Extensions described in this article allow you to block the execution and wait until certain condition, based on OPC UA item data, is fulfilled. 

Waiting until item data satisfy some condition

With methods in this group, you can subscribe to specified OPC UA nodes and attributes, and wait by monitoring their data until specified conditions (predicates) become true, and then automatically unsubscribe. The methods also return when the specified time elapses. The item data that satisfied the specified condition is returned by the method.

In order to use the described functionality, call the IEasyUAClientExtension2.WaitForMultipleValues method. There are multiple overloads.

Waiting until item data have some status level (severity)

It is not uncommon that an OPC server needs considerable time before it can provide the data requested. When you Read a node or Subscribe to a monitored item with some OPC servers, they will first deliver data with "bad" or "uncertain" severity in their status, and only after a while, data with "good" severity in their status are provided. This behavior, of course, creates a problem if all your code needs is that actual, "good" data value, for further processing. You cannot just do a Read; you need additional logic to overcome the possibility of these initial "uncertain" or "bad" results or updates.

OPC Data Client has ready-made methods that contain the necessary logic. The IEasyUAClientExtension2.WaitForMultipleValues method is similar in its usage to ReadMultipleValues Method , and the IEasyUAClientExtension2.WaitForValue method is similar in its usage to ReadValue Method. Internally, however, they use subscriptions to possibly observe the data over a period of time, and automatically overcome the initial "unreliable" updates of the value.

The methods have various overloads that allow you to leave out some of the arguments and use their default values instead. In the full form, you need to specify:

• Monitoring item parameters, which contain the sampling rate, used by the subscription to observe the incoming data.
• The maximum required status severity.
• The timeout for the operation.

Example

.NET

// This example shows how to wait on an item until a value with "good" status severity becomes available.
//
// Find all latest examples here: https://opclabs.doc-that.com/files/onlinedocs/OPCLabs-OpcStudio/Latest/examples.html .
// OPC client and subscriber examples in C# on GitHub: https://github.com/OPCLabs/Examples-QuickOPC-CSharp .
// Missing some example? Ask us for it on our Online Forums, https://www.opclabs.com/forum/index ! You do not have to own
// a commercial license in order to use Online Forums, and we reply to every post.

using System;
using OpcLabs.EasyOpc.UA;
using OpcLabs.EasyOpc.UA.Extensions;
using OpcLabs.EasyOpc.UA.OperationModel;

namespace UADocExamples._EasyUAClientExtension
{
    class WaitForValue
    {
        public static void Main1()
        {
            UAEndpointDescriptor endpointDescriptor =
                "opc.tcp://opcua.demo-this.com:51210/UA/SampleServer";
            // or "http://opcua.demo-this.com:51211/UA/SampleServer" (currently not supported)
            // or "https://opcua.demo-this.com:51212/UA/SampleServer/"

            // Instantiate the client object
            var client = new EasyUAClient();

            Console.WriteLine("Waiting until an item value with \"good\" status severity becomes available...");
            object value;
            try
            {
                value = client.WaitForValue(endpointDescriptor, "nsu=http://test.org/UA/Data/ ;i=10221",
                    monitoringParameters: 100,  // this is the sampling rate
                    millisecondsTimeout: 60*1000);
            }
            catch (UAException uaException)
            {
                Console.WriteLine("*** Failure: {0}", uaException.GetBaseException().Message);
                return;
            }

            // Display the obtained value.
            Console.WriteLine("value: {0}", value);
        }
    }
}

' This example shows how to wait on an item until a value with "good" status severity becomes available.
'
' Find all latest examples here: https://opclabs.doc-that.com/files/onlinedocs/OPCLabs-OpcStudio/Latest/examples.html .
' OPC client and subscriber examples in VB.NET on GitHub: https://github.com/OPCLabs/Examples-QuickOPC-VBNET .
' Missing some example? Ask us for it on our Online Forums, https://www.opclabs.com/forum/index ! You do not have to own
' a commercial license in order to use Online Forums, and we reply to every post.

Imports Opc.Ua
Imports OpcLabs.EasyOpc.UA
Imports OpcLabs.EasyOpc.UA.Extensions
Imports OpcLabs.EasyOpc.UA.OperationModel

Namespace _EasyUAClientExtension
    Friend Class WaitForValue
        Public Shared Sub Main1()

            ' Define which server we will work with.
            Dim endpointDescriptor As UAEndpointDescriptor =
                    "opc.tcp://opcua.demo-this.com:51210/UA/SampleServer"
            ' or "http://opcua.demo-this.com:51211/UA/SampleServer" (currently not supported)
            ' or "https://opcua.demo-this.com:51212/UA/SampleServer/"

            ' Instantiate the client object
            Dim client = New EasyUAClient()

            Console.WriteLine("Waiting until an item value with ""good"" status severity becomes available...")
            Dim value As Object
            Try
                value = client.WaitForValue(endpointDescriptor, "nsu=http://test.org/UA/Data/ ;i=10221",
                    monitoringParameters:=100,  ' this is the sampling rate
                    millisecondsTimeout:=60 * 1000)
            Catch uaException As UAException
                Console.WriteLine("*** Failure: {0}", uaException.GetBaseException.Message)
                Exit Sub
            End Try

            ' Display the obtained value.
            Console.WriteLine("value: {0}", value)
        End Sub
    End Class
End Namespace

Python

# This example shows how to wait on an item until a value with "good" status severity becomes available.
#
# Find all latest examples here: https://opclabs.doc-that.com/files/onlinedocs/OPCLabs-OpcStudio/Latest/examples.html .
# OPC client and subscriber examples in Python on GitHub: https://github.com/OPCLabs/Examples-QuickOPC-Python .
# Missing some example? Ask us for it on our Online Forums, https://www.opclabs.com/forum/index ! You do not have to own
# a commercial license in order to use Online Forums, and we reply to every post.
# The QuickOPC package is needed. Install it using "pip install opclabs_quickopc".
import opclabs_quickopc

# Import .NET namespaces.
from OpcLabs.EasyOpc.UA import *
from OpcLabs.EasyOpc.UA.Extensions import *
from OpcLabs.EasyOpc.UA.OperationModel import *


endpointDescriptor = UAEndpointDescriptor('opc.tcp://opcua.demo-this.com:51210/UA/SampleServer')
# or 'http://opcua.demo-this.com:51211/UA/SampleServer' (currently not supported)
# or 'https://opcua.demo-this.com:51212/UA/SampleServer/'

# Instantiate the client object.
client = EasyUAClient()

print('Waiting until an item value with "good" status severity becomes available...')
try:
    value = IEasyUAClientExtension2.WaitForValue(client,
                                                 endpointDescriptor,
                                                 UANodeDescriptor('nsu=http://test.org/UA/Data/ ;i=10221'),
                                                 UAMonitoringParameters(100),   # this is the sampling rate
                                                 60*1000)   # millisecondsTimeout
except UAException as uaException:
    print('*** Failure: ' + uaException.GetBaseException().Message)
    exit()

# Display the obtained value.
print('value: ', value, sep='')

print()
print('Finished.')