html 简单 div 拖动

解决:   div的 onclike ( this )  得到选择
            由 window.event 得到 鼠标 x y
            由 this.style.top/left 改变 div 位置

        var x,y;
        function mousedown(obj)
        {
            obj.onmousemove = mousemove;
            obj.onmouseup = mouseup;
           
            oEvent = window.event ? window.event : event;
            x = oEvent.clientX;
            y = oEvent.clientY;
        }
        function mousemove()
        {
            oEvent = window.event ? window.event : event;
            var _top = oEvent.clientY - y + parseInt(this.style.top) + "px"; // oEvent.clientY - y  为div 上 移动的位置 再反映到                                                                                //parseInt(this.style.top)
            var _left = oEvent.clientX - x + parseInt(this.style.left) +"px";
            this.style.top = _top;
            this.style.left = _left;
            x =  oEvent.clientX;
            y =  oEvent.clientY
        }
        function mouseup()
        {
            this.onmousemove = null;
            this.onmouseup = null;
        }