Class events

ABL class events and .NET class events (also called .NET events) are very similar in concept, and OpenEdge allows you to manage both of them using the same ABL mechanism. Each class or interface can define any number of class events. Thus, class events are members of the class or interface for which they are defined, like methods, properties, and data members. Like any class member, a class event can be defined as either an instance or a static event, and an instance event can also be defined as abstract. A class event also has an access mode (private, protected, or public), which determines the application scope that can respond to the event.

A class event has a signature, similar to a method, that can define parameters for passing data in response to event notification. An ABL application receives notice of a class event when the object that defines the event publishes it. In an ABL application, you can subscribe one or more ABL class methods or internal procedures as event handlers, which execute when a given event is published. These event handlers must all be defined with a signature that is compatible with the event that they are subscribed to handle.

Class events provide similar features for class-based objects that named events provide for procedure-based objects, but using different mechanisms. You publish a named event using the PUBLISH statement and manage named event handler subscriptions using the SUBSCRIBE statement and UNSUBSCRIBE statement. However, you publish a class event using the Publish( ) event method and manage class event handler subscriptions using the Subscribe( ) event method and Unsubscribe( ) event method. Among the differences between the two types of events, a named event is defined when it is published and without a strongly-typed signature, while a class event must be defined before it is published and with a strongly-typed signature.