docking容器的特别之处

最近看了几个选择屏幕上用Docking容器做PARENT的例子，发现Docking容器只是定义了，并没有实例，就可以拿来用。把下例红色的部分注释掉，完全不会影响程序的执行。

 

REPORT  ztest.
TABLES tdwst.

PARAMETERS p TYPE i.

DATA:BEGIN OF itab OCCURS 0.
        INCLUDE STRUCTURE tdwst.
DATA:mark TYPE i,
     END OF itab.

DATA:wa_fcat TYPE lvc_s_fcat,
     it_fcat TYPE lvc_t_fcat,
     init.

DATA:l_dock TYPE REF TO cl_gui_docking_container,
     l_grid TYPE REF TO cl_gui_alv_grid.

AT SELECTION-SCREEN OUTPUT.
  IF init IS INITIAL.
    init = 'X'.
    CREATE OBJECT l_dock
      EXPORTING
         side = 4
         ratio = 80.

    CREATE OBJECT l_grid
      EXPORTING
        i_parent = l_dock.

    SELECT * INTO TABLE itab FROM tdwst UP TO 10 ROWS.

***不显示结构中某些字段
    CLEAR wa_fcat.
    wa_fcat-fieldname = 'CVLANG'.
    wa_fcat-no_out = 'X'.
    APPEND wa_fcat TO it_fcat.

***增加结构中没有的字段
    CLEAR wa_fcat.
    wa_fcat-fieldname = 'MARK'.
    wa_fcat-coltext = 'MARK'.
    wa_fcat-col_pos = 1.
    wa_fcat-emphasize =  'C510'.
    APPEND wa_fcat TO it_fcat.

    CALL METHOD l_grid->set_table_for_first_display
      EXPORTING
        i_structure_name              = 'TDWST'
      CHANGING
        it_outtab                     = itab[]
        it_fieldcatalog               = it_fcat
      EXCEPTIONS
        invalid_parameter_combination = 1
        program_error                 = 2
        too_many_lines                = 3
        OTHERS                        = 4.
    IF sy-subrc <> 0.
      MESSAGE ID sy-msgid TYPE sy-msgty NUMBER sy-msgno
                 WITH sy-msgv1 sy-msgv2 sy-msgv3 sy-msgv4.
    ENDIF.
  ENDIF.