Re: [Boost-users] [Statechart] How to decide between post_eventand process_eventatrun time ?

From: Philippe DAVID (philippe.a.david_at_[hidden])
Date: 2007-11-15 04:29:37

>
> Yes, it is slightly more work than if there was a 
> state_machine::is_processing()
> function. I'm just not sure whether your use case is sufficiently common to
> justify adding such a function. Maybe you could provide more details 
> so that I
> understand it better?

Sure, I'll try.
My state machine is coupled to a connection on which I must comply with some
protocol. My layer is a middleware. Each time I have something interesting,
like data or events, I call a user-supplied handler on a corresponding method
from a react method of the state machine.
If the user wants to close the connection I have the problem I described
because:
- either he decides to close the connection from a callback, which means the
state machine is in the middle of processing the event which caused the 
call to
the handler, then I must use post_event
- or he decided to close the connection based on an event that didn't 
come from
the connection, in that case I must use process_event.

I simplified the problem but the idea is there. If my state machine calls a
handler on which I don't have total control, and I supply a few methods that
need to be processed using the state machine, then I need to decide between
process and post_event.

In the simple case I could also provide 2 close methods for the handler, one
that must be called in callbacks and one for the other methods of handler. I
don't think this is a clean way to solve this issue. Each method in my
interface would have 2 versions, and the stability becomes really weak because
calling the wrong method at the wrong place results in a crash.

At this point,
maybe my design is weak, and I shouldn't call the handlers callback directly
from the react methods. Instead I could use an event queue to talk to the
handler. In the react method I would post events to the queue, and make sure I
dequeue it from outside the state machine. But it involves (much) more work...

Hope my explanations are clear ;)

What is your point of view on this ?

Regards,
Philippe