code: public void startMonitoringNetworkChanges() {
          if (networkReceiver != null) {
              IntentFilter filter = new IntentFilter(ConnectivityManager.CONNECTIVITY_ACTION);
              context.registerReceiver(networkReceiver, filter);
          } else {
              logger.warn("Cannot monitor device network changes. Make sure that a NetworkConnectionListener was passed to the NetworkMonitor constructor.");
          }
      }
docs: Begin monitoring changes in the device's network connection.
  Before using this method, be sure to pass a {@link NetworkConnectionListener} to
  {@link #NetworkMonitor(Context, NetworkConnectionListener)}.