在FLEX中添加新的datagridRow，对datagrid进行排序并提供序列号

I have a datagrid. I add a row to the datagrid using an ADD button. Once I add, I sort the datagrid based on a column. I also provide the serial numbers i.e. row numbers as first column to the datagrid. But, the serial number function does not apply after sorting. Hence, a new row added for e.g. row 5, based on sorting should be row 1, the serial number displayed is still row 5. The UI looks bad as numbers are not in correct order. the code is :I have a datagrid. I add a row to the datagrid