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From: tmoran@acm.org
Newsgroups: comp.lang.ada
Subject: Re: [OT] Best way to isolate a GUI?
References: <3E5121BD.4010200@cogeco.ca>
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> > Callbacks are an excellent example of a tool's limitations affecting the
> > users' way of thinking (everything looking like a nail if you only have
> > a hammer).
>...
> I can't speak for CLAW,
  Claw handles window event callbacks by calling tagged object procedures,
which may be overridden by the app program.  It's all type-checked etc
Ada.  It's like writing all your code inside interrupt handlers.  But an
app that isn't basically a set of event responders can be done differently.
  One of the Claw demo programs, Bigjob, has a main task that just does a
(dummy) big computation, and a separate windows' event handler task.  The
windows routines put user command stuff into a message and the computation
task periodically checks there to see if it needs to modify its computation.
Similarly, the computation task periodically puts results in a status
record.  The window task inspects that and updates the display as needed.
In the Bigjob example, if the main task wants to force an immediate screen
update, it simply invalidates the window, which forces a redraw, which
will use the updated status info.