text field 입력/미입력시마다 버튼 활성화 여부 구현하기

 ValueListenableBuilder<TextEditingValue>(
                          valueListenable: _nickNameController,
                          builder: (context, value, child) {
                            return ElevatedButton(
                              style: ElevatedButton.styleFrom(
                                minimumSize: const Size(double.infinity, 50),
                                foregroundColor: Colors.white,
                                backgroundColor: const Color(0xff2666f6),
                                shape: RoundedRectangleBorder(
                                  borderRadius: BorderRadius.circular(8.0),
                                ),
                              ),
                              onPressed: value.text.isNotEmpty
                                  ? _handleButtonPress
                                  : null,
                              child: const Text("완료",
                                  style: TextStyle(fontSize: 20.0)),
                            );
                          },
                        )

위와같이 버튼을 설정해주면 내가 원하는데로
내부에 글자가 없을때는 버튼을 비활성화, 글자가 있을때는 활성화 시켜준다.

전체코드

import 'package:flutter/material.dart';
import 'package:get/get.dart';
import 'package:yakal/viewModels/Profile/user_view_model.dart';

class ProfileHeaderText extends StatefulWidget {
  const ProfileHeaderText({
    super.key,
    required this.userViewModel,
  });

  final UserViewModel userViewModel;

  @override
  _ProfileHeaderTextState createState() => _ProfileHeaderTextState();
}

class _ProfileHeaderTextState extends State<ProfileHeaderText> {
  final TextEditingController _nickNameController = TextEditingController();
  @override
  void dispose() {
    _nickNameController.dispose();
    super.dispose();
  }

  _handleButtonPress() {
    // no nickname
    if (_nickNameController.text.isEmpty) {
      return;
    }
    String newNickName = _nickNameController.text;
    widget.userViewModel.updateNickName(newNickName);
    _nickNameController.clear(); // Clear the text field
    Navigator.pop(context);
  }

  @override
  Widget build(BuildContext context) {
    return Row(
      children: [
        // 프로필 사진
        Obx(
          // Wrap the TextSpan with Obx to automatically update when nickName changes
          () => RichText(
            text: TextSpan(
              children: [
                TextSpan(
                  text: widget.userViewModel.user.value.nickName,
                  style: const TextStyle(
                    fontSize: 20,
                    fontWeight: FontWeight.bold,
                    color: Color.fromRGBO(8, 8, 8, 1.0),
                  ),
                ),
                const TextSpan(
                  text: "님,\n약알과 함께 건강하세요!",
                  style: TextStyle(
                    fontSize: 20,
                    color: Color.fromRGBO(8, 8, 8, 1.0),
                  ),
                ),
              ],
            ),
          ),
        ),

        const Spacer(),
        // 닉네임 수정 네모 버튼 안에는 수정 이라고 글자 적혀있다.
        OutlinedButton(
          style: OutlinedButton.styleFrom(
            padding: const EdgeInsets.symmetric(vertical: 8, horizontal: 12),
            foregroundColor: Colors.black,
            backgroundColor: const Color.fromRGBO(233, 233, 238, 1),
            shape: RoundedRectangleBorder(
              borderRadius: BorderRadius.circular(8),
            ),
            side: const BorderSide(
              color: Color.fromRGBO(233, 233, 238, 1),
            ),
          ),
          onPressed: () {
            double screenHeight = MediaQuery.of(context).size.height;
            double targetHeight = 0.7 * screenHeight; // 70% of screen height
            showModalBottomSheet<void>(
              isScrollControlled: true,
              context: context,
              builder: (BuildContext context) {
                return Container(
                  //keyboard 보다 높게 올라오게 하기 위해
                  height: targetHeight,
                  width: double.infinity,
                  decoration: const BoxDecoration(
                    color: Colors.white,
                    borderRadius: BorderRadius.only(
                      topLeft: Radius.circular(30.0),
                      topRight: Radius.circular(30.0),
                    ),
                  ),
                  child: Padding(
                    padding: const EdgeInsets.fromLTRB(20, 26, 20, 20),
                    child: Column(
                      mainAxisAlignment: MainAxisAlignment.start,
                      crossAxisAlignment: CrossAxisAlignment.start,
                      children: <Widget>[
                        const Text(
                          '닉네임 수정',
                          style: TextStyle(
                            fontSize: 20.0,
                            fontWeight: FontWeight.w700,
                          ),
                        ),
                        const Text(
                          '약알이 어떻게 불러드릴까요?',
                          style: TextStyle(fontSize: 16.0),
                        ),
                        // text input box
                        const SizedBox(height: 20),
                        TextField(
                          controller: _nickNameController,
                          onChanged: (text) {
                            setState(() {
                              print(_nickNameController.text.isNotEmpty);
                            });
                          },
                          decoration: const InputDecoration(
                            border: OutlineInputBorder(
                              borderRadius:
                                  BorderRadius.all(Radius.circular(8.0)),
                              borderSide: BorderSide(
                                color: Color.fromRGBO(84, 135, 252,
                                    1), // UIColor(red: 0.33, green: 0.53, blue: 0.99, alpha: 1)
                              ),
                            ),
                            labelText: '닉네임',
                          ),
                        ),
                        const SizedBox(height: 20),

                        ValueListenableBuilder<TextEditingValue>(
                          valueListenable: _nickNameController,
                          builder: (context, value, child) {
                            return ElevatedButton(
                              style: ElevatedButton.styleFrom(
                                minimumSize: const Size(double.infinity, 50),
                                foregroundColor: Colors.white,
                                backgroundColor: const Color(0xff2666f6),
                                shape: RoundedRectangleBorder(
                                  borderRadius: BorderRadius.circular(8.0),
                                ),
                              ),
                              onPressed: value.text.isNotEmpty
                                  ? _handleButtonPress
                                  : null,
                              child: const Text("완료",
                                  style: TextStyle(fontSize: 20.0)),
                            );
                          },
                        )
                      ],
                    ),
                  ),
                );
              },
            );
          },
          child: const Text(
            '수정',
            style: TextStyle(
                fontSize: 14,
                color: Color.fromRGBO(144, 144, 159, 1),
                fontWeight: FontWeight.w500),
          ),
        ),
      ],
    );
  }
}

참고 : https://stackoverflow.com/questions/52468987/how-to-turn-disabled-button-into-enabled-button-depending-on-conditions