js鼠标滑轮滚动事件绑定（兼容主流浏览器）

   /** Event handler for mouse wheel event.
             *鼠标滚动事件
             */
            var wheel = function(event) {
                var delta = 0;
                if (!event) /* For IE. */
                    event = window.event;
                if (event.wheelDelta) { /* IE/Opera. */
                    delta = event.wheelDelta / 120;
                } else if (event.detail) {
                    /** Mozilla case. */
                    /** In Mozilla, sign of delta is different than in IE.
                     * Also, delta is multiple of 3.
                     */
                    delta = -event.detail / 3;
                }
                /** If delta is nonzero, handle it.
                 * Basically, delta is now positive if wheel was scrolled up,
                 * and negative, if wheel was scrolled down.
                 */
                if (delta)
                    handle(delta);
                /** Prevent default actions caused by mouse wheel.
                 * That might be ugly, but we handle scrolls somehow
                 * anyway, so don't bother here..
                 */
                if (event.preventDefault)
                    event.preventDefault();
                event.returnValue = false;
            }  

            /** Initialization code.
             * If you use your own event management code, change it as required.
             */
            if (window.addEventListener) {
                /** DOMMouseScroll is for mozilla. */
                window.addEventListener('DOMMouseScroll', wheel, false);
            }
            /** IE/Opera. */
            window.onmousewheel = document.onmousewheel = wheel;  

            /** This is high-level function.
             * It must react to delta being more/less than zero.
             */
            var handle = function(delta) {
                var random_num = Math.floor((Math.random() * 100) + 50);
                if (delta < 0) {
                    // alert("鼠标滑轮向下滚动：" + delta + "次！"); // 1
                    $("btn_next_pic").onclick(random_num);
                    return;
                } else {
                    // alert("鼠标滑轮向上滚动：" + delta + "次！"); // -1
                    $("btn_last_pic").onclick(random_num);
                    return;
                }
            }