Run Moor within a background isolate

This basically follows the guidelines at
https://moor.simonbinder.eu/docs/advanced-features/isolates/. In the
future we will be able to have multiple simultaneous database
connections.

lib/backend.dart

@@ -1,10 +1,63 @@
+import 'dart:io';
+import 'dart:isolate';
+
 import 'package:flutter/services.dart';
 import 'package:flutter/widgets.dart';
+import 'package:moor/isolate.dart';
+import 'package:moor/moor.dart';
+import 'package:moor_ffi/moor_ffi.dart';
+import 'package:path/path.dart' as p;
+import 'package:path_provider/path_provider.dart' as pp;
 import 'package:rxdart/rxdart.dart';
 import 'package:shared_preferences/shared_preferences.dart';
 
 import 'database.dart';
 
+// The following code was taken from
+// https://moor.simonbinder.eu/docs/advanced-features/isolates/ and just
+// slightly modified.
+
+Future<MoorIsolate> _createMoorIsolate() async {
+  // This method is called from the main isolate. Since we can't use
+  // getApplicationDocumentsDirectory on a background isolate, we calculate
+  // the database path in the foreground isolate and then inform the
+  // background isolate about the path.
+  final dir = await pp.getApplicationDocumentsDirectory();
+  final path = p.join(dir.path, 'db.sqlite');
+  final receivePort = ReceivePort();
+
+  await Isolate.spawn(
+    _startBackground,
+    _IsolateStartRequest(receivePort.sendPort, path),
+  );
+
+  // _startBackground will send the MoorIsolate to this ReceivePort.
+  return (await receivePort.first as MoorIsolate);
+}
+
+void _startBackground(_IsolateStartRequest request) {
+  // This is the entrypoint from the background isolate! Let's create
+  // the database from the path we received.
+  final executor = VmDatabase(File(request.targetPath));
+  // We're using MoorIsolate.inCurrent here as this method already runs on a
+  // background isolate. If we used MoorIsolate.spawn, a third isolate would be
+  // started which is not what we want!
+  final moorIsolate = MoorIsolate.inCurrent(
+    () => DatabaseConnection.fromExecutor(executor),
+  );
+  // Inform the starting isolate about this, so that it can call .connect().
+  request.sendMoorIsolate.send(moorIsolate);
+}
+
+// Used to bundle the SendPort and the target path, since isolate entrypoint
+// functions can only take one parameter.
+class _IsolateStartRequest {
+  final SendPort sendMoorIsolate;
+  final String targetPath;
+
+  _IsolateStartRequest(this.sendMoorIsolate, this.targetPath);
+}
+
 enum BackendStatus {
   loading,
   setup,
@@ -36,6 +89,7 @@ class BackendState extends State<Backend> {
   Database db;
   String musicLibraryUri;
 
+  MoorIsolate _moorIsolate;
   SharedPreferences _shPref;
 
   @override
@@ -53,7 +107,10 @@ class BackendState extends State<Backend> {
   }
 
   Future<void> _load() async {
-    db = Database('musicus.sqlite');
+    _moorIsolate = await _createMoorIsolate();
+    final dbConnection = await _moorIsolate.connect();
+    db = Database.connect(dbConnection);
+
     _shPref = await SharedPreferences.getInstance();
     musicLibraryUri = _shPref.getString('musicLibraryUri');
     
@@ -110,6 +167,12 @@ class BackendState extends State<Backend> {
       }
     }
   }
+
+  @override
+  void dispose() {
+    super.dispose();
+    _moorIsolate.shutdownAll();
+  }
 }
 
 class _InheritedBackend extends InheritedWidget {

lib/database.dart

@@ -1,10 +1,6 @@
-import 'dart:io';
 import 'dart:math';
 
 import 'package:moor/moor.dart';
-import 'package:moor_ffi/moor_ffi.dart';
-import 'package:path/path.dart' as p;
-import 'package:path_provider/path_provider.dart' as pp;
 
 part 'database.g.dart';
 
@@ -38,13 +34,8 @@ class PerformanceModel {
     'database.moor',
   },
 )
-class Database extends _$Database {
-  Database(String fileName)
-      : super(LazyDatabase(() async {
-          final dir = await pp.getApplicationDocumentsDirectory();
-          final file = File(p.join(dir.path, fileName));
-          return VmDatabase(file);
-        }));
+class Database extends _$Database {  
+  Database.connect(DatabaseConnection connection) : super.connect(connection);
 
   @override
   int get schemaVersion => 1;