Q176951: HOWTO: Use Events to Generate Asynchronous Callbacks

Article: Q176951
Product(s): Microsoft Visual Basic for Windows
Version(s): 
Operating System(s): 
Keyword(s): kbGrpDSVBDB
Last Modified: 11-JAN-2001

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The information in this article applies to:

- Microsoft Visual Basic Learning Edition for Windows, versions 5.0, 6.0 
- Microsoft Visual Basic Professional Edition for Windows, versions 5.0, 6.0 
- Microsoft Visual Basic Enterprise Edition for Windows, versions 5.0, 6.0 
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SUMMARY
=======

In asynchronous processing, the method that starts a task is returned
immediately without waiting for a result. The client can continue to do what it
was originally doing while the task is completed. The server needs a way to
notify the client asychronously when the task is done. One way of doing this is
to create an event that the client will handle.

MORE INFORMATION
================

Following is a small code sample demonstrating how a client application can
start another server application and wait for its completion notification via an
event. Clicking the Start button on the client's form starts up an instance of
the server and returns immediately. The server can then begin some long
calculation or other work. Upon completion of the task, the server will fire the
Notify event. The client receives this event and responds accordingly.

Server Application
------------------

The server application is an ActiveX .exe file consisting of one class module
(clsTimer) and one standard module. Set the instancing property of the Class
module to Single Use. In the Project Properties dialog box, make sure that the
Startup Object is set to None:

clsTimer code
-------------

       Event Notify(ByVal somenum As Double)

       Public Sub StartTimer()
           Call BeginTimer(Me)  'Start the timer
       End Sub

       Public Sub Notifyme(ByVal anum As Double)
           RaiseEvent Notify(anum)  'Create an event with the result
       End Sub                      'as a parameter

       Public Sub StopTimer()
           Call EndTimer
       End Sub

       Private Sub Class_Terminate()
           Call EndTimer  'Make sure the timer is stopped
       End Sub

Standard module code:
---------------------

       Global timerid As Long
       Dim thetimer As clsTimer

       Public Declare Function SetTimer Lib "user32" _
               (ByVal hwnd As Long, _
               ByVal nIDEvent As Long, _
               ByVal uElapse As Long, _
               ByVal lpTimerFunc As Long) As Long

       Public Declare Function KillTimer Lib "user32" _
               (ByVal hwnd As Long, _
               ByVal nIDEvent As Long) As Long

       Public Sub EndTimer()
           Dim retval As Long
           retval = KillTimer(0, timerid)
       End Sub

       Public Sub BeginTimer(ByRef Timerobj As clsTimer)
           Set thetimer = Timerobj
           timerid = SetTimer(0, 0, 1, AddressOf TimerProc)
       End Sub

       Public Sub TimerProc(ByVal hwnd As Long, ByVal umsg As Long, _
           ByVal idEvent As Long, ByVal dwtime As Long)

           Dim counter As Long
           Dim theanswer As Double

           KillTimer 0, timerid  'Stop the timer

           For counter = 1 To 15000000  'Do some work here
               theanswer = theanswer + counter
           Next counter
           thetimer.Notifyme (theanswer)  'Work is done. Pass the result
       End Sub                            'as a parameter.

Client Application
------------------

The client application is a Standard .exe file consisting of one class module
(clsNotify) and one Form. The form has a Listbox control (lstTimes) and a
CommandButton (cmdStart).

Make sure you add a reference to your server application in your client project:

Class Module code:
------------------

       Public WithEvents obj As clsTimer

       Private Sub Class_Terminate()
           Set obj = Nothing
       End Sub

       Private Sub obj_Notify(ByVal somenum As Double)
           Form1.lstTimes.AddItem somenum
           obj.StopTimer
           Set obj = Nothing  'Shut down the server when it has completed
       End Sub                'the task.

       Public Sub startit()
           Set obj = New clsTimer
           obj.StartTimer
       End Sub

Form Code:
----------

       Dim notifyobject As clsNotify

       Private Sub cmdStart_Click()
           notifyobject.startit
       End Sub

       Private Sub Form_Load()
           Set notifyobject = New clsNotify
       End Sub

Additional query words: callback kbVBp500 kbVBp600 kbActiveX kbdse kbDSupport kbVBp

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Keywords          : kbGrpDSVBDB 
Technology        : kbVBSearch kbAudDeveloper kbZNotKeyword6 kbZNotKeyword2 kbVB500Search kbVB600Search kbVBA500 kbVBA600 kbVB500 kbVB600
Hardware          : x86
Issue type        : kbhowto

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