Fixed issue #150, by better supporting background/multiple `FlutterEn…

…gine`s

lib/src/backend/impls/objectbox/backend/backend.dart

@@ -7,6 +7,7 @@ import 'dart:io';
 import 'dart:isolate';
 
 import 'package:collection/collection.dart';
+import 'package:flutter/material.dart';
 import 'package:flutter/services.dart';
 import 'package:meta/meta.dart';
 import 'package:path/path.dart' as path;
@@ -44,19 +45,25 @@ final class FMTCObjectBoxBackend implements FMTCBackend {
   /// [the ObjectBox docs](https://docs.objectbox.io/getting-started) for
   /// details.
   ///
+  /// [rootIsolateToken] should only be used in exceptional circumstances where
+  /// this backend is being initialised in a seperate isolate (or background)
+  /// thread.
+  ///
   /// Avoid using [useInMemoryDatabase] outside of testing purposes.
   @override
   Future<void> initialise({
     String? rootDirectory,
     int maxDatabaseSize = 10000000,
     String? macosApplicationGroup,
+    RootIsolateToken? rootIsolateToken,
     @visibleForTesting bool useInMemoryDatabase = false,
   }) =>
       FMTCObjectBoxBackendInternal._instance.initialise(
         rootDirectory: rootDirectory,
         maxDatabaseSize: maxDatabaseSize,
         macosApplicationGroup: macosApplicationGroup,
         useInMemoryDatabase: useInMemoryDatabase,
+        rootIsolateToken: rootIsolateToken,
       );
 
   /// {@macro fmtc.backend.uninitialise}

lib/src/backend/impls/objectbox/backend/internal.dart

@@ -118,9 +118,19 @@ class _ObjectBoxBackendImpl implements FMTCObjectBoxBackendInternal {
     required int maxDatabaseSize,
     required String? macosApplicationGroup,
     required bool useInMemoryDatabase,
+    required RootIsolateToken? rootIsolateToken,
   }) async {
     if (_sendPort != null) throw RootAlreadyInitialised();
 
+    // Obtain the `RootIsolateToken` to enable the worker isolate to use
+    // ObjectBox
+    rootIsolateToken ??= ServicesBinding.rootIsolateToken ??
+        (throw StateError(
+          'Unable to start FMTC in a background thread without access to a '
+          '`RootIsolateToken`',
+        ));
+
+    // Construct the root directory path
     if (useInMemoryDatabase) {
       this.rootDirectory = Store.inMemoryPrefix + (rootDirectory ?? 'fmtc');
     } else {
@@ -137,16 +147,11 @@ class _ObjectBoxBackendImpl implements FMTCObjectBoxBackendInternal {
     _workerResOneShot[0] = Completer();
     final workerInitialRes = _workerResOneShot[0]!
         .future // Completed directly by handler below
-        .then<({ByteData? storeRef, Object? err, StackTrace? stackTrace})>(
+        .then<({Object err, StackTrace stackTrace})?>(
       (res) {
         _workerResOneShot.remove(0);
         _sendPort = res!['sendPort'];
-
-        return (
-          storeRef: res['storeReference'] as ByteData,
-          err: null,
-          stackTrace: null,
-        );
+        return null;
       },
       onError: (err, stackTrace) {
         _workerHandler.cancel();
@@ -156,7 +161,7 @@ class _ObjectBoxBackendImpl implements FMTCObjectBoxBackendInternal {
         _workerResOneShot.clear();
         _workerResStreamed.clear();
 
-        return (storeRef: null, err: err, stackTrace: stackTrace);
+        return (err: err, stackTrace: stackTrace);
       },
     );
 
@@ -209,22 +214,19 @@ class _ObjectBoxBackendImpl implements FMTCObjectBoxBackendInternal {
         rootDirectory: this.rootDirectory,
         maxDatabaseSize: maxDatabaseSize,
         macosApplicationGroup: macosApplicationGroup,
-        rootIsolateToken: ServicesBinding.rootIsolateToken!,
+        rootIsolateToken: rootIsolateToken,
       ),
       onExit: receivePort.sendPort,
       debugName: '[FMTC] ObjectBox Backend Worker',
     );
 
-    // Wait for initial response from isolate
-    final initResult = await workerInitialRes;
-
-    // Check whether initialisation was successful
-    if (initResult.storeRef case final storeRef?) {
+    // Check whether initialisation was successful after initial response
+    if (await workerInitialRes case (:final err, :final stackTrace)) {
+      Error.throwWithStackTrace(err, stackTrace);
+    } else {
       FMTCBackendAccess.internal = this;
       FMTCBackendAccessThreadSafe.internal =
-          _ObjectBoxBackendThreadSafeImpl._(storeReference: storeRef);
-    } else {
-      Error.throwWithStackTrace(initResult.err!, initResult.stackTrace!);
+          _ObjectBoxBackendThreadSafeImpl._(rootDirectory: this.rootDirectory);
     }
   }
 

lib/src/backend/impls/objectbox/backend/internal_workers/standard/worker.dart

@@ -25,11 +25,18 @@ Future<void> _worker(
   // Open database, kill self if failed
   late final Store root;
   try {
-    root = await openStore(
-      directory: input.rootDirectory,
-      maxDBSizeInKB: input.maxDatabaseSize, // Defaults to 10 GB
-      macosApplicationGroup: input.macosApplicationGroup,
-    );
+    // If already opened, attach existing instance
+    // This can occur when a background `FlutterEngine` is in use, keeping the
+    // database open
+    if (Store.isOpen(input.rootDirectory)) {
+      root = Store.attach(getObjectBoxModel(), input.rootDirectory);
+    } else {
+      root = await openStore(
+        directory: input.rootDirectory,
+        maxDBSizeInKB: input.maxDatabaseSize, // Defaults to 10 GB
+        macosApplicationGroup: input.macosApplicationGroup,
+      );
+    }
 
     // If the database is new, create the root statistics object
     final rootBox = root.box<ObjectBoxRoot>();
@@ -47,7 +54,7 @@ Future<void> _worker(
   // Respond with comms channel for future cmds
   sendRes(
     id: 0,
-    data: {'sendPort': receivePort.sendPort, 'storeReference': root.reference},
+    data: {'sendPort': receivePort.sendPort},
   );
 
   //! UTIL METHODS !//
@@ -58,8 +65,6 @@ Future<void> _worker(
   /// Should be run within a transaction.
   ///
   /// Specified values may be negative.
-  ///
-  /// Handles cases where there is no root statistics object yet.
   void updateRootStatistics({int deltaLength = 0, int deltaSize = 0}) =>
       root.box<ObjectBoxRoot>().put(
             root.box<ObjectBoxRoot>().get(1)!
@@ -836,8 +841,7 @@ Future<void> _worker(
               (numExportedTiles) {
                 if (numExportedTiles == 0) {
                   throw ArgumentError(
-                    'must include at least one tile in any of the specified '
-                        'stores',
+                    'Specified stores must include at least one tile total',
                     'storeNames',
                   );
                 }

lib/src/backend/impls/objectbox/backend/internal_workers/thread_safe.dart

@@ -5,20 +5,20 @@ part of '../backend.dart';
 
 class _ObjectBoxBackendThreadSafeImpl implements FMTCBackendInternalThreadSafe {
   _ObjectBoxBackendThreadSafeImpl._({
-    required this.storeReference,
+    required this.rootDirectory,
   });
 
   @override
   String get friendlyIdentifier => 'ObjectBox';
 
-  final ByteData storeReference;
+  final String rootDirectory;
   Store get expectInitialisedRoot => _root ?? (throw RootUnavailable());
   Store? _root;
 
   @override
   void initialise() {
     if (_root != null) throw RootAlreadyInitialised();
-    _root = Store.fromReference(getObjectBoxModel(), storeReference);
+    _root = Store.attach(getObjectBoxModel(), rootDirectory);
   }
 
   @override
@@ -29,7 +29,7 @@ class _ObjectBoxBackendThreadSafeImpl implements FMTCBackendInternalThreadSafe {
 
   @override
   _ObjectBoxBackendThreadSafeImpl duplicate() =>
-      _ObjectBoxBackendThreadSafeImpl._(storeReference: storeReference);
+      _ObjectBoxBackendThreadSafeImpl._(rootDirectory: rootDirectory);
 
   @override
   Future<ObjectBoxTile?> readTile({