code: void start() {
          synchronized (lock) {
              if (query.getDatabase() == null) {
                  throw new IllegalArgumentException("associated database should not be null.");
              }
  
              observing = true;
              releaseResultSet();
              query.getDatabase().getActiveLiveQueries().add(this);
              // NOTE: start() method could be called during LiveQuery is running.
              // Ex) Query.setParameters() with LiveQuery.
              if (dbListenerToken == null) { dbListenerToken = query.getDatabase().addChangeListener(this); }
              update(0);
          }
      }
docs: Starts observing database changes and reports changes in the query result.