Jump to content

Numeric 'Value Change' event still triggers when coerced minimum and maximum are defined

dhakkan

By dhakkan,
in User Interface

Followers 1
Go to solution Solved by hooovahh,

Recommended Posts

dhakkan

dhakkan 3

Posted
Posted

Hello,

 

I searched within this site and via Google for any related queries. Couldn't find any. Hence this post...

 

Example of Situation:

I configured an I8 Numeric with non-default limits in its 'Properties >> Data Entry' window...

Minimum of -2 (coerced); Maximum of 2 (coerced). I set up a simple event that looks for 'Value Change' of this Numeric.

 

Expectation:

No matter what the 'Increment' value and 'Response to value outside limits' are set to, the 'Value Change' must NOT trigger if the numeric is already at the limit and cannot be changed further. To explain, If current value of Numeric is 2 and I try to increment it, its value must remain at 2 and no event should be triggered. Similarly, with current value of Numeric at -2, if I try to decrement it, its value must remain at -2 and no event should be triggered.

 

What I found was that the value does remain at the limit, but an event is still triggered when the 'Increment' setting is non-zero. Is this normal behavior?

 

Test Numeric Event Triggering.vi coded in LV2014

Link to post
  • Solution
hooovahh

hooovahh 768

Posted
  • Solution
Posted

What I found was that the value does remain at the limit, but an event is still triggered when the 'Increment' setting is non-zero. Is this normal behavior?

Yes, a value change event does not necessarily mean a different value than the previous one.  You can use the Value (Sig) property to set a control to the current value and the event is still triggered, same with this.

 

The solution seems like an easy one.  If the old value, is equal to the new value, do nothing.  The old and new values are available in the event that is triggered.

Link to post
dhakkan

dhakkan 3

Posted
  • Author
Posted

Yes, a value change event does not necessarily mean a different value than the previous one.  You can use the Value (Sig) property to set a control to the current value and the event is still triggered, same with this.

 

The solution seems like an easy one.  If the old value, is equal to the new value, do nothing.  The old and new values are available in the event that is triggered.

 

Thanks for the response, Hoovahh. Understood and agreed regarding the 'Val (Sig)' property. In this case, the LV Developer is forcing LV to trigger an event.

 

However, my expectation from the Front Panel standpoint was for LabVIEW to not even trigger the event, i.e. LV checks for value actually changing before deciding to generate the event internally.

 

IMHO, if one is using 'value change' events, the current implementation pretty much defeats the purpose of the minimum and maximum limits set in the Numeric's properties - depending on data validation requirements, the Developer would be forced to perform a check in the event structure to discard further action.

 

To give you a background, my thought process for this started when I realized some subVI was coercing numeric values coming in even though no 'in range and coerce' equivalent functionality was implemented in its block diagram. That led me to realize that it's obviously not a good practice to validate inputs' range via front panel properties, unless some in-code documentation clearly states so. More importantly, as several sources rightly state, coercing values without alerting the user is not a good practice. Then, I wanted to see if this coercion was OK for front panels meant to be direct user interfaces.

 

Ummm, I guess I refuted my own statement... with the event triggered even when there is no value change, it would after all be good programming to alert the user that a 'limit' has been reached and offer alternatives (instead of 'discarding' further action as I mis-stated above).

 

Thanks! for the mini-discussion.  :)

Link to post
Rolf Kalbermatter

Rolf Kalbermatter 505

Posted
Posted

Thanks for the response, Hoovahh. Understood and agreed regarding the 'Val (Sig)' property. In this case, the LV Developer is forcing LV to trigger an event.

 

However, my expectation from the Front Panel standpoint was for LabVIEW to not even trigger the event, i.e. LV checks for value actually changing before deciding to generate the event internally.

 

IMHO, if one is using 'value change' events, the current implementation pretty much defeats the purpose of the minimum and maximum limits set in the Numeric's properties - depending on data validation requirements, the Developer would be forced to perform a check in the event structure to discard further action.

 

To give you a background, my thought process for this started when I realized some subVI was coercing numeric values coming in even though no 'in range and coerce' equivalent functionality was implemented in its block diagram. That led me to realize that it's obviously not a good practice to validate inputs' range via front panel properties, unless some in-code documentation clearly states so. More importantly, as several sources rightly state, coercing values without alerting the user is not a good practice. Then, I wanted to see if this coercion was OK for front panels meant to be direct user interfaces.

 

Ummm, I guess I refuted my own statement... with the event triggered even when there is no value change, it would after all be good programming to alert the user that a 'limit' has been reached and offer alternatives (instead of 'discarding' further action as I mis-stated above).

 

Thanks! for the mini-discussion.  :)

 

I'm not really understanding what you are saying. For one, the case that UI controls configured to coerce their values would do the coercion also when the VI is used as subVI: that was true in LabVIEW 3.0 and maybe 4.0 but was removed after that because it was considered indeed a bad idea. I should say that quite a few people got upset at this back then :D, but you seem to agree that this is not desirable.

 

As to using the fact that the Value Change event does not get triggered to allow you to alert a user that there was a coercion??? Sorry but I'm not following you here at all. How can you use an event that does NOT occur, to trigger any action?? That sounds so involved and out of our space-time continium that my limited brain capacity can't comprehend it.  :P

 

But you can't use the fact that the value is the same to mean that a limit has been reached. The user is free to go to the control, type in exactly the same value as is seen already, and hit enter or click on something else in the panel and the Value Change event will be triggered. Value Changed simply doesn't mean that there is a different value, only that the user did something with the control to change the control to the same or a different value. Sounds involved I know but there have been many discussions both in the LabVIEW devlopment team as well as in many other companies who design UI widget libraries and they generally all agree that you want to trigger on user interaction with the control for maximum functionality and leave the details about if an equal value should mean something or not to the actual implementor. The name for the event may indeed be somewhat misleading here. In LabWindows CVI NI used the VALUE_COMMIT term for the same event. However I suppose the word "commit" was considered to technical for use in LabVIEW.

Link to post
dhakkan

dhakkan 3

Posted
  • Author
Posted

Hello Rolf,

 

... As to using the fact that the Value Change event does not get triggered to allow you to alert a user that there was a coercion??? ...

 

Not sure where I implied that... Sorry, if the write-up was not clear. I was of a different opinion when I started writing that post as compared to when I finished it a few minutes later.  :rolleyes:

 
Should I retract or re-write that post?

 

In any case, the rest of your post indicates we are on the same page.

 

Yes, the event name could be considered misleading. Variations of VALUE_ENTER or VALUE_SUBMIT may have helped. But, that's up to the NI gurus, isn't it.

Link to post
Rolf Kalbermatter

Rolf Kalbermatter 505

Posted
Posted

Hello Rolf,

 

Not sure where I implied that... Sorry, if the write-up was not clear. I was of a different opinion when I started writing that post as compared to when I finished it a few minutes later.  :rolleyes:

 

Yeah after rereading it a few more times I got the feeling that something like that had happened. :D

 

 

Should I retract or re-write that post?

 

I think it's not worth the effort. Things are pretty clear now and editing posts substantially after the fact is something that is generally not considered helpful nor correct. We all write sometimes things that turn out later to be badly written. For myself I usually limit editing for posts to fixing typos and adding an occasional extra information that seems useful.

Link to post
dhakkan

dhakkan 3

Posted
  • Author
Posted

...

I think it's not worth the effort. Things are pretty clear now and editing posts substantially after the fact is something that is generally not considered helpful nor correct. We all write sometimes things that turn out later to be badly written. For myself I usually limit editing for posts to fixing typos and adding an occasional extra information that seems useful.

 

Gotcha. Thanks for the tips too! (I'm pretty new to posting on discussion forums in general, let alone LAVA.)

Link to post

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

×
Followers 1
Go to topic listing

  • Similar Content

    • Mode_Locked
      How do I de-register a Windows WMI Event watcher before timeout in LV?
      By Mode_Locked
      My code watches for an event where my Toshiba USB stick is unplugged. If the event does not occur within 30 secs, the code times out. However, in the case that there is no disconnection of the USB stick (no event generated), I can't end the program before the timeout period of 30 sec. Is there a way to end the program before the timeout period? Any help is appreciated. Thanks in advance!
      WMI_USBStorage_Event_withDeviceIDandTimeOut.vi
    • _Mike_
      IMAQdx session "Attribute updated" event
      By _Mike_
      Hello!
      I am running a system with GigE camera with NI framegrabber  in continuous acquisition mode - in which I change some attributes every now and then.
      So far I have been using following scheme to assert I am getting correct image after attribute change:
      change attribute read "Last buffer number" property -> n run IMAQdx get image.vi set to "buffer' and parameter n+2 (+2 being due to the fact that i assumed attribute change arrives to camere during running exposure that will be unaffected by attribute change - hence i needed to discard "next" buffer) Unfortunately I still every now and then acquired image that obviously was aquired with previous attributes (e.g. I've dramatically increased.decreased exposure rate, while acquired image was very similar to previously acquired one). Guessing that it may have something to do with buffers I have increased "magic number" from +2 to +3. It indeed helped, but after longer usage I have discovered, that it only reduced the frequency of the error.
      Hence I needed to design more "bulletproof" solution that will satisfy my timing requirements (stoping and starting acquisition is out of question as it takes more than 100ms what is unacceptable for me)
      I would like to:
      change attribute acquire information from camera that will allow me to fetch image acquired with changed attribute For such purpose I have discovered IMAQdx events - especially "Attribute updated" and "frameDone". Unfortunately, I could not find any detailed documentation about those functions. Therefore I would like to ask you for help to determine when 'Attribute updated" is triggered. Is it when:
      driver receives the command and pushes it to camera? (pretty useless for me as I cannot relate it to any particular exposure) camera confirms getting it (then assuming it arrives during an ongoing exposure, I'll discard "next' image and expect second next to be correct camera confirms applying it (then I assume that next image should be obtained with correct settings) camera confirms it managed to acquire a frame with new parameter (pretty unlikely scenario - but the i'd expect correct image to reside in "last" buffer) Could you help me determine which case is true? Also should I be concerned about the fact that there may be a frame "in transition" between camera and IMAQdx driver which will desynchronize my efforts here?
       
    • takanoha
      Event structure and case structure in the same loop
      By takanoha
      Hi
      I am writing an application to update the numeric value given user's input value and depending on the user's input value the program checks if it is greater than 10 if it is greater than 10 then the program waits for 1 second and then will have a popup message says "true".
      My intention was to call the conditional check, printing true every one second if the user's input value is greater than 10; in other words, the case structure was to be called regardless of the event structure in the same loop infinitely.
      But it doesn't seem to work the way I expected. Unless there is user's new input value, getting into the event structure, the program doesn't get to the case structure even though the case structure is in a loop.
      Is there any way to call the case structure not dependent to the event structure but I want to use a shift register for the numerical value and also having an event structure and a case structure in parallel ? 
      Thank you.
       
       


      test.vi
    • Manudelavega
      Bad behavior with tree control
      By Manudelavega
      I have been trying to understand what's going on with my tree control and I'd like to share this with the community to see if somebody has seen this behavior before. Maybe there is something obvious that I am not seeing...
       
      I attached the VI (LV2011) with instructions inside (will take you 20s to perform the experiment, I dismantled the VI to its bare minimum). Please don't comment on the overall architecture (state machine...) since I modified the VI just for the sake of posting it here and removed all advanced features.
       
      Long story short, after programmatically modifying the value of the tree, I modify its value a second time by clicking directly on one of the tree items. I then expect the NewVal value given through the left node of the value change event case to match the value of the tree terminal. Depending on the code that first modified the value programmatically, sometimes this expectation is met and sometimes it isn't. Run the code, it's easier to understand that way...
       
      Thanks for your feedback!
      Tree value issue.vi
    • Roryriffic
      Catch Session Timeout Events in LV Web Service
      By Roryriffic
      Hi all,
       
      new to Lava, so (kindly) let me know if I screwed up my post
       
      I have a multi-user web application controlled by a LV web service.  I keep track of all the users through the web service > sessions VI's.  This is pretty cool and works really great. I can give each user different levels of authentication, force them to use SSL, track where they go and when throughout the website, etc.
       
      Now what I'd like to do is keep track of when their session expires.  Obviously I can provide a "Logout" link on the client-side which ultimately calls the "Destroy Session" VI, but I can't ensure that all clients will click the link to log out (e.g. how many times do you really sign out of gmail verse just close the browser?!)
       
      What's the best way to catch the session timeout event that is specified during "Create Session" VI?  It appears as if I need a "LV Web Service Request" object to even call the Destroy Session VI, which makes matters much more difficult it seems. Is this even possible?
       
      Thanks for the help
×
×
  • Create New...

Important Information

By using this site, you agree to our Terms of Use.

  I accept