How can I get the "fnRecordsTotal" if I use an AjaxSource ?

rcmeng93

      var lastRow;
      var table = $('#example1').DataTable({

          "sAjaxSource" : 'assets/ajax/page_main_online.txt'
      });

      var oSettings = table.fnSettings();
      lastRow = oSettings.fnRecordsTotal() ;

      alert(lastRow);

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When I use an AjaxSource ,the result of lastRow is 0, so how can I get the value of "fnRecordsTotal" ?

Thanks.