What is Triggering and Handling Events in ABAP OOPs

Question

Gowri

Answer 1

) TYPE type ..
To declare static events, use the following statement:
CLASS-EVENTS ... ...
It links a list of handler methods with corresponding trigger methods.  There are four different types of event.
It can be
·        An instance event declared in a class
·        An instance event declared in an interface
·        A static event declared in a class
·        A static event declared in an interface
The syntax and effect of the SET HANDLER depends on which of the four cases listed above applies. 
For an instance event, you must use the FOR addition to specify the instance for which you want to register the handler.  You can either specify a single instance as the trigger, using a reference variable
After the RAISE EVENT statement, all registered handler methods are executed before the next statement is processed (synchronous event handling).  If a handler method itself triggers events, its handler methods are executed before the original handler method continues. To avoid the possibility of endless recursion, events may currently only be nested 64 deep.
Handler methods are executed in the order in which they were registered.  Since event handlers are registered dynamically, you should not assume that they will be processed in a particular order. Instead, you should program as though all event handlers will be executed simultaneously.
"Example  :
REPORT demo_class_counter_event.
CLASS counter DEFINITION.
  PUBLIC SECTION.
    METHODS increment_counter.
    EVENTS  critical_value EXPORTING value(excess) TYPE i.
  PRIVATE SECTION.
    DATA: count     TYPE i,
          threshold TYPE i VALUE 10.
ENDCLASS.
CLASS counter IMPLEMENTATION.
  METHOD increment_counter.
    DATA diff TYPE i.
    ADD 1 TO count.
    IF count > threshold.
      diff = count - threshold.
      RAISE EVENT critical_value EXPORTING excess = diff.
    ENDIF.
  ENDMETHOD.
ENDCLASS.
CLASS handler DEFINITION.
  PUBLIC SECTION.
    METHODS handle_excess
            FOR EVENT critical_value OF counter
            IMPORTING excess.
ENDCLASS.
CLASS handler IMPLEMENTATION.
  METHOD handle_excess.
    WRITE: / 'Excess is', excess.
  ENDMETHOD.
ENDCLASS.
DATA: r1 TYPE REF TO counter,
      h1 TYPE REF TO handler.
START-OF-SELECTION.
  CREATE OBJECT: r1, h1.
  SET HANDLER h1->handle_excess FOR ALL INSTANCES.
  DO 20 TIMES.
    CALL METHOD r1->increment_counter.
  ENDDO.
The class COUNTER implements a counter. It triggers the event CRITICAL_VALUE when a threshold value is exceeded, and displays the difference. HANDLER can handle the exception in COUNTER. At runtime, the handler is registered for all reference variables that point to the object.
Reward  points if it is usefull ...
Girish