Examples - OPC XML-DA - Subscribe with a callback

// This example shows how subscribe to changes of a single item in an OPC XML-DA server and display the value of the item 
// with each change, using a callback method specified using lambda expression.
//
// Find all latest examples here: https://opclabs.doc-that.com/files/onlinedocs/OPCLabs-OpcStudio/Latest/examples.html .

using System;
using System.Threading;
using System.Diagnostics;
using OpcLabs.EasyOpc.DataAccess;

namespace DocExamples.DataAccess.Xml
{
    partial class SubscribeItem
    {
        public static void CallbackLambdaXml()
        {
            // Instantiate the client object.
            var client = new EasyDAClient();

            Console.WriteLine("Subscribing item...");
            // The callback is a lambda expression the displays the value
            client.SubscribeItem(
                "http://opcxml.demo-this.com/XmlDaSampleServer/Service.asmx",
                "Dynamic/Analog Types/Int",
                1000,
                (sender, eventArgs) =>
                {
                    Debug.Assert(eventArgs != null);

                    if (eventArgs.Succeeded)
                    {
                        Debug.Assert(eventArgs.Vtq != null);
                        Console.WriteLine(eventArgs.Vtq.ToString());
                    }
                    else
                        Console.WriteLine($"*** Failure: {eventArgs.ErrorMessageBrief}");
                },
                state: null);

            Console.WriteLine("Processing item changed events for 30 seconds...");
            Thread.Sleep(30 * 1000);

            Console.WriteLine("Unsubscribing items...");
            client.UnsubscribeAllItems();

            Console.WriteLine("Waiting for 2 seconds...");
            Thread.Sleep(2 * 1000);

            Console.WriteLine("Finished.");
        }
    }
}

# This example shows how subscribe to changes of a single item in an OPC XML-DA server and display the value of the item
# with each change, using a callback method.
#
# 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 .
# The QuickOPC package is needed. Install it using "pip install opclabs_quickopc".
import opclabs_quickopc
import time

# Import .NET namespaces.
from OpcLabs.EasyOpc import *
from OpcLabs.EasyOpc.DataAccess import *
from OpcLabs.EasyOpc.DataAccess.OperationModel import *


# Item changed callback
def itemChanged(sender, e):
    assert e is not None
    if e.Succeeded:
        assert e.Vtq is not None
        print(e.Vtq)
    else:
        print('*** Failure: ', e.ErrorMessageBrief, sep='')


# Instantiate the client object
client = EasyDAClient()

print('Subscribing item changes...')
IEasyDAClientExtension.SubscribeItem(client,
    ServerDescriptor('http://opcxml.demo-this.com/XmlDaSampleServer/Service.asmx'),
    DAItemDescriptor('Dynamic/Analog Types/Int'),
    DAGroupParameters(1000),
    EasyDAItemChangedEventHandler(itemChanged),
    None)

print('Processing item change callbacks for 10 seconds...')
time.sleep(10)

print('Unsubscribing item changes...')
client.UnsubscribeAllItems()

print('Waiting for 2 seconds...')
time.sleep(2)

print('Finished.')

' This example shows how subscribe to changes of a single item in an OPC XML-DA server and display the value of the item 
' with each change, using a callback method specified using lambda expression.
'
' Find all latest examples here: https://opclabs.doc-that.com/files/onlinedocs/OPCLabs-OpcStudio/Latest/examples.html .

Imports OpcLabs.EasyOpc.DataAccess

Namespace DataAccess.Xml
    Partial Friend Class SubscribeItem
        Shared Sub CallbackLambdaXml()
            ' Instantiate the client object
            Dim client = New EasyDAClient()

            Console.WriteLine("Subscribing...")
            ' The callback is a lambda expression the displays the value
            client.SubscribeItem(
                    "http://opcxml.demo-this.com/XmlDaSampleServer/Service.asmx",
                    "Dynamic/Analog Types/Int",
                    1000,
                    Sub(sender, eventArgs)
                        Debug.Assert(eventArgs IsNot Nothing)
                        If eventArgs.Succeeded Then
                            Debug.Assert(eventArgs.Vtq IsNot Nothing)
                            Console.WriteLine(eventArgs.Vtq.ToString())
                        Else
                            Console.WriteLine("*** Failure: {0}", eventArgs.ErrorMessageBrief)
                        End If
                    End Sub,
                    state:=Nothing)

            Console.WriteLine("Processing item changed events for 30 seconds...")
            Threading.Thread.Sleep(30 * 1000)

            Console.WriteLine("Unsubscribing...")
            client.UnsubscribeAllItems()

            Console.WriteLine("Waiting for 2 seconds...")
            Threading.Thread.Sleep(2 * 1000)

            Console.WriteLine("Finished.")
        End Sub
    End Class
End Namespace

It is also possible to specify lambda function as a callback in Python.NET, but because lambda functions are limited to an expression in Python, the usability of this approach is limited.

QuickOPC supports OPC XML-DA also on Linux and macOS.

Examples - OPC Data Access

Examples - OPC Data Access - Callback using lambda or regular method

Examples - OPC Data Access - Subscribe to a single item

Examples - OPC Data Access - Subscribe to multiple items

Conceptual

Using Callback Methods Instead of Event Handlers (OPC Data)