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Another Simple Wait Page

, 11 Oct 2006
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An article to demonstrate to use asynchronous delegate call and client script callback to build long wait page

Sample Image - screenshot.jpg

Introduction

Sometime you need to process a lengthy job in your web form. It is a better idea to build a wait page instead of letting your user just stare at the screen doing nothing.

Background

There are a lot of solutions over Internet to accomplish this purpose. My solution isn't new, the initial idea is from Brian Dunnington's article Building a Better Wait Page in code project. I have been using this approach for long time in ASP.NET 1.1. Since the release of ASP.NET 2.0, Microsoft makes a lot easier to implement client side callback, I decided to expand this solution to include AJAX function, so that we don't need to refresh the page from time to time.

Using the code

There are 3 main areas you need to consider in this solution

  • Asynchronous delegate call
  • Hook up with client callback
  • Client script

Asynchronous delegate call

    /// <span class="code-SummaryComment"><summary>
</span>    /// Definition of delegate
    /// <span class="code-SummaryComment"></summary>
</span>    /// <span class="code-SummaryComment"><param name="minute"></param>
</span>    /// <span class="code-SummaryComment"><returns></returns>
</span>    private delegate bool DoJobDelegate (int minute);

    /// <span class="code-SummaryComment"><summary>
</span>    /// To invoke the long process function
    /// <span class="code-SummaryComment"></summary>
</span>    /// <span class="code-SummaryComment"><param name="minute"></param>
</span>    /// <span class="code-SummaryComment"><returns></returns>
</span>    private IAsyncResult DoJobAsync(int minute) {
        DoJobDelegate doDelegate = new DoJobDelegate(doLongJob);
        IAsyncResult ar = doDelegate.BeginInvoke(minute, 
                          new AsyncCallback(MyCallback), null);
        return ar;
    }

    /// <span class="code-SummaryComment"><summary>
</span>    /// The server side callback handler
    /// <span class="code-SummaryComment"></summary>
</span>    /// <span class="code-SummaryComment"><param name="ar"></param>
</span>    private void MyCallback (IAsyncResult ar) {
        AsyncResult aResult = (AsyncResult)ar;
        DoJobDelegate doDelegate = (DoJobDelegate)aResult.AsyncDelegate;
        // Session object is used to tell if process finishes or not
        Session["NewOrderResult"] = doDelegate.EndInvoke(ar);
    }

    /// <span class="code-SummaryComment"><summary>
</span>    /// The main function to run long process
    /// <span class="code-SummaryComment"></summary>
</span>    /// <span class="code-SummaryComment"><param name="minute"></param>
</span>    /// <span class="code-SummaryComment"><returns></returns>
</span>    private bool doLongJob (int minute) {
        System.Threading.Thread.Sleep(minute * 1000 * 60);
        if (minute % 2 == 0) {
            return true;
        } else {
            return false;
        }
    }

Hook up with client callback

In order to hook with client callback, you have to implement ICallbackEventHandler interface

public partial class Process : System.Web.UI.Page, ICallbackEventHandler {
 protected string CallBackEventReference;
...
}

Then we need to prepare script to reference client function

        string ScriptRef = this.ClientScript.GetCallbackEventReference(
        this,
        "'CheckStatus'",
        "ClientCallBack",
        "this",
        "ClientCallBack",
        true);

        CallBackEventReference = ScriptRef;

This will create a client function like WebForm_DoCallback('__Page','CheckStatus',ClientCallBack,this,ClientCallBack,true);

From this point, you need to implement 2 functions of this interface

    /// <span class="code-SummaryComment"><summary>
</span>    /// Capture the event argument in this field, in this case I don't use it.
    /// <span class="code-SummaryComment"></summary>
</span>    string eventArgument = "";

    /// <span class="code-SummaryComment"><summary>
</span>    /// Returns result to client side
    /// <span class="code-SummaryComment"></summary>
</span>    /// <span class="code-SummaryComment"><returns></returns>
</span>    string ICallbackEventHandler.GetCallbackResult() {
        if (Session["NewOrderResult"] != null) {
            // after async call finishes,
            // it sets this Session object to some value,
            // this method will capture the status
            if (Convert.ToBoolean(Session["NewOrderResult"])) {
                return "even.htm";
            } else {
                return "odd.htm";
            }
        } else {
            return "";
        }
    }

    /// <span class="code-SummaryComment"><summary>
</span>    /// Gets parameter passed from client side
    /// <span class="code-SummaryComment"></summary>
</span>    /// <span class="code-SummaryComment"><param name="eventArgument"></param>
</span>    void ICallbackEventHandler.RaiseCallbackEvent (string eventArgument) {
        this.eventArgument = eventArgument;
    }

Client script

On the client side, we need to prepare some javascript functions with talk with server.

function ClientCallBack(Result, Context) {
    if (Result != "") {
        window.location.href = Result;
    }
}

In order to make broswer to check server process status, we also need a timer to make client script to run at some certain interval.

<body onload="startClock();">
// check server every 5 seconds, adjust this value to your own preference
var interval = 5
var x = interval
function startClock(){
    x = x - 1;
    setTimeout("startClock()", 1000);
    if(x == 0){
       <%= this.CallBackEventReference %>

So what is happnening now? The client script will be triggered every 5 seconds to talk with server. And ICallbackEventHandler.GetCallbackResult() method on server side will be called every 5 seconds. In this method it checks Session object value, if asynchronous call finishes the process, it will call the MyCallback to set Session object to a not null value returned from process result to make ICallbackEventHandler.GetCallbackResult() be able to capture the result.

Points of Interest

My first explore of client callback, I hope to add more features later.

History

  • 2006-10-11 Initial version.

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

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About the Author

Hardy Wang
Architect
Canada Canada
No Biography provided

Comments and Discussions

 
GeneralIs it possible to use this in a wizard PinmemberD.Sridhar5-May-08 8:26 
GeneralRe: Is it possible to use this in a wizard PinmemberHardy Wang5-May-08 8:37 
Generalthe job is load a new page Pinmemberjrmora7-Mar-08 5:02 
GeneralRe: the job is load a new page PinmemberHardy7-Mar-08 6:07 
QuestionSQL Server Session State PinmemberSimon Byrne16-May-07 4:37 
AnswerRe: SQL Server Session State PinmemberHardy4-Jun-07 3:38 
Generalbrowser back button PinmemberJoaquin Luna9-Dec-06 10:36 
GeneralVB.net PinmemberPapichulo.NET18-Oct-06 11:04 
GeneralRe: VB.net PinstaffChristian Graus18-Oct-06 11:39 
GeneralRe: VB.net PinmemberHardy19-Oct-06 5:15 
AnswerRe: VB.net PinmemberJoaquin Luna8-Dec-06 11:33 
Well I've never used C# and I only started using Visual Basic last week, but I think I've got the wait page working under VB.   Sorry if there are any bugs I didn't catch, I'm new to the whole 'making websites' business.   Good Luck!
 
Here is the code for Default.aspx.vb
 
Imports System
Imports System.Data
Imports System.Configuration
Imports System.Web
Imports System.Web.Security
Imports System.Web.UI
Imports System.Web.UI.WebControls
Imports System.Web.UI.WebControls.WebParts
Imports System.Web.UI.HtmlControls
 

Partial Class defaultclass
      Inherits System.Web.UI.Page
 
      Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
 
      End Sub
 

      Protected Sub btnSubmit_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles btnSubmit.Click
            If Not Me.IsValid Then
                  Return
            End If
      End Sub
 

      Private Min As Integer
 
      Public ReadOnly Property Minute() As Integer
            Get
                  Min = 0
                  Integer.TryParse(txtMinute.Text, Min)
                  Return Min
            End Get
      End Property
 
End Class
 

 

 

 

And here is the code for Process.aspx.vb
 
Imports System
Imports System.Data
Imports System.Configuration
Imports System.Collections
Imports System.Web
Imports System.Web.Security
Imports System.Web.UI
Imports System.Web.UI.WebControls
Imports System.Web.UI.WebControls.WebParts
Imports System.Web.UI.HtmlControls
Imports System.Runtime.Remoting.Messaging
 

Partial Class Process
      Inherits System.Web.UI.Page
      Implements ICallbackEventHandler
 
      Protected CallBackEventReference As String
      Private Delegate Function DoJobDelegate(ByVal minute As Integer) As Boolean
      Dim eventArgument As String = ""
 

      Private Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
            If Not IsPostBack And Not IsCallback Then
                  Session("NewOrderResult") = Nothing
                  Dim prevPage As defaultclass = PreviousPage
                  If prevPage IsNot Nothing Then
                        lMinute.Text = "Your request will finish in " & prevPage.Minute & " minute(s)."
                        setcallbackstring()
                        DoJobAsync(prevPage.Minute)
                  Else
                        Response.Redirect("default.aspx")
                  End If
            End If
      End Sub
 
      Private Function DoJobAsync(ByVal minute As Integer) As IAsyncResult
            Dim doDelegate As New DoJobDelegate(AddressOf dolongjob)
            Dim ar As IAsyncResult = doDelegate.BeginInvoke(minute, New AsyncCallback(AddressOf mycallback), Nothing)
            Return ar
      End Function
 
      Private Sub mycallback(ByVal ar As IAsyncResult)
            Dim aResult As AsyncResult = ar
            Dim doDelegate As DoJobDelegate = aResult.AsyncDelegate
            Session("NewOrderResult") = doDelegate.EndInvoke(ar)
      End Sub
 
      Private Function dolongjob(ByVal seconds As Integer) As Boolean
            System.Threading.Thread.Sleep(seconds * 1000)
            If (seconds Mod 2 = 0) Then
                  Return True
            Else
                  Return False
            End If
 
      End Function
 
      Protected Sub setcallbackstring()
            Dim ScriptRef As String = ClientScript.GetCallbackEventReference( _
                        Me, "'CheckStatus'", "ClientCallBack", "this", "ClientCallBack", True)
 
            CallBackEventReference = ScriptRef
      End Sub
 

 

      Function GetCallbackResult() As String Implements ICallbackEventHandler.GetCallbackResult
            If Session("NewOrderResult") IsNot Nothing Then
                  If Convert.ToBoolean(Session("NewOrderResult")) Then
                        Return "even.htm"
                  Else
                        Return "odd.htm"
                  End If
            Else
                  Return ""
            End If
 
      End Function
 

      Sub RaiseCallbackEvent(ByVal eventArgument As String) Implements ICallbackEventHandler.RaiseCallbackEvent
            Me.eventArgument = eventArgument
      End Sub
 
End Class
 

 
Joaquin Luna
Roanoke VA
Questionempty source files? Pinmemberalex_-_12-Oct-06 4:42 
AnswerRe: empty source files? PinmemberHardy12-Oct-06 4:46 
GeneralThanks ! Pinmemberdapoussin12-Oct-06 2:19 

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