Flutter输入框TextField光标不在内容最后面

通过controller控制TextField时，默认状态下光标会跑到内容最前面。
通过以下方式，使光标保持在内容最后面；

class _FormmState extends State<Formm> {
  TextEditingController _username = new TextEditingController();
  void initState() {
    super.initState();
    _username.text = "初始值";
  }

  @override
  Widget build(BuildContext context) {
    return Container(
      height: 600,
      child: Column(
        children: <Widget>[
          TextField(
            controller: _username,
            decoration: InputDecoration(
              icon: Icon(Icons.people),
              labelText: "用户名",
              helperText: "输入长度2个字以上的汉字",
              hintText: "请输入用户名",
              errorText: "errorText",
            ),
            onChanged: (value) {
              setState(() {
                _username.text = value;
                _username.value = TextEditingValue(
                  text: _username.text,
                  selection: TextSelection.fromPosition(
                    TextPosition(
                      affinity: TextAffinity.downstream,
                      offset: _username.text.length,
                    ),
                  ),
                );
              });
            },
          ),
          Container(
            child: TextButton(
              child: Text("提交"),
              onPressed: () {
                print(_username.text);
              },
            ),
          )
        ],
      ),
    );
  }