An Easy Way to Implement ASP.NET Callback


Partial Class CallBackHandler
    Inherits System.Web.UI.UserControl
    Implements ICallbackEventHandler

#Region "Conts"

    Private Const C_CallbackHandlerNumberSpliter As String = "###"
    Private Const C_CallbackArgsSpliter As String = "$$"
    Private Const C_CallbackResultSpliter As String = "$$"

#End Region

#Region "Declarations"

    Dim returnValue As String = String.Empty

#End Region

#Region "Events And Delegates"

    Public Delegate Sub RaiseCallbackEventHandler(ByVal eventArgs() As String, ByVal HandlerNumber As Integer, ByRef CallBackResult As String, ByRef ExcutableClientScript As String, ByVal RaiseException As Exception)

    Public Event RaiseCustomCallback As RaiseCallbackEventHandler


#End Region

#Region "Page Load"

    Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load

        If Not Me.IsPostBack Then

            InitJustOnFirstLoad()

        End If

        InitOnEveryPostBack()

    End Sub

#End Region

#Region "Callback  Events"

    Public Function GetCallbackResult() As String Implements System.Web.UI.ICallbackEventHandler.GetCallbackResult

        Return returnValue

    End Function

    Public Sub RaiseCallbackEvent(ByVal eventArgument As String) Implements System.Web.UI.ICallbackEventHandler.RaiseCallbackEvent

        '===============  declarations  ==================
        Dim tempArgs As String(), HandlerNo As Integer, RealArgs As String() = Nothing

        Dim ExcutableClientScript As String = String.Empty, CallbakcResult As String = String.Empty

        Dim RaisedException As Exception = Nothing


        Try

            '===========  Extracting Arguments ==========================
            'sample  eventArgument with 3 parameter  :-->   param1$$param2$$param3###handlerno:1
            'there are 2 part which splited by ### . part 1 is parameters and part 2 is handler number


            tempArgs = eventArgument.Replace(C_CallbackHandlerNumberSpliter, "~").Split("~")

            HandlerNo = tempArgs(1).Split(":")(1)

            ' removing extra spliters
            If tempArgs(0).StartsWith(C_CallbackArgsSpliter) Then tempArgs(0) = tempArgs(0).Remove(0, C_CallbackArgsSpliter.Length)
            If tempArgs(0).EndsWith(C_CallbackArgsSpliter) Then tempArgs(0) = tempArgs(0).Remove(tempArgs(0).Length - C_CallbackArgsSpliter.Length, C_CallbackArgsSpliter.Length)

            'extracting parameters which send from client
            RealArgs = tempArgs(0).Replace(C_CallbackArgsSpliter, "~").Split("~")


        Catch ex As Exception
            RaisedException = ex
        End Try

        Try

            RaiseEvent RaiseCustomCallback(RealArgs, HandlerNo, CallbakcResult, ExcutableClientScript, RaisedException)

            returnValue = String.Format("{0}{1}{2}", CallbakcResult, C_CallbackResultSpliter, ExcutableClientScript)

        Catch ex As Exception

            returnValue = String.Format("{0}{1}{2}", "unexpected error!!!", C_CallbackResultSpliter, "")

        End Try

    End Sub

#End Region

#Region "Init"

    Protected Sub InitJustOnFirstLoad()

        'to do ...

    End Sub

    Protected Sub InitOnEveryPostBack()

        Dim cm As ClientScriptManager = Page.ClientScript

        Dim cbReferenceGeneral As String = cm.GetCallbackEventReference(Me, "arg", "ReceiveCustomCallBackData", "context")

        Dim callbackScript1 As String = "function  AsynCallServer(arg, context) {" & cbReferenceGeneral & "; };"

        Dim callbackScript2 As String = "function  asyncall(args,handlernumber ,receiverdivid) {var _arg = args + '" + C_CallbackHandlerNumberSpliter + "' + 'handlerno:' + handlernumber ; AsynCallServer(_arg, receiverdivid); };"

        cm.RegisterClientScriptBlock(Me.GetType(), "AsynCallKey1", callbackScript1, True)

        cm.RegisterClientScriptBlock(Me.GetType(), "AsynCallKey2", callbackScript2, True)


    End Sub

#End Region

End Class