How to pop up error message when printing invoice / delivery note

Question

i have a problem that when I'm going to print the invoice (VF03), i have to check the field displayed e.g. port of loading, final destination should not be empty.If both are empty, when user is ready to print, then alert message is displayed (e.g. Please return to the invoice to fill in the port of loading / final destination) and is not allowed to print it out
i'm using Zxxxxx program (including the subroutine)  and SAPScript form Zxxxxxx ,in the form, i use "PERFORM XXX IN PROGRAM ZXXXX" to call the parameter,so how can I do the alert message and stop the printing process and Do i write a checking subroutine in the program and perform that subroutine , so that when that required fields are empty, then show alert when printing?
PLs advise

Answer 1

Hi Sky,
i wont suggest you to give a pop up as if you try and execute the invoice in background it would cause termination.
Standard way of doing it is by updating nast entries. You can refer below mentioned pseudo code. You can also refer to any standard driver program & form attached to a output type for  more code.
The below code would show your output condition in error status if your condition fails.
**Do you validations in the print program and  check for return code.
  IF SY-SUBRC NE 0.
    PERFORM PROTOCOL_UPDATE.
    RETCODE = 1.
    EXIT.
  ENDIF.
      FORM PROTOCOL_UPDATE                                          *
      The messages are collected for the processing protocol.       *
FORM PROTOCOL_UPDATE.
  CALL FUNCTION 'NAST_PROTOCOL_UPDATE'
       EXPORTING
            MSG_ARBGB = SYST-MSGID
            MSG_NR    = SYST-MSGNO
            MSG_TY    = SYST-MSGTY
            MSG_V1    = SYST-MSGV1
            MSG_V2    = SYST-MSGV2
            MSG_V3    = SYST-MSGV3
            MSG_V4    = SYST-MSGV4
       EXCEPTIONS
            OTHERS    = 1.
ENDFORM.
Hope this Helps you.
Sumit