Microsoft.NET

……………………………………………….Expertise in .NET Technologies

Calling Postback Events from JavaScript

Posted by Ravi Varma Thumati on December 16, 2009

Introduction

Postback is a concept introduced in ASP.NET and is a very handy method. Postback is built into the ASP.NET and most of the web controls support it without writing any code.

Calling Postback event from Javascript

There may be some scenario where you may want to explicitly postback to the server using some clientside javascript. It is pretty simple to do this.

ASP.NET already creates a client side javascript method as shown below to support Postbacks for the web controls:

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

So, all you have to do is, just call this method with appropriate arguments. You may call this as shown below:

<script language=’Javascript’>
__doPostBack (‘__Page’, ‘MyCustomArgument’);
</script>

However, it is not recommended to use this method name directly in the client side. The best approach is, generate this piece of code from the code behind file using ASP.NET. This way, you are safe even if Microsoft later change the name of the method ‘__doPostBack’ to something else in a future release.

In your code behind file, declare a protected variable as shown below:

Protected PostBackStr As String

Now, in the page load event, write the following code:

PostBackStr = Page.ClientScript.GetPostBackEventReference(Me, “MyCustomArgument”)

The method GetPostBackEventReference() will generate the same piece of client side code that you need to use to call the Postback method. Instead of hard coding the method name __doPostBack, we are asking ASP.NET to tell us what the method name is.

Now insert the following code in your Aspx page:

<script language=’Javascript’>

</script>

at runtime, it will be evaluated as:

<script language=’Javascript’>
__doPostBack(‘__Page’, ‘MyCustomArgument’);
</script>

Remember to insert the above script into some Javascript method/event where you want to call the postback, instead of simply inserting into the page as shown above.

How to identify and handle the postback in code behind?

You found how to call the postback from javascript. Now you need a way to identify your postback in the code behind file. The second argument the doPostback method becomes helpful here.

Go to the code behind file and write the following code in the Page Load event:

If Page.IsPostBack Then
Dim eventArg As String = Request(“__EVENTARGUMENT”)
If eventArg = “MyCustomArgument” Then
Response.Write(“You got it !”)
End If
End If

Did you notice how we identify if the page is loaded as part of our postback? We used the second argument in the __doPostBack method to pass a value and used that in PageLoad to identify if it is called as a result of our PostBack.

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