LabVIEW has a number of features that will enable you to do exactly what you describe. One is the Event Structure -- this is something that just sits around, not using any CPU time, until "something interesting" happens, such as a Control changes, or a key is pressed, or the programmer just "raises a User Event flag". This runs inside a While "Do-Forever" loop, but as noted above, it simply waits until one or more of the Events you have designated fires.
The other piece of the puzzle is something called the Consumer/Producer Design. This consists of two While loops, running in parallel. One, the Producer, is the Event loop described above -- it sits idle until an Event happens, then "produces" a signal that is sent to the other, Consumer loop. One way to convey information from Producer to Consumer is via a Queue, a data structure that lets you put data in one end and have it come out the other. The nice thing about Queues is that they also have a "blocking" function -- if there is nothing in the Queue (because no Event has fired), then the Consumer loop trying to process the next element in the Queue will simply wait, again taking no CPU time.
What all of this means is that you can have a third loop doing something that needs to be constantly updated (like maybe updating a display) while the Consumer and Producer "talk to each other" in the background, only requesting computing cycles when (a) a Event occurs (Producer) or (b) there is something new to process (Consumer).
I realize this may be a slight overkill, but it really illustrates the power of LabVIEW -- the ability to do two or three things in parallel, the natural way "time" is allocated among processes, and the flexibility of the possible designs. To learn more, you should be able to Find Examples, particularly if you look for Consumer/Producer (or maybe Producer/Consumer) and Event Structure.
BS
