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EasyUAClient.SubscribeDataChange() behaviour

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support
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25 Sep 2019 13:41 #7758 by support

Replied by support on topic EasyUAClient.SubscribeDataChange() behaviour

Hello,

QuickOPC simply delivers you the notifications (data changes) that the server provided. And, the server is providing an initial data change when a subscription is made - it actually *has* to do it, according to OPC UA specifications. This, among other reasons, allows to use subscriptions without having to issue an additional initial "Read" (which, besides being highly impractical, would cause unsolvable race conditions).

If you need to act upon a transition from "false" to "true", you need to add a logic to your event handler and do it yourself. Also beware of error conditions (e.g. after a server disconnection) to make the logic right.

Best regards

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Giuseppe12
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25 Sep 2019 13:11 - 25 Sep 2019 13:17 #7757 by Giuseppe12

EasyUAClient.SubscribeDataChange() behaviour was created by Giuseppe12

testing EasyUAClient.SubscribeDataChange() with dataChangeCallback, the event raise also on startup program

_client.SubscribeDataChange(
	endpointDescriptor: _endpointDescriptor,
      nodeDescriptor: $"{UANamespace.UriString};s={attributes.BrowsePath.Replace("/", "")}",
      samplingInterval: 1000,
      dataChangeCallback: notificationEvent,
      dataChangeFilter: new UADataChangeFilter()
      {
      	Trigger = UADataChangeTrigger.StatusValue,
      },
      state: null);

where the subscribed variable on PLC (and C#) is a boolean type.

Is there a way to filter the event only when variable change to "true" but not in startup application ?

Otherwise in the event handler understand not a true changes ?

Thanks

Last edit: 25 Sep 2019 13:17 by support.

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