Steps For Event Handler :-

Triggering Method :- A method which will raise the event OR a method which will be raising the event will be called as triggering method.
Event Handler Method :- A method which will handle the event.
Register the event handler method using SET Handler statement.

Syntax :-

SET HANDLER lo_object1→ EVENT_HANDLER for lo_object2.
- where lo_object1 = Class Object in which event handler method is defined.
- EVENT_HANDLER = event handler method name.
- lo_object2 = Class Object in which triggering method is defined.

let’s create a program to understand it.

Requirement :-

We will create a executable program where we will pass a input parameter for user.
And if user do not pass any input, We will give a message on the Selection screen to pass the input to the parameter.
And we will achieve this requirement using events.

Solution :-

Step 1 :- Go to ABAP Editor ( SE38 ) and create a executable program.
Step 2 :- Now we need to create few fields from VBAK as a local variable in which we will get the details on the basis of Sales document number provided by user as a input.
Step 3 :- Create a class with one method and one event.
Step 4 :- Now we need to implement the method and raise the event of the above class.
Step 5 :- Now we will create a class which will handle the event of the Class1 which we have created above.
Step 6 :- In Start of selection create the object of both classes.
Step 7 :- Handle the event.
Step 8 :- call method display and use the write statement to display the output.

Code :-

*&---------------------------------------------------------------------*
*& Report ZAR_OOPS_EVENTS
*&---------------------------------------------------------------------*
*&
*&---------------------------------------------------------------------*
REPORT zar_oops_events.

DATA : perdat TYPE erdat,
       perzet TYPE erzet,
       pernam TYPE ernam,
       pvbtyp TYPE vbtypl.

PARAMETERS : pvbeln TYPE vbeln_va.

CLASS class1 DEFINITION.
  PUBLIC SECTION.
    METHODS display IMPORTING pvbeln TYPE vbeln_va
                    EXPORTING perdat TYPE erdat
                              perzet TYPE erzet
                              pernam TYPE ernam
                              pvbtyp TYPE vbtypl.
    EVENTS : no_input.
ENDCLASS.

CLASS CLASS1 IMPLEMENTATION.
  METHOD DISPLAY.
    if pvbeln is INITIAL.
      raise event no_input.
    else.
      Select single erdat erzet ernam vbtyp
        from vbak into ( perdat, perzet, pernam, pvbtyp ).

    endif.
  ENDMETHOD.
ENDCLASS.

CLASS CLASS2 DEFINITION.
  PUBLIC SECTION.
  METHODS MESSAGE FOR EVENT no_input of class1.

ENDCLASS.

class class2 IMPLEMENTATION.
  method message.
    WRITE : 'Please enter Sales Document Number'.

  ENDMETHOD.
ENDCLASS.

START-OF-SELECTION.
DATA(lo_obj1) = new CLASS1( ).
DATA(lo_obj2) = new CLASS2( ).

SET HANDLER lo_obj2->message for lo_obj1.

lo_obj1->display( EXPORTING pvbeln = pvbeln
                  IMPORTING perdat = perdat
                            perzet = perzet
                            pernam = pernam
                            pvbtyp = pvbtyp ).

write :/ perdat, perzet, pernam, pvbtyp.

*&-------------------------------------------------------------------
*&End Of Program
*&------------------------------------------------------------------