chore: screenshots

lib/data/model/app/sync.dart

@@ -0,0 +1,24 @@
+import 'dart:async';
+
+class SyncResult<T, E> {
+  final List<T> up;
+  final List<T> down;
+  final Map<T, E> err;
+
+  const SyncResult({
+    required this.up,
+    required this.down,
+    required this.err,
+  });
+
+  @override
+  String toString() {
+    return 'SyncResult{up: $up, down: $down, err: $err}';
+  }
+}
+
+abstract class SyncIface<T> {
+  /// Merge [other] into [this], return [this] after merge.
+  /// Data in [other] has higher priority than [this].
+  FutureOr<void> sync(T other);
+}