_onFetch(page = 1, callback, options) {
    setTimeout(() => {
      var header = 'Header '+page;
      var rows = {};
      rows[header] = ['row '+((page - 1) * 3 + 1), 'row '+((page - 1) * 3 + 2), 'row '+((page - 1) * 3 + 3)];
      if (page === 5) {
        callback(rows, {
          allLoaded: true, // the end of the list is reached
        });
      } else {
        callback(rows);
      }
    }, 1000); // simulating network fetching
  }
Will be called when refreshing
Should be replaced by your own logic
@param {number} page Requested page to fetch
@param {function} callback Should pass the rows
@param {object} options Inform if first load