OPC Studio User's Guide and Reference
Using Callback Methods Instead of Event Handlers (OPC Data)
Client and Subscriber Development > Development Models > Imperative Programming Model > Imperative Programming Model for OPC Data (Classic and UA) > Subscribing to Information (OPC Data) > Using Callback Methods Instead of Event Handlers (OPC Data)
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Using event handlers for processing notifications is a standard way with many advantages. There also situations, however, where event handlers are not very practical. For example, if you want to do fundamentally different processing on different kinds of subscriptions, you end up with all notifications being processed by the same event handler, and you need to put in extra code to distinguish between different kinds of subscriptions they come from. Event handlers also require additional code to set up and tear down.

In order to overcome these problems, OPC Data Client components allow you to pass in a delegate for a callback method to subscription methods. There are subscription methods overloads that accept the callback method parameter. The callback method has the same signature (arguments) as the event handler, and is called by the component in addition to invoking the event handler (if you do not hook a handler to the event, only the callback method will be invoked). You can therefore pass in the delegate for the callback method right into the subscription method call, without setting up event handlers.

The callback method can also be specified using an anonymous delegate or a lambda expression, i.e. without having to declare the method explicitly elsewhere in your code. This is especially useful for short callback methods.

Examples for OPC Classic and XML-DA:

// This example shows how subscribe to changes of a single item 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.Diagnostics;
using System.Threading;
using OpcLabs.EasyOpc.DataAccess;

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

            Console.WriteLine("Subscribing...");
            // The callback is a lambda expression the displays the value
            client.SubscribeItem("", "OPCLabs.KitServer.2", "Simulation.Random", 1000,
                (sender, eventArgs) =>
                {
                    Debug.Assert(eventArgs != null);

                    if (eventArgs.Succeeded)
                    {
                        Debug.Assert(eventArgs.Vtq != null);
                        Console.WriteLine(eventArgs.Vtq.ToString());
                    }
                    else
                        Console.WriteLine("*** Failure: {0}", eventArgs.ErrorMessageBrief);
                });

            Console.WriteLine("Processing item changed events for 10 seconds...");
            Thread.Sleep(10 * 1000);

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

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

# This example shows how subscribe to changes of a single item and display the value of the item with each change,
# using a regular 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.DataAccess import *


# Item changed callback.
def itemChangedCallback(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...')
# The callback is a regular method that displays the value.
IEasyDAClientExtension.SubscribeItem(client,
                                     '', 'OPCLabs.KitServer.2', 'Simulation.Random', 1000,
                                     EasyDAItemChangedEventHandler(itemChangedCallback))

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

print('Unsubscribing all items...')
client.UnsubscribeAllItems()

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

print('Finished.')

' This example shows how subscribe to changes of a single item 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._EasyDAClient
    Partial Friend Class SubscribeItem
        Shared Sub CallbackLambda()
            ' Instantiate the client object
            Dim client = New EasyDAClient()

            Console.WriteLine("Subscribing...")
            ' The callback is a lambda expression the displays the value
            client.SubscribeItem("", "OPCLabs.KitServer.2", "Simulation.Random", 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)

            Console.WriteLine("Processing item changed events for 10 seconds...")
            Threading.Thread.Sleep(10 * 1000)

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

            Console.WriteLine("Waiting for 2 seconds...")
            Threading.Thread.Sleep(2 * 1000)
        End Sub
    End Class
End Namespace

 

// 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.

 

OPC Data Client supports OPC XML-DA also on Linux and macOS.

Examples for OPC UA:

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

using System;
using OpcLabs.EasyOpc.UA;

namespace UADocExamples._EasyUAClient
{
    partial class SubscribeDataChange
    {
        public static void CallbackLambda()
        {
            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("Subscribing...");
            // The callback is a lambda expression the displays the value
            client.SubscribeDataChange(endpointDescriptor, "nsu=http://test.org/UA/Data/ ;i=10853", 1000,
                (sender, eventArgs) =>
                {
                    if (eventArgs.Succeeded)
                        Console.WriteLine("Value: {0}", eventArgs.AttributeData.Value);
                    else
                        Console.WriteLine("*** Failure: {0}", eventArgs.ErrorMessageBrief);
                });

            Console.WriteLine("Processing data change events for 10 seconds...");
            System.Threading.Thread.Sleep(10 * 1000);

            Console.WriteLine("Unsubscribing...");
            client.UnsubscribeAllMonitoredItems();

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

// This example shows how to subscribe to changes of a single monitored item, and display the value of the item with each change
// using a callback method that is provided as a function delegate.
//
// Find all latest examples here: https://opclabs.doc-that.com/files/onlinedocs/OPCLabs-OpcStudio/Latest/examples.html .

module _EasyUAClient.SubscribeDataChange

open OpcLabs.EasyOpc.UA
open System
open System.Threading

let CallbackFunction =

    let endpointDescriptor =
        new 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
    let client = new EasyUAClient()

    Console.WriteLine("Subscribing...");
    // The callback is a delegate that displays the value
    let handle = 
        client.SubscribeDataChange(
            endpointDescriptor,
            new UANodeDescriptor("nsu=http://test.org/UA/Data/;i=10853"),
            1000,
            new EasyUADataChangeNotificationEventHandler(
                fun sender eventArgs -> 
                    if eventArgs.Succeeded then Console.WriteLine("Value: {0}", eventArgs.AttributeData.Value)
                    else Console.WriteLine("*** Failure: {0}", eventArgs.ErrorMessageBrief)))

    Console.WriteLine("Processing data change events for 10 seconds...")
    Thread.Sleep(10 * 1000)

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

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

# This example shows how to subscribe to changes of a single monitored item, and display the value of the item with each
# change using a regular 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.UA import *
from OpcLabs.EasyOpc.UA.OperationModel import *


def dataChangeNotification(sender, eventArgs):
    # Display value.
    if eventArgs.Succeeded:
        print('Value: ', eventArgs.AttributeData.Value, sep='')
    else:
        print('*** Failure: ', eventArgs.ErrorMessageBrief, sep='')


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('Subscribing...')
IEasyUAClientExtension.SubscribeDataChange(client,
    endpointDescriptor,
    UANodeDescriptor('nsu=http://test.org/UA/Data/ ;i=10853'),
    1000,
    EasyUADataChangeNotificationEventHandler(dataChangeNotification))

print('Processing data change events for 10 seconds...')
time.sleep(10)

print('Unsubscribing...')
client.UnsubscribeAllMonitoredItems()

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

print('Finished.')

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

Imports OpcLabs.EasyOpc.UA

Namespace _EasyUAClient
    Partial Friend Class SubscribeDataChange
        Public Shared Sub CallbackLambda()

            ' 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("Subscribing...")
            ' The callback is a lambda expression the displays the value
            client.SubscribeDataChange( _
                endpointDescriptor, _
                "nsu=http://test.org/UA/Data/ ;i=10853", _
                1000, _
                Sub(sender, eventArgs)
                    If eventArgs.Succeeded Then
                        Console.WriteLine("Value: {0}", eventArgs.AttributeData.Value)
                    Else
                        Console.WriteLine("*** Failure: {0}", eventArgs.ErrorMessageBrief)
                    End If
                End Sub)

            Console.WriteLine("Processing monitored item changed events for 10 seconds...")
            Threading.Thread.Sleep(10 * 1000)

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

            Console.WriteLine("Waiting for 2 seconds...")
            Threading.Thread.Sleep(2 * 1000)
        End Sub
    End Class
End Namespace

 

More...

For subscription methods that work with multiple subscriptions at once, there is also a Callback (in OPC Classic) or DataChangeCallback (in OPC-UA) property in the arguments objects that you can use for the same purpose.

Note that if you specify a non-null callback parameter to the subscription method, the callback method will be invoked in addition to the event handlers. If you use both event handlers and callback methods in the same application, and you do not want the event handlers to process the notifications that are also processed by the callback methods, you can either

 

See Also
OPC UA Monitored Item Changed Event or Callback
OPC Classic Item Changed Event or Callback
Obtaining Subscription Information (OPC Data)
Unsubscribing (OPC Data)
Changing Existing Subscription (OPC Data)
Subscribing to OPC UA Data Changes
Subscribing to OPC Classic Items
Subscribing to Information (OPC Data)
Imperative Programming Model for OPC Data (Classic and UA)
Imperative Programming Model

Knowledge Base

QuickOPC.NET: How to log OPC Data Access item changes into an XML file
QuickOPC.NET: How to log OPC Data Access item changes into a SQL Server database

 

 

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