Javascript 生成excel文件时显示等待加载程序弹出窗口，下载完成后隐藏

我想在生成文件时添加modalpoup或wait loader，一旦下载，我想隐藏弹出窗口。这只是为了锁定屏幕，以便在生成文件和下载文件响应时不受干扰

我的html中有一个aspx按钮，它“onclick”运行大量代码来生成excelPackage文件

<div> 
    <asp:Button ID="Button1" runat="server" Text="Download" onclick="Button1_Click"/>
</div>

ExcelPackage excel = new ExcelPackage();
var file = get_file();
//long code which writes the content of file to excel taking time
using (var memoryStream = new MemoryStream())
{
    Response.ContentType = "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet";
    Response.AddHeader("content-disposition", "attachment;  filename=file.xlsx");
    excel.SaveAs(memoryStream);
    memoryStream.WriteTo(Response.OutputStream);
    Response.Flush();
    Response.End();
}