OPC Studio User's Guide and Reference
Subscribing to OPC A&E Events
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Concepts > OPC Data Client Concepts > OPC Data Client Development Models > Imperative Programming Model > Imperative Programming Model for OPC Classic A&E > Subscribing to Information (OPC A&E) > Subscribing to OPC A&E Events
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General

Subscription is initiated by calling the  SubscribeEvents method. The component will call handlers for Notification event for each event that satisfies the filter criteria of the created subscription. Obviously, you first need to hook up event handler for that event, and in order to prevent event loss, you should do it before subscribing.

Events may be generated quite rapidly. Your application needs to specify the notification rate, which effectively tells the OPC Alarms and Events server that you do not need to receive event notifications any faster than that.

If you want to subscribe to particular set of OPC Events, call the SubscribeEvents method. You can pass in individual arguments for machine name, server class, and notification rate.

.NET

// This example shows how to subscribe to events and display the event message with each notification. It also shows how to 
// unsubscribe afterwards.
//
// 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 System.Threading;
using OpcLabs.EasyOpc.AlarmsAndEvents;
using OpcLabs.EasyOpc.AlarmsAndEvents.OperationModel;

namespace DocExamples.AlarmsAndEvents._EasyAEClient
{
    partial class SubscribeEvents
    {
        public static void Main1()
        {
            // Instantiate the client object.
            using (var client = new EasyAEClient())
            {
                var eventHandler = new EasyAENotificationEventHandler(client_Notification);
                client.Notification += eventHandler;

                int handle = client.SubscribeEvents("", "OPCLabs.KitEventServer.2", 1000);

                Console.WriteLine("Processing event notifications for 1 minute...");
                Thread.Sleep(60 * 1000);

                client.UnsubscribeEvents(handle);
            }
        }

        // Notification event handler
        static void client_Notification(object sender, EasyAENotificationEventArgs e)
        {
            if (!e.Succeeded)
            {
                Console.WriteLine("*** Failure: {0}", e.ErrorMessageBrief);
                return;
            }
            if (!(e.EventData is null))
                Console.WriteLine(e.EventData.Message);
        }
    }
}

COM

// This example shows how to subscribe to events and display the event message with each notification. It also shows how to
// unsubscribe afterwards.
//
// Find all latest examples here: https://opclabs.doc-that.com/files/onlinedocs/OPCLabs-OpcStudio/Latest/examples.html .
// OPC client and subscriber examples in Object Pascal (Delphi) on GitHub: https://github.com/OPCLabs/Examples-QuickOPC-OP .
// 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.

type
  TClientEventHandlers = class
    // Notification event handler
    procedure OnNotification(
      ASender: TObject;
      sender: OleVariant;
      const eventArgs: _EasyAENotificationEventArgs);
  end;

procedure TClientEventHandlers.OnNotification(
  ASender: TObject;
  sender: OleVariant;
  const eventArgs: _EasyAENotificationEventArgs);
begin
  if not eventArgs.Succeeded then
    WriteLn(Format('*** Failure: %s', [eventArgs.ErrorMessageBrief]));
  if eventArgs.EventData <> nil then
      WriteLn(eventArgs.EventData.Message);
end;

class procedure SubscribeEvents.Main;
var
  Client: TEasyAEClient;
  ClientEventHandlers: TClientEventHandlers;
  Handle: Integer;
  ServerDescriptor: _ServerDescriptor;
  State: OleVariant;
  SubscriptionParameters: _AESubscriptionParameters;
begin
  ServerDescriptor := CoServerDescriptor.Create;
  ServerDescriptor.ServerClass := 'OPCLabs.KitEventServer.2';

  // Instantiate the client object and hook events
  Client := TEasyAEClient.Create(nil);
  ClientEventHandlers := TClientEventHandlers.Create;
  Client.OnNotification := ClientEventHandlers.OnNotification;

  WriteLn('Subscribing events...');
  SubscriptionParameters := CoAESubscriptionParameters.Create;
  SubscriptionParameters.NotificationRate := 1000;
  Handle := Client.SubscribeEvents(ServerDescriptor, SubscriptionParameters, true, State);

  WriteLn('Processing event notifications for 1 minute...');
  PumpSleep(60*1000);

  WriteLn('Unsubscribing events...');
  Client.UnsubscribeEvents(Handle);

  WriteLn('Finished.');
  FreeAndNil(Client);
  FreeAndNil(ClientEventHandlers);
end;

Python

# This example shows how to subscribe to events and display the event message with each notification. It also shows how
# to unsubscribe afterwards.
#
# 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 time

# Import .NET namespaces.
from OpcLabs.EasyOpc import *
from OpcLabs.EasyOpc.AlarmsAndEvents import *


# Notification event handler
def notification(sender, e):
    if not e.Succeeded:
        print('*** Failure: ', e.ErrorMessageBrief, sep='')
        return
    else:
        if e.EventData is not None:
            print(e.EventData.Message)


# Instantiate the client object
client = EasyAEClient()

client.Notification += notification

print('Subscribing events...')
handle = IEasyAEClientExtension.SubscribeEvents(client, '', 'OPCLabs.KitEventServer.2', 1000)

print('Processing event notifications for 1 minute...')
time.sleep(60)

print('Unsubscribing events...')
client.UnsubscribeAllEvents()

client.Notification -= notification

print('Finished.')

 

Optionally, you can specify a subscription filter; it is a separate object of AESubscriptionFilterType type. Other optional parameters are attributes that should be returned in event notifications (separate set of attributes for each event category is needed), and the “active” and “refresh when active” flags. You can also pass in a State argument of any type. When any event notification is generated, the State argument is then passed to the Notification event handler in the EasyAENotificationEventArgs.Arguments.State object.

You can alternatively pass in a ServerDescriptor in place of machine name and server class arguments. You can also replace the individual notification rate, subscription filter, and returned attributes arguments by passing in an AESubscriptionParameters object.

The State argument is typically used to provide some sort of correlation between objects in your application, and the event notifications. For example, if you are programming an HMI application and you want the event handler to update the control that displays the event messages, you may want to set the State argument to the control object itself. When the event notification arrives, you simply update the control indicated by the State property of EasyAENotificationEventArgs, without having to look it up.

The “refresh when active” flag enables a functionality that is useful if you want to keep a “copy” of condition states (that primarily exist in the OPC server) in your application. When this flag is set, the component will automatically perform a subscription Refresh (see further below) after the connection is first time established, and also each time it is reestablished (after a connection loss). This way, the component assures that your code will get notifications that allow you to “reconstruct” the state of event conditions at any given moment.

Note: It is NOT an error to subscribe to the same set of events twice (or more times), even with precisely the same parameters. You will receive separate subscription handles, and with regard to your application, this situation will look no different from subscribing to different set of events.

More examples

The example below logs OPC Alarms and Events notifications into an XML file.

The main program:

// Logs OPC Alarms and Events notifications into an XML file.
//
// 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 System.Diagnostics;
using System.Xml;
using System.Xml.Serialization;
using OpcLabs.BaseLib.Runtime.InteropServices;
using OpcLabs.EasyOpc.AlarmsAndEvents;
using OpcLabs.EasyOpc.AlarmsAndEvents.OperationModel;

namespace XmlEventLogger
{
    class Program
    {
        static void Main()
        {
            ComManagement.Instance.AssureSecurityInitialization();

            Console.WriteLine("Starting up...");
            var xmlSerializer = new XmlSerializer(typeof(EasyAENotificationEventArgs));
            var xmlWriter = XmlWriter.Create("OpcEvents.xml", new XmlWriterSettings
            {
                Indent = true,
                CloseOutput = true
            });
            // The root element can have any name you need, but the name below also allows reading the log back as .NET array
            xmlWriter.WriteStartElement("ArrayOfEasyAENotificationEventArgs");

            Console.WriteLine("Logging events for 30 seconds...");
            int handle = EasyAEClient.SharedInstance.SubscribeEvents("", "OPCLabs.KitEventServer.2", 100,
               (_, eventArgs) =>
                   {
                       Debug.Assert(!(eventArgs is null));
                       Console.Write(".");
                       xmlSerializer.Serialize(xmlWriter, eventArgs);
                   });
            System.Threading.Thread.Sleep(30 * 1000);

            Console.WriteLine();
            Console.WriteLine("Shutting down...");
            EasyAEClient.SharedInstance.UnsubscribeEvents(handle);
            xmlWriter.WriteEndElement();    // not really necessary - XmlWriter would write the end tag for us anyway
            xmlWriter.Close();

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

 

 

See Also

Installed Examples - Client Console