Design thought for callbacks

[Marko Rauhamaa]

Cem Karan <cfkaran2 at gmail.com>:

> On Feb 21, 2015, at 12:08 PM, Marko Rauhamaa <marko at pacujo.net> wrote:
>> Maybe the logic of the receiving object isn't prepared for the callback
>> anymore after an intervening event.
>> 
>> The problem then, of course, is in the logic and not in the callbacks.
>
> This was PRECISELY the situation I was thinking about. My hope was to
> make the callback mechanism slightly less surprising by allowing the
> user to track them, releasing them when they aren't needed without
> having to figure out where the callbacks were registered. However, it
> appears I'm making things more surprising rather than less.

When dealing with callbacks, my advice is to create your objects as
explicit finite state machines. Don't try to encode the object state
implicitly or indirectly. Rather, give each and every state a symbolic
name and log the state transitions for troubleshooting.

Your callbacks should then consider what to do in each state. There are
different ways to express this in Python, but it always boils down to a
state/transition matrix.

Callbacks sometimes cannot be canceled after they have been committed to
and have been shipped to the event pipeline. Then, the receiving object
must brace itself for the impending spurious callback.


Marko