问JQuery ui drop事件？如何知道丢弃的是什么？

EN

$(function() {
        $( "#draggable,#draggable2,#draggable3" ).draggable({ revert: "invalid" });
        $( "#draggable,#draggable2,#draggable3" ).droppable({
            accept: "#draggable,#draggable2,#draggable3",
            activeClass: "ui-state-hover",
            hoverClass: "ui-state-active",
            drop: function( event, ui ) {


                alert("destination:"+ this.id+" source:"+event.target.id);
                            alert("source is "+"???");
        });
    });