功能
监听目标元素与祖先元素（默认是根元素）的交叉情况
使用
const intersectionObserver = new IntersectionObserver((entries) => {
// entries.forEach(i => {
// if (i.isIntersecting) {
// console.log(i)
// }
// })
console.log(entries)
}, {
root: document.getElementsByClassName("box")[0],
rootMargin:"10px 10px",
threshold:[0,0.5]
})
root:指定祖先元素，没有指定时默认为根元素rootMargin：指定观察区域的扩展大小或收缩大小，定义格式类似margin- 默认观察区域为祖先元素的大小
threshold：指定交叉比例，当元素与root交叉比例等于threshold时，触发回调函数
entries常使用的属性
isIntersecting：当前元素与root交叉是否满足threshold,是返回true，否返回falsetarget：监听的目标元素
例子
监听元素是否进入父级元素
<!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 {
width: 300px;
height: 300px;
background: mediumaquamarine;
position: relative;
}
.box1 {
width: 100px;
height: 100px;
background: red;
position: absolute;
left: 350px;
}
</style>
</head>
<body>
<div class="box">
<div class="box1"></div>
</div>
<script>
const box = document.getElementsByClassName("box")[0]
const box1 = document.getElementsByClassName("box1")[0]
const intersectionObserver = new IntersectionObserver((entries) => {
entries.forEach(i => {
if (i.isIntersecting) {
console.log(i)
}
})
}, {
root: box,
threshold: 0.01
})
intersectionObserver.observe(box1)
setInterval(() => {
box1.style.left = box1.offsetLeft - 20 + 'px'
}, 1000);
</script>
</body>
</html>
