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