Events and error handling in ABAP Objects

System variable check(sy_subrc) for event handling.
If sy-subrc = 0 then all event handler methods have been properly registered
SY-SUBRC = 4: then you tried to register the same combination of triggering event, event

handling method, and handler more than once.

If sy-subrc = 8: then you tried to deregister an event handler that was not registered.
Non-cacheable runtime error during event handling.

SET_HANDLER_DISP_OVERFLOW: Unable to register any more handlers.
SET_HANDLER_E_NO_FOR: Handlers of instance methods need the FOR addition.
SET_HANDLER_FOR_CE: Event handler registered for a static event.
SET_HANDLER_FOR_NULL: An event may not be triggered using NULL.

Note
If an instance triggers an event, the registration is implicitly deleted if the automatic
memory management (garbage collection) deletes the triggering instance.
When you register instance methods as event handlers, note that the registration refers to the current instance and not to the reference variable that is used for registration. Even when the reference variable takes another value after SET HANDLER, the registration of the object remains unchanged. This also affects to the lifetime of objects - an object exists for as long as it is registered as an event handler, even if no more reference variables point to it. The object is either explicitly deregistered using the ACTIVATION addition, or implicitly when the triggering instance no longer exists.