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From: "Jeffrey R. Carter" <spam.not.jrcarter@acm.not.spam.org>
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Subject: Re: Custom model in gtkada?
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Stephen Leake wrote:
> 
> I think Gtk is the best model, although the C underpinnings make
> things difficult sometimes. In particular, it is difficult to detect
> some errors at compile time, that the fully Ada systems can detect. At
> root, the C pointers are not strongly typed.

I think neither callbacks nor dispatching are good models. Callbacks are 
a way to get around C's lack of concurrency, since GUIs are inherently 
concurrent. Let the window manager control your code so you only have 1 
thread of control. The result violates locality, tends to result in lots 
of state variables global to the callbacks, is unintuitive, and is 
difficult to understand.

Dispatching is simply an attempt to model callbacks with something other 
than pointers. It has the same problems as callbacks. You can do 
something similar with generics, but it's still no improvement.

If you were designing a windowing system from scratch for a concurrent 
language such as Ada, you'd probably have a concurrent queue of events 
for each window, with a way to control what events are reported. That's 
far more straightforward and easy to understand. JEWL is an example of 
this, though very basic.

-- 
Jeff Carter
"Monsieur Arthur King, who has the brain of a duck, you know."
Monty Python & the Holy Grail
09