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From Christopher Giblin <...@zurich.ibm.com>
Subject Re: [SCXML] Querying if an event would cause a transition
Date Tue, 24 Jul 2007 13:33:17 GMT
Hi,
Update: In the  meantime, I have implemented a TransitionDetector class 
which implements SCXMLListener and is registered with SCXMLExecutor.
It is only slightly odd to program:

        transitionDetector.reset();
        fsmExecutor.triggerEvent(ev);
        System.out.println("Transition : " + transitionDetector
.isTransitionOccurred());

That gives me a tactical solution for the time being. If I find that a 
query, as suggested in my original post, really is required, I will get 
back.

Thanks, chris


Christopher Giblin/Zurich/IBM wrote on 07/24/2007 12:00:36 PM:

> Hi,
> 
> Sorry for the delayed response. My remarks below.
> 
> > Please prefix the email subject with [SCXML], since this mailing list
> > is shared across all Commons components. I've added the prefix here.
> 
> Thanks - I will be sure to in the future.
> 
> > On 6/27/07, Christopher Giblin <CGI@zurich.ibm.com> wrote:
> >>
> >> Hi,
> >> I am new to Commons SCXML.
> >> Is it possible to determine whether an event would result in a 
transition,
> >> without that transition occuring? Including evaluation of transition
> >> conditions?
> ><snip/>
> >
> > While the idea is interesting, we do not have such an "introspection 
mode".
> > 
> > It wouldn't take too much effort to implement it, but:
> 
> > a) Would probably increase the code smell factor a tad (if in
> > introspection mode, then follow transition else just report back etc.)
> 
> > b) Would require some API additions that might warrant a major 
release.
> 
> > If you don't mind sharing in this public forum, why do you need such
> > introspection? The SCXML specification doesn't talk about this.
> > Understanding that interaction pattern may or may not help better
> > answer the question, we will have to see :-)
> 
> I am using SCXML to describe allowable states for an application. 
> Every event should result in a state transition. If no transition, 
> then I need to raise an error. Adding error states to the state 
> machine complicates it significantly. By comparison, simply 
> expressing all allowable transitions is more elegant. This requires 
> that the engine can be queried with  an event to determine if it 
> would result in a transition. By default, SCXML ignores non-
> applicable events, remaining in the current state. Remaining in the 
> same state is ok, but I need to know if an event transition occurred
> or not, as said. One approach would be to register as a listener to 
> determine if a transition occurred. This results in an awkward 
> programming style, however, further compounded by my app having 
> perhaps hundreds of state machines. A program would be easier to 
> read if one could ask the engine if the event leads to a transition.
> 
> I also considered clone the state machine, listening and then 
> triggering the event. Too much overhead, though. My code must also 
> be efficient.
> 
> Or do you see a better way of dealing with this? 
> 
> Thanks, chris
> 
> 
> 
> > -Rahul
> 

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