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From Rahul Akolkar <rahul.akol...@gmail.com>
Subject Re: [scxml] Target of transition as variable?
Date Thu, 23 Jun 2011 21:54:12 GMT
On Tue, Jun 21, 2011 at 3:42 PM, Artem Vovk <vovk.artem@googlemail.com> wrote:
> Can I use dynamic transition targets? Something like this?
>
> <datamodel>
>        <data id="dyn_state" expr="State1">
> </datamodel>
>
> ...
>
> <transition  ... target="dyn_state"/>
>
> If I use it on this way, I receive an exception: Transition target with ID "dyn_state"
not found -> Interpreter thinks that it is a name of the state, but not the variable...
Is there a way to define such transitions?
>
<snip/>

Not directly as you are illustrating above. Most state machines in
general, and SCXML in particular, requires you specify fixed target(s)
for each transition. This is actually quite useful because it allows
static analysis to determine whether all transition targets are legal
even before the state machine is executed. It is also not as limiting
as it may seem at first, because of the existence of things like guard
conditions on transitions and history states.

For the above, a literal translation may appear to be as follows
(replacing dynamic target with static ones):

  <transition  cond="dyn_state eq 'State1'" target="State1"/>
  <transition  cond="dyn_state eq 'State2'" target="State2"/>
  ...

The above pattern can certainly be used, and may seem more verbose.
But note that there are usually two <assign> or similar statements
elsewhere that are updating the data 'dyn_state'. Often, these can
instead be replaced as appropriate guard conditions on the two
transitions above so the net effect is no change in verbosity.

-Rahul

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