拖拽效果实现原理

    oDiv.onmousedown = function(){
        document.onmousemove = function(){
            var e = e || window.event;
            var diffX = e.clientX - oDiv.offdsetLeft;
            var diffY = e.clientY - oDiv.offdsettop;

            var _this = this;

            _this.style.left = e.clientX - diffX + ‘px‘;     // 鼠标点击哪里就可以从哪里开始拖拽
            _this.style.top = e.clientY - diffY + ‘px‘;
        }
        document.onmouseup = function(){
            this.onmousemove = null;
            this.onmouseup = null;
        }
    }