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From: Matthew Heaney <matthew_heaney@acm.org>
Subject: Re: "Classes" as packages in Ada
Date: 1999/03/28
Message-ID: <m3n20xtnf2.fsf@mheaney.ni.net>#1/1
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References: <73f0p1$4iu8$1@prime.imagin.net>
 <MtE62.27$8X3.208255@news.rdc1.az.home.com> <1103_911962898@DZOG-CHEN>
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NNTP-Posting-Date: Sun, 28 Mar 1999 11:34:32 PDT
Newsgroups: comp.lang.ada,comp.object
Date: 1999-03-28T00:00:00+00:00
List-Id: <comp.lang.ada>

"John Goodsen" <jgoodsen@radsoft.com> writes:

> So my conclusion is that Ada code for an interface is not even
> close in simplicity (from the programmer's point of view) as
> something like:
> 
> public class ObserverEvent {
>     ...
> }
> public interface Observer {
>     public void Update(Observable);
> }
> public interface Observerable {
>     public void AddObserver(Observer);
>     public void RemoveObserver(Observer);
>     public void NotifyObservers(Observer, ObserverEvent).
> }


There are a few variations of the Observer pattern theme in the March 99
ACM patterns archive.

<mailto:patterns@acm.org>
<http://www.acm.org/archives/patterns.html>


The example in the article "Observer Is Subject" has a spec very similar
to what you wrote above:


package Subjects_And_Observers is

   type Root_Subject_Type is
     abstract tagged limited private;

   procedure Notify (Subject : in out Root_Subject_Type'Class);


   type Root_Observer_Type is
     abstract new Root_Subject_Type with private;

   procedure Update (Observer : access Root_Observer_Type) is abstract;


   procedure Attach
     (Observer : access Root_Observer_Type'Class;
      To       : access Root_Subject_Type'Class);

   procedure Detach
     (Observer : access Root_Observer_Type'Class;
      From     : access Root_Subject_Type'Class);
...
end Subjects_And_Observers;


See also my versions of the Mediator pattern, which use the Observer
pattern for their implementation.

I have converted every example in the GoF Design Patterns book to
Ada95.  All the work is archived at the URL above.

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Matt