Dart: How to mock and stub Sqflite transaction (inner callback)? - flutter

Question

i am trying to mock the following method of sqlite_api.dart by (https://pub.dev/packages/sqflite):
Future<T> transaction<T>(Future<T> Function(Transaction txn) action, {bool? exclusive});
my implementation/adapting of the method is like:
Future<void> _transaction(Set<DatabaseLocalRequest> payload) async {
await this._api.transaction((txn) async => {
for (final req in payload) {
await txn.rawInsert(req.query.sql, req.query.arguments)
}
});
}
my db_test.dart using Mocktail (https://pub.dev/packages/mocktail):
test('if [single] put succeeds', () async {
// SETUP
sut = DatabaseLocalProvider(db: mockDb);
final query = Statement(sql: 'INSERT INTO Test(name, value, num) VALUES("some name", 1234, 456.789)');
final req = DatabaseLocalRequest(query: query);
// MOCK
when(() => mockDb.transaction((txn) => txn.rawInsert(req.query.sql, req.query.arguments)))
.thenAnswer((_) async => 1);
// ACT, ASSERT
await sut.put(req: req, bulkReq: null).then((response) => {
expect(response, ...
});
}); // test end
I got the following response from the console ERROR:
🚨🚨
type 'Null' is not a subtype of type 'Future<Set<Set<int>>>'
How do I stub the inner txn.rawInsert() method that should respond with the Future<Set<Set<int>>> with {{1}}?
Thanks in advance!

Answer 1

I might not respond exactly to your question but you can mock sqflite by using a real implementation with sqflite_common_ffi since it works on all desktop (MacOS, Linux, Windows) on the dart VM so also in flutter and dart unit tests:
More information here: https://pub.dev/packages/sqflite_common_ffi#unit-test-code
One solution is open a database in memory for each test so that you start with an empty database.
import 'package:test/test.dart';
import 'package:sqflite_common/sqlite_api.dart';
import 'package:sqflite_common_ffi/sqflite_ffi.dart';
void main() {
// Init ffi loader if needed.
sqfliteFfiInit();
test('simple sqflite example', () async {
var db = await databaseFactoryFfi.openDatabase(inMemoryDatabasePath);
expect(await db.getVersion(), 0);
await db.close();
});
}

Answer 2

when(() => mockDb.transaction(any())).thenAnswer((_) async => {{1}});
when(() => mockDb.rawInsert(any())).thenAnswer((_) async => 1);
this did the trick! but it is not 100 solution, because the closure is not stubbed but bypassed.