jquery sortable插件問題

$('.A').sortable({
        stop : function (event , ui ) {
        ///這裏面ui代表的是什麼對象呢?有什麼用法?
                   },
});

ui.helper - the current helper element (most often a clone of the item)
ui.position - current position of the helper
ui.offset - current absolute position of the helper
ui.item - the current dragged element
ui.placeholder - the placeholder (if you defined one)
ui.sender - the sortable where the item comes from (only exists if you move from one connected list to another)

例如你拖动的对象就是ui.item

sortable({
         stop: funcion (event, ui ) {
                   alert(ui.position);  
         },
});
我這樣用時瀏覽器報錯,而且變量undifinded