鼠标移动事件，第二次无效，求解

Question

这段代码第一次执行的时候正常，但第二次就无效，求解var cir = document.getElementsByClassName("circle")[0];cir.onmousedown = function( event ){    var disX = event.clientX - cir.offsetLeft;    var disY = event.clientY - cir.offsetTop;    document.onmousemove = function( event ){            cir.style.left = ( event.clientX - disX ) + "px";            cir.style.top = ( event.clientY - disY ) + "px";            return false;        }    }    document.onmouseup = function(){        cir.onmousedown = null;        document.onmousemove = null;    }