drjdpowell

My first thought is that a listbox is just a UI element for display purposes, and your real classes are “Users” and “Test Info”, which don’t had any obvious reason to inherit from the same parent class. Design them that way, and include a method in each called “Listbox Entries” that returns the necessary info to fill a listbox.

There are a few LabVIEW examples that might help, including “DateServerUsingStartAsynchronousCall.vi” that shows how to pass off new connections to a dynamically launched VI as ned mentioned.

I had wondered about that, once I understood interfaces to have strict rules about private data, as one can’t make an interface the descendant of any class that has any private data. Oh, it was just an idea I threw together in response to this conversation. Anyway, criticism is better than apathy.

Thanks Daklu, that helps me understand “Interfaces” better. I hadn’t appreciated the fact that interfaces should have no private data themselves, and I see now how your implementation can work with by-value objects (I suggest adding a by-value example if you ever have the time). My design above was motivated by what I perceived as a desire to create some kind of combined object that can simultaneously belong to two independent inheritance trees (complete with the private data of both trees).

Depends how many default values you have. If you’re carrying along a dozen constants you’ll have complex “Create” methods and generic probes which are well nigh unreadable. Note that a custom probe can call the classes methods in order to display the values. I usually put a method called “Text description” in any class hierarchy that gives a clear-english description of only the important information of the object; this makes making custom probes or other debug or logging tools easier. Yes. And that method will work with parameters that don’t have non-physical values (like −1 wheels).

As a different take on it, if “Number of Wheels” is a constant of the class (2 for Motorbike, etc.) then it shouldn’t be a “data” item at all. It should be an overridable method that returns the constant value: Car.lvclass would have a similar override with the constant 4. Use this method wherever you need the number of wheels. — James Added later: if the number of wheels isn’t a constant, you can still have different defaults. Make the default number of wheels “-1”, and have the methods return the default value for the class only if the data item is “-1”. That way you can change the value if you want, but the default exists without needing an “Init” method.

Hi Daklu, What do you think of my collection-of-objects-that-can-switch-between-type-identities idea posted above?

There was a big discussion about this on the 2012 Beta forum at NI.com, with a very strong recommendation from most beta testers that conditional indexing behave like regular indexing. I guess it was too late to make the change for 2012.

OK, but why do you you call the Init.vi in a loop over an array of objects? Why not take the individual objects, call Init on them, then call the Parameter setting VI, and only then put them in an array. Like this (modifying your “old style” example): Here “Init.vi” can be dynamically dispatched, while “Write String.vi” can be a unique method for each child class. You could even branch the wire and reuse the preinitialized object with different parameters. Basically I don’t understand the reasoning behind your initial example, where you put the objects in a array to call Init in a loop, then strip them all out again to treat them individually, then put them all back in an array again. Seems pointlessly over complicated. — James (sorry for the late reply, I’ve been on holiday without internet access)

If you’d like to stay by-value and avoid the DVRs, you can instead keep your objects in an array, and store the object’s array index in the variant attribute look up tables.

Why are you using a dynamically-dispatched “Init” method at all? Why not just have a “Create <commandname>” specific method in each command class that initializes it (with specific parameter inputs) and then send it off to be executed?

He’s using a Property Node: Right-click>>Create>>Property Node>>Value Should exist in 8.0. You could also use a local variable (Right-click>>Create>>Local Variable)

Other than for TCP and UDP, how many different “Is Valid” methods can you possibly need? Note that you can make a method “protected”, which would allow your base class to call child-class methods without making the methods public. Also note this conversation and consider if you really need an “Is Valid” method.

I wish it were easier to change namespacing once code is used in multiple projects. Or if it were possible to define a “display name” and a separate unique identifier that one could leave unchanged. So my “Actor” class could secretly be “47D44584FGHT” or whatever. No collisions then.

The actual parent-class data of the object. An object of a child class is a collection of clusters, one for each level of class hierarchy. You can call any parent-class method on any child-class object. Note that I am being careful to call it a “child-class object”, not a “child object”, as the whole “parent/child” metaphor really only applies to classes. No actual object is a “child” of any other object.

Seven months later and I’m working on getting my reuse messaging code in a VIPM package. And another issue occurs to me. I now have most of my classes outside of any lvlib library, which prevents unnecessary loading. However, that means I have rather generic class names like “ErrorMessage”, “Messenger”, and “Actor” with no further namespacing that I could easily imagine another package using. What do other people do about this potential conflict? — James

In the end I decided to add a “Text Encoding” property to the "SQL Statement” class, with choices of UTF-8, system (converted to UTF-8 with the primitives Shaun linked to), and UTF-16. System is the default choice. I also added the system-to-UTF-8 conversion primitives on all things like SQL text or database names (thanks Shaun). I also used the sqlite3_errmsg text to give more useful errors (thanks Matt).

Are you being sure to throw away any bits > 25 when the encoder rolls over? The code below should give the correct U25 difference even though it’s using U32’s: Added later: actually, the fact that the “random values” alternate with the good ones suggests some other problem, unless you are reading the encoder about two times a revolution.

The heartbeat can be built in to your reusable TCP component (I’ve considered adding it to my background processes running the TCP connection). You don’t have to make them part of the actor code itself.

I was a reaching a bit with the “any actor” thing. Though my actors tend to be a quite “server” like already, much more than your “SlaveLoops” or the Actor Framework “Actors”. They publish information and/or reply to incoming messages, and don’t have an “output” or “caller” queue or any hardwired 1:1 relationship with an actor at a higher level. They serve clients. The higher-level code that launches them is usually the main, and often only, client, but this isn’t hardcoded in. Thus, they are much more suitable to be sitting behind a TCP server. — James

Well, my current TCPMessenger is set up as a TCP Server, and is really only suitable for an Actor that itself acts as a server: an independent process that waits for Clients to connect. A break in the connection throws an error in the client, but the server continues waiting for new connections. This is the behavior I have in the past needed, where I had Real-Time controllers that must continue operating even when the UI computer goes down. However, most of my actors (in non-network, single-app use) are intended to shutdown if their launching code/actor shutdown for any reason. For this I have the communication reference (queue, mostly) be created in the caller, so it goes invalid if the caller quits, which triggers shutdown as in your case 1. Case 2 doesn’t apply in my system as there is no output queue. Now, if I wanted auto-shutdown behavior in a remote actor, then I would probably need to make a different type of TCPMessenger that worked more like Network Streams than a TCP server. So a break in the connection is an error on both ends, and the remote actor is triggered to shutdown. — James

That’s the route I took in adding TCP message communication to my system, with “loop D” being a reusable component dynamically launched in the background. Incoming TCP messages are placed on a local queue, which can also have local messages placed on it directly. Because “TCPMessenger” is a child of “QueueMessenger”, I can substitute it into any pre-existing component that uses QueueMessenger, making that component available on the network without modifying it internally. — James

I use variants mostly for simple values; to avoid having 101 different simple message types. I have four or five polymorphic VIs for simple messages, and having so many different message types would be unmanageable. But for any complex message, I usually have a custom message class. It’s no more complex to make a new message class then make a typedef cluster to go inside a variant message. — James I’m hoping to work up to testing my object messages on an sbRIO next week. I will be sure to run a memory-leak test.

My limited testing was mostly functional (and over a slow-uploading home broadband connection) so I can’t answer for throughput. My messages vary in size, but common small ones have at least two objects in them (the message itself and a “reply address” in the message) and so flatten into quite a long string: a “Hello World” message is 75 bytes. I wrote a custom flattening function for the more common classes of message which greatly reduced that size. Also, for larger messages that can involve several objects I found that ZLIB compression (OpenG) works quite well, due to all the repeated characters like “.lvclass” and long strings of mostly zeroes in the class version info. I use “Flatten to String”, which works great communicating between LabVIEW instances. If you need one end to be non-LabVIEW then you’ll want something like XML or JSON. — James

I believe AMC uses UDP, instead of TCP, for network communication. Depending on what you are doing, you might require the lossless nature of TCP. You should also have a look at ShaunR’s “Transport” and “Dispatcher” packages in the Code Repository; they do TCP. I’ve done some (limited) testing of sending objects over the network (not Lapdog, but very similar), and the only concern I had was the somewhat verbose nature of flattened objects. I used the OpenG Zip tools to compress my larger messages and that worked quite well.