For the very first post on this blog, I presented a modest proposal for an alternative implementation of the LabVIEW version of the producer/consumer design pattern. I also said that we would be back to talk about it a bit more — and here we are!
The point of the original post was to present the modified design pattern in a form similar to that used for the version of the pattern that ships with LabVIEW. The problem is that while it demonstrates the interaction between the two loops, the structure cannot be expanded very far before you start running into obstacles. For example, it’s not uncommon to have the producer and consumer loops in separate VIs. Likewise, as with a person I recently dealt with on the forums, you might want to have more than one producer loop passing data to the same consumer. In either case, explicitly passing around a reference complicates matters because you have to come up with ways for all the separate processes to get the references they need.
The way around this conundrum lies in the concept of information hiding and the related process of encapsulation.
The idea behind information hiding is that you want to hide from a function any information that it doesn’t need to do its job. Hiding information in this sense makes code more robust because what a routine doesn’t know about it can’t break. Encapsulation is an excellent way if implementing information hiding.
In the case of our design pattern, the information that is complicating things is the detailed logic of how the user event is implemented, and the resulting event reference. What we need is a way to hide the event logic, while retaining the functionality. We can accomplish this goal by encapsulating the data passing logic in a set of VIs that hide the messy details about how they do their job.
Standardizing User-Defined Events
The remainder of this post will present a technique that I have used for several years to implement UDEs. The code is not particularly difficult to build, but if you are a registered subscriber the code can be downloaded from the site’s Subversion SCC server.
The first thing we need to do is think a bit and come up with a list of things that a calling program would legitimately need to do with an event — and it’s actually a pretty short list.
- Register to Receive the Event
- Generate the Event
- Destroy the Event When the Process Using it Stops
This list tells us what VIs the calling VI will need. However, there are a couple more objects that those VIs will be needed internally. One is a VI that will generate and store the event reference, the other is a type definition defining the event data.
Finally, if we are going to be creating 4 VIs and a typedef for each event in a project, we are going to need some way of keeping things organized. So let’s define a few conventions for ourselves.
Convention Number 1
To make it easy to identify what event VI performs a given function, let’s standardize the names. Thus, any VI that creates an event registration will be called Register for Event.vi. The other two event interface VI will, likewise, have simple, descriptive names: Generate Event.vi and Destroy Event.vi. Finally, the VI that gets the event reference for the interface VIs, shall be called Get Event Reference.vi and the typedef that defines the event data will be Event Data.ctl.
But doesn’t LabVIEW require unique VI names? Yes, you are quite right. LabVIEW does indeed require unique VI names. So you can’t have a dozen VIs all named Generate Event.vi. Thus we define:
Convention Number 2
All 5 files associated with an event shall be associated with a LabVIEW library that is named the same as the event. This action solves the VI Name problem because LabVIEW creates a fully-qualified VI name by concatenating the library name and the VI file name. For example, the name of the VI that generates the Pass Data event would have the name:
Pass Data.lvlib:Generate Event.vi
While the VI Name of the VI that generates the Stop Application event would be:
Stop Application.lvlib:Generate Event.vi
The result also reads pretty nice. Though, it still doesn’t help the OS which will not allow two files with the same name to coexist in the same directory. So we need:
Convention Number 3
The event library file, as well as the 5 files associated with the event, will reside in a directory with the same name as the event — but without the lvlib file extension. Hence Pass Data.lvlib, and the 5 files associated with it would reside in the Pass Data directory, while Stop Application.lvlib and its 5 files would be found in the directory Stop Application.
So do you have to follow these conventions? No of course not, but as conventions go, they make a lot of sense logically. So why not just use them and save your creative energies for other things…
The UDE Files
So now that we have places for our event VIs to be saved, and we don’t have to worry about what to name them, what do the VIs themselves look like? As I mentioned before, you can grab a working copy from our Subversion SCC server. The repository resides at:
To get started, you can simply click on the link and the Subversion web client will let you grab copies of the necessary files. You’ll notice that when you get to the SCC directory, it only has two files in it: UDE.zip and readme.pdf. The reason for the zip file is that I am going to be using the files inside the archive as templates and don’t want to get them accidentally linked to a project. The readme file explains how to use the templates, and you should go through that material on your own. What I want to cover here is how the templates work.
Get Event Reference.vi
This VI’s purpose is to create, and store for reuse, a reference to the event we are creating. Given that description, you shouldn’t be too surprised to see that it is built around the structure of a functional global variable, or FGV. However, instead of using an input from the caller to determine whether it needs to create a reference, it tests the reference in the shift-register and if it is invalid, creates a reference. If the reference is valid, it passes out the one already in the shift-register.
If you consider the constant that is defining the event datatype, you observe two things. First, you’ll see that it is a scalar variant. For events that essentially operate like triggers and so don’t have any real data to pass, this configuration works fine. Second, there is a little black rectangle in the corner of the constant indicating that it is a typedef (Event Data.ctl). This designation is important because it significantly simplifies code modification.
If the constant were not a typedef, the datatype of the event reference would be a scalar variant and any change to it would mean that the output indicator would have to be recreated. However, with the constant as a typedef, the datatype of the event is the type definition. Consequently you can modify the typedef any way you want and every place the event reference is used will automatically track the change.
Register for Event.vi
This VI is used wherever a VI needs to be able to respond to the associated event. Due to the way events operate, multiple VIs can, and often will, register to receive the same event. As you look at the template block diagram, however, you’ll something is missing: the registration output. The reason for this omission lies in how LabVIEW names events.
When LabVIEW creates an event reference it naturally needs to generate a name for the event. This name is used in event structures to identify the specific particular event handler that will be responding to an event. To obtain the name that it will use, LabVIEW looks for a label associated with the event reference wire. In this case, the event reference is coming from a subVI, so LabVIEW uses the label of the subVI indicator as the event name. Unfortunately, if the name of this indicator changes after the registration reference indictor is created, the name change does not get propagated. Consequently, this indicator can only be created after you have renamed the output of the Get Event Reference.vi subVI to the name that you wish the event to have.
The event naming process doesn’t stop with the event reference name. The label given to the registration reference can also become important. If you bundle multiple references together before connecting them to an event structure’s input dynamic event terminal, the registration indicator is added to the front of the event name. This fact has two implications:
- You should keep the labels short
- You can use these labels to further identify the event
You could, for example, identify events that are just used as triggers with the label trig. Alternately, you could use this prefix to identify the subsystem that is generating the event like daq or gui.
The logic of this template is pretty straight-forward. The only noteworthy thing about it is that the event data (right now a simple variant) is a control on the front panel. I coded it this way to save a couple steps if I need to modify it for an event that is passing data. Changing the typedef will modify the front panel control, so all I have to do is attach it to a terminal on the connector pane.
Again, this is very simple code. Note that this VI only destroys the event reference. If it has been registered somewhere, that registration will need to be destroyed separately.
Putting it all Together
So how would all this fit into our design pattern? The instructions in the readme file give the step-by-step procedure, but here is the result.
As explained in the instructions, I intend to use this example as a testbed of sorts to demonstrate some things, so I also modified the event data to be a numeric, and changed the display into a chart. You’ll also notice that the wire carrying the event reference is no longer needed. With the two loops thus disconnected from each other logically, it would be child’s play to restructure this logic to have multiple producer loops, or to have the producer loop(s) and the consumer loop(s) in separate VIs.
By the way, there’s no reason you can’t have multiple consumer loops too. You might find a situation where, for example, the data is acquired once a second, but the consumer loops takes a second and a half to do the necessary processing. The solution? Multiple consumer loops.
However, there is still one teeny-weensy problem. If you now have an application that consists of several parallel processes running in memory, how do you get them all launched in the proper order?
Until next time…