Improve dart demo

packages/powersync_core/example/getting_started.dart

@@ -1,7 +1,12 @@
+import 'dart:ffi';
 import 'dart:io';
 
 import 'package:powersync_core/powersync_core.dart';
 import 'package:path/path.dart';
+import 'package:powersync_core/sqlite3.dart' as sqlite;
+import 'package:powersync_core/sqlite3_common.dart';
+import 'package:powersync_core/sqlite_async.dart';
+import 'package:powersync_core/sqlite3_open.dart' as sqlite_open;
 
 const schema = Schema([
   Table('customers', [Column.text('name'), Column.text('email')])
@@ -25,13 +30,73 @@ class BackendConnector extends PowerSyncBackendConnector {
   }
 }
 
-openDatabase() async {
+/// Custom factory to load the PowerSync extension.
+/// This is required to load the extension from a custom location.
+/// The extension is required to sync data with the backend.
+/// On macOS and Linux, the default sqlite3 library is overridden to load the extension.
+class PowerSyncDartOpenFactory extends PowerSyncOpenFactory {
+  PowerSyncDartOpenFactory({required super.path, super.sqliteOptions});
+
+  @override
+  CommonDatabase open(SqliteOpenOptions options) {
+    sqlite_open.open.overrideFor(sqlite_open.OperatingSystem.linux, () {
+      return DynamicLibrary.open('libsqlite3.so.0');
+    });
+    sqlite_open.open.overrideFor(sqlite_open.OperatingSystem.macOS, () {
+      return DynamicLibrary.open('libsqlite3.dylib');
+    });
+    return super.open(options);
+  }
+
+  @override
+  void enableExtension() {
+    var powersyncLib = DynamicLibrary.open(getLibraryForPlatform());
+    sqlite.sqlite3.ensureExtensionLoaded(sqlite.SqliteExtension.inLibrary(
+        powersyncLib, 'sqlite3_powersync_init'));
+  }
+
+  @override
+  String getLibraryForPlatform({String? path = "."}) {
+    switch (Abi.current()) {
+      case Abi.androidArm:
+      case Abi.androidArm64:
+      case Abi.androidX64:
+        return '$path/libpowersync.so';
+      case Abi.macosArm64:
+      case Abi.macosX64:
+        return '$path/libpowersync.dylib';
+      case Abi.linuxX64:
+      case Abi.linuxArm64:
+        return '$path/libpowersync.so';
+      case Abi.windowsX64:
+        return '$path/powersync.dll';
+      case Abi.androidIA32:
+        throw PowersyncNotReadyException(
+          'Unsupported processor architecture. X86 Android emulators are not '
+          'supported. Please use an x86_64 emulator instead. All physical '
+          'Android devices are supported including 32bit ARM.',
+        );
+      default:
+        throw PowersyncNotReadyException(
+          'Unsupported processor architecture "${Abi.current()}". '
+          'Please open an issue on GitHub to request it.',
+        );
+    }
+  }
+}
+
+Future<String> getDatabasePath() async {
   const dbFilename = 'powersync-demo.db';
   final dir = (Directory.current.uri).toFilePath();
-  var path = join(dir, dbFilename);
+  return join(dir, dbFilename);
+}
 
+openDatabase() async {
   // Setup the database.
-  db = PowerSyncDatabase(schema: schema, path: path);
+  final psFactory = PowerSyncDartOpenFactory(path: await getDatabasePath());
+  db = PowerSyncDatabase.withFactory(psFactory, schema: schema);
+
+  // Initialise the database.
   await db.initialize();
 
   // Run local statements.