerPushNotifications() {
    Messaging.messaging().delegate = self
    if #available(iOS 10.0, *) {
        // For iOS 10 display notification (sent via APNS)
        UNUserNotificationCenter.current().delegate = UIApplication.shared.delegate as? UNUserNotificationCenterDelegate

        let authOptions: UNAuthorizationOptions = [.alert, .badge, .sound]
        UNUserNotificationCenter.current().requestAuthorization(
            options: authOptions,
            completionHandler: {granted, _ in
                if granted {
                    DispatchQueue.main.async {
                        UIApplication.shared.registerForRemoteNotifications()
                    }
                }
        })
    } else {
        let settings: UIUserNotificationSettings =
            UIUserNotificationSettings(types: [.alert, .badge, .sound], categories: nil)
        UIApplication.shared.registerUserNotificationSettings(settings)
        UIApplication.shared.registerForRemoteNotifications()
    }
}

当然，你必须在didFinishLaunchingWithOptions方法中保留FirebaseApp.configure()。
稍后编辑：您可能会收到关于从主线程调用registerForRemoteNotifications()的警告，因此我调整了代码以实现这一点。

添加类

import Foundation
import FirebaseMessaging
import FirebaseInstanceID
import UserNotifications

class Notifications {

    private init() { }
    static let shared = Notifications()

    func requestNotifications() {
        UNUserNotificationCenter.current().requestAuthorization(options: [.alert, .sound, .badge]) { (isAuthorized, error) in
            if error != nil {
                print(error as Any)
                return
            } else {
                print("set up successfully")
                // Completion Code Here:
            }
        }
    }
}

添加到您喜欢的VC

override func viewDidAppear(_ animated: Bool) {
        super.viewDidAppear(animated)
        Notifications.shared.requestNotifications()
    }