ASP.NET postback with JavaScript

Question

I have several small divs which are utilizing jQuery draggable. These divs are placed in an UpdatePanel, and on dragstop I use the _doPostBack() JavaScript function, where I extract necessary information from the page's form.

My problem is that when I call this function, the whole page is re-loaded, but I only want the update panel to be re-loaded.

Answer 1

Here is a complete solution

Entire form tag of the asp.net page

<form id="form1" runat="server">     <asp:LinkButton ID="LinkButton1" runat="server" /> <%-- included to force __doPostBack javascript function to be rendered --%>      <input type="button" id="Button45" name="Button45" onclick="javascript:__doPostBack('ButtonA','')" value="clicking this will run ButtonA.Click Event Handler" /><br /><br />     <input type="button" id="Button46" name="Button46" onclick="javascript:__doPostBack('ButtonB','')" value="clicking this will run ButtonB.Click Event Handler" /><br /><br />      <asp:Button runat="server" ID="ButtonA" ClientIDMode="Static" Text="ButtonA" /><br /><br />     <asp:Button runat="server" ID="ButtonB" ClientIDMode="Static" Text="ButtonB" /> </form> 

Entire Contents of the Page's Code-Behind Class

Private Sub ButtonA_Click(sender As Object, e As System.EventArgs) Handles ButtonA.Click     Response.Write("You ran the ButtonA click event") End Sub  Private Sub ButtonB_Click(sender As Object, e As System.EventArgs) Handles ButtonB.Click     Response.Write("You ran the ButtonB click event") End Sub 

• The LinkButton is included to ensure that the __doPostBack javascript function is rendered to the client. Simply having Button controls will not cause this __doPostBack function to be rendered. This function will be rendered by virtue of having a variety of controls on most ASP.NET pages, so an empty link button is typically not needed

What's going on?

Two input controls are rendered to the client:

<input type="hidden" name="__EVENTTARGET" id="__EVENTTARGET" value="" /> <input type="hidden" name="__EVENTARGUMENT" id="__EVENTARGUMENT" value="" /> 

• __EVENTTARGET receives argument 1 of __doPostBack
• __EVENTARGUMENT receives argument 2 of __doPostBack

The __doPostBack function is rendered out like this:

function __doPostBack(eventTarget, eventArgument) {     if (!theForm.onsubmit || (theForm.onsubmit() != false)) {         theForm.__EVENTTARGET.value = eventTarget;         theForm.__EVENTARGUMENT.value = eventArgument;         theForm.submit();     } } 

• As you can see, it assigns the values to the hidden inputs.

When the form submits / postback occurs:

• If you provided the UniqueID of the Server-Control Button whose button-click-handler you want to run (javascript:__doPostBack('ButtonB',''), then the button click handler for that button will be run.

What if I don't want to run a click handler, but want to do something else instead?

You can pass whatever you want as arguments to __doPostBack

You can then analyze the hidden input values and run specific code accordingly:

If Request.Form("__EVENTTARGET") = "DoSomethingElse" Then     Response.Write("Do Something else")  End If 

Other Notes

• What if I don't know the ID of the control whose click handler I want to run?
  • If it is not acceptable to set ClientIDMode="Static", then you can do something like this: __doPostBack('<%= myclientid.UniqueID %>', '').
  • Or: __doPostBack('<%= MYBUTTON.UniqueID %>','')
  • This will inject the unique id of the control into the javascript, should you wish it