What is the use of selection -screen on field

Question

Hi,
Can anyone tell me the use of at selection-screen on field and its syntax?
Regards,
Hema

Answer 1

Hi Hema,
AT SELECTION-SCREEN selscreen_event.
<b>Effect</b>
This statement defines event blocks for different events selscreen_event that are triggered by the ABAP runtime environment during selection screen processing.
Selection screen events occur immediately before sending a selection screen and after certain user actions on a displayed selection screen. They assist in selection screen processing in the ABAP program.
AT SELECTION-SCREEN is triggered at least twice during actions on selection screens that are linked into another selection screen as a subscreen - first for the linked selection screen itself, and then for the linking selection screens.
Note
The event blocks after AT SELECTION-SCREEN are implemented internally as procedures. Declarative statments in these event blocks create local data.
<b>Differenct Options:</b>
  | { ON {para|selcrit} }
  | { ON END OF selcrit }
  | { ON BLOCK block }
  | { ON RADIOBUTTON GROUP radi }
  | { }
  | { ON {HELP-REQUEST|VALUE-REQUEST}
  |   FOR {para|selcrit-low|selcrit-high} }
  | { ON EXIT-COMMAND }.
<b>Example</b>
REPORT demo_at_selection_screen.
Global data
DATA: sflight_tab TYPE TABLE OF sflight,
      sflight_wa  LIKE LINE  OF sflight_tab.
Selection screens
PARAMETERS p_carrid TYPE spfli-carrid.
SELECTION-SCREEN BEGIN OF SCREEN 500.
  SELECT-OPTIONS s_conn FOR sflight_wa-connid.
  DATA s_conn_wa LIKE LINE OF s_conn.
SELECTION-SCREEN END OF SCREEN 500.
Handling selection screen events
AT SELECTION-SCREEN ON p_carrid.
  IF p_carrid IS INITIAL.
    MESSAGE 'Please enter a value' TYPE 'E'.
  ENDIF.
  AUTHORITY-CHECK OBJECT 'S_CARRID'
                      ID 'CARRID' FIELD p_carrid
                      ID 'ACTVT'  FIELD '03'.
  IF sy-subrc = 4.
    MESSAGE 'No authorization for carrier' TYPE 'E'.
  ELSEIF sy-subrc <> 0.
    MESSAGE 'Error in authority check' TYPE 'A'.
  ELSE.
    IF sy-ucomm = 'ONLI'.
      CALL SELECTION-SCREEN '0500'.
    ENDIF.
  ENDIF.
AT SELECTION-SCREEN.
  IF sy-dynnr = '0500'.
    IF s_conn IS INITIAL.
      MESSAGE 'Please enter values' TYPE 'W'.
    ELSE.
      SELECT *
             FROM sflight
             INTO TABLE sflight_tab
             WHERE carrid = p_carrid AND
                   connid IN s_conn.
      IF sy-subrc <> 0.
        MESSAGE 'No flights found' TYPE 'E'.
      ENDIF.
    ENDIF.
  ENDIF.
Main program
START-OF-SELECTION.