UIActivityIndicator无法正常工作Xamarin IOS

我包括UIActivityIndicator在我的UIViewController。我需要从FTP使用一个文件。

我的按钮上有一个按钮ViewController，单击该按钮后，下载将开始。我在UIActivityIndicator（aiReceive）上放了一个（）ViewController，它在视图控制器上以停止状态显示，但只有在完成文件下载后才能进行动画处理。

我正在Xamarin IOS中制作iPad App。

我添加了异步方法，但是现在出现以下错误。仅当其包含的lambda表达式用async修饰符标记时，才可以使用await运算符

public partial class FirstViewController : UIViewController
{
    public override void ViewDidLoad()
    {
        base.ViewDidLoad();
        btnReceiveData.TouchUpInside += (object sender, EventArgs e) =>
        {
            //BTProgressHUD.Show("Receiving Data...");    --> This Component is also not working,
             aiReceive.StartAnimating();
             await GetFileAsync());

            BTProgressHUD.Dismiss();

        };

    }

async Task GetFileAsync()
        {
  using (var client = new HttpClient())
            {
                try
                {
                    aiReceive.StartAnimating();   /* --> Inimation is not starting at this point. Animation Start After Downloading file, download takes time, i want to show animation in the mean while.*/
                        using (WebClient ftpclient = new WebClient())
                        {
                            try
                            {
                                ftpclient.Credentials = new System.Net.NetworkCredential("UserName", "Password");
                                string sourceFilePath = @"ftp://ftp.google.com/FileName.pdf";

                                        var FileDownloadStart = UIAlertController.Create("Info", "Data file received.", UIAlertControllerStyle.Alert);
                             FileDownloadStart.AddAction(UIAlertAction.Create("Ok", UIAlertActionStyle.Default, null));
                                        UIApplication.SharedApplication.KeyWindow.RootViewController.PresentViewController(FileDownloadStart, true, null);

                                        //Downloading file from FTP.
                                        ftpclient.DownloadFile(sourceFilePath, "C:\");

                                        var FileDownloadAlert = UIAlertController.Create("Info", "Data file received.", UIAlertControllerStyle.Alert);
                                        FileDownloadAlert.AddAction(UIAlertAction.Create("Ok", UIAlertActionStyle.Default, null));
                                        UIApplication.SharedApplication.KeyWindow.RootViewController.PresentViewController(FileDownloadAlert, true, null);
                            }
                            catch (Exception ex)
                            {
                                File.Delete(EpazzDirectory + AuditorId.ToString());
                                var ExceptionAlert = UIAlertController.Create("Exception", ex.Message, UIAlertControllerStyle.Alert);

                                ExceptionAlert.AddAction(UIAlertAction.Create("Ok", UIAlertActionStyle.Default, null));
                                UIApplication.SharedApplication.KeyWindow.RootViewController.PresentViewController(ExceptionAlert, true, null);
                            }

                        }
                }
            }  
            }

}