function Alex() {

    //给予video.js的页面滚动到视频元素范围内自动播放/出范围暂停播放-----01

    this.video_autoplay = function (box) {

        var video_box = box.getElementsByTagName('video')[0];

        //能否自动播放

        var canAuto = true;

        //元素距离顶部高度//上滚此距离元素完全出现

        var top = box.offsetTop;

        //元素高度

        var height = parseFloat((window.getComputedStyle ? window.getComputedStyle(box, null) : null || box.currentStyle).height);

        //网页可见区域高度

        var _height = window.innerHeight || document.documentElement.clientHeight || document.body.clientHeight;

        //下滚此距离元素完全出现

        var min_scroll_height = top - _height + height;

        //下滚超过此距离元素完全消失

        var max_scroll_height = top + height;

        //上滚此距离元素完全出现

        var up_max_height = max_scroll_height - height;

        //上滚此距离元素完全消失

        var up_min_height = top - _height;

        //人为点击之后不再自动播放

        box.addEventListener('click', function () {

            canAuto = false;

        });

        //判断是上滚还是下滚

        var scroll = function (fn) {

            var beforeScrollTop = document.documentElement.scrollTop || document.body.scrollTop,

                fn = fn || function () {

                    };

            window.addEventListener("scroll", function () {

                var afterScrollTop = document.documentElement.scrollTop || document.body.scrollTop,

                    delta = afterScrollTop - beforeScrollTop;

                if (delta === 0) return false;

                fn(delta > 0 ? "down" : "up");

                beforeScrollTop = afterScrollTop;

            }, false);

        }

        var video_scroll = function () {

            //可以自动播放

            scroll(function (direction) {

                var _top = document.documentElement.scrollTop || document.body.scrollTop;

                if ((direction == 'down') && (_top > min_scroll_height) && canAuto) {

                    video_box.play();

                }

                if ((direction == 'down') && (_top > max_scroll_height)) {

                    video_box.pause();

                }

                if ((direction == 'up') && (_top < up_max_height) && canAuto) {

                    video_box.play();

                }

                if ((direction == 'up') && (_top < up_min_height)) {

                    video_box.pause();

                }

            });

        }

        video_scroll();

    };

    //元素滚动到一定距离后固定在页面顶部------------------------------02

    this.menu_fixed = function (menu_container) {

        var _top = menu_container.offsetTop;

        var scroll_div = function () {

            var top = document.documentElement.scrollTop || document.body.scrollTop;

            if (top > _top) {

                menu_container.style.position = 'fixed';

                menu_container.style.top = '0';

            } else if (top < _top) {

                menu_container.style.position = 'relative';

                menu_container.style.top = '0';

            }

        }

        scroll_div();

    }

}

var alex = new Alex();

var box = document.querySelector('.index_video');

alex.video_autoplay(box);