Re: "Classes" as packages in Ada

[Matthew Heaney <matthew_heaney@acm.org>]

eachus@spectre.mitre.org (Robert I. Eachus) writes:

>    But if I wanted to simply implement the Ada equivalent to John's
> classes, I'd probably use an Ada protected object.  (Note to Ada 95
> programmers, this is a perfect case for using requeue to insure that
> each observer is notified once for each event.)
> 
>     protected type Observed_Object is
>       procedure Set(Observed: in Object);
>       function Value return Object;
>       entry Modified(Observed: out Object);
>     private
>       entry Waiting(Observed: out Object);
>       Current: Object;
>       Reporting: Boolean := False;
>     end Observed_Object;
>        
>     protected body Observed_Object is
>       procedure Set (Observed: in Object) is
>       begin
>         Current := Observed;
>         Reporting := True;
>       end Set;
> 
>       function Value return Object is begin return Current; end Value;
> 
>       entry Modiified(Observed: out Object) when not Reporting is  
>       begin requeue Waiting; end;
> 
>       entry Waiting(Observed: out Object) when Reporting is
>       begin
>         Observed := Current;
>         if Waiting'Count = 0 then Reporting := False; end if;
>       end; 
> 
>     end Observed_Object;
> 
>     Now I probably could spend pages describing why I made certain
> choices in this version, and how to modify it to deal with other
> cases, but I'll refrain.  (Knowing the list though, I'm sure there
> will be a dozen variations posted by Monday ;-)


Oh, they were posted by Monday, just not the Monday you had in mind. ;^)

In the March 99 ACM patterns archive, you'll find these examples of the
Observer pattern:

  observer (see also revised version)
  observer is subject
  observers of multiple subjects
  mediator
  mediator (alternate version)


I have converted every example in the GoF Design Patterns book to Ada95,
and posted them to the ACM patterns list.

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Matt