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paul_cardinale

XControl with Latching Action

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Have you ever wanted to create an XControl that has a Latching mechanical action?  For a long time I thought it was impossible because there's no event that occurs when the block diagram terminal is read.  But it can be done by launching a background task that monitors the value in the wire connected to the terminal.  See attachment.

Latching XControl.zip

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Nifty trick!
I'm impressed with the ingenuity. But two minor caveats...

Warning 1: This XControl allows local variables and Value property nodes. LabVIEW prevents the built-in latching Boolean controls from having either a local variable of the latched Boolean (compile time error) or a Value property node (run time error). Those things would mess with latching, and will interfere with this code executing correctly. You should not use either of those things with a latching XControl.

Warning 2: This XControl uses an unpublished private method of scripting that is known to have problems with thread synchronization. It was created to handle some very specific editor operations. It is not a safe thing to use generally, which is why it has never been made public, because of inplaceness optimizations that make multiple wires share the same memory.

I think the use in this case is safe because there isn't any optimization that I know of that will make LabVIEW avoid the data copy if a downstream node modifies the value. For example:

image.png.f0c5d4f40c713a8e68c7c66e975ab273.png

The above image shows the Buffer Allocation dots... and we can see that the Not copies the Bool, even when debugging is turned off. I will ask my team if there are any scenarios where inplaceness will elide that dot. I don't think there can be (because

So it seems to be ok here, but I would not advise freely using that method in code generally.

 

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