Delegate and Callback

Call Back With Delegate Explained

Public Class Form1

    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click

        Dim RP As New RunningProcess

        RP.SomeLongRunningProcess()

        'With Above Logic We can;t get Update from Loop inside.Means Proceess which is Inside will not

       able to update to 'Current Click Event which File is been Processed or File is been Downloaded or

       Email is been Sent. 'Which record of Database is been Processed

        'To Over come this Issue DELEGATE can be used with CallBack feature of Delegate.

        Dim X As Boolean = RP.SomeLongRunningProcessWithCallBack(AddressOf GetCallBackDelegateValue)

    End Sub

    Public Sub GetCallBackDelegateValue(Value As Integer)

        'Console.Write(Value)

        Me.ListBox1.Items.Add(Value)

    End Sub

End Class

Public Class RunningProcess

    Public Function SomeLongRunningProcess()

        For intloop As Integer = 0 To 1000000

            'Some Long Running Process. File Downloading, Email Sending Creating Files,

     'Complex Database Operartion

            Dim i As Integer

            i = intloop * 110 + 10

        Next

        Return True

    End Function

    Public Delegate Sub CallBackDelegate(Value As Integer)

    Public Function SomeLongRunningProcessWithCallBack(Current As CallBackDelegate)

        For intloop As Integer = 0 To 100

            'Some Long Running Process. File Downloading, Email Sending Creating Files, Complex Database

            'Operartion

            Dim i As Integer

            i = intloop * 110 + 10

            Current(i)

        Next

        Return True

    End Function

End Class

NOTE: If not delegate. You will need to pass Control as parameter to function which will update Value of [i] in Listbox. That will make you Class Dependent to Return Value in Listbox only. If control has to be changed or DLL will have to be changed also if Logic is in different Class/DLL Or if same data is to be represented in other form using GRID or Textbox.

Logic with Listbox as Parameter will not work. Above example make output control free and any change in user interface will not effect the DLL or working.

Threading Call Back With Delegate Explained

    Private Delegate Sub AddValueToList(Value As Integer)

    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click

        Dim Thrd As New Threading.Thread(AddressOf StartProcess)

        Thrd.Start()

    End Sub

  

    Private Sub StartProcess()

        Dim RP As New RunningProcess

        Dim X As Boolean = RP.SomeLongRunningProcessWithCallBack(AddressOf GetCallBackDelegateValue)

    End Sub

    Private Sub GetCallBackDelegateValue(Value As Integer)

        ListBox1.Invoke(New AddValueToList(AddressOf AddValueToListBoxControl), Value)

    End Sub

Public Class RunningProcess

    Public Delegate Sub CallBackDelegate(Value As Integer)

    Public Function SomeLongRunningProcessWithCallBack(Current As CallBackDelegate)

        For intloop As Integer = 0 To 10000

            'Some Long Running Process. File Downloading, Email Sending Creating Files, Complex Database Operartion

            Dim i As Integer

            i = intloop * 110 + 10

            Current(i)

        Next

        Return True

    End Function

End Class