ASP.NET postback with JavaScript
asp.net, javascript, postback, updatepanel, webforms
Solution
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
Problem
I have several small `div`s which are utilizing `jQuery` draggable. These `div`s 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.