feat(linux): Get the isolate implementation somewhat working

lib/pigeon/background_service.g.dart

@@ -20,10 +20,10 @@ class MoxxyBackgroundServiceApi {
 
   Future<String> getExtraData() async {
     final BasicMessageChannel<Object?> channel = BasicMessageChannel<Object?>(
-        'dev.flutter.pigeon.moxxy_native.MoxxyBackgroundServiceApi.getExtraData', codec,
+        'dev.flutter.pigeon.moxxy_native.MoxxyBackgroundServiceApi.getExtraData',
+        codec,
         binaryMessenger: _binaryMessenger);
-    final List<Object?>? replyList =
-        await channel.send(null) as List<Object?>?;
+    final List<Object?>? replyList = await channel.send(null) as List<Object?>?;
     if (replyList == null) {
       throw PlatformException(
         code: 'channel-error',
@@ -47,7 +47,8 @@ class MoxxyBackgroundServiceApi {
 
   Future<void> setNotificationBody(String arg_body) async {
     final BasicMessageChannel<Object?> channel = BasicMessageChannel<Object?>(
-        'dev.flutter.pigeon.moxxy_native.MoxxyBackgroundServiceApi.setNotificationBody', codec,
+        'dev.flutter.pigeon.moxxy_native.MoxxyBackgroundServiceApi.setNotificationBody',
+        codec,
         binaryMessenger: _binaryMessenger);
     final List<Object?>? replyList =
         await channel.send(<Object?>[arg_body]) as List<Object?>?;
@@ -69,7 +70,8 @@ class MoxxyBackgroundServiceApi {
 
   Future<void> sendData(String arg_data) async {
     final BasicMessageChannel<Object?> channel = BasicMessageChannel<Object?>(
-        'dev.flutter.pigeon.moxxy_native.MoxxyBackgroundServiceApi.sendData', codec,
+        'dev.flutter.pigeon.moxxy_native.MoxxyBackgroundServiceApi.sendData',
+        codec,
         binaryMessenger: _binaryMessenger);
     final List<Object?>? replyList =
         await channel.send(<Object?>[arg_data]) as List<Object?>?;
@@ -93,8 +95,7 @@ class MoxxyBackgroundServiceApi {
     final BasicMessageChannel<Object?> channel = BasicMessageChannel<Object?>(
         'dev.flutter.pigeon.moxxy_native.MoxxyBackgroundServiceApi.stop', codec,
         binaryMessenger: _binaryMessenger);
-    final List<Object?>? replyList =
-        await channel.send(null) as List<Object?>?;
+    final List<Object?>? replyList = await channel.send(null) as List<Object?>?;
     if (replyList == null) {
       throw PlatformException(
         code: 'channel-error',

lib/src/service/background/isolate.dart

@@ -0,0 +1,53 @@
+import 'dart:convert';
+import 'dart:isolate';
+import 'dart:ui';
+import 'package:logging/logging.dart';
+import 'package:moxlib/moxlib.dart';
+import 'package:moxxy_native/src/service/background/base.dart';
+import 'package:moxxy_native/src/service/config.dart';
+import 'package:moxxy_native/src/service/datasender/types.dart';
+import 'package:uuid/uuid.dart';
+
+class IsolateBackgroundService extends BackgroundService {
+  IsolateBackgroundService(this._sendPort);
+  final SendPort _sendPort;
+  final ReceivePort receivePort = ReceivePort();
+
+  /// A logger.
+  final Logger _log = Logger('IsolateBackgroundService');
+
+  @override
+  Future<void> send(BackgroundEvent event, {String? id}) async {
+    final data = DataWrapper(
+      id ?? const Uuid().v4(),
+      event,
+    );
+
+    _sendPort.send(jsonEncode(data.toJson()));
+  }
+
+  @override
+  Future<void> init(
+    ServiceConfig config,
+  ) async {
+    // Ensure that the Dart executor is ready to use plugins
+    // NOTE: We're not allowed to use this here. Maybe reusing the RootIsolateToken
+    //       (See IsolateForegroundService) helps?
+    // WidgetsFlutterBinding.ensureInitialized();
+    DartPluginRegistrant.ensureInitialized();
+
+    // Register the channel for Foreground -> Service communication
+    receivePort.listen((data) async {
+      // TODO(Unknown): Maybe do something smarter like pigeon and use Lists instead of Maps
+      await config
+          .handleData(jsonDecode(data! as String) as Map<String, dynamic>);
+    });
+
+    // Start execution
+    _log.finest('Setup complete. Calling main entrypoint...');
+    await config.entrypoint(config.initialLocale);
+  }
+
+  @override
+  void setNotificationBody(String body) {}
+}

lib/src/service/datasender/isolate.dart

@@ -0,0 +1,15 @@
+import 'dart:convert';
+import 'dart:isolate';
+import 'package:moxlib/moxlib.dart';
+import 'package:moxxy_native/src/service/datasender/types.dart';
+
+class IsolateForegroundServiceDataSender
+    extends AwaitableDataSender<BackgroundCommand, BackgroundEvent> {
+  IsolateForegroundServiceDataSender(this._port);
+  final SendPort _port;
+
+  @override
+  Future<void> sendDataImpl(DataWrapper<JsonImplementation> data) async {
+    _port.send(jsonEncode(data.toJson()));
+  }
+}

lib/src/service/datasender/types.dart

@@ -1,8 +1,4 @@
-import 'dart:io';
 import 'package:moxlib/moxlib.dart';
-import 'package:moxxy_native/pigeon/service.g.dart';
-import 'package:moxxy_native/src/service/datasender/pigeon.dart';
-import 'package:moxxy_native/src/service/exceptions.dart';
 
 typedef ForegroundServiceDataSender
     = AwaitableDataSender<BackgroundCommand, BackgroundEvent>;
@@ -10,11 +6,3 @@ typedef ForegroundServiceDataSender
 abstract class BackgroundCommand implements JsonImplementation {}
 
 abstract class BackgroundEvent implements JsonImplementation {}
-
-ForegroundServiceDataSender getForegroundDataSender(MoxxyServiceApi api) {
-  if (Platform.isAndroid) {
-    return PigeonForegroundServiceDataSender(api);
-  } else {
-    throw UnsupportedPlatformException();
-  }
-}

lib/src/service/entrypoints/base.dart

@@ -0,0 +1,28 @@
+import 'dart:io';
+import 'package:moxxy_native/src/service/config.dart';
+import 'package:moxxy_native/src/service/entrypoints/isolate.dart';
+import 'package:moxxy_native/src/service/entrypoints/pigeon.dart';
+import 'package:moxxy_native/src/service/exceptions.dart';
+
+typedef PlatformEntrypointCallback = Future<void> Function(dynamic);
+
+ServiceConfig getServiceConfig(
+  HandleEventCallback srvHandleData,
+  HandleEventCallback uiHandleData,
+  String initialLocale,
+) {
+  PlatformEntrypointCallback entrypoint;
+  if (Platform.isAndroid) {
+    entrypoint = pigeonEntrypoint;
+  } else if (Platform.isLinux || Platform.isWindows || Platform.isMacOS) {
+    entrypoint = isolateEntrypoint;
+  } else {
+    throw UnsupportedPlatformException();
+  }
+
+  return ServiceConfig(
+    entrypoint,
+    srvHandleData,
+    initialLocale,
+  );
+}

lib/src/service/entrypoints/isolate.dart

@@ -0,0 +1,30 @@
+import 'dart:isolate';
+import 'package:flutter/services.dart';
+import 'package:get_it/get_it.dart';
+import 'package:moxxy_native/src/service/background/base.dart';
+import 'package:moxxy_native/src/service/background/isolate.dart';
+import 'package:moxxy_native/src/service/config.dart';
+
+@pragma('vm:entry-point')
+Future<void> isolateEntrypoint(dynamic parameters) async {
+  parameters as List<dynamic>;
+
+  final sendPort = parameters[0] as SendPort;
+  final config = ServiceConfig.fromString(parameters[1] as String);
+
+  // This allows us to use the root isolate's method channels.
+  // See https://medium.com/flutter/introducing-background-isolate-channels-7a299609cad8
+  BackgroundIsolateBinaryMessenger.ensureInitialized(
+    parameters[2] as RootIsolateToken,
+  );
+
+  // Set up the background service
+  final srv = IsolateBackgroundService(sendPort);
+  GetIt.I.registerSingleton<BackgroundService>(srv);
+
+  // Reply back with the new send port
+  sendPort.send(srv.receivePort.sendPort);
+
+  // Run the entrypoint
+  await srv.init(config);
+}

lib/src/service/entrypoints/pigeon.dart

@@ -8,7 +8,7 @@ import 'package:moxxy_native/src/service/config.dart';
 /// An entrypoint that should be used when the service runs
 /// in a new Flutter Engine.
 @pragma('vm:entry-point')
-Future<void> pigeonEntrypoint() async {
+Future<void> pigeonEntrypoint(dynamic _) async {
   // ignore: avoid_print
   print('androidEntrypoint: Called on new FlutterEngine');
 

lib/src/service/foreground/base.dart

@@ -3,6 +3,7 @@ import 'package:moxlib/moxlib.dart';
 import 'package:moxxy_native/src/service/config.dart';
 import 'package:moxxy_native/src/service/datasender/types.dart';
 import 'package:moxxy_native/src/service/exceptions.dart';
+import 'package:moxxy_native/src/service/foreground/isolate.dart';
 import 'package:moxxy_native/src/service/foreground/pigeon.dart';
 
 /// Wrapper API that is only available to the UI isolate.
@@ -40,6 +41,8 @@ ForegroundService getForegroundService() {
   if (_service == null) {
     if (Platform.isAndroid) {
       _service = PigeonForegroundService();
+    } else if (Platform.isLinux || Platform.isWindows || Platform.isMacOS) {
+      _service = IsolateForegroundService();
     } else {
       throw UnsupportedPlatformException();
     }

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,
+    );
+  }
+}