ex9.md

Let's change the take_off method and declare it as an event.

CLASS lcl_flight DEFINITION.

    EVENTS : take_off EXPORTING VALUE(ev_plane_number) TYPE string.
  

Now we will create a new class to listen to the event and react to this event: lcl_controller

CLASS lcl_controller DEFINITION.
  PUBLIC SECTION.
    METHODS : on_takeoff FOR EVENT take_off OF lcl_flight IMPORTING ev_plane_number sender.
ENDCLASS.

CLASS lcl_controller IMPLEMENTATION.
  METHOD on_takeoff.
    WRITE:/ 'Good flight to you' && ' ' && ev_plane_number.
 
  ENDMETHOD.
ENDCLASS.

In the report part, we will declare an instance on controller and we will make it listen to the flight's event.

DATA(lo_controller) = NEW lcl_controller( ).

Now let's see what happens if we run the following code.

LOOP AT lot_flights INTO DATA(lo_flight).
	SET HANDLER lo_controller->on_takeoff( ) FOR lo_flight ACTIVATION abap_true.
	lo_flight->leave_aiport( ).
ENDLOOP.

Now we will call the leave_airport method a two times in the same loop.

LOOP AT lot_flights INTO DATA(lo_flight).
	lo_flight->leave_aiport( ).
	lo_flight->leave_aiport( ).
ENDLOOP.

Are the flights leaving the airport a second time ? Why ?

Now, we will deactivate the listening after the event processing.

  METHOD on_takeoff.
    WRITE:/ 'Good flight to you' && ev_plane_number.
    SET HANDLER on_takeoff FOR sender ACTIVATION space.
  ENDMETHOD.

Let's run again our code.

Better ! Right ?