code: private static void callback(long handle) {
          final C4DatabaseObserver obs = REVERSE_LOOKUP_TABLE.get(handle);
          if (obs != null && obs.listener != null) { obs.listener.callback(obs, obs.context); }
      }
docs: Callback invoked by a database observer.
  <p>
  NOTE: Two parameters, observer and context, which are defined for iOS:
  observer -> this instance
  context ->  maintained in java layer