How can I create a subVI that has state?

Question

I'm trying to simplify a VI that I inherited and I noticed a recurring pattern that IMHO should be a subVI.  The pattern is that we have a digital output line that is controlled by a timer.  Each output has a start time for it to activate and a duration after which it is turned off.  When the state changes, a single line entry is added to a common log file and an indicator shared variable is set so that the remote host VI updates.  We have a big while loop that checks the current time against each of the activation conditions.  To detect when the state has changed, we're using a shift register (for each actuator).  It's one thing to see the same pattern 3 times, but it's worse -- we want to sometimes use a different number of actuators.  If I was writing this in C++ (with which I'm far more comfortable than LabView), I'd write a class for the actuator and instantiate it N times.  The class would have member data to keep track of its state.  I can create a subVI that does the meat of the design pattern, but I can't figure out how to store the previous state in the subVI without requiring it to be passed in.  How do I keep the state and behavior of a VI together?  Surely there's a way to do this because otherwise I can't see how you could implement state-ful algorithms like, say, a PID controller.  But I'm at a loss to figure it out ...
Thanks.
Keith.

Answer 1

To save state in a subVI and not use that state in other subVI calls, make the subVI reentrant.  When a subVI that is reentrant is called it creates a unique memory space for that subVI.  This is called a clone.  That memory space is where state is held for that subVI.  Keep in mind that it also means that the memory footprint is going to be larger.
To set a subVI to be reentrant, open it and go to File -> VI Properties and select execution.  It will be a choice on the bottom left.
Hope that this helps,
Bob Young
Bob Young - Test Engineer - Lapsed Certified LabVIEW Developer
DISTek Integration, Inc. - NI Alliance Member
mailto:[email protected]