Displays the current balance of a given ethereum address

main.dart

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

void main() {
  runApp(MaterialApp(
    home: EthApp(),
    theme: ThemeData(
      primaryColor: Colors.orange,
      typography: Typography(
        englishLike: Typography.englishLike2018,
      ),
    ),
    title: 'Balance app',
  ));
}

class EthApp extends StatefulWidget {
  @override
  _EthAppState createState() => _EthAppState();
}

class _EthAppState extends State<EthApp> {
  Web3Client _client;
  final TextEditingController _addressController = TextEditingController();

  bool _loading = false;
  String _error;
  String _balance;

  @override
  void initState() {
    _client = Web3Client(
        'https://mainnet.infura.io/v3/e8b3da9156fd4b4f8f8cdac7e085e1e0',
        Client());
    super.initState();
  }

  void _loadBalance() async {
    setState(() {
      _loading = true;
      _error = null;
      _balance = null;
    });

    try {
      final address = EthereumAddress(_addressController.text);
      final ethAmount = await _client.getBalance(address);

      setState(() {
        _balance = '${ethAmount.getValueInUnit(EtherUnit.ether)} Eth';
      });
    } catch (e, s) {
      print(e);
      print(s);

      setState(() {
        _error = e.toString();
      });
    } finally {
      setState(() {
        _loading = false;
      });
    }
  }

  @override
  Widget build(BuildContext context) {
    final content = <Widget>[
      Text(
        'Enter an address to show its balance',
        style: Theme.of(context).textTheme.title,
      ),
      Padding(
        padding: const EdgeInsets.all(8.0),
        child: TextField(
          controller: _addressController,
          decoration: InputDecoration(
            labelText: 'Address',
            hintText: '',
            prefix: Text('0x'),
            border: OutlineInputBorder(),
          ),
          maxLength: 40,
          maxLengthEnforced: true,
          onSubmitted: (_) => _loadBalance(),
          enabled: !_loading,
        ),
      ),
    ];

    if (_loading) {
      content.add(CircularProgressIndicator());
    }

    if (_error != null) {
      content.add(Text(_error, style: TextStyle(color: Colors.red)));
    }

    if (_balance != null) {
      content.add(Text(_balance));
    }

    return Scaffold(
      appBar: AppBar(
        title: const Text('Ethereum Balance'),
      ),
      body: Padding(
        padding: const EdgeInsets.all(8.0),
        child: Align(
          alignment: Alignment.center,
          child: Column(
            mainAxisSize: MainAxisSize.min,
            children: content,
          ),
        ),
      ),
    );
  }
}

Thanks for the help Simon

thanks a lot

I got this message, how to resolve it? thx
Compiler message:
lib/main.dart:47:58: Error: The argument type 'String' can't be assigned to the parameter type 'Uint8List'.

• 'Uint8List' is from 'dart:typed_data'.
  final address = EthereumAddress(_addressController.text);
  ^
  Target kernel_snapshot failed: Exception: Errors during snapshot creation: null

Yes, good point. It should be updated to EthereumAddress.fromHex @petergaoxl