enqueue method
Implementation
Future<void> enqueue(NotificationActionEvent event) {
_serial = _serial.then((_) async {
final queue = await _read();
final identity = _identity(event);
if (queue.any((item) => item.identity == identity)) return;
queue.add(_QueuedAction(identity: identity, event: event, attempts: 0));
await _write(queue);
});
return _serial;
}