滚轮事件onmousewheel (event.wheelDelta<0:滚轮向下，event.wheelDelta>0:滚轮向上)【firefox不支持】DOMMouseScroll(使用addEventListener事件绑定) (event.detail < 0:滚轮向上，event.detail > 0:滚轮向下)【firefox支持方法】滚轮事件兼容写法：function mouseWheelFn(element, upFn, downFn){   element.onmousewheel = fn;   if(window.addEventListener){        element.addEventListener("DOMMouseScroll", fn, false);   }   function fn(e){            var e = e || window.event;            // 判断滚轮方向(向上)        if(e.wheelDelta > 0 || e.detail < 0){            upFn();        }else{            downFn();        }        // 阻止冒泡        e.cancelBubble = true;        e.stopPropagation();    }}