如何发送静默推送通知IOS13？更新后不起作用(Xamarin.ios)

if (UIDevice.CurrentDevice.CheckSystemVersion(10, 0))
{
    UNUserNotificationCenter.Current.RequestAuthorization(UNAuthorizationOptions.Alert | UNAuthorizationOptions.Sound | UNAuthorizationOptions.Badge, (granted, error) =>
    {
        if (granted)
        {
            InvokeOnMainThread(UIApplication.SharedApplication.RegisterForRemoteNotifications);
        }
    });
}
else if (UIDevice.CurrentDevice.CheckSystemVersion(8, 0))
{
    var settings = UIUserNotificationSettings.GetSettingsForTypes(UIUserNotificationType.Alert | UIUserNotificationType.Sound | UIUserNotificationType.Badge, null);
    UIApplication.SharedApplication.RegisterUserNotificationSettings(settings);
    UIApplication.SharedApplication.RegisterForRemoteNotifications();
}

public override void RegisteredForRemoteNotifications(UIApplication application, NSData deviceToken)
{
    var token = deviceToken.Description.Trim('<', '>').Replace(" ", "");
    // 将设备令牌发送到服务器进行存储或处理
}

public override void DidReceiveRemoteNotification(UIApplication application, NSDictionary userInfo, Action<UIBackgroundFetchResult> completionHandler)
{
    // 处理接收到的推送通知
    completionHandler(UIBackgroundFetchResult.NewData);
}

using (var client = new WebClient())
{
    client.Headers.Add("Content-Type", "application/json");
    client.Headers.Add("Authorization", "Bearer YOUR_AUTH_TOKEN");

    var payload = new
    {
        aps = new
        {
            contentAvailable = true
        }
    };

    var jsonPayload = JsonConvert.SerializeObject(payload);
    var response = client.UploadString("https://api.push.apple.com/3/device/YOUR_DEVICE_TOKEN", "POST", jsonPayload);
}