form提交后如何获取提交完成事件？

//通过form.submit的方式实现导出execl的功能

function _exportExecl(params){

   ...//弹出正在导出对话框

    var downloadHelper = $('<iframe style="display:none;" id="downloadHelper"></iframe>').appendTo('body')[0];

    var doc = downloadHelper.contentWindow.document;

    if (doc) {

        doc.open();

        doc.write('')//微软为doc.clear()有时会出bug

        doc.writeln("<html><body><form id='downloadForm' name='downloadForm' method='post' action='" + params.url + "'>");

        for (var key in params) 

            doc.writeln("<input type='hidden' name='" + key + "' value='" +  params[key] + "'>");

        doc.writeln('<\/form><\/body><\/html>');

        doc.close();

        var form = doc.forms[0];

        if (form) {

            form.submit();   //提交表单后如何得到完成事件？

        }

    }

}

public void ProcessDownloadRequest(HttpContext context, Dictionary<string, string> dic)

        {

            .....//通过NPOI生成EXECL



            #region 下载文件

            Stream iostream = exporter.SaveAsStream();

            byte[] buffer = new byte[iostream.Length];

            iostream.Position = 0;

            iostream.Read(buffer, 0, buffer.Length);

            iostream.Close();



            var res = HttpContext.Current.Response;

            res.ContentType = "application/octet-stream";

            //通知浏览器下载文件而不是打开

            res.AddHeader("Content-Disposition", "attachment; filename=" + HttpUtility.UrlEncode(Decode("exportname"), System.Text.Encoding.UTF8));

            res.BinaryWrite(buffer);

            res.Flush();

            res.End();

            #endregion

        }