WebSockets in Flutter: solved exercise with web_socket_channel

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# pubspec.yaml
dependencies:
  flutter:
    sdk: flutter
  web_socket_channel: ^3.0.1

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import 'dart:async';
import 'dart:convert';
import 'dart:math';
import 'package:flutter/material.dart';
import 'package:web_socket_channel/web_socket_channel.dart';

void main() => runApp(const MaterialApp(home: PriceTicker()));

// ── Models ─────────────────────────────────────────────────────────────────────
enum ConnectionState { connecting, connected, disconnected, error }

class PriceUpdate {
  final String symbol;
  final double price;
  final double change;  // percentage change from previous price
  final DateTime timestamp;

  PriceUpdate({
    required this.symbol,
    required this.price,
    required this.change,
    required this.timestamp,
  });

  factory PriceUpdate.fromJson(Map<String, dynamic> json) => PriceUpdate(
        symbol: json['symbol'] as String,
        price: (json['price'] as num).toDouble(),
        change: (json['change'] as num).toDouble(),
        timestamp: DateTime.parse(json['ts'] as String),
      );
}

// ── WebSocket service with reconnection ────────────────────────────────────────
class PriceWebSocketService {
  // For real use you can connect to wss://stream.binance.com:9443/ws
  // Here we use a public echo server and simulate data locally
  static const _wsUrl = 'wss://echo.websocket.org';

  WebSocketChannel? _channel;
  StreamController<PriceUpdate>? _priceController;
  Timer? _reconnectTimer;
  Timer? _simulatorTimer;
  int _retryCount = 0;
  bool _disposed = false;

  // Connection state exposed as ValueNotifier
  final connectionState = ValueNotifier(ConnectionState.disconnected);

  Stream<PriceUpdate> get priceStream =>
      (_priceController ??= StreamController.broadcast()).stream;

  final _subscriptions = <String>{};
  final _prices = <String, double>{'BTC': 65000, 'ETH': 3200, 'SOL': 145};

  void connect() {
    if (_disposed) return;
    _priceController ??= StreamController.broadcast();
    connectionState.value = ConnectionState.connecting;

    try {
      _channel = WebSocketChannel.connect(Uri.parse(_wsUrl));
      _channel!.ready.then((_) {
        if (_disposed) return;
        connectionState.value = ConnectionState.connected;
        _retryCount = 0;
        // Re-send active subscriptions
        for (final symbol in _subscriptions) {
          _sendSubscribe(symbol);
        }
        // On a real server we would listen to _channel!.stream
        // Here we simulate updates locally for the example
        _startSimulator();
      }).catchError(_handleError);
    } catch (e) {
      _handleError(e);
    }
  }

  void subscribe(String symbol) {
    _subscriptions.add(symbol);
    if (connectionState.value == ConnectionState.connected) {
      _sendSubscribe(symbol);
    }
  }

  void unsubscribe(String symbol) {
    _subscriptions.remove(symbol);
    if (connectionState.value == ConnectionState.connected) {
      _channel?.sink.add(jsonEncode({'action': 'unsubscribe', 'symbol': symbol}));
    }
  }

  void _sendSubscribe(String symbol) {
    _channel?.sink.add(jsonEncode({'action': 'subscribe', 'symbol': symbol}));
  }

  void _startSimulator() {
    _simulatorTimer?.cancel();
    final rng = Random();
    _simulatorTimer = Timer.periodic(const Duration(seconds: 1), (_) {
      for (final symbol in _subscriptions) {
        final prev = _prices[symbol]!;
        final delta = (rng.nextDouble() - 0.49) * prev * 0.005;
        final newPrice = prev + delta;
        _prices[symbol] = newPrice;
        _priceController?.add(PriceUpdate(
          symbol: symbol,
          price: newPrice,
          change: delta / prev * 100,
          timestamp: DateTime.now(),
        ));
      }
    });
  }

  void _handleError(dynamic error) {
    if (_disposed) return;
    connectionState.value = ConnectionState.error;
    _simulatorTimer?.cancel();
    _scheduleReconnect();
  }

  void _scheduleReconnect() {
    _reconnectTimer?.cancel();
    final delay = Duration(seconds: min(30, pow(2, _retryCount).toInt()));
    _retryCount++;
    _reconnectTimer = Timer(delay, connect);
  }

  void disconnect() {
    _reconnectTimer?.cancel();
    _simulatorTimer?.cancel();
    _channel?.sink.close();
    connectionState.value = ConnectionState.disconnected;
  }

  void dispose() {
    _disposed = true;
    disconnect();
    _priceController?.close();
  }
}

// ── Main screen ────────────────────────────────────────────────────────────────
class PriceTicker extends StatefulWidget {
  const PriceTicker({super.key});
  @override
  State<PriceTicker> createState() => _PriceTickerState();
}

class _PriceTickerState extends State<PriceTicker> {
  final _service = PriceWebSocketService();
  final _latestPrices = <String, PriceUpdate>{};
  StreamSubscription<PriceUpdate>? _sub;

  @override
  void initState() {
    super.initState();
    _service.connect();
    for (final s in ['BTC', 'ETH', 'SOL']) {
      _service.subscribe(s);
    }
    _sub = _service.priceStream.listen((update) {
      if (mounted) setState(() => _latestPrices[update.symbol] = update);
    });
  }

  @override
  void dispose() {
    _sub?.cancel();
    _service.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('Real-time ticker'),
        actions: [
          ValueListenableBuilder(
            valueListenable: _service.connectionState,
            builder: (_, state, __) => Padding(
              padding: const EdgeInsets.only(right: 16),
              child: _ConnectionBadge(state: state),
            ),
          ),
        ],
      ),
      body: Column(
        children: [
          Expanded(
            child: _latestPrices.isEmpty
                ? const Center(child: CircularProgressIndicator())
                : ListView(
                    padding: const EdgeInsets.all(12),
                    children: _latestPrices.values
                        .map((u) => _PriceCard(update: u))
                        .toList(),
                  ),
          ),
          _SubscriptionPanel(service: _service),
        ],
      ),
    );
  }
}

// ── Widgets ────────────────────────────────────────────────────────────────────
class _ConnectionBadge extends StatelessWidget {
  final ConnectionState state;
  const _ConnectionBadge({required this.state});

  @override
  Widget build(BuildContext context) {
    final (label, color) = switch (state) {
      ConnectionState.connected => ('● Connected', Colors.green),
      ConnectionState.connecting => ('● Connecting…', Colors.orange),
      ConnectionState.error => ('● Error', Colors.red),
      ConnectionState.disconnected => ('● Disconnected', Colors.grey),
    };
    return Text(label, style: TextStyle(color: color, fontSize: 12));
  }
}

class _PriceCard extends StatelessWidget {
  final PriceUpdate update;
  const _PriceCard({required this.update});

  @override
  Widget build(BuildContext context) {
    final isUp = update.change >= 0;
    return Card(
      child: ListTile(
        leading: CircleAvatar(child: Text(update.symbol[0])),
        title: Text(update.symbol,
            style: const TextStyle(fontWeight: FontWeight.bold)),
        subtitle: Text(
            '${TimeOfDay.fromDateTime(update.timestamp).format(context)}'),
        trailing: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          crossAxisAlignment: CrossAxisAlignment.end,
          children: [
            Text(
              '\$${update.price.toStringAsFixed(2)}',
              style: const TextStyle(fontSize: 18, fontWeight: FontWeight.bold),
            ),
            Text(
              '${isUp ? '+' : ''}${update.change.toStringAsFixed(3)}%',
              style: TextStyle(
                  color: isUp ? Colors.green : Colors.red, fontSize: 12),
            ),
          ],
        ),
      ),
    );
  }
}

class _SubscriptionPanel extends StatefulWidget {
  final PriceWebSocketService service;
  const _SubscriptionPanel({required this.service});
  @override
  State<_SubscriptionPanel> createState() => _SubscriptionPanelState();
}

class _SubscriptionPanelState extends State<_SubscriptionPanel> {
  final _active = {'BTC', 'ETH', 'SOL'};
  static const _available = ['BTC', 'ETH', 'SOL', 'ADA', 'DOT', 'AVAX'];

  @override
  Widget build(BuildContext context) {
    return Container(
      padding: const EdgeInsets.all(12),
      decoration: BoxDecoration(
        border: Border(top: BorderSide(color: Colors.grey.shade300)),
      ),
      child: Wrap(
        spacing: 8,
        children: _available.map((symbol) {
          final isActive = _active.contains(symbol);
          return FilterChip(
            label: Text(symbol),
            selected: isActive,
            onSelected: (selected) {
              setState(() {
                if (selected) {
                  _active.add(symbol);
                  widget.service.subscribe(symbol);
                } else {
                  _active.remove(symbol);
                  widget.service.unsubscribe(symbol);
                }
              });
            },
          );
        }).toList(),
      ),
    );
  }
}

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void _scheduleReconnect() {
  // Waits 1s, 2s, 4s, 8s… up to 30s maximum
  final delay = Duration(seconds: min(30, pow(2, _retryCount).toInt()));
  _retryCount++;
  _reconnectTimer = Timer(delay, connect);
}