mobile-music/lib/remote/discovery.dart

import 'dart:async';
import 'dart:convert';
import 'dart:io';
import 'dart:typed_data';

import 'package:nsd/nsd.dart';

import 'messages.dart';

/// A host advertising itself on the LAN, resolved enough to connect to.
class DiscoveredDevice {
  const DiscoveredDevice({
    required this.name,
    required this.host,
    required this.port,
    required this.serverKey,
    required this.version,
  });

  final String name;
  final String host;
  final int port;

  /// The host's Subsonic [serverKey]; only matches are shown/controllable.
  final String serverKey;
  final int version;

  /// `ws://host:port` — the WebSocket endpoint `remote_session.dart` dials.
  Uri get wsUri => Uri(scheme: 'ws', host: host, port: port);

  /// Stable identity for list diffing / selection (mDNS instance names are
  /// unique per network).
  String get id => name;
}

/// Wraps `nsd` mDNS registration (advertise) + discovery (browse) behind a
/// small, app-shaped API. One instance owns at most one active registration
/// and one active browse at a time.
///
/// Advertising and browsing are independent: a device that is playing
/// advertises so others can find it, and any device can browse to control one.
/// TXT records carry the [serverKey] and protocol version so the browser can
/// filter to same-server, same-version peers before ever opening a socket.
class RemoteDiscovery {
  static const _kServerKey = 'sk';
  static const _kVersion = 'v';

  Registration? _registration;
  Discovery? _discovery;
  ServiceListener? _listener;

  /// The final (possibly conflict-renamed) name of our own advertisement, so
  /// the browser can filter it out of its own results.
  String? _ownName;
  String? _browseServerKey;

  final StreamController<List<DiscoveredDevice>> _devices =
      StreamController<List<DiscoveredDevice>>.broadcast();
  List<DiscoveredDevice> _current = const [];

  /// Latest discovered, filtered device list (also replayed as a stream).
  List<DiscoveredDevice> get current => _current;
  Stream<List<DiscoveredDevice>> get devices => _devices.stream;

  bool get isAdvertising => _registration != null;
  bool get isBrowsing => _discovery != null;

  // ---- Advertise --------------------------------------------------------

  /// Publish this device as a controllable player on [port]. Replaces any
  /// existing advertisement.
  Future<void> advertise({
    required String deviceName,
    required int port,
    required String serverKey,
  }) async {
    await stopAdvertising();
    final reg = await register(Service(
      name: deviceName,
      type: kRemoteServiceType,
      port: port,
      txt: {
        _kServerKey: _bytes(serverKey),
        _kVersion: _bytes('$kRemoteProtocolVersion'),
      },
    ));
    _registration = reg;
    _ownName = reg.service.name;
  }

  Future<void> stopAdvertising() async {
    final reg = _registration;
    _registration = null;
    _ownName = null;
    if (reg != null) {
      try {
        await unregister(reg);
      } catch (_) {
        // Already gone / platform hiccup — nothing to recover.
      }
    }
  }

  // ---- Browse -----------------------------------------------------------

  /// Start browsing for players on [serverKey]'s network. Emits filtered
  /// [DiscoveredDevice] lists on [devices]. Replaces any existing browse.
  Future<void> startBrowsing(String serverKey) async {
    await stopBrowsing();
    _browseServerKey = serverKey;
    // v4 lookup so we get a routable address to dial rather than a bare
    // `.local` hostname the socket layer may not resolve.
    final discovery = await startDiscovery(
      kRemoteServiceType,
      ipLookupType: IpLookupType.v4,
    );
    _discovery = discovery;
    void listener(Service service, ServiceStatus status) => _rebuild();
    _listener = listener;
    discovery.addServiceListener(listener);
    _rebuild();
  }

  Future<void> stopBrowsing() async {
    final discovery = _discovery;
    final listener = _listener;
    _discovery = null;
    _listener = null;
    _browseServerKey = null;
    _current = const [];
    if (discovery != null) {
      if (listener != null) discovery.removeServiceListener(listener);
      try {
        await stopDiscovery(discovery);
      } catch (_) {
        // Already stopped.
      }
    }
  }

  Future<void> dispose() async {
    await stopAdvertising();
    await stopBrowsing();
    await _devices.close();
  }

  void _rebuild() {
    final discovery = _discovery;
    if (discovery == null) return;
    final out = <DiscoveredDevice>[];
    for (final s in discovery.services) {
      final dev = _toDevice(s);
      if (dev == null) continue;
      if (_browseServerKey != null && dev.serverKey != _browseServerKey) {
        continue; // different Subsonic server — its song ids won't resolve here
      }
      if (dev.version != kRemoteProtocolVersion) continue;
      if (_ownName != null && s.name == _ownName) continue; // ourselves
      out.add(dev);
    }
    _current = out;
    if (!_devices.isClosed) _devices.add(out);
  }

  DiscoveredDevice? _toDevice(Service s) {
    final port = s.port;
    if (port == null) return null;
    final host = _hostOf(s);
    if (host == null) return null;
    final txt = s.txt ?? const <String, Uint8List?>{};
    return DiscoveredDevice(
      name: s.name ?? host,
      host: host,
      port: port,
      serverKey: _text(txt, _kServerKey) ?? '',
      version: int.tryParse(_text(txt, _kVersion) ?? '') ?? 0,
    );
  }

  /// Prefer a resolved IPv4 address; fall back to the reported hostname.
  static String? _hostOf(Service s) {
    final addrs = s.addresses;
    if (addrs != null && addrs.isNotEmpty) {
      final v4 = addrs.firstWhere(
        (a) => a.type == InternetAddressType.IPv4,
        orElse: () => addrs.first,
      );
      return v4.address;
    }
    return s.host;
  }

  static Uint8List _bytes(String s) => Uint8List.fromList(utf8.encode(s));

  static String? _text(Map<String, Uint8List?> txt, String key) {
    final v = txt[key];
    if (v == null || v.isEmpty) return null;
    return utf8.decode(v, allowMalformed: true);
  }
}