1 $(function () {
 2     var scrollTop = 0, //缓存上一次触发scroll的时候的scrollTop值
 3         appendIndex = 0, //由于第23行append这个操作需要时间，在这个时间内可能多次触发scroll事件，为了不会append多次，这里缓存一个appendIndex变量，标识是否已经进行append操作
 4         removeIndex = 0, //同上，为了第46行不会detach多次
 5         firstTitle = $("p.contentTitle").eq(0);
 6     if (firstTitle != undefined) {
 7         $("#topTitle p.titleNormal").html(firstTitle.html());
 8     }
 9     $(window).scroll(function () {
10         var topTitle = $("#topTitle p"), //顶部title
11                     topTitleWrap = $("#topTitle"), //顶部title的容器
12                     curIndex = parseInt(topTitle.last().attr("data-index")), //顶部title最后的索引值
13                     nextTitle = $("p.contentTitle").eq(curIndex + 1), //向上滑动时下一个title
14                     prevTitle = $("p.contentTitle").eq(curIndex - 1), //向下滑动时上一个title
15                     curTitle = $("p.contentTitle").eq(curIndex),
16                     curScrollTop = $("body").scrollTop(); //取得当前scrollTop的值
17
18         if (curScrollTop > scrollTop) {//如果向上滑动
19             if (nextTitle[0] != undefined) {
20                 if (nextTitle[0].getBoundingClientRect().top <= 52) {//如果下一个title与顶部的title贴到一起或者超过它
21                     var nextTitleIndex = parseInt(nextTitle.attr("data-i")) - 1; //下一个title的索引值
22                     var lastTitleIndex = parseInt(topTitle.last().attr("data-index")); //顶部最后一个title的索引值
23                     if (nextTitleIndex == lastTitleIndex + 1) {
24                         if (appendIndex == lastTitleIndex) {
25                             topTitleWrap.html("<p class='titleNormal' data-index=" + nextTitleIndex + ">" + nextTitle.html() + "</p>");
26                         }
27                         removeIndex = nextTitleIndex;
28                         appendIndex = nextTitleIndex;
29                     }
30                 }
31             }
32         }
33         else {//如果向下滑动
34             if (prevTitle[0] != undefined && curTitle[0] != undefined) {
35                 if (curTitle[0].getBoundingClientRect().top >= 0) {
36                     var prevTitleIndex = parseInt(curTitle.attr("data-i")) - 1;
37                     var lastTitleIndex = parseInt(topTitle.last().attr("data-index"));
38                     if (prevTitleIndex == lastTitleIndex) {
39                         if (removeIndex == lastTitleIndex && removeIndex != 0) {
40                             topTitleWrap.html("<p class='titleNormal' data-index=" + parseInt(curIndex - 1) + ">" + prevTitle.html() + "</p>");
41                         }
42                         if (curTitle.attr("data-i") - 1 > 0) {
43                             appendIndex = prevTitleIndex - 1;
44                             removeIndex = prevTitleIndex - 1;
45                         }
46                     }
47                 }
48             }
49         }
50         scrollTop = curScrollTop;
51     });
52 });