Controls not updated by callback event

D

Dave Weeden

Hi all,

We have developed a custom web control that implements ICallbackEventHandler
and fires an event when the callback is invoked.

We had hoped that subscribers to this event would be able to update the
state of other web controls but have been unable to make this work.

I have distilled the control down to the minimum required to illustrate the
issue and have shown a minimal consumer web form (both are inline below).

In the web form, both Button1_Click and Wrapper1_Save are called (verified
through breakpoints) but the latter does not update the label properly.

Is this a limitation of the modified callback lifecycle or is there some way
to make it work?

Thanks,

Dave


--- start of control ---

using System;
using System.Web.UI;
using System.Web.UI.WebControls;

namespace MyControls
{
[ToolboxData("<{0}:MyControl runat=\"server\"></{0}:MyControl>")]
public class MyControl : WebControl, ICallbackEventHandler
{
public event EventHandler MyEvent;

private string result;

protected override void OnInit(EventArgs e)
{
base.OnInit(e);

Page.ClientScript.RegisterClientScriptBlock(typeof(MyControl),
"OnSuccess", "function OnSuccess(result, context) {}", true);

string callback =
Page.ClientScript.GetCallbackEventReference(this, "result", "OnSuccess",
"context");

Page.ClientScript.RegisterClientScriptBlock(typeof(MyControl),
ID, string.Concat("function ", ID, "Callback(result, context) { ", callback,
"; }"), true);
}

public void RaiseCallbackEvent(string eventArgument)
{
result = eventArgument;

if (MyEvent != null)
{
MyEvent(this, EventArgs.Empty);
}
}

public string GetCallbackResult()
{
return result;
}

}
}

--- end of control ---

--- start of web page ---

<%@ Page Language="C#" %>
<%@ Register TagPrefix="custom" Namespace="MyControls" %>

<script runat="server">

protected void Button1_Click(object sender, EventArgs e)
{
Label1.Text = "PostBack button clicked";
}

protected void Wrapper1_Save(object sender, EventArgs e)
{
Label1.Text = "CallBack button clicked";
}

</script>

<html>
<head>
<title>Test Page</title>
</head>
<body>
<form id="form1" runat="server">
<div>
<custom:MyControl ID="MyControl1" OnMyEvent="Wrapper1_Save"
runat="server" />
<asp:Button ID="Button1" Text="PostBack" runat="server"
OnClick="Button1_Click" />
<asp:Button ID="Button2" Text="CallBack"
OnClientClick="MyControl1Callback('Testing123', null)" runat="server" />
<asp:Label ID="Label1" runat="server" />
</div>
</form>
</body>
</html>

--- end of web page ---
 
W

Walter Wang [MSFT]

Hi Dave,

In a client callback, a client script function sends a request to the
ASP.NET Web page, which then runs an abbreviated version of its normal life
cycle to process the callback: the page is initiated and its controls and
other members are created, and then a specially marked method is invoked.
The method performs the processing that you have coded and then returns a
value to the browser that can be read by another client script function.

Besides implementing ICallbackEventHandler and its methods
RaiseCallbackEvent, GetCallbackResult, the page must contain three client
script functions that perform the following actions:

* One function calls a helper method that performs the actual request to
the server. In this function, you can perform custom logic to prepare the
event arguments first, and then you can send a string as a parameter to the
server-side callback event handler.
* Another function receives (is called by) the result from the server code
that processed the callback event, accepting a string that represents the
results. This is called the client callback function.
* A third function is the helper function that performs the actual request
to the server, which is generated automatically by ASP.NET when you
generate a reference to this function using the GetCallbackEventReference
method in server code.


The key point here is that it's not a postback, the page will not be
rendered again; if you change the server-side controls' properties, the
changes will not be rendered.

Therefore, in your code, if we make following changes:

(In MyControl)

protected override void OnInit(EventArgs e)
{
base.OnInit(e);


//Page.ClientScript.RegisterClientScriptBlock(typeof(MyControl),
"OnSuccess", "function OnSuccess(result, context) { }", true);

string callback =
Page.ClientScript.GetCallbackEventReference(this, "result",
string.Concat(ID, "OnSuccess"), "context");

Page.ClientScript.RegisterClientScriptBlock(typeof(MyControl),
ID, string.Concat("function ", ID, "Callback(result, context) { ",
callback, "; }"), true);
}


(In ASPX)

<head>
<title>Test Page</title>

<script language="javascript" type="text/javascript">
function MyControl1OnSuccess(text) {
alert(text);
}
</script>
</head>



References:

#Implementing Client Callbacks Without Postbacks in ASP.NET Web Pages
http://msdn2.microsoft.com/en-us/library/ms178208.aspx

#Client-Callback Implementation (C#) Example
http://msdn2.microsoft.com/en-us/library/ms178210.aspx



Sincerely,
Walter Wang ([email protected], remove 'online.')
Microsoft Online Community Support

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W

Walter Wang [MSFT]

Hi Dave,

Some updates:

<html>
<head>
<title>Test Page</title>

<script language="javascript" type="text/javascript">
function MyControl1OnSuccess(text) {
//alert(text);
document.getElementById("Label1").innerHTML = text;
}
</script>
</head>
<body>
<form id="form1" runat="server">
<div>
<custom:MyControl ID="MyControl1" OnMyEvent="Wrapper1_Save"
runat="server" />
<asp:Button ID="Button1" Text="PostBack" runat="server"
OnClick="Button1_Click" />
<asp:Button ID="Button2" Text="CallBack"
OnClientClick="MyControl1Callback('Testing123', null); return false;"
runat="server" />
<asp:Label ID="Label1" runat="server" />
</div>
</form>
</body>
</html>


1) To update the label's content on callback, use DHTML dom elements to
change its content.

2) You need to add "return false" in button2's OnClientClick, otherwise
there's an additional normal postback at the end (which will re-render the
page content according to server-side controls' states -- which means the
label1's content that we changed via script callback will be overwritten
again).


Hope this helps.


Regards,
Walter Wang ([email protected], remove 'online.')
Microsoft Online Community Support

==================================================
When responding to posts, please "Reply to Group" via your newsreader so
that others may learn and benefit from your issue.
==================================================

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D

Dave Weeden

Hey Walter,

Thanks for that very good explanation. We are in fact trying to wrap some
AJAX type behaviour in a control and wanted to insulate the consumer of the
wrapper from having to code any javascript.

Looks like that is not possible so we'll be using something similar to what
you have suggested.

Thanks again!

Dave
 
W

Walter Wang [MSFT]

Hi Dave,

I'm glad that the reply helped.

Thank you for using MSDN Managed Newsgroup support service!

Regards,
Walter Wang ([email protected], remove 'online.')
Microsoft Online Community Support

==================================================
When responding to posts, please "Reply to Group" via your newsreader so
that others may learn and benefit from your issue.
==================================================

This posting is provided "AS IS" with no warranties, and confers no rights.
 

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