At selection-screen output and at selection-screen on

Question

Hi Experts,
I had coded in both at selection-screen output for disabling input of certain fields and at selection-screen for validation like this:
AT SELECTION-SCREEN output.
  IF p_amt = 'X'.
    LOOP AT SCREEN.
      CASE screen-name.
        WHEN 'P_AMT'.
          screen-input = '1'.
        WHEN 'P_TST'.
          screen-input = '1'.
        WHEN 'P_B4C'.
          screen-input = '1'.
        WHEN 'P_AFC'.
          screen-input = '1'.
        WHEN 'P_COM'.
          screen-input = '1'.
        WHEN 'P_COM1'.
          screen-input = '1'.
        WHEN 'P_COM2'.
          screen-input = '1'.
        WHEN OTHERS.
          screen-input = '0'.
      ENDCASE.
      MODIFY SCREEN.
    ENDLOOP.
  ENDIF.
  IF p_amt = 'X'.
    LOOP AT SCREEN.
      CASE screen-name.
        WHEN 'P_AMT'.
          screen-input = '1'.
        WHEN 'P_TST'.
          screen-input = '1'.
        WHEN 'P_B4C'.
          screen-input = '1'.
        WHEN 'P_AFC'.
          screen-input = '1'.
        WHEN 'P_COM'.
          screen-input = '1'.
        WHEN 'P_COM1'.
          screen-input = '1'.
        WHEN 'P_COM2'.
          screen-input = '1'.
        WHEN OTHERS.
          screen-input = '0'.
      ENDCASE.
      MODIFY SCREEN.
    ENDLOOP.
  ENDIF.
  IF p_pdf = 'X'.
    LOOP AT SCREEN.
      CASE screen-name.
        WHEN 'P_PDF'.
          screen-input = '1'.
        WHEN 'P_TST'.
          screen-input = '1'.
        WHEN 'P_PATH'.
          screen-input = '1'.
        WHEN OTHERS.
          screen-input = '0'.
      ENDCASE.
      MODIFY SCREEN.
    ENDLOOP.
  ENDIF.
  IF p_afc = 'X'.
    LOOP AT SCREEN.
      CASE screen-name.
        WHEN 'P_TST'.
          screen-input = '1'.
        WHEN 'S_FID-LOW' OR 'S_FID-HIGH'.
          screen-input = '0'.
        WHEN 'S_ORG-LOW' OR 'S_ORG-HIGH'.
          screen-input = '0'.
        WHEN 'S_OFF-LOW' OR 'S_OFF-HIGH'.
          screen-input = '0'.
        WHEN 'S_DAT-LOW' OR 'S_DAT-HIGH'.
          screen-input = '0'.
      ENDCASE.
      MODIFY SCREEN.
    ENDLOOP.
  ENDIF.
AT selection-screen on block b5.
if p_tst is initial.
  message text-016 type 'E'.
endif.
AT selection-screen on block b2.
if p_amt ne 'X' and p_pdf ne 'X'.
if s_fid is initial and s_org is initial.
message text-000 type 'E'.
endif.
if s_dat is initial.
message text-000 type 'E'.
endif.
endif.
when i comment out at selection-screen validations the input is disabled for the screen fields as set, else the validations are executed first.
Any solution for this???
Thanks in advance.

Answer 1

AT SELECTION-SCREEN
Syntax
AT SELECTION-SCREEN selscreen_event.
Effect
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.