Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!apple!well!tmh
From: tmh@well.UUCP (Todd M. Hoff)
Newsgroups: comp.lang.c++
Subject: Combining event driven and object orient programming
Message-ID: <13928@well.UUCP>
Date: 4 Oct 89 03:37:57 GMT
Reply-To: tmh@well.UUCP (Todd M. Hoff)
Distribution: all
Organization: Whole Earth 'Lectronic Link, Sausalito, CA
Lines: 45

With the question of C++ having boolean types out of our blood,
let's move on to something really challenging.

QUESTION:

In an event driven environment, how are events associated with 
the correct method within the correct object instance? How is
argument parsing handled?

SITUATION:

For our project each process is completely event driven. A set of
libraries implements event handling through a set of callback routines
and work/response queues. There are a number of clients and servers
using and providing various services. Applications process events from
multiple sources including a user interface manager, several database
managers, processes on remote machines asnchronously emitting events, 
and a few others. In each application are multiple objects handling
these events and controlling remote objects.

The problem is how to associate an event with a particular method within
a particular object instance. Events arrive and they must be processed.
Use which object? Which method? How is the message translated into
method arguments? The situation is complicated even more by the heterogenaity 
of the event message formats. Some messages are basically just serialized C 
structures and others are CMIP messages encoded in ASN.1, needing A LOT of 
decoding. So there is no standard message format or standard set of arguments 
to pass.

SOLUTION:

The problem seems to stem around the inability to send a message to
an object, something like: objID selector data. There's function
overloading, but function overloading assumes you're in a context 
such that you know what method to call. In an event driven environment 
the former is rarely the case.

I have a couple of possible solutions, but I am hoping someone has
faced this problem before and has an elegant painfully-obvious-on-
further-reflection solution. I don't consider endless switch statements 
a real solution.

Thanks in advance

tmh@well