code: public void addMonitorNotifier(@NonNull MonitorNotifier notifier) {
          if (determineIfCalledFromSeparateScannerProcess()) {
              return;
          }
          //noinspection ConstantConditions
          if (notifier != null) {
              monitorNotifiers.add(notifier);
          }
      }
docs: Specifies a class that should be called each time the <code>BeaconService</code> sees or
  stops seeing a Region of beacons.
  <p/>
  Permits to register several <code>MonitorNotifier</code> objects.
  <p/>
  Unregister the notifier using {@link #removeMonitoreNotifier}
  
  @param notifier The {@link MonitorNotifier} to register.
  @see MonitorNotifier
  @see #startMonitoringBeaconsInRegion(Region)
  @see Region