Prevent a control from updating during user interaction

I have a slow (couple of kBauds) serial link to a device that I want to control from within LabView.
There's an event loop that takes care of processing the control changes. When a control changes, the thread acquires a semaphore to access the serial link, sends out the new value and releases the semaphore again. In another thread, the device is periodically polled for actual values. All that works fine.
The problem is that, due to the lag of the link (and polling and sending happening in two different threads), the newly set parameters are returned with a delay. So let's say, I have a slider from 0-1000 and I slide it down from 1000 towards 0. While I hold the mouse down on the slider, dragging it, the communication thread returns a value that is different from where the slider is at and therefore sets the slider to an outdated value. That makes the slider jitter and that's what I need to get rid of.
I think this is a common problem, yet I couldn't find a straightforward solution. Basically, what I was looking for was a property "isEditing" or "isDragging" which I could check and then not update the control when this is true, to prevent that race between events. Does something like that exist? How do you guys tackle such a problem?
Thanks,
z

zeeed wrote:
I have a slow (couple of kBauds) serial link to a device that I want to control from within LabView.
...and therefore sets the slider to an outdated value. That makes the slider jitter and that's what I need to get rid of.
I think this is a common problem, yet I couldn't find a straightforward solution. Basically, what I was looking for was a property "isEditing" or "isDragging" which I could check and then not update the control when this is true, to prevent that race between events. Does something like that exist? How do you guys tackle such a problem?
Thanks,
z
THe first part of this post got wiped out by the goffy forum editor. I do not have time to retype it....
applied when you get a new value from a queue or by polling the current value.
While changing...
THe control must act as control and must over-ride the widget value. The natural thought would be to put that code in the value change event... but how to do it such that the value does not get over-riden?
Put SOMETHING inbetween that can mange the value. You could go as far as to develop an QMH (Queued Messae handler) or similar inplace to handle the logic but I don't think we have to go that far. I would first concider developing an Action Engine to act as the traffic control you need.
Applying an Action Engine
i would use the AE to intercat with the widget and store it curent value. YOu will need an Init Action and a Close action as is normal but I would include two mtehods that would help you with the Over-ride.
Get Value - This action will read (using serial query) the current value of the widget and return the value to the caller. You could also have a "Read' or similar that would retireve the value from a Shift Register inside the AE. This action would be called regularly in your T.O. event and will keep the contro lupdated as to the widget value.
Set Value - Would be the key action that can help you out. THe details depend on the widget but in general the Action would acept the new over-ride value and use it to send the comand to the widget to set the new value AND will query the widget to ensure the widget took the seeting or reached that value. After the widget is done, the new value is cached in a SR in the AE so that the latter checks by the TO event give you the value you set.
If you read the Nugget i linked you will see how an Action Engine blocks calls while it is running. By putting the code to set the widget value in the AE, you can ensure the widget went to the new value before any update code can read an old value.
The above is just one idea and there are others. I would think other solutions would all involve putting some type of traffic control inplace so the AE is a natural solution.
Note:
I have used similar approaches in the past to enforce safety logic to ensure the automatic PID updates do not over-ride a safety-shutdwon scenario.
I hope this helped more than it hurt,
Ben
Ben Rayner
I am currently active on.. MainStream Preppers
Rayner's Ridge is under construction

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Message Edited by pincpanter on 03-09-2009 03:39 PM
Paolo
LV 7.0, 7.1, 8.0.1, 2011
Solved!
Go to Solution.

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Attachments:
Provablocco.vi ‏23 KB

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