flutter create eth_balance_checker
cd eth_balance_checker

dependencies:
  flutter:
    sdk: flutter
  web3dart: ^2.6.0 # 最新版本可能不同
  http: ^1.1.0 # 最新版本可能不同

import 'package:flutter/material.dart';
import 'package:http/http.dart';
import 'package:web3dart/web3dart.dart';

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

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'ETH 余额查询',
      theme: ThemeData(
        primarySwatch: Colors.blueGrey,
      ),
      home: const EthBalanceCheckerScreen(),
    );
  }
}

class EthBalanceCheckerScreen extends StatefulWidget {
  const EthBalanceCheckerScreen({super.key});

  @override
  State<EthBalanceCheckerScreen> createState() => _EthBalanceCheckerScreenState();
}

class _EthBalanceCheckerScreenState extends State<EthBalanceCheckerScreen> {
  final TextEditingController _addressController = TextEditingController();
  String _balance = '0.0 ETH';
  bool _isLoading = false;
  String? _errorMessage;

  // Infura 或 Alchemy 的 RPC URL。请替换为你的实际项目 ID。
  // 建议使用环境变量或配置文件管理敏感信息。
  final String _rpcUrl = 'YOUR_INFURA_OR_ALCHEMY_RPC_URL'; // 例如: https://mainnet.infura.io/v3/YOUR_PROJECT_ID

  late Web3Client _web3client;

  @override
  void initState() {
    super.initState();
    _web3client = Web3Client(_rpcUrl, Client());
  }

  @override
  void dispose() {
    _web3client.dispose();
    _addressController.dispose();
    super.dispose();
  }

  Future<void> _checkBalance() async {
    setState(() {
      _isLoading = true;
      _errorMessage = null;
    });

    try {
      if (_rpcUrl == 'YOUR_INFURA_OR_ALCHEMY_RPC_URL') {
        throw Exception('请在代码中配置您的 Infura 或 Alchemy RPC URL。');
      }

      final String addressText = _addressController.text.trim();
      if (!EthereumAddress.isValid(addressText)) {
        throw Exception('请输入有效的 Ethereum 地址。');
      }

      final EthereumAddress address = EthereumAddress.fromHex(addressText);
      final EtherAmount balance = await _web3client.getBalance(address);

      setState(() {
        _balance = '${balance.getValueInUnit(EtherUnit.ether).toStringAsFixed(4)} ETH';
      });
    } catch (e) {
      setState(() {
        _errorMessage = '查询失败: ${e.toString()}';
      });
    } finally {
      setState(() {
        _isLoading = false;
      });
    }
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('ETH 余额查询'),
      ),
      body: Padding(
        padding: const EdgeInsets.all(16.0),
        child: Column(
          crossAxisAlignment: CrossAxisAlignment.stretch,
          children: <Widget>[
            TextField(
              controller: _addressController,
              decoration: const InputDecoration(
                labelText: 'Ethereum 地址',
                hintText: '0x...', // 提示用户输入地址格式
                border: OutlineInputBorder(),
              ),
            ),
            const SizedBox(height: 16),
            ElevatedButton(
              onPressed: _isLoading ? null : _checkBalance,
              child: _isLoading
                  ? const CircularProgressIndicator(color: Colors.white)
                  : const Text('查询余额'),
            ),
            const SizedBox(height: 24),
            if (_errorMessage != null)
              Text(
                _errorMessage!,
                style: const TextStyle(color: Colors.red, fontSize: 16),
                textAlign: TextAlign.center,
              ),
            const SizedBox(height: 16),
            Text(
              '余额: $_balance',
              style: const TextStyle(fontSize: 24, fontWeight: FontWeight.bold),
              textAlign: TextAlign.center,
            ),
          ],
        ),
      ),
    );
  }
}