Client callback is one way to update the webpage without performing the full page postback. It maintains the Client state while updating the Webpage. During Client callback the webpage runs through modifies version of Page Life Cycle.
The LifeCycle of a page in Client Call Back is as
If you want to learn more about Client Callback, click here.
But the main drawback of Client Callback, The input data is not posted from Client to Server. Also, It does not maintain the view state during partial postback as you can see the Page life cycle of Client callbak it is not saved or SaveViewState event is not fired.
Lets see it with an example.
I have created a simple form to collect some information of an person data. And it has a submit button, which initiates a Callback. As
<table> <tr><td><asp:Label id="lblFName" runat="server" Text="First Name" /> </td> <td><asp:TextBox ID="tbFName" runat="server"></asp:TextBox></td></tr><tr> <td><asp:Label id="lblMName" runat="server" Text="Middile Name" /> </td> <td><asp:TextBox ID="tbMName" runat="server"></asp:TextBox></td></tr><tr> <td><asp:Label id="lblLName" runat="server" Text="Last Name" /></td><td><asp:TextBox ID="tbLName" runat="server"></asp:TextBox></td></tr><tr> <td><asp:Label id="lblAge" runat="server" Text="Age" /></td><td><asp:TextBox ID="tbAge" runat="server"></asp:TextBox></td></tr><tr> <td><asp:Label id="lblMailId" runat="server" Text="Email" /></td><td><asp:TextBox ID="tbEMail" runat="server"></asp:TextBox></td></tr><tr> <td><asp:Label id="lblSex" runat="server" Text="Sex" /></td><td><asp:TextBox ID="tbSex" runat="server"></asp:TextBox></td></tr><tr> <td colspan="2"><input id="btnCallBack" type="button" value="Submit" onclick="InitiateCallBack();"/></td></tr> </table>
Server side code is as
public partial class Callback : System.Web.UI.Page,ICallbackEventHandler
{
string result;
protected void Page_Load(object sender, EventArgs e)
{
if (Page.IsCallback)
return;
//Creating a reference of Client side Method, that is called after callback on server
String cbReference = Page.ClientScript.GetCallbackEventReference(this, "arg",
"ReceiveServerData", "");
//Putting the reference in the method that will initiate Callback
String callbackScript = "function CallServer(arg, context) {" +
cbReference + "; }";
//Registering the method
Page.ClientScript.RegisterClientScriptBlock(this.GetType(),
"CallServer", callbackScript, true);
}
public void RaiseCallbackEvent(String eventArgument)
{
string firstName = tbFName.Text;
//Read other data as well
}
public string GetCallbackResult()
{
return result;
}
}
And the client side method is
function InitiateCallBack() {
//Intiate the callback
CallServer("",'');
}
// Called after the server side processing is done
function ReceiveServerData(arg, context) {
//arg: hold the result
//Updating the UI
document.getElementById('divCallbacl').innerHTML = arg;
}
After filling this form, I submitted it. Now if you try to access the entered using control’s property, you would not get it.
Let’s dig it more and check the form collection, whether the data is passed from Client to server or not
So as you can see that the data is not passed in form collection
But let’s put the following lines of code in java-script, just before initiating the call for Callback as
function InitiateCallBack() {
__theFormPostCollection.length = 0;
__theFormPostData = "";
WebForm_InitCallback();
//Intiate the callback
CallServer("",'');
}
Now let’s run the code and submit the form as earlier.
Now let’s check the form collection.
What a surprise, we got all the input data in form collection. That’s nice.
Now access the data using control’s property.
and you got it.
Now let’s try to see, what does the extra lines of code does
//This Clear the earlier the old data in form collection. __theFormPostCollection.length = 0; //It sets the forms data to empty __theFormPostData = ""; //This method builds the body of the POST request when the page loads. The body of the page is a string //(that is __theFormPostData) filled with the contents of the view state and all of the input fields in the form. WebForm_InitCallback();
But I would suggest to use these line at caution. Use only when you need to collect the update from Page during Callback. If you don’t need to get some data or it is in readonly page, don’t use it it introduce extra overhead to your page and costs at performance point of view.
Hope you all have enjoyed the post.
Code Wala 