Re: [Boost-users] returning values in Boost.StateChart

From: Federico J. Fernández (ffernandez_at_[hidden])
Date: 2007-03-28 16:11:47

> Ok, what will happen with this event? Should it be processed by the same
> state machine object or a different one? I guess a different one,
> correct? (If its the same FSM object then you should call post_event
> from an action).

In this particular case using the same event for both places is only
to simplify the approach. To be concrete, this returned event will be
used not for another FSM but for pushing it in an event queue... maybe
there is a way to implement this event queue with FSMs but this is the
current approach.. the idea of the event queue is to communicate
different threads of a multithreaded program (related to
communications).

> > We tried with different approaches. For example,
> > modifying the passed event (but it's const) and trying to do a custom
> > reaction that returns something (but I had problems too.. don't
> > remember exactly now).
>
> The easiest way to get back a result from a state_machine is to add a
> boost::function object as a member of the event:
>
> 1. Add either an in_state_reaction or a transition with a transition
> action to your state_machine. The triggering event should contain a
> boost::function object as member. The action should call this object.
> 2. Construct the event and set the boost::function member to a suitable
> callback function. In your case that function would accept an event
> parameter.
> 3. Call process_event, passing the event object.
> 4. Your callback function is called.

Sounds good.

> This is the easiest way to do it but also the most dangerous, please see
> "Why asynchronous state machines are necessary":
>
> http://www.boost-consulting.com/boost/libs/statechart/doc/tutorial.html#AsynchronousStateMachines
>
> IOW, you might want to use an asynchronous state machine instead of your
> synchronous one.

I am not sure about that. I evaluated using that kind of FSM but as
the other use of the state is not in a FSM (and is something simpler
that not synchronizes) maybe this is enough. I recall, using the event
in another place is only to reuse the struct, no more than that.

Thanks!