【jquery ui】sort 存儲存值及限制個數

$( "ul.droptrue" ).sortable({
 connectWith: "ul"
});

$( "ul.dropfalse" ).sortable({
 connectWith: "ul",
 update: function( event, ui ) {
  var result = $(this).sortable('toArray');
  alert(result);
  if (result.length > 2){
   alert('最多只能放2個');
   $( ".droptrue" ).sortable( "cancel" );
  }
 }
});