无法将“AppDelegate”分配给“UNUserNotificationCenterDelegate”

我将学习本教程: https://medium.com/flawless-app-stories/ios-remote-push-notifications-in-a-nutshell-d05f5ccac252

但出于某种原因,我

无法将“AppDelegate”分配给“UNUserNotificationCenterDelegate”。

那条线是干什么的?如果我将其注释掉,其余代码将正常工作,并提示用户是否允许通知。

func registerForPushNotifications() {
    UNUserNotificationCenter.current().delegate = self // line in question
    UNUserNotificationCenter.current().requestAuthorization(options: [.alert, .sound, .badge]) {
        (granted, error) in
        print("Permission granted: \(granted)")
        // 1. Check if permission granted
        guard granted else { return }
        // 2. Attempt registration for remote notifications on the main thread
        DispatchQueue.main.async {
            UIApplication.shared.registerForRemoteNotifications()
        }
    }
}