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Subscription to datachange in visual foxpro

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09 Nov 2021 15:02 #10343 by support
There are two ways to do it.

One is with "true" events. That is, you will write an event handler that gets called when there is a notification. That will probably require the use of the EVENTHANDLER() function. I have not done this yet because it is bit longer.

The second approach is with "event pull". That is, your code asks whether there is a new notification available. I have made an example for that, and it is further below.

Note that internally in QuickOPC, BOTH these approaches are equivalent - and they both use genuine OPC UA subscriptions (change-based). The event pull approach just maintains an additional queue and gives you a different way of accessing the same flow of data.

* This example shows how to subscribe to changes of a single monitored item, pull events, and display each change.
 
DECLARE INTEGER GetTickCount IN kernel32 
 
THISFORM.OutputEdit.Value = ""
 
* Instantiate the client object.
oClient = CREATEOBJECT("OpcLabs.EasyOpc.UA.EasyUAClient")
 
* In order to use event pull, you must set a non-zero queue capacity upfront.
oClient.PullDataChangeNotificationQueueCapacity = 1000
 
THISFORM.OutputEdit.Value = THISFORM.OutputEdit.Value + "Subscribing..." + CHR(13) + CHR(10)
oClient.SubscribeDataChange("http://opcua.demo-this.com:51211/UA/SampleServer", "nsu=http://test.org/UA/Data/ ;i=10853", 1000)
 
THISFORM.OutputEdit.Value = THISFORM.OutputEdit.Value + "Processing data change notification events for 1 minute..." + CHR(13) + CHR(10)
 
EndTick = GetTickCount() + 60000
DO WHILE GetTickCount() < EndTick
    oEventArgs = oClient.PullDataChangeNotification(2*1000)
    IF NOT ISNULL(oEventArgs)
		THISFORM.OutputEdit.Value = THISFORM.OutputEdit.Value + oEventArgs.DisplayString + CHR(13) + CHR(10)
    ENDIF
ENDDO
 
THISFORM.OutputEdit.Value = THISFORM.OutputEdit.Value + "Unsubscribing..." + CHR(13) + CHR(10)
oClient.UnsubscribeAllMonitoredItems()
 
THISFORM.OutputEdit.Value = THISFORM.OutputEdit.Value + "Finished." + CHR(13) + CHR(10)

If you need the other approach (event handlers), let me know.

Best regards
The following user(s) said Thank You: info@software2000.it

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09 Nov 2021 09:27 #10341 by support
Hello,

I will try to write such an example. Please allow some time for this. I will post here again when I have an update.

Best regards

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08 Nov 2021 16:53 #10340 by info@software2000.it
Can anyone post me some code snippet to be able to make an OPC-UA subscription with visual foxpro to be able to receive an event that indicates that a value of a tag on a particular node has changed?
I only found examples for reading a single value or reading multiple values ​​but no examples for subscribing and receiving updated values.
Just a small snippet to subscribe and show a message with the vsalue changed.
A thousand thanks

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