在TextFormField验证器颤动中调用异步函数

Future<bool> isUsernameAvailable(String username) async {
  // 模拟异步操作，例如从数据库获取数据
  await Future.delayed(Duration(seconds: 1));
  
  // 返回验证结果
  return username != "existingUser";
}

Future<void> validateUsername(String value) async {
  if (value.isNotEmpty) {
    bool isAvailable = await isUsernameAvailable(value);
    if (!isAvailable) {
      throw Exception("Username already exists");
    }
  }
}

final TextEditingController _controller = TextEditingController();
final FocusNode _focusNode = FocusNode();

@override
Widget build(BuildContext context) {
  return TextFormField(
    controller: _controller,
    focusNode: _focusNode,
    decoration: InputDecoration(labelText: "Username"),
    onChanged: (value) {
      validateUsername(value).then((_) {}).catchError((error) {
        // 处理验证错误，例如显示一个SnackBar
        ScaffoldMessenger.of(context).showSnackBar(
          SnackBar(content: Text(error.toString())),
        );
      });
    },
  );
}

_focusNode.addListener(() {
  if (!_focusNode.hasFocus) {
    validateUsername(_controller.text).then((_) {}).catchError((error) {
      // 处理验证错误
    });
  }
});