Over the years, I've grown to like the First Call? primitive on the Synchronization palette. It's a simple, useful tool if you want to run a subVI, or a section of a block diagram in a case structure, only once when the VI first starts up. I find it particularly useful for initializing the behavior of modules that I've implemented as functional globals. Recently, though, I found that several of the modules and applications I've been working on needed the ability to do what I call a "soft restart." That is, they need to be able to be returned almost completely to their start-up conditions, without completely stopping and restarting the applications. This is particularly true for my RT applications, which, lacking a GUI, can't easily and directly be stopped and restarted. Herein lies the quandary: First Call? returns true only once, but to perform a soft restart we need the ability to effectively reset it so it that returns true again, on demand. The solution: Bob's handy-dandy Retriggerable First Call routines.
First Call Retriggerable.vi
Used in a manner similar to the First Call primitive, this reentrant VI will return a TRUE the first time that it is called. It also will obtain a reference to a named Notifier (and keep said reference on an uninitialized shift register) which is used to accept the retriggering notification. There is an accessory VI, called Retrigger First Call Nodes, which when called will cause all instances of this VI to again return TRUE the first time they are called after the Retrigger VI is invoked. In this manner, we can use this VI for initialization functions, while retaining the ability to restart processes if necessary.
Retrigger First Call Nodes.vi
Pretty straight-forward: obtain the Notifier reference on first call, keep it on a shift register, and send notification every time the VI is called. Unless, of course, an error is passed in, in which case do nothing.
For completeness and cleanliness, I suppose there should also be a third component to this utility: a clean-up routine to destroy the notifier reference. Also, looking again a little more critically at the Retrigger First Call Nodes.vi, we could make the diagram a tad simpler. What we really want is to get rid of the error-handling case structure, and instead wire the error in and out cluster through the Send Notification. As my professors in college used to say, "I will leave this as an exercise for the interested reader."
LabVIEW 2016 and NI Technology Preview
8 months ago