make transaction return value

moor/lib/src/runtime/database.dart

@@ -280,22 +280,23 @@ mixin QueryEngine on DatabaseConnectionUser {
   ///     inside a transaction returns the parent transaction.
   @protected
   @visibleForTesting
-  Future transaction(Future Function() action) async {
+  Future<T> transaction<T>(Future<T> Function() action) async {
     final resolved = _resolvedEngine;
     if (resolved is Transaction) {
       return action();
     }
 
     final executor = resolved.executor;
-    await executor.doWhenOpened((executor) {
+    return await executor.doWhenOpened((executor) {
       final transactionExecutor = executor.beginTransaction();
       final transaction = Transaction(this, transactionExecutor);
 
       return _runEngineZoned(transaction, () async {
         var success = false;
         try {
-          await action();
+          final result = await action();
           success = true;
+          return result;
         } catch (e) {
           await transactionExecutor.rollback();
 

moor/lib/src/runtime/executor/before_open.dart

@@ -13,7 +13,7 @@ class BeforeOpenEngine extends DatabaseConnectionUser with QueryEngine {
 
   @override
   @alwaysThrows
-  Future transaction(Function action) {
+  Future<T> transaction<T>(Function action) {
     throw UnsupportedError("Transactions can't be started inside beforeOpen");
   }
 }

moor/test/transactions_test.dart

@@ -99,4 +99,9 @@ void main() {
       verify(executor.doWhenOpened(any));
     });
   });
+
+  test('transaction return value', () async {
+    final actual = await db.transaction(() async => 1);
+    expect(actual, 1);
+  });
 }