jquery mobile 的 panel中的事件绑定怎么用？

Question

jqm API中的panel关闭前的事件beforeclose怎么用，API的文档照着写不起作用：

Code examples:

Initialize the panel with the beforeclose callback specified:

$( ".selector" ).panel({ beforeclose: function( event, ui ) {}});

Bind an event listener to the panelbeforeclose event:

$( ".selector" ).on( "panelbeforeclose", function( event, ui ) {} );