A simple widget for validating password conditions like length >= 8, hasNumber etc

password_validator.dart

import 'package:flutter/material.dart';

const Color darkBlue = Color.fromARGB(255, 18, 32, 47);

void main() {
  runApp(MyApp());
}

class MyApp extends StatefulWidget {
  @override
  State<MyApp> createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  
  final _passwordController = TextEditingController();
  
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData.dark().copyWith(
        scaffoldBackgroundColor: darkBlue,
      ),
      debugShowCheckedModeBanner: false,
      home: Scaffold(
        body: Column(
          children: [
            TextField(
              controller: _passwordController,
            ),
            ValueListenableBuilder(
              valueListenable: _passwordController,
              builder: (context, textEditingValue, _) {
                final password = textEditingValue.text; 
                final hasAtLeastEightChars = password.length >= 8;
                final hasUpperAndLowerCaseChars = password.contains(RegExp('[A-Z]'));
                final hasNumber = password.contains(RegExp('[0-9]'));
                final hasSpecialChar = password.contains(RegExp(r'[!@#$%^&*(),.?":{}|<>]'));
                return  PasswordValidator(
                  hasAtLeastEightChars: hasAtLeastEightChars, 
                  hasUpperAndLowerCaseChars: hasUpperAndLowerCaseChars, 
                  hasNumber: hasNumber,
                  hasSpecialChar: hasSpecialChar,
                );
              }
            ),
          ],
        ),
      ),
    );
  }
}

class PasswordValidator extends StatelessWidget{
  final bool hasAtLeastEightChars;
  final bool hasUpperAndLowerCaseChars;
  final bool hasNumber;
  final bool hasSpecialChar;

  const PasswordValidator({
    super.key,
    required this.hasAtLeastEightChars, 
    required this.hasUpperAndLowerCaseChars, 
    required this.hasNumber, 
    required this.hasSpecialChar
  });
  @override
  Widget build(BuildContext context) {
    return Column(
      mainAxisSize: MainAxisSize.min,
      children: [
        PasswordConditionIndicator(
          isValid: hasAtLeastEightChars,
          title: 'At least 8 characters',
        ),
        PasswordConditionIndicator(
          isValid: hasUpperAndLowerCaseChars,
          title: 'Upper and lower case characters',
        ),
        PasswordConditionIndicator(
          isValid: hasNumber,
          title: '1 or more numbers',
        ),
        PasswordConditionIndicator(
          isValid: hasSpecialChar,
          title: '1 or more special characters',
        ),
      ],
    );
  }
}

class PasswordConditionIndicator extends StatelessWidget{
  final bool isValid;
  final String title;

  const PasswordConditionIndicator({
    super.key, 
    required this.isValid,
    required this.title,
  });
  
  @override
  Widget build(BuildContext context) {
    return Row(
          children: [
            if(isValid) const Icon(Icons.check),
            Text(title),
          ]
     );
  }
  
}