題目1：如何判斷一個元素是否出現在窗口可視范圍（瀏覽器的上邊緣和下邊緣之間，肉眼可視）。寫一個函數 isVisible實現
function isVisible($node){
var windowHeight = $(window).height();
var scrollTop = $(window).scrollTop();
var nodeHeight = $node.height();
var offsetHeight = $node.offset().top;
if((scrollTop>offsetHeight-windowHeight)&&(scrollTop<offsetHeight+nodeHeight)){
return true;
}else{
return false;
}
}
題目2：當窗口滾動時，判斷一個元素是不是出現在窗口可視范圍。每次出現都在控制臺打印 true 。用代碼實現
var $box = $('.box');
$(window).on('scroll',function(){
print($box);
})
function print($nodes){
$nodes.each(function(){
if(isVisible($(this))){
console.log(true);
}
})
}
function isVisible($node){
var windowHeight = $(window).height();
var scrollTop = $(window).scrollTop();
var nodeHeight = $node.height();
var offsetHeight = $node.offset().top;
if((scrollTop>offsetHeight-windowHeight)&&(scrollTop<offsetHeight+nodeHeight)){
return true;
}else{
return false;
}
}
題目3：當窗口滾動時，判斷一個元素是不是出現在窗口可視范圍。在元素第一次出現時在控制臺打印 true，以后再次出現不做任何處理。用代碼實現
var $box = $('.box');
$(window).on('scroll',function(){
printOne($box);
})
function printOne($nodes){
$nodes.not('.appear').each(function(){
if(isVisible($(this))){
$(this).addClass('appear');
console.log(true);
}
})
}
function isVisible($node){
var windowHeight = $(window).height();
var scrollTop = $(window).scrollTop();
var nodeHeight = $node.height();
var offsetHeight = $node.offset().top;
if((scrollTop>offsetHeight-windowHeight)&&(scrollTop<offsetHeight+nodeHeight)){
return true;
}else{
return false;
}
}
4.圖片懶加載的原理是什么？
- 一個網站需要加載的圖片會很多，如果要求瀏覽器在用戶請求打開該網站的那一瞬間，加載完所有的圖片，會導致加載的時間非常長，且效果不一定好，如果遇到傳輸問題，有的圖片甚至可能出現顯示不全的情況，這樣的話用戶的體驗就很差了。
對于這個問題，我們想到了一個解決辦法：直到該圖片出現在用戶的視線范圍內時，才加載該圖片。
- 關鍵的步驟：
①判斷圖片是否可見:如果屏幕滾動的高度+窗口高度 > 圖片到頁面頂部的距離&&屏幕滾動的高度<圖片的總高度+圖片到頁面頂部的距離，那么該圖片即是可見的；
②轉換圖片的可見狀態:修改圖片原本的src值。我們可以給每一個圖片的src設一個相同的初始值，這樣只會發起一次請求，如果不設置，可能會出現“X”就很難看。當圖片是可見時，再修改圖片的src屬性，這時，圖片的內容就能被加載出來了。
5
//router.js
app.get('/getImgs', function(req, res) {
var arr = ["http://cdn.jirengu.com/book.jirengu.com/img/1.jpg",
"http://cdn.jirengu.com/book.jirengu.com/img/2.jpg",
"http://cdn.jirengu.com/book.jirengu.com/img/3.jpg",
"http://cdn.jirengu.com/book.jirengu.com/img/4.jpg",
"http://cdn.jirengu.com/book.jirengu.com/img/5.jpg",
"http://cdn.jirengu.com/book.jirengu.com/img/6.jpg",
"http://cdn.jirengu.com/book.jirengu.com/img/7.jpg",
"http://cdn.jirengu.com/book.jirengu.com/img/8.jpg",
"http://cdn.jirengu.com/book.jirengu.com/img/9.jpg",
"http://cdn.jirengu.com/book.jirengu.com/img/10.jpg",
"http://cdn.jirengu.com/book.jirengu.com/img/11.jpg",
"http://cdn.jirengu.com/book.jirengu.com/img/12.jpg",
"http://cdn.jirengu.com/book.jirengu.com/img/13.jpg",
"http://cdn.jirengu.com/book.jirengu.com/img/14.jpg",
"http://cdn.jirengu.com/book.jirengu.com/img/15.jpg",
"http://cdn.jirengu.com/book.jirengu.com/img/16.jpg",
"http://cdn.jirengu.com/book.jirengu.com/img/17.jpg",
"http://cdn.jirengu.com/book.jirengu.com/img/18.jpg",
"http://cdn.jirengu.com/book.jirengu.com/img/19.jpg",
"http://cdn.jirengu.com/book.jirengu.com/img/20.jpg",
"http://cdn.jirengu.com/book.jirengu.com/img/21.jpg",
"http://cdn.jirengu.com/book.jirengu.com/img/22.jpg",
"http://cdn.jirengu.com/book.jirengu.com/img/23.jpg",
"http://cdn.jirengu.com/book.jirengu.com/img/24.jpg",
"http://cdn.jirengu.com/book.jirengu.com/img/25.jpg",
"http://cdn.jirengu.com/book.jirengu.com/img/26.jpg",
"http://cdn.jirengu.com/book.jirengu.com/img/27.jpg"];
res.send(arr);
});
//index.html
<!doctype html>
<html>
<head>
<meta charset="utf-8">
<title>gaoyang</title>
<style>
.ct {
width: 620px;
padding: 0;
margin: 0 auto;
border: 1px solid #ccc;
}
ul {
list-style: none;
margin: 0;
padding: 0;
margin-left: -10px;
margin-bottom: -10px;
}
ul:after {
content: '';
display: block;
clear: both;
}
ul>li {
width: 200px;
height: 400px;
cursor: pointer;
float: left;
margin: 0;
padding: 0;
margin-left: 10px;
margin-bottom: 10px;
font-size: 0;
}
ul>li>img {
width: 100%;
height: 100%;
margin: 0;
padding: 0;
/*vertical-align: bottom;*/
}
</style>
</head>
<body>
<div class="ct">
<ul class="box">
</ul>
</div>
<script src='./jquery.js'></script>
<script>
var $box = $('.box');
var $li;
var $img;
//監聽滾動事件
$(window).on('scroll',function(){
loadImg($img);
})
//加載圖片
function loadImg($nodes){
$nodes.not('.appear').each(function(){
if(isVisible($(this))){
$(this).addClass('appear');
//取出data-src數據
var imgUrl = $(this).attr('data-src')
console.log(imgUrl)
//賦值src
$(this).attr('src',imgUrl);
}
})
}
//判斷圖片是否一出現在可視范圍
function isVisible($node){
var windowHeight = $(window).height();
var scrollTop = $(window).scrollTop();
var nodeHeight = $node.height();
var offsetHeight = $node.offset().top;
if((scrollTop>offsetHeight-windowHeight)&&(scrollTop<offsetHeight+nodeHeight)){
return true;
}else{
return false;
}
}
//ajax請求
$.get('/getImgs').done(function(ret){
for(var i=0 ;i<ret.length;i++){
$box.append('<li><img src="" data-src='+ret[i]+'></li>')
}
$img = $('.box>li>img');
loadImg($img);
}).fail(function(){
})
</script>
</body>
</html>
6
//router.js
app.get('/getNews', function (req, res) {
var page = parseInt(req.query.page);
var len = parseInt(req.query.len);
var arr = [];
if (page < 5) {
for (var i = page * len; i < page * len + len; i++) {
arr.push('數據' + i);
}
}
res.send(arr);
});
//index.html
<!doctype html>
<html>
<head>
<meta charset="utf-8">
<title>gaoyang</title>
<style>
.box {
list-style: none;
padding: 0;
margin: 0;
width: 600px;
}
.box>li {
border: 1px solid #ccc;
margin: 10px;
}
.box>li>a:hover {
background-color: blue;
color: #fff;
}
.box>li>a {
display: block;
text-decoration: none;
padding: 10px;
color: #000;
}
p {
visibility: hidden;
}
</style>
</head>
<body>
<div class="ct">
<ul class="box">
</ul>
<p>加載數據</p>
</div>
<script src='./jquery.js'></script>
<script>
var pageIndex = 0;
var loading = false;
//開始就加載數據
getData()
$('.box').on('click', 'li', function (event) {
event.preventDefault();
})
//window滾動事件
$(window).on('scroll', function () {
//加載數據
getData();
})
//請求數據
function getData() {
if (!loading && isVisible($('.ct>p'))) {
$.get('/getNews', { page: pageIndex, len: 5 }).done(function (ret) {
handleSuccess(ret)
}).fail(function (error) {
loading = false;
})
loading = true;
}
}
//處理成功的回調
function handleSuccess(ret) {
//拼接顯示
if (ret.length > 0) {
handleData(ret)
} else {
$('.box').append('<li><a href="#">' + '沒有更多數據了。。。' + '</a></li>');
}
}
//處理返回數據不為0條
function handleData(ret) {
$.each(ret, function () {
$('.box').append('<li><a href="#">' + this + '</a></li>');
});
pageIndex++;
loading = false;
//繼續加載數據
getData();
}
//判斷p(加載數據)是否一出現在可視范圍
function isVisible($node) {
var windowHeight = $(window).outerHeight(true);
var scrollTop = $(window).scrollTop();
var nodeHeight = $node.outerHeight(true);
var offsetHeight = $node.offset().top;
if ((scrollTop > offsetHeight - windowHeight) && (scrollTop < offsetHeight + nodeHeight)) {
return true;
} else {
return false;
}
}
</script>
</body>
</html>
