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
