RE: SCXML: Fundamental questions

Andrew,

 Your statements are all correct.  For simple (= flat) state machines of
the kind you would see in an automata theory class, things can be stated
very straightforwardly: "at any point, a state machine is in one and
only one state, which is called the 'active' state.  When an event
occurs, the state machine may take a transition, instantaneously leaving
its current active state and entering another. "  Things are more
complicated in SCXML due to nested and parallel states, but the same
basic idea applies: at any point an SCXML state machine is either a) in
a single _atomic_ (innermost) state, and the other active states are the
ancestors of the atomic state, or b) in multiple atomic states, in which
case the atomic states are descendents of a single parallel element, and
the other active states are the ancestors of those atomic states.  

 

- Jim

 

 

________________________________

From: www-voice-request@w3.org [mailto:www-voice-request@w3.org] On
Behalf Of Andrew Emmons
Sent: Friday, April 11, 2008 3:50 PM
To: www-voice@w3.org
Subject: SCXML: Fundamental questions

 

Hello,

 

Quickoffice has been looking at using SCXML in combination with our SVG
implementation.

 

We have a few quick questions about the very basics of state machines.
We were trying to find a clear definition in the spec about what an
"active" state is. Or what it means to "activate" a state. Or "being in"
a state. This might be an oversight of the spec, or might be something
so fundamentally obvious that it was not deemed necessary to
specifically describe it, in which case we apologize for the questions.

 

Consider the following state machine excerpt (transitions omitted):

 

<scxml> 

  <state id="s">

    <state id="s0"/> 

    <state id="s1"/> 

  </state>

  <state id="t">

    <state id="t0"/> 

    <state id="t1"/> 

  </state>

</scxml>

 

Are those statements correct :

 

- "being in s" means that s is active

- "entering s" means that s becomes active

- generally speaking, if a state machine "is in" a state, it means that
state is active (and vice-versa)

- when a substate is active, all its ancestor states are necessarily
also active

- thus, if the state machine "is in" s0, it is also necessarily "in" s

- a transition from s0 to t0 necessarily exits s0 and s (they both
become inactive) and enters t and t0 (which both become active)

 

A straightforward answer to those questions will go a long way towards
helping us to implement some aspects of the SCXML spec and towards
designing proper state machines using SCXML.

 

Thanks in advance,

Andrew

 

Andrew Emmons 
Architect and Manager, SVG Technologies
Co-Chair, W3C SVG Working Group
Quickoffice, BitFlash Division
Web: www.bitflash.com <http://www.bitflash.com/> , 

www.quickoffice.com