How do i abort Ajax TabContainer Tab Change?

I have a Ajax TabContainer that has 3 tabs and each of the tab contains a formview to display/update data. When the user modifies any data in the edit mode in the formview and tries to change tab without updating the data, i would like to display a warning message to alert the user that he/she has some unsaved data in the form and abort the tab change. I sort of have an idea how to do it but somehow it's giving me a runtime javascript object reference = null runtime error on sender.set_activeTabIndex(0); in tabChanged(). Can someone help please? Thanks.

Current Code:

<script type="text/javascript">

var isDirty = false;

var prevTabIndex = 0;

function tabChanged(sender, args)

{

if (isDirty)

{

sender.set_activeTabIndex(0);

prevTabIndex = parseInt(sender.get_activeTabIndex());

}

}

function setDirty(objID)

{

if (document.getElementById(objID).value != document.getElementById(objID).defaultValue)

{

//alert("Old Value: "+document.getElementById(objID).defaultValue+", New Value: "+document.getElementById(objID).value);

isDirty=true;

}

}

</script>

<cc1:TabContainerID="TabContainer1"runat="server"Width="818px"Height="260px"ActiveTabIndex="0"OnClientActiveTabChanged="tabChanged">

<cc1:TabPanelID="TabPanel1"runat="server"HeaderText="TabA">

<HeaderTemplate>TabA</HeaderTemplate>

<ContentTemplate>

<asp:UpdatePanelID="UpdatePanel1"runat="server">

<ContentTemplate>

<asp:FormViewID="fvA"runat="server"DefaultMode="readonly"DataSourceID="dsA"OnModeChanging="changeFormViewMode">

<ItemTemplate>

FieldA :<%# Eval("FieldA") %>

</ItemTemplate>

<EditItemTemplate>

FieldA:

<asp:TextBoxID="TextBoxA"runat="server"Text='<%# Bind("FieldA") %>'onchange="setDirty(this.id)"></asp:TextBox>

</EditItemTemplate>

</asp:FormView>

</ContentTemplate>

</asp:UpdatePanel>

</ContentTemplate>

</cc1:TabPanel>

<cc1:TabPanelID="TabPanel2"runat="server"HeaderText="TabB">

<HeaderTemplate>TabB</HeaderTemplate>

<ContentTemplate>

<asp:UpdatePanelID="UpdatePanel2"runat="server">

<ContentTemplate>

<asp:FormViewID="fvB"runat="server"DefaultMode="readonly"DataSourceID="dsB"OnModeChanging="changeFormViewMode">

<ItemTemplate>

FieldB :<%# Eval("FieldB") %>

</ItemTemplate>

<EditItemTemplate>

FieldB:

<asp:TextBoxID="TextBoxB"runat="server"Text='<%# Bind("FieldB") %>'onchange="setDirty(this.id)"></asp:TextBox>

</EditItemTemplate>

</asp:FormView>

</ContentTemplate>

</asp:UpdatePanel>

</ContentTemplate>

</cc1:TabPanel>

Hi Sazabi,

After a quick glance, I found that there maybe a endless loop in your code. When you change the actived Tab, it will raise OnClientActiveTabChanged event and there's set_activeTabIndex() in your code, then it will raise OnClientActiveTabChanged once more.

You should do some modification. For example:

var flag = false;
function tabChanged(sender, args){
if(flag) {flag = false; return;}
flag = true;
sender.set_activeTabIndex(0);
prevTabIndex = sender.get_activeTabIndex();
}

Best regards,

Jonathan