feat(linux): Get the isolate implementation somewhat working

lib/src/service/foreground/isolate.dart

@@ -0,0 +1,98 @@
+import 'dart:async';
+import 'dart:convert';
+import 'dart:isolate';
+import 'dart:ui';
+import 'package:logging/logging.dart';
+import 'package:moxxy_native/src/service/config.dart';
+import 'package:moxxy_native/src/service/datasender/isolate.dart';
+import 'package:moxxy_native/src/service/datasender/types.dart';
+import 'package:moxxy_native/src/service/entrypoints/isolate.dart';
+import 'package:moxxy_native/src/service/foreground/base.dart';
+
+class IsolateForegroundService extends ForegroundService {
+  /// The port on which we receive data from the isolate.
+  final ReceivePort _receivePort = ReceivePort();
+
+  /// The port on which we send data to the isolate.
+  late final SendPort _sendPort;
+
+  /// A completer that indicates when _sendPort has been set.
+  /// For more notes, see the comment in [start].
+  Completer<void>? _sendPortCompleter = Completer<void>();
+
+  /// The data sender backing this class.
+  late final IsolateForegroundServiceDataSender _dataSender;
+
+  /// A logger.
+  final Logger _log = Logger('IsolateForegroundService');
+
+  @override
+  Future<void> attach(
+    HandleEventCallback handleData,
+  ) async {
+    _receivePort.asBroadcastStream().listen((data) async {
+      if (data is SendPort) {
+        // Set the send port.
+        _sendPort = data;
+
+        // Resolve the waiting future.
+        assert(
+          _sendPortCompleter != null,
+          '_sendPort should only be received once!',
+        );
+        _sendPortCompleter?.complete();
+        return;
+      }
+
+      await handleData(
+        jsonDecode(data! as String) as Map<String, dynamic>,
+      );
+    });
+  }
+
+  @override
+  Future<void> start(
+    ServiceConfig config,
+    HandleEventCallback uiHandleData,
+  ) async {
+    // Listen for events
+    await attach(uiHandleData);
+
+    await Isolate.spawn(
+      isolateEntrypoint,
+      [
+        _receivePort.sendPort,
+        config.toString(),
+        RootIsolateToken.instance!,
+      ],
+    );
+
+    // Wait for [_sendPort] to get set.
+    // The issue is that [_receivePort] provides a stream that only one listener can listen to.
+    // This means that we cannot do `await _receivePort.first`. To work around this, we just cram
+    // an approximation of `_receivePort.first` into the actual listener.
+    await _sendPortCompleter!.future;
+    _sendPortCompleter = null;
+
+    // Create the data sender
+    _dataSender = IsolateForegroundServiceDataSender(_sendPort);
+    _log.finest('Background service started...');
+  }
+
+  @override
+  Future<bool> isRunning() async => false;
+
+  @override
+  ForegroundServiceDataSender getDataSender() => _dataSender;
+
+  @override
+  Future<BackgroundEvent?> send(
+    BackgroundCommand command, {
+    bool awaitable = true,
+  }) {
+    return _dataSender.sendData(
+      command,
+      awaitable: awaitable,
+    );
+  }
+}