addPendingOperation method
Add a new pending operation to the queue.
Implementation
@override
Future<void> addPendingOperation(String userId, DatumSyncOperation<T> operation) async {
final ops = _pendingOps.putIfAbsent(userId, () => []);
final i = ops.indexWhere((o) => o.id == operation.id);
if (i != -1) {
ops[i] = operation;
} else {
ops.add(operation);
}
}