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From Hallvard Trætteberg (JIRA) <j...@apache.org>
Subject [jira] Commented: (SCXML-27) Improve possibility to extending (subclassing) the default semantics
Date Mon, 02 Oct 2006 20:42:20 GMT
    [ http://issues.apache.org/jira/browse/SCXML-27?page=comments#action_12439307 ] 
            
Hallvard Trætteberg commented on SCXML-27:
------------------------------------------

My suggestion was not to directly support subclassing of Transition or caching of regexp patterns,
but to refactor SCXMLSemanticsImpl so extending the event matching strategy is easy for a
subclass. As mentioned, for such a simple case, I had to copy the whole filterTransitionsSet
method from SCXMLSemanticsImpl to get hold of the Transition, as it was needed by the event
matching strategy. If the eventMatch method took the Transition as the first argument, instead
of its event, an overriding method would have more information.

I have a second use case for this. I also use the scripting language for event matching. More
specifically, I allow referring to contextual objects, so the event matching method may match
against "any event generated from object x", where object x is an object in the context of
the transition. Without knowing which Transition the event is attached to, the expression
cannot be evaluated. 

> Improve possibility to extending (subclassing) the default semantics
> --------------------------------------------------------------------
>
>                 Key: SCXML-27
>                 URL: http://issues.apache.org/jira/browse/SCXML-27
>             Project: Commons SCXML
>          Issue Type: Improvement
>    Affects Versions: 0.5
>            Reporter: Hallvard Trætteberg
>
> I've extended the default SCXML semantics (subclassed SCXMLSemanticsImpl) with a more
flexible event matching method. However, this required copying a lot of code from SCXMLSemanticsImpl,
just to replace the event matching method. The problem was that the eventMatch method takes
String and Set arguments, with no way to get at the Transition who's event is to be tested.
I copied the relevant code and replaced eventMatch(String,Set) with eventMatch(Transition,Set),
to make it more flexible. I ask that this change is made to SCXMLSemanticsImpl, to better
support overriding.
> The flexibility I've added is to use regular expressions for matching, and since this
happens often, I wanted to cache the compiled pattern. The natural place to do this is the
Transition, so I subclassed the Transition class, included a field for holding the compiled
pattern and made sure to use this subclass whenever a regexp was needed. Then in the modified
eventMatch code, I check whether the Transition is of this subclass and used the cache to
improve performance.

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