That does give us a couple of options. One or the other of these would certainly be fine for a case where relatively few events need to be handled in this way. I think, though, that it could add significant complexity to a large statechart where all or most reactions had to involve an indirection through a transition action plus possibly an additional in-state reaction. Though it would work, I think the extra mechanism would obscure the fundamental structure of the statechart.

In our case, we have an additional option, since we have objects that live externally to the state objects and which get attached to them as they come and go. These external objects can offer an interface for setting/getting the triggering event; then each transit<xyz>() call can be preceded by a call that sets the event object, and if the event object is needed it will be available when xyz is constructed. This has some disadvantages - we lose type safety, of course, and we become dependent on these external objects to handle these situations - but we have a need for these external objects anyway, and this situation is so common in our case that at the moment it seems preferable to the option of adding additional actions/reactions to the statechart (which will already be fairly complex).

So, I would still lobby for the addition of simple_state::triggering_event(), but we do have options in the meantime.

Thanks,
Bill

On Thu, Sep 3, 2009 at 1:56 AM, Andreas Huber <ahd6974-spamboostorgtrap@yahoo.com> wrote:
This can be determined with the token, perhaps supplemented with queries to
other objects.

Ok, in this case one easy option would be to make the query within the transition action. Contrary to the entry and exit actions, a transition action does have type-safe access to the triggering event. If other objects need to be queried then these could be accessed from the transition action as well. The data that is gathered in the query could be put into a new event which is passed to simple_state::post_event. The destination state would then define an in_state_reaction triggered by the new event.

Or, if you want to make the query after entering the destination state: Repost the event carrying the token in the transition action and define an in-state reaction triggered by the same event. The in-state reaction would then execute the query and store the data in the destination state.


I should have mentioned that in our system this is a general pattern, with
different token and data types that need to be responded to differently in
different states - this is why it seems that the most reliable and general
solution would be to pass the event data into the new state. It's true that
this removes type safety, since I now have to dynamic_cast or equivalent in
order to get to the actual data, but I'm not terribly worried about this
(or, maybe more accurately, I don't see a better alternative).

See above, an alternative is the "transition action posts event that triggers an in-state reaction"-pattern (what a name :-)). With this pattern type-safety is upheld, at the cost of needing to process an additional event, which is probably negligible is your case. Another (IMO minor) problem is that you might need move some of the things that you planned to to in the destination state entry action to the in-state reaction.

HTH,


--
Andreas Huber

When replying by private email, please remove the words spam and trap
from the address shown in the header.


_______________________________________________
Boost-users mailing list
Boost-users@lists.boost.org
http://lists.boost.org/mailman/listinfo.cgi/boost-users