Page Buttons Not Responding After Getting PDF in SharePoint

Today I have been working on a SharePoint 2010 WebPart in which the user can click a button to get a PDF report. Open source iTextSharp library is used to generate the PDF report. The code is quite straightforward:

        void GeneratePDF()
        {
            Document doc = new Document(PageSize.A4);
            MemoryStream pdfStream = new MemoryStream();
            PdfWriter.GetInstance(doc, pdfStream);
            doc.Open();
            // Populate document with business data
            doc.Close();

            Response.Clear();
            Response.ClearHeaders();
            Response.ContentType = "application/pdf";
            Response.AddHeader("Content-Disposition", "attachment;filename=report.pdf");
            Response.BinaryWrite(pdfStream.ToArray());
            Response.Flush();
            Response.End();   
        }

That PDF function works fine, but all other buttons on the same page are not responding (postback doesn't occur) after the PDF button is click. Such behavior only happens in SharePoint environment and everything is okay in a regular ASP.NET page. It looks like some special validation in SharePoint causing the problem. I debugged into the JavaScript and found the setting of "__spFormOnSubmitCalled" variable is the culprit.

ASP.NET validation process triggered by the click of a button includes invocation of JavaScript function called WebForm_OnSubmit. SharePoint overrides this function for each page:

<script type="text/javascript">
//<![CDATA[
    if (typeof(Sys) === 'undefined') 
        throw new Error('ASP.NET Ajax client-side framework failed to load.');
    if (typeof(DeferWebFormInitCallback) == 'function') 
        DeferWebFormInitCallback();
    function WebForm_OnSubmit() {
        UpdateFormDigest('webUrl..', 1440000);
        if (typeof(vwpcm) != 'undefined') {
            vwpcm.SetWpcmVal();
        };
        return _spFormOnSubmitWrapper();
    }
//]]>
</script>

The JavaScript function __spFormOnSubmitWrapper is defined in /_layouts/1033/init.js:

function _spFormOnSubmitWrapper() {
    if (_spSuppressFormOnSubmitWrapper)
    {
        return true;
    }
    if (_spFormOnSubmitCalled)
    {
        return false;
    }
    if (typeof(_spFormOnSubmit) == "function")
    {
        var retval = _spFormOnSubmit();
        var testval = false;
        if (typeof(retval) == typeof(testval) && retval == testval)
        {
            return false;
        }
    }
    _spFormOnSubmitCalled=true;
    return true;
}

The "_spFormOnSubmitCalled" field is false by default when the page is loaded. It's set to true when you click a button on the page. This machanism ensures only the first button click will take action and prevents other clicks from posting back to the server. The "_spFormOnSubmitCalled" field is reset to false once the page is reloaded. A postback will usually result in a page reloading, but not in above PDF out case where the server writes the PDF attachment to the client then ends the interaction. So the "_spFormOnSubmitCalled" field remains true which blocks any future postback.

So theoretically the issue is not limited to PDF output. Directly writing and ending on the Response object in the server side would result in the same problem. There're a few approaches to resolve the problem:

• 1. Reset "_spFormOnSubmitCalled" to false after the PDF button is clicked. Note that the reset timing is important, and it must be later then the submission process (after the WebForm_OnSubmit method is called), for example:

     function resetSharePointSubmitField() {
          setTimeout(function () { _spFormOnSubmitCalled = false; }, 1000); // set the field after 1 second
          return true;
      }

• 2. Overide the WebForm_OnSubmit function and make it always return true:

      function resetSharePointSubmitField() {
          window.WebForm_OnSubmit = function() {return true;};
      }

Apply the JavaScript to a button:

    <asp:Button ID="btnGeneratePDF" runat="server" Text="Get PDF" OnClientClick="resetSharePointSubmitField();" />

The other option is add a client side script "JavaScript: _spFormOnSubmitCalled = false;" for all buttons on the page, but that is not scalable and not recommended.

Bonus tip The regular pdf export function by Response.Write() won't work inside a modal dialog (the popup window open by window.showModalDialog() from JavaScript). To resolve this particular problem you can replace the PDF export button with a hyper link and set its target to an empty iframe:

    <a id="popupPDF" target="pdfTarget" runat="server">Get PDF</a>
    <iframe name="pdfTarget" id="pdfTarget" width="0" height="0" style="display:none;"></iframe>

Then simply assign a query string the anchor which tells the server to generate the PDF:

    protected void Page_Load(object sender, EventArgs e)
    {
        popupPDF.HRef = Request.Url.AbsoluteUri + "&pdf=true";
        if (Request.QueryString["pdf"] == "true")
            GeneratePDF();
    }