An excellent usage of FIELD-SYMBOL

Simply,we can use field symbol  to identify a specify memory area. Below is the example.

There is a requirement about the enhancement of account document that generated after saving MIRO.The user want to write invice party(Vendor) to the account document item text. Which item-buzid equal 'T' which indicate taxing.

I found the BADI:AC_DOCUMENT to implement this requirement.The method is CHANGE_INITIAL.The exporting parameter of this method is EX_DOCUMENT ,data type is ACC_DOCUMENT_SUBST.So i must use importing parameter IM_DOCUMENT to construct EX_DOCUMENT.But very unluckily,the field PRZNR of ex_document-item-prznr is RAW. And the value will always be '0000'. After executing this BADI ,the field value of prznr of internal table t_accit[]  of calling program is also set to '0000'.The MIRO throw an error message about the definition of BUSSINESS PROCESS.After several hours study i found that the field symbol can achieve my requirement by pass the exporting parameter EX_DOCUMENT.

The method CHANGE_INITIAL OF BADI AC_DOCUMENT is called in FM:AC_DOCMENT_CREATE.
The FM: AC_DOCUMENT_CREATE is called in FM:MRM_INVOICE_POST.
AND the FM:  MRM_INVOICE_POST is called in MIRO when SAVING.

The TABLES parameter T_ACCIT of FM:AC_DOCUMENT_CREATE contain the line items of account document. And this table is transfered from MRM_INVOICE_POST. The correspond global data is XACCIT in program SAPLMRMP.So we can use field symbol to access data XACCIT in program SAPLMRMP.Below is the code.


method IF_EX_AC_DOCUMENT~CHANGE_INITIAL.


  DATA: l_v_lifnr  TYPE LIFNR.


  CHECK SY-TCODE EQ 'MIRO'.


  FIELD-SYMBOLS <ft> TYPE ACCIT_T.


  FIELD-SYMBOLS <fs> TYPE ACCIT.


  DATA: field TYPE CHAR18 VALUE '(SAPLMRMP)xaccit[]'.


  ASSIGN (field) TO <ft>.


  CHECK SY-SUBRC EQ 0.


  LOOP AT <fT> ASSIGNING <fs>.


    IF SY-TABIX EQ 1.


      l_v_lifnr = <fs>-lifnr.


    ENDIF.


    CHECK <fs>-buzid EQ 'T'.


    <fs>-sgtxt = l_v_lifnr.


  ENDLOOP.


endmethod.

I think this usage of field-symbol is very useful in enhancement.But before using this way to change data , we must check the consistency of the data.