[AccessD] Implements vs WithEvents

Charlotte Foust cfoust at infostatsystems.com
Tue Jan 13 19:16:56 CST 2004


John,

Would you have understood your response when *you* were first getting
your feet wet in this kind of programming?  I wasn't trying to provide
the definitive answer, just a frame of reference.  VB/VBA isn't fully
object-oriented, so it can't really support Implements in all its glory.

Charlotte Foust

-----Original Message-----
From: John W. Colby [mailto:jwcolby at colbyconsulting.com] 
Sent: Tuesday, January 13, 2004 4:54 PM
To: Access Developers discussion and problem solving
Subject: [AccessD] Implements vs WithEvents


Charlotte,

>They have somewhat different purposes.

No, they have completely different purposes.

>Implements allows you to subclass an existing object/class and give it
additional or different functionality, so you can have a class Dog and a
subclass Spaniel that has a property Retriever.

Well... true but not exactly.  My issue with that statement is the term
subclass as you use it.  Yes you are subclassing the original but ONLY
the interface.  IMHO that is almost useless

Implements is an interface ONLY inheritance, i.e. it allows (forces) you
to start with the same public data and method DEFINITION but does not
actually inherit the behaviors (methods) themselves.  I.e. the code that
runs in the class that is being subclassed is NOT inherited into the
subclass.

Access' inheritance (interface) is not very useful except as a template
kind of thing.  I find it useless enough that I personally don't bother.
On the other hand Shamil is a proponent so it must have its uses.

When you move to VB.NET (or any of the other .net languages I believe)
TRUE inheritance is available.  I.e. if you have a method with a
thousand lines of code, when you subclass that object the new object
inherits that code as well as the data.  In our Implements subclassing,
you would have to cut and paste that thousand lines of code into the
subclass.  Can you say maintenance nightmare?

>You can intercept and respond to specific events when they happen in 
>the
object.  Did the Bark event trigger?  WithEvents
lets you find out and respond from another class/object.

Exactly correct.  One of the MAJOR shortcomings of Access / VBA (in fact
VB prior to VB.Net) is that Inheritance is virtually non-existent, and
even worse, objects other than forms don't have a built in class that
you can get at to add functionality directly to.  What WithEvents really
does is allow you to build class wrappers for objects so that you can
encapsulate behaviors (functionality) in your class.  In essence you
still have two objects, the control (or even another class or OCX) and
your class, but now you can put all of the common functionality in your
class along with the event stubs of the control/class/ocx that generates
the event.

This does not get you any further down the road towards inheritance,
sadly it simply doesn't appear to be possible in Access / VBA.  But it
does provide a huge step up in encapsulation.

John W. Colby
www.ColbyConsulting.com

-----Original Message-----
From: accessd-bounces at databaseadvisors.com
[mailto:accessd-bounces at databaseadvisors.com]On Behalf Of Charlotte
Foust
Sent: Tuesday, January 13, 2004 4:56 PM
To: Access Developers discussion and problem solving
Subject: RE: [AccessD] =function() in .onclick


They have somewhat different purposes.  Implements allows you to
subclass an existing object/class and give it additional or different
functionality, so you can have a class Dog and a subclass Spaniel that
has a property Retriever.  WithEvents allows you to monitor the events
of an object/class.  You can intercept and respond to specific events
when they happen in the object.  Did the Bark event trigger?  WithEvents
lets you find out and respond from another class/object.

Charlotte Foust

-----Original Message-----
From: Robert Gracie [mailto:Subscriptions at servicexp.com]
Sent: Tuesday, January 13, 2004 1:45 PM
To: Access Developers discussion and problem solving
Subject: RE: [AccessD] =function() in .onclick


Hello All,
 Since I'm just learning this type of programming technique I thought I
would ask a question...

  Why not use Implements instead of WithEvents?  My limited understand
of WithEvents is that they will not return functionality unit they (the
event) has completed processing... Since I'm just at the "door" so to
speak with learning how to use and incorporate these new (to me)
programming techniques into my work, I really want to start out using
the "best" technique...


Robert Gracie
www.servicexp.com


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