Re: Properties

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On Wed, 1 Dec 2010 11:36:57 -0800 (PST), Shark8 wrote:

>>> Agreed, but I don't believe we're stuck with that solution.
>>> All you need is procedure'Access for the callback.
>>
>>No, because events have parameters and those must be typed.
>>The standard solution is that you instantiate a generic (and
>>there is one for each number of parameters and the results).
>>That is extremely boring.
> 
> Er, why not use OO/tagged-records for the events themselves and let
> the base-handler have that as a parameter?
> 
> i.e. the handler would be an access to something like this:
> Procedure On_Event( Object : in out : GUI_Object; --'Class?
>                     Event  : In Base_Event'Class );

Yes, event handlers are naturally doubly dispatching, this is why you were
not sure if On_Event should be a primitive operation of GUI_Object. It
should and it also should be one of the Base_Event. Unless you downcast in
the handler body.

Another issue with the above is that you will have to derive from
Base_Event somewhere this might be actually forbidden. On_Event is a
closure, which is tend to be a subprogram rather than a primitive
operation.

Event parameters usually marshaled, which might be difficult.

Yet another issue. Handlers actually connect the GUI object and the context
of the handler. The context, if not of a closure, then another GUI object.
E.g. normally one object catches events of another. In a GUI objects are
usually collected. So you need a mechanism which disconnects the handler if
any of the objects get destroyed. In any case it is probably triply
dispatching:

procedure On_Event
   (Emitter, Observer : in out : GUI_Object; Event  : In Base_Event);

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de