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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: Properties
Date: Thu, 2 Dec 2010 22:43:19 -0600
Organization: Jacob Sparre Andersen
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Date: 2010-12-02T22:43:19-06:00
List-Id: <comp.lang.ada>

"Maciej Sobczak" <see.my.homepage@gmail.com> wrote in message 
news:2a3219cb-5793-4e5b-97aa-036da124ccba@y31g2000vbt.googlegroups.com...
...
>Still, good design principles call for decoupling of
>application logic and display (maybe with some intermediate "data
>model" module?) and this decoupling blurs that distinction
>significantly.

When it comes to GUI programming, this supposedly "good design principle" 
often doesn't make any sense.

For example, consider the series of SLED (Simple Little EDitor) examples in 
Claw. These show how a Notepad-like editor could be constructed using Claw. 
In this case, the "application logic" is editing of text (that is, handling 
insertions, deletions, cut, paste, etc.) If you strictly decouple that from 
the GUI, you then cannot use the prexisting controls that automatically 
provide that editing functionality. Several of the SLED versions show how 
that can be done, but the versions using the Rich_Edit control are a lot 
shorter and also a lot more powerful.

You will have that problem with any GUI program that manages text. And there 
are lots of other similar sorts of examples out there as well.

The problem, of course is that there is no such thing as a clear boundary 
between "application logic" and "display". The application logic has a 
strong effect on what can be displayed and how; similarly, what the user 
does with the GUI has a strong effect on what application logic is invoked. 
They can't be completely independent.

                                   Randy.