利用getBoundingClientRect
-
优点：容易理解，操作easy
-
缺点：只能用于全屏滚动的列表，不适用于局部滚动，譬如下图
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
<style>
#box{
height: 300px;
overflow: scroll;
}
img {
width: 100%;
}
</style>
</head>
<body>
<div id="box">
<img src="./213.jpg"
data-src="https://img1.baidu.com/it/u=2490614914,3569829953&fm=253&fmt=auto&app=138&f=JPEG?w=591&h=500"
alt="">
<img src="./213.jpg"
data-src="https://img1.baidu.com/it/u=2490614914,3569829953&fm=253&fmt=auto&app=138&f=JPEG?w=591&h=500"
alt="">
<img src="./213.jpg"
data-src="https://img1.baidu.com/it/u=2490614914,3569829953&fm=253&fmt=auto&app=138&f=JPEG?w=591&h=500"
alt="">
<img src="./213.jpg"
data-src="https://img1.baidu.com/it/u=2490614914,3569829953&fm=253&fmt=auto&app=138&f=JPEG?w=591&h=500"
alt="">
<img src="./213.jpg"
data-src="https://img1.baidu.com/it/u=2490614914,3569829953&fm=253&fmt=auto&app=138&f=JPEG?w=591&h=500"
alt="">
<img src="./213.jpg"
data-src="https://img1.baidu.com/it/u=2490614914,3569829953&fm=253&fmt=auto&app=138&f=JPEG?w=591&h=500"
alt="">
<img src="./213.jpg"
data-src="https://img1.baidu.com/it/u=2490614914,3569829953&fm=253&fmt=auto&app=138&f=JPEG?w=591&h=500"
alt="">
</div>
</body>
<script>
window.onload = function () {
let box = document.getElementById('box')
let img = [...document.querySelectorAll('img')]
box.onscroll=function(){
lazyLoad()
}
lazyLoad()
function lazyLoad(){
for(let i = 0;i<img.length;i++){
let {top,bottom} = img[i].getBoundingClientRect()
// 判断是否在box显示界限内
if(top>0&&bottom<box.offsetHeight+img[i].offsetHeight/2){
console.log(top,bottom)
img[i].src = img[i].dataset.src
img.splice(i,1)
i--
}
}
}
}
</script>
</html>
利用api IntersectionObserver
优点：容易使用，适用于各种场景
缺点：兼容性不是很好
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
<style>
#box{
height: 300px;
overflow: scroll;
}
img {
width: 100%;
}
</style>
</head>
<body>
<div id="box">
<img src="./213.jpg"
data-src="https://img1.baidu.com/it/u=2490614914,3569829953&fm=253&fmt=auto&app=138&f=JPEG?w=591&h=500"
alt="">
<img src="./213.jpg"
data-src="https://img1.baidu.com/it/u=2490614914,3569829953&fm=253&fmt=auto&app=138&f=JPEG?w=591&h=500"
alt="">
<img src="./213.jpg"
data-src="https://img1.baidu.com/it/u=2490614914,3569829953&fm=253&fmt=auto&app=138&f=JPEG?w=591&h=500"
alt="">
<img src="./213.jpg"
data-src="https://img1.baidu.com/it/u=2490614914,3569829953&fm=253&fmt=auto&app=138&f=JPEG?w=591&h=500"
alt="">
<img src="./213.jpg"
data-src="https://img1.baidu.com/it/u=2490614914,3569829953&fm=253&fmt=auto&app=138&f=JPEG?w=591&h=500"
alt="">
<img src="./213.jpg"
data-src="https://img1.baidu.com/it/u=2490614914,3569829953&fm=253&fmt=auto&app=138&f=JPEG?w=591&h=500"
alt="">
<img src="./213.jpg"
data-src="https://img1.baidu.com/it/u=2490614914,3569829953&fm=253&fmt=auto&app=138&f=JPEG?w=591&h=500"
alt="">
</div>
</body>
<script>
window.onload = function () {
let box = document.getElementById('box')
let img = document.querySelectorAll('img')
let io = new IntersectionObserver(function (entries) {
entries.forEach(entry => {
console.log(entry.intersectionRatio)
if (entry.intersectionRatio > 0) {
entry.target.src = entry.target.dataset.src
io.unobserve(entry.target)
}
})
})
img.forEach(item => {
io.observe(item)
})
}
</script>
</html>
借用offsetTop
- 优点：适用各种情况
- 缺点：需要对原生代码熟悉，过后容易忘记
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
<style>
#box{
height: 300px;
overflow: scroll;
position: relative;
}
img {
width: 100%;
}
</style>
</head>
<body>
<div style="height: 300px;width: 200px;"></div>
<div id="box">
<img src="./213.jpg"
data-src="https://img1.baidu.com/it/u=2490614914,3569829953&fm=253&fmt=auto&app=138&f=JPEG?w=591&h=500"
alt="">
<img src="./213.jpg"
data-src="https://img1.baidu.com/it/u=2490614914,3569829953&fm=253&fmt=auto&app=138&f=JPEG?w=591&h=500"
alt="">
<img src="./213.jpg"
data-src="https://img1.baidu.com/it/u=2490614914,3569829953&fm=253&fmt=auto&app=138&f=JPEG?w=591&h=500"
alt="">
<img src="./213.jpg"
data-src="https://img1.baidu.com/it/u=2490614914,3569829953&fm=253&fmt=auto&app=138&f=JPEG?w=591&h=500"
alt="">
<img src="./213.jpg"
data-src="https://img1.baidu.com/it/u=2490614914,3569829953&fm=253&fmt=auto&app=138&f=JPEG?w=591&h=500"
alt="">
<img src="./213.jpg"
data-src="https://img1.baidu.com/it/u=2490614914,3569829953&fm=253&fmt=auto&app=138&f=JPEG?w=591&h=500"
alt="">
<img src="./213.jpg"
data-src="https://img1.baidu.com/it/u=2490614914,3569829953&fm=253&fmt=auto&app=138&f=JPEG?w=591&h=500"
alt="">
</div>
</body>
<script>
window.onload = function () {
let box = document.getElementById('box')
let img = [...document.querySelectorAll('img')]
box.onscroll=function(){
lazyLoad()
}
lazyLoad()
function lazyLoad(){
for(let i = 0;i<img.length;i++){
// 记得父元素需要设置定位
let offsetTop = img[i].offsetTop
if(box.scrollTop+box.offsetHeight/2>=offsetTop){
img[i].src = img[i].dataset.src
img.splice(i,1)
i--
}
}
}
}
</script>
</html>