我得到的错误：

我怎样才能解决这个问题？

Cannot assign value of type ‘AppDelegate’ to type ‘UNUserNotificationCenterDelegate?’

第一个场景 – 我一步一步地做了什么(按照教程)：

> pod init具有以下内容：
pod’Firebase / Core’
pod’Firebase / Messaging’
> pod安装
>在AppDelegate类之上“导入Firebase”

第二个场景 – 通过github存储库下载谷歌演示iOS客户端应用程序(https://github.com/firebase/quickstart-ios下的“messaging”文件夹)

>编译他们的应用程序……工作正常.
>根据以下步骤,将我们现有的XCode项目的逻辑编译为：
> pod init包含以下内容：pod’Firebase / Messaging’
> pod安装
>导入以下内容：UIKit,UserNotifications,Firebase,FirebaseInstanceID,FirebaseMessaging

AppDelegate中的代码：

func application(_ application: UIApplication,didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {
        // [START register_for_notifications]
        if #available(iOS 10.0,*) {
            let authOptions : UNAuthorizationOptions = [.alert,.badge,.sound]
            UNUserNotificationCenter.current().requestAuthorization(
                options: authOptions,completionHandler: {_,_ in })
            // For iOS 10 display notification (sent via APNS)
            UNUserNotificationCenter.current().delegate = self
            // For iOS 10 data message (sent via FCM)
            FIRMessaging.messaging().remoteMessageDelegate = self
        } else {
            let settings: UIUserNotificationSettings =
                UIUserNotificationSettings(types: [.alert,.sound],categories: nil)
            application.registerUserNotificationSettings(settings)
        }
        application.registerForRemoteNotifications()
        // [END register_for_notifications]
        FIRApp.configure()
        // Add observer for InstanceID token refresh callback.
        NotificationCenter.default.addObserver(self,selector: #selector(self.tokenRefreshNotification),name: .firInstanceIDTokenRefresh,object: nil)
        return true
    }