To SET HANDLER for parameter value

Question

Dear Gurus,
When the user enters a plant value in PARAMETER then i need to hide some of the Screen fields.
But the system has to trigger the PAI automatically
I know this can be acheived using CL_GUI_CFW but how to implement the SET HANDLER method for a parameter
I searched and found the HANDLER METHODS for ALV, TREE, TIMER etc but I want it for  SELECTION-SCREEN elements.
I searched WIKI.SDN.SAP.COM and SDN and other stuff.
Please provide me sample code possibly
Please guide me.
Regards
Ramchander Rao.K

Answer 1

for selecion screens, I have this:
PARAMETERS p_cebe TYPE char01 RADIOBUTTON GROUP grp1 DEFAULT 'X' USER-COMMAND flag.
           PARAMETERS p_ceco TYPE char01 RADIOBUTTON GROUP grp1.
PARAMETERS: p_kokrs LIKE cobk-kokrs DEFAULT 'CORP' MODIF ID cec,
                                    p_gjahr LIKE cobk-gjahr                MODIF ID cec.
         PARAMETERS : p_rbukrs LIKE faglflexp-rbukrs MODIF ID ceb,
                                   p_ryear LIKE faglflexp-ryear   MODIF ID ceb,
AT SELECTION-SCREEN OUTPUT.
  LOOP AT SCREEN.
    IF p_cebe = 'X' AND screen-group1 = 'CEC'.
      screen-active = '0'.
    ENDIF.
    IF p_ceco = 'X' AND screen-group1 = 'CEB'.
      screen-active = '0'.
    ENDIF.
    MODIFY SCREEN.
  ENDLOOP.