Add methods for exchanging database connections

Author: simolus3

packages/sqlite_async/lib/src/web/database.dart

@@ -5,11 +5,12 @@ import 'package:sqlite3/common.dart';
 import 'package:sqlite3_web/sqlite3_web.dart';
 import 'package:sqlite_async/sqlite_async.dart';
 import 'package:sqlite_async/src/utils/shared_utils.dart';
+import 'package:sqlite_async/web.dart';
 import 'protocol.dart';
 
 class WebDatabase
     with SqliteQueries, SqliteDatabaseMixin
-    implements SqliteDatabase {
+    implements SqliteDatabase, WebSqliteConnection {
   final Database _database;
   final Mutex? _mutex;
 
@@ -56,6 +57,11 @@ class WebDatabase
   /// Not relevant for web.
   Never get openFactory => throw UnimplementedError();
 
+  @override
+  Future<SqliteWebEndpoint> exposeEndpoint() async {
+    return await _database.additionalConnection();
+  }
+
   @override
   Future<T> readLock<T>(Future<T> Function(SqliteReadContext tx) callback,
       {Duration? lockTimeout, String? debugContext}) async {

packages/sqlite_async/lib/web.dart

@@ -0,0 +1,35 @@
+///
+library sqlite_async.web;
+
+import 'package:sqlite3_web/sqlite3_web.dart';
+import 'sqlite_async.dart';
+import 'src/web/database.dart';
+
+/// A [SqliteConnection] interface implemented by opened connections when
+/// running on the web.
+///
+/// This adds the [exposeEndpoint], which uses `dart:js_interop` types not
+/// supported on native Dart platforms. The method can be used to access an
+/// opened database across different JavaScript contexts
+/// (e.g. document windows and workers).
+abstract class WebSqliteConnection implements SqliteConnection {
+  /// Returns a [SqliteWebEndpoint] from `package:sqlite3/web.dart` - a
+  /// structure that consists only of types that can be transferred across a
+  /// `MessagePort` in JavaScript.
+  ///
+  /// After transferring this endpoint to another JavaScript context (e.g. a
+  /// worker), the worker can call [connectToEndpoint] to obtain a connection to
+  /// the same sqlite database.
+  Future<SqliteWebEndpoint> exposeEndpoint();
+
+  /// Connect to an endpoint obtained through [exposeEndpoint].
+  ///
+  /// The endpoint is transferrable in JavaScript, allowing multiple JavaScript
+  /// contexts to exchange opened database connections.
+  static Future<WebSqliteConnection> connectToEndpoint(
+      SqliteWebEndpoint endpoint) async {
+    final rawSqlite = await WebSqlite.connectToPort(endpoint);
+    final database = WebDatabase(rawSqlite, null);
+    return database;
+  }
+}