What is the need of Field Catalog in ABAP?

Question

Can someone explain the importance of having the code below? I'm new to ABAP and currently trying to create an ALV in a docking container. Thanks.

* Field Catalog

    wa_fieldcat   TYPE lvc_s_fcat.

http://saptechnical.com/Tutorials/ALV/Docking/Index.htm

Answer 1

The code in the example has this subroutine:

*&--------------------------------------------------------------*
*&      Form  FILL_FIELDCAT                                     *
*&--------------------------------------------------------------*
*       To Fill the Field Catalog                               *
*---------------------------------------------------------------*
*  Three Parameters are passed                                  *
*  pv_field   TYPE any for Field                                *
*  pv_tabname TYPE any for Table Name                           *
*  pv_coltext TYPE any for Header Text                          *
*---------------------------------------------------------------*
FORM fill_fieldcat  USING   pv_field   TYPE any
                            pv_tabname TYPE any
                            pv_coltext TYPE any .

  wa_fieldcat-fieldname  = pv_field.
  wa_fieldcat-tabname    = pv_tabname.
  wa_fieldcat-coltext    = pv_coltext.

  APPEND wa_fieldcat TO t_fieldcat.
  CLEAR  wa_fieldcat.
ENDFORM.                               " FILL_FIELDCAT   

The structure wa_fieldcat is used as a container for the information that is about to be added to t_fieldcat.

Since pv_field, pv_tabname, and pv_coltext are three disjoint variables, you cannot APPEND them to t_fieldcat before putting them in a central, unified structure.

That being said, I see no reason why the variable declaration you pointed out couldn't be placed inside of the subroutine FILL_FIELDCAT rather than being a global variable. Keeping the scope of variables only as large as they need to be is good coding practice.