sap ui5 table conditional row selection

Hi experts

I am trying to enable row selection only when my max selected row count = 19

my table is as follows

var oTable = new sap.ui.table.Table("tableid", {

    selectionMode :  sap.ui.table.SelectionMode.MultiToggle,

  visibleRowCount : 60,

}

but the attach row selection takes place even after that ,

please help

here selection mode is a global variable

if(selectionmode == true )

    {

    oTable.attachRowSelectionChange(function(){

       var arr=[];

       arr =  oTable.getSelectedIndices();

        var selectedrowcount  = arr.length;

        var rat = parseInt(selectedrowcount);

       if((selectedrowcount)==19)

         {  overlay_test.open();

    selectionmode = false;

         }   

  });

    }