Dialog programming, dynpro screen field validation

* Validating a single field on a dynpro screen via a PAI module call
  FIELD scr_field-ebeln
   MODULE validate_screen_field     "ABAP code for validation contained in PAI module
  ON INPUT.
*Validating multiple field on a dynpro screen via a PAI module call
  CHAIN.
    FIELD: scr_field-ebeln, scr_field-ebelp.
    MODULE validate_screen_fields.   "ABAP code for validation contained in PAI module
  ENDCHAIN.
*Validating a dynpro screen field via a direct ABAP table selection
  FIELD scr_field-ebeln
    SELECT *
      FROM ekko
     WHERE ebeln = scr_field-ebeln
      INTO ekko
       WHENEVER NOT FOUND SEND ERRORMESSAGE 001
                         WITH ' Document Number '
 ON INPUT.