Flutter TextField输入内容后 保持光标在最后

/// 输入框控制器
TextEditingController _textEditingController = new TextEditingController();

/// 给控制器赋值
textSetting(textStr){
    _textEditingController.value = TextEditingValue(
      text: textStr,
      selection: TextSelection.fromPosition(
          TextPosition( /// 保持光标在最后
            affinity: TextAffinity.downstream,
            offset: textStr.length,
          ),
      ),
   );
}

/// 输入框组件
TextField(
  controller: _textEditingController
)