这是我的代码，我使用的是textformfield，这是用于otp输入的，所以每当用户点击清除按钮时，我想把注意力集中在前一个文本字段上。

TextFormField(
    controller: controller,
    autofocus: true,
    focusNode: focusNod,
    validator: (String value) {
      if (value.isEmpty) {
        return "";
      } else {
        setState(() {
          otp = one.text + two.text + three.text + 
          four.text;
          print("otp : " + otp.toString());
        });
        return null;
      }
    },
    onChanged: (String number) {
      if (number.isNotEmpty) {
        focus.requestFocus();
      }

      navigation();
    },
    keyboardType: TextInputType.number,
    inputFormatters: [
      FilteringTextInputFormatter.digitsOnly,
      LengthLimitingTextInputFormatter(1)
    ],
    style: TextStyle(color: Colors.white),
    decoration: InputDecoration(
      helperText: '',
      border: OutlineInputBorder(
          borderRadius: BorderRadius.circular(12),
          borderSide: BorderSide(color: Colors.green)),
      focusedBorder: OutlineInputBorder(
          borderRadius: BorderRadius.circular(12),
          borderSide: BorderSide(color: Colors.white)),
      errorBorder: OutlineInputBorder(
          borderRadius: BorderRadius.circular(12),
          borderSide: BorderSide(color: Colors.white)),
      focusedErrorBorder: OutlineInputBorder(
          borderRadius: BorderRadius.circular(12),
          borderSide: BorderSide(color: Colors.white)),
      enabledBorder: OutlineInputBorder(
          borderRadius: BorderRadius.circular(12),
          borderSide: BorderSide(color: Colors.white)),
    ),
    textInputAction: action,
    cursorColor: Color(0xFFFFFFFF),
  ),