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<p class=MsoNormal>Hi all,<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>I’m still getting my ‘sea legs’ with
Twisted, so this may be a novice question, but I’m wondering what would
be a good way to create a state machine with Twisted? For instance I’ve
written quite a bit of code in different languages that creates co-operative,
context switching schemes using state machines. Here would be a primative example
in psuedocode:<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>Switch(state):<o:p></o:p></p>
<p class=MsoNormal> Case
INIT:<o:p></o:p></p>
<p class=MsoNormal> Initialize()<o:p></o:p></p>
<p class=MsoNormal> state
= CMD1<o:p></o:p></p>
<p class=MsoNormal> Case
CMD1:<o:p></o:p></p>
<p class=MsoNormal> CommandOne()<o:p></o:p></p>
<p class=MsoNormal> state
= WAITCMD1<o:p></o:p></p>
<p class=MsoNormal> Case
WAITCMD1:<o:p></o:p></p>
<p class=MsoNormal> If
CommandOneDone()<o:p></o:p></p>
<p class=MsoNormal> state
= DONE<o:p></o:p></p>
<p class=MsoNormal> Else<o:p></o:p></p>
<p class=MsoNormal> state
= WAITCMD1<o:p></o:p></p>
<p class=MsoNormal> Case
DONE:<o:p></o:p></p>
<p class=MsoNormal> state
= EXIT<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>This state machine initializes a system, issues an
asynchronous command that takes time to complete (say a motor move command) and
then waits for that command to be done before exiting. In the context of a
framework that is calling this state machine, the WAITCMD1 is executed
repeatedly (polling) while the asynchronous command completes. A system can be
constructed with lots of little state machines like this and be made to look
like it is doing many things at once even though the execution of the program
is single threaded.<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>I understand (pretty much) the Twisted framework is like
this and implmenting event handlers like connectionMade(), etc., is a state
machine, but I’m wondering how to implement what I’ve outlined
above in one of Twisted’s state event handlers, or inside something like
callLater(). For example, let’s say I want to use Twisted to run a long
running daemon process that has an XMLRPC interface. That XMLRPC interface is
an interface to a state machine inside Twisted that allows the caller to change
state, or start new state machines, or get the status of a running state
machine. In that case I’m thinking I want a state machine the runs
continuously in the Twisted loop, structured like the example above;
co-operatively yielding back to Twisted, but running non-stop. Something like
callLater(1, stateMachine), but non-stop, without even the 1 second call loop.<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>Is this something I can do with callLater()? Would using a
deferred be a better option? I see deferred’s callBack() system as a
connecting states together, but I’m not sure how a WAITONCMD1 kind of
thing would work, where the deferred callBack(d, waitTillDone) would have to
keep coming back to itself till some condition is met?<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>Thanks in advance for your help,<o:p></o:p></p>
<p class=MsoNormal>Doug<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
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