test: add unit test complex demo

sembast_test/test/demo_on_snapshot_query_unit_test.dart

@@ -0,0 +1,81 @@
+// Example on how to perform a complex unit test with Sembast.
+import 'dart:async';
+
+import 'package:sembast/sembast_memory.dart';
+import 'package:test/test.dart';
+
+void main() {
+  late Database db;
+  setUp(() async {
+    /// Open the database (use memory)
+    var factory = databaseFactoryMemory;
+    db = await factory.openDatabase(sembastInMemoryDatabasePath);
+  });
+  tearDown(() async {
+    await db.close();
+  });
+  test('onSnapshot', () async {
+    // Current records
+    late List<RecordSnapshot> currentQueryRecords;
+
+    // Key is an int, value is a map
+    var store = intMapStoreFactory.store();
+
+    // Create a query to test records with 'key1' = 'value1'
+    // Ordered by keys
+    var query = store.query(
+        finder: Finder(
+            filter: Filter.equals('key1', 'value1'),
+            sortOrders: [SortOrder(Field.key)]));
+
+    // Add some data
+    await db.transaction((txn) async {
+      // matches
+      await store.record(1).put(txn, {'key1': 'value1'});
+      // does not match
+      await store.record(2).put(txn, {'key1': 'value2'});
+      // matches
+      await store.record(3).put(txn, {'key1': 'value1'});
+    });
+
+    // create test completers when we have 2 records
+    var count2Completer = Completer<void>();
+    // create test completers when we have 3 records
+    var count3Completer = Completer<void>();
+
+    /// Listen to the query snapshots
+    var subscription = query.onSnapshots(db).listen((records) {
+      // For debugging purpose
+      print('records $records');
+      currentQueryRecords = records;
+      if (records.length == 2 && !count2Completer.isCompleted) {
+        count2Completer.complete();
+      } else if (records.length == 3 && !count3Completer.isCompleted) {
+        count3Completer.complete();
+      }
+    });
+
+    // Wait for the first 2 records
+    await count2Completer.future;
+    // Record 1 and 3 matches
+    expect(currentQueryRecords.map((snapshot) => snapshot.key), [1, 3]);
+
+    // Modify 3 records, modify record 2 to match, delete record 3, add record 4
+    // 3 rcords should match then
+    await db.transaction((txn) async {
+      // matches
+      await store.record(2).put(txn, {'key1': 'value1'});
+      // matches
+      await store.record(3).delete(txn);
+      // matches
+      await store.record(4).put(txn, {'key1': 'value1'});
+    });
+
+    // Wait for the query to match 3 records
+    await count3Completer.future;
+    // Record 1, 2 and 4 matches
+    expect(currentQueryRecords.map((snapshot) => snapshot.key), [1, 2, 4]);
+    await subscription.cancel();
+    await db.close();
+  });
+}