CallBacks

Has anyone implemented a callback mechanism in Vb 6? I have tried but have not been able to implement a solution. I have been using "Me" to send a reference to another object to invoke a reference but this fails.

I would be very grateful for any feedback on this particularly whether it can be done or not. I have some simple source below to highlight the problem. you should get

"object doesn;t support the method..........."

Don't forget to set the instancing of IState to 2 ( not creatable ), this is acting as an interface.


IState:
--------
Public Function talk() As Boolean

End Function

Public Function changeState(myContext As cls_Context) As Boolean

End Function


cls_State2
-------------
Implements IState

Private Function IState_changeState(myContext As cls_Context) As Boolean
myContext.setState (New cls_State1)
End Function

Private Function IState_talk() As Boolean

MsgBox "I'm 2"

End Function



cls_State1
-------------
Implements IState

Private Function IState_changeState(myContext As cls_Context) As Boolean
myContext.setState (New cls_State2)
End Function

Private Function IState_talk() As Boolean
MsgBox "I'm 1"
End Function



cls_Context
---------------
Private myState As IState

Public Function setState(myNewState As IState)
myState = myNewState
End Function

Public Function Speak() As Boolean
myState.talk
End Function

Private Sub Class_Initialize()
Set myState = New cls_State1
End Sub

Public Sub AlterState()
myState.changeState (Me)
End Sub

Lenin

That is all a bit confusing, what exactly are you trying to use callbacks with, a set of classes it looks like?

Well one thing you need to use set in the setState function of cls_Context.

You should check out the MSDN Coffee example among other things it has a good example of callbacks, at least I think that is the one.

in the class:

VB Code:

Option Explicit
 
Public Event myCallbackEvent(ByVal myCallbackValue As Integer)
 
Sub EventTest()
 
RaiseEvent myCallbackEvent(19)
 
End Sub

behind your form:

VB Code:

Option Explicit
Dim WithEvents cls As Class1
 
Private Sub cls_myCallbackEvent(ByVal myCallbackValue As Integer)
 
Debug.Print myCallbackValue
 
End Sub
 
Private Sub Command1_Click()
 
 
Set cls = New Class1
 
cls.EventTest
 
End Sub

hth,

Helger

Also have a look at this thread if you should be interested in asynchronous communication. (The timer is needed because it is one of the few functions that gives control back to the calling thread right after it got triggered - just in case you should wonder).

regards,

Helger

Helger thanks.

Edneeis, yes I am try to incorporate callbacks with "a set of classes". I am trying to use callbacks to implement a behavioural patern ( the State Pattern ).

I have been able to do this quite easily in Java, but had some issues in VB.

So did Helger fix your problem?

Another thing that may help is making the methods the other object will call Friend instead of private.