View source for OPC A&E Notification Handler Pseudo-Code

[[Category:OPC A&E]] [[Category:Procedural Coding]] 
In QuickOPC for OPC Alarms&Events, the '''Notification''' handler can be called for several different reasons. Your code needs to recognize the notification reason, and act accordingly. This article explains the various situations in which the '''Notification''' handler may be called, and suggests a pseudo-code that you can use to weite your own event handler.

When an OPC Alarms and Events server generates an event, the '''EasyAEClient''' object generates a '''Notification''' event. For subscription mechanism to be useful, you should hook one or more event handlers to this event.

To be more precise, the '''Notification''' event is actually generated in other cases, too - if there is any significant occurrence related to the event subscription. This can be for three reasons:

# You receive the '''Notification''' when a successful connection (or re-connection) is made. In this case, the '''Exception''' and '''EventData''' properties of the event arguments are null references.
# You receive the '''Notification''' when there is a problem with the event subscription, and it is disconnected. In this case, the '''Exception''' property contains information about the error. The '''EventData''' property is a null reference.
# You receive one additional '''Notification''' after the component has sent you all notifications for the forced “refresh”. In this case, the '''RefreshComplete''' property of the event arguments is set to '''True''', and the '''Exception''' and '''EventData''' properties contain null references.

The notification for the '''Notification''' event contains an '''EasyAENotificationEventArgs''' argument. You will find all kind of relevant data in this object. Some properties in this object contain valid information under all circumstances. These properties are '''ServerDescriptor''', '''SubscriptionParameters''', and '''State'''. Other properties, such as '''EventData''', contain null references when there is no associated information for them. When the '''Event''' property is not a null reference, it contains an '''AEEventData''' object describing the detail of the actual OPC event received from the OPC Alarms and Events server.

Before further processing, your code should always inspect the value of '''Exception''' property of the event arguments. If this property is not a null reference, there has been an error related to the event subscription, the '''Exception''' property contains information about the problem, and the '''Event''' property does not contain a valid object.

If the '''Exception''' property is a null reference, the notification may be informing you about the fact that a “forced” refresh is complete (in this case, the '''RefreshComplete''' property is True), or that an event subscription has been successfully connected or re-connected (in this case, the '''Event''' property is a null reference). If none of the previous applies, the '''EventData''' property contains a valid '''AEEventData''' object with details about the actual OPC event generated by the OPC server.

Pseudo-code for the full '''Notification''' event handler may look similar to this:

 '''if''' notificationEventArgs.Exception is not null '''then'''
        ''An error occurred and the subscription is disconnected, handle it (or ignore)''
 
 '''else if''' notificationEventArgs.RefreshComplete '''then'''
        ''A “refresh” is complete; handle it (only needed if you are invoking a refresh explicitly)''
 
 '''else if''' notificationEventArgs.EventData is null '''then'''
        ''Subscription has been successfully connected or re-connected, handle it (or ignore)''
 
 '''else'''
        ''Handle the OPC event, details are in notificationEventArgs.EventData. You may use notificationEventArgs.Refresh flag for distinguishing refreshes from original notifications.''

The '''Notification''' event handler is called on a thread determined by the '''EasyAEClient''' component. For details, please refer to “Multithreading and Synchronization” chapter under “Advanced Topics” in the "Concepts" document.