code: public void onResume() {
          Session session = Session.getActiveSession();
          if (session != null) {
              if (callback != null) {
                  session.addCallback(callback);
              }
              if (SessionState.CREATED_TOKEN_LOADED.equals(session.getState())) {
                  session.openForRead(null);
              }
          }
  
          // add the broadcast receiver
          IntentFilter filter = new IntentFilter();
          filter.addAction(Session.ACTION_ACTIVE_SESSION_SET);
          filter.addAction(Session.ACTION_ACTIVE_SESSION_UNSET);
  
          // Add a broadcast receiver to listen to when the active Session
          // is set or unset, and add/remove our callback as appropriate
          broadcastManager.registerReceiver(receiver, filter);
      }
docs: To be called from an Activity or Fragment's onResume method.