On 9/5/2010 12:36 PM, Christophe Henry wrote:
Hi,
The problem is that UML does not specify how often an anonymous transition should be "tried" and I probably chose the wrong solution (to try again and again). So, as your guard always returns false, msm keeps trying, which means calls to process_event() until the stack overflows. The solution probably is to try just once but it will force you to manually try to reprocess by calling, say a process() function without event, but it arguably simply means passing the buck to you.
Please let me think about it a bit more, I'd like to think this out a bit more. For the moment, I suggest you to either write a useful guard or revert to a named event, which you will process when it makes sense in your application.
Regards,
Christophe
Hi Christophe - I hope you are well. You know how I like to dig into this stuff... so I'll give my thoughts. They are tainted by the the UML predecessor; however, I think there is good logic and possibly even a specification argument for my point of view. UML 2.2 Superstructure section 15.3.14 [Transition] provides the associations for: trigger : Trigger[0..*] -- I am assuming that no trigger is what you are referring to as an anonymous transition. I don't see that term used in current specifications anymore. guard: Constraint[0..1] -- "A guard is a constraint that provides a fine-grained control over the firing of the transition. The guard is evaluated when an event occurrence is dispatched by the state machine. If the guard is true at that time, the transition may be enabled; otherwise, it is disabled. Guards should be pure expressions without side effects. Guard expressions with side effects are ill formed." I would argue that the definition of the guard constraint requires that the transition is evaluated when an event is dispatched and is only evaluated once for that event. Because the transition is disabled if the guard evaluates false, then the transition cannot be evaluated again until another event is dispatched. I would also argue that the system is considered stable once evaluation of an event begins. Guards are not allowed to cause side effects; therefore, the 'state' of the system when evaluation begins should be the same as when evaluation has completed if no transition is taken. Personally, the least surprising thing (imnsho) would be that: 1. event is dispatched 2. internal transitions are evaluated for current state 3. external transitions are evaluated for current state 4. hierarchy is followed up and 2/3 repeated 5. if top is reached without satisfying event ... an error has occurred and is acted upon as normal I do not think whether a trigger is defined or not for a transition will affect this behavior. Transitions without a trigger are simply candidates for any event. The steps outlined above are those used for transitions with triggers, no? Take care - michael -- ---------------------------------- Michael Caisse Object Modeling Designs www.objectmodelingdesigns.com