Javascript面向对象拖拽

function Drag(id)
{    
    var _this=this;
    this.disX=0;
    this.disY=0;    
    this.oDiv=document.getElementById(id);
    this.oDiv.onmousedown=function(e)
    {
        _this.fnDown(e);
    }
}

Drag.prototype.fnDown=function(e)
{    
    var _this=this;
    var e=e||event;
    this.disX=e.pageX-this.oDiv.offsetLeft;
    this.disY=e.pageY-this.oDiv.offsetTop;
    document.onmousemove=function(e)
    {
        _this.fnMove(e);
    }
    document.onmouseup=function()
    {
        _this.fnUp();
    }
}

Drag.prototype.fnMove=function(e)
{
    var e=e||event;
    this.oDiv.style.left=e.pageX-this.disX+'px';
    this.oDiv.style.top=e.pageY-this.disY+'px';
}

Drag.prototype.fnUp=function()
{
    document.onmousemove=null;
    document.onmouseup=null;
}

function Drag2(id)
{
    Drag.call(this,id);
}

for(var i in Drag.prototype)
{
    Drag2.prototype[i]=Drag.prototype[i];
}

window.onload=function(){
    var a = new Drag('div1');
    var b = new Drag2('div2');
}