[AccessD] Implements vs WithEvents

Yes, please send.

And if that's a 135, then I never get above about a 20 or so.

;-)

Thanks,

John W. Colby
www.ColbyConsulting.com

-----Original Message-----
From: accessd-bounces at databaseadvisors.com
[mailto:accessd-bounces at databaseadvisors.com]On Behalf Of Ken Ismert
Sent: Thursday, January 15, 2004 6:40 PM
To: 'Access Developers discussion and problem solving'
Subject: RE: [AccessD] Implements vs WithEvents



>One thing you say intrigues me though.  You mention Implements allowing a
>custom callback scheme and sending messages.  How does this work?  How does
>the callback communicate between class instances?

John,

Interesting, the difference in perspectives: after your experience with true
object languages, you find VBA limiting, while I cut my teeth on non-object
languages, and find Implements a revelation. I'd love to work in a true OOP
language that supports inheritance, but feel it would take some time to
fully grasp its power and use it effectively.

The callback scheme can be summarized as follows:

1. The callbacks are defined as methods in the ICallback interface:
      ' Insert this code in Class module ICallback
      Public Sub Message(sBody As Text)
      End Sub

2. Class CWannaMessage, wanting to receive messages, implements ICallback:
      Implements ICallback
      Private Sub ICallback_Message(sBody As Text)
          Debug.Print sBody
      End Sub

3. The CWannaMessage object makes a CCaller object, and passes a reference
to itself:
      Set rCaller = New CCaller
      rCaller.Setup Me

4. CCaller's setup code looks like this:
      Private mrCallback as ICallback
      Public Sub Setup(CallbackObj As ICallback)
          Set mrCallback = CallbackObj
      End Sub

5. When CCaller wants to send a message:
      mrCallback.Message "Hello, you!"

6. Since you now have circular object references, CCaller must also provide
a way to release its callback reference:
      Public Sub Shutdown
          Set mrCallback = Nothing
      End Sub

7. CWannaMessage must shutdown its caller when it isn't needed, or neither
object will terminate:
      rCaller.Shutdown

This example shows a one-on-one conversation, but it could easily be
extended for multiple callers. CWannaMessage could maintain a collection of
CCallers, and pass each CCaller its name or index on Setup, which CCaller
could then pass back when it sends its Message. Or CCaller could just pass a
reference to itself (Me) back to CWannaMessage. In any case, ICallback would
have to be suitably extended.

I calculate the obtuseness of this post at 135 degrees, a 5 degree
improvement over the last.

-Ken

ps: I have some relatively simple form management code using interfaces. It
allows a Parent form to open a Child, and be notified when it closes. I'll
send you it if you're interested.

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