div层的移动及性能优化

function Endrag(source,target){
source=typeof(source)=="object" ? source:document.getElementById(source);
target=typeof(target)=="object" ? target:document.getElementById(target);
var x0=0,y0=0,x1=0,y1=0,moveable=false,index=100;
var timer,flag=false;
var i=0;
source.onmousedown=function(e){
e = e ? e : (window.event ? window.event : null);
x0 = e.clientX ;
y0 = e.clientY ;
x1 = isNaN(parseInt(source.style.left))?0:parseInt(source.style.left);
y1 = isNaN(parseInt(source.style.top))?0:parseInt(source.style.top);
moveable = true;
//当鼠标按下时，定时器开始工作，每隔50ms执行一次mousemove事件
timer=setInterval(function(){flag=true;},30);
};
//拖动;
source.onmousemove=function(e){
e = e ? e : (window.event ? window.event : null);
if(moveable){
if(flag){
i++;
flag=false;
target.style.left = (e.clientX + x1 - x0 ) + "px";
target.style.top = (e.clientY + y1 - y0 ) + "px";
}
}
};
//停止拖动;
source.onmouseup=function (e){
if(moveable) {
moveable = false;
clearInterval(timer);
//alert(i);
}
};
//停止拖动;
source.onmouseout=function (e){
if(moveable) {
moveable = false;
clearInterval(timer);
//alert(i);
}
};
}