[SCXML] Implementation of system variables in a separate context?

[Ingmar Kliche <in...@googlemail.com>]

Rahul,

the latest SCXML WD [1] describes a set of system variables which are
read-only for the application. See section 5.5 [2]: "The Data Module
maintains a protected portion of the data model containing information that
may be useful to applications...."

This could be implemented in commons-scxml using an additional context and
set it as the parent of the current root context. Some expression languages
(such as ECMAScript) support scope chains and allow to attach scopes read
only. Other expression languages may require a special treatment of this
"system variable context". In this case it could be necessary to implement a
special context which handles the read-only behavior.

At the same time we could change the "eventname" and "eventvalue" variables
to "_event.name" and "_event.value".

What do you think?

- Ingmar.

[1] http://www.w3.org/TR/2008/WD-scxml-20080516/
[2] http://www.w3.org/TR/2008/WD-scxml-20080516/#SystemVariables

Re: [SCXML] Implementation of system variables in a separate context?

[Rahul Akolkar <ra...@gmail.com>]

On Fri, Dec 5, 2008 at 8:02 AM, Ingmar Kliche
<in...@googlemail.com> wrote:
> Rahul,
>
> the latest SCXML WD [1] describes a set of system variables which are
> read-only for the application. See section 5.5 [2]: "The Data Module
> maintains a protected portion of the data model containing information that
> may be useful to applications...."
>
> This could be implemented in commons-scxml using an additional context and
> set it as the parent of the current root context. Some expression languages
> (such as ECMAScript) support scope chains and allow to attach scopes read
> only.
<snip/>

We may need to abstract out the notion of a partitioned root context,
just like we have a notion of ordinary context.


> Other expression languages may require a special treatment of this
> "system variable context". In this case it could be necessary to implement a
> special context which handles the read-only behavior.
>
<snap/>

Yes, its read-only in a limited sense (for the application developer
i.e. within the SCXML document -- not for the implementation i.e.
_event for example keeps changing with each event so the executor
needs to be able to write to it).


> At the same time we could change the "eventname" and "eventvalue" variables
> to "_event.name" and "_event.value".
>
<snip/>

Yeah, that needs to be done (though I think you mean "_eventdata"
instead of "eventvalue").

If you feel like it, do open a JIRA issue to track this for the next release.

-Rahul



> What do you think?
>
> - Ingmar.
>
> [1] http://www.w3.org/TR/2008/WD-scxml-20080516/
> [2] http://www.w3.org/TR/2008/WD-scxml-20080516/#SystemVariables
>

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