如下代码将会输出一个P_U的字段
PARAMETERS p_u TYPE string VISIBLE LENGTH 12 MEMORY ID m1.
AT SELECTION-SCREEN OUTPUT.
  LOOP AT SCREEN.
    IF screen-name = 'P_U'.
      screen-invisible = '1'.
      MODIFY SCREEN.
    ENDIF.
  ENDLOOP.
        如果我们给这个字段设置一个默认值,参考如下代码,这时将会输出什么呢?
DATA: lv_xs TYPE xstring.
PARAMETERS p_u TYPE string VISIBLE LENGTH 12 MEMORY ID m1.
INITIALIZATION.
  lv_xs =  '0000'.
  p_u = cl_bcs_convert=>xstring_to_string( iv_cp = '4103' iv_xstr = lv_xs ).
AT SELECTION-SCREEN OUTPUT.
  LOOP AT SCREEN.
    IF screen-name = 'P_U'.
      screen-invisible = '1'.
      MODIFY SCREEN.
    ENDIF.
  ENDLOOP.
        会发现输入框消失了

此时将代码修改到上一个版本,并在AT SELECTION-SCREEN OUTPUT下清除该值,仍然没有输入框
DATA: lv_xs TYPE xstring.
PARAMETERS p_u TYPE string VISIBLE LENGTH 12 MEMORY ID m1.
AT SELECTION-SCREEN OUTPUT.
  LOOP AT SCREEN.
    IF screen-name = 'P_U'.
      screen-invisible = '1'.
      MODIFY SCREEN.
    ENDIF.
  ENDLOOP.
  CLEAR: p_u.
START-OF-SELECTION.
  WRITE:/ p_u.
        SAPKB74024 SAP Basis Component