# IntersectionObserver
<div id="scrollArea" style="height:900px">
<div style="height:1000px"></div>
<div id="title">观察我吧</div>
<div style="height:1000px"></div>
<div>
<script>
let options = {
root: document.querySelector('#scrollArea'), // 根( root ) 元素，用于检查目标的可见性。必须是目标元素的父级元素。如果未指定或者为null，则默认为浏览器视窗。
rootMargin: '0px', // 属性（root）元素的外边距。类似于 CSS 中的margin属性，“ 10px 20px 30px 40px"（上、右、下、左。如果有根参数，则 rootMargin 也可以使用指定来取值。该根是如何使用的）根节点和元素的发生范围是什么时候的计算集，使用该属性可以控制每一边的边界或范围目标的发生范围。交接元素默认为0。
threshold: 1.0, //元素是元素的编号，也是元素的根值，也是目标和根的相交程度。如果你想要的属性值超过 25% 就在 root 的每一个级别执行一次，那么你可以指定一个范围[0, 0.25, 0.5, 0.75, 1]。默认值是 0（元素含义）。有一个目标）元素中，出现功能在该根执行。
}
// 创建一个观察者之后需要给定一个目标元素进行观察。
let observer = new IntersectionObserver(callback, options);
// 每当该目标满足 IntersectionObserver 指定的阈值，就被调用。
let title = document.querySelector('#title')
observer.observe(title)
</script>
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# 可以运行的案例
<html>
<head>
<meta charset="utf-8">
<meta name="robots" content="noindex, nofollow">
<style type="text/css">
body {
padding: 0;
margin: 0;
}
svg:not(:root) {
display: block;
}
.playable-code {
background-color: #f4f7f8;
border: none;
border-left: 6px solid #558abb;
border-width: medium medium medium 6px;
color: #4d4e53;
height: 100px;
width: 90%;
padding: 10px 10px 0;
}
.playable-canvas {
border: 1px solid #4d4e53;
border-radius: 2px;
}
.playable-buttons {
text-align: right;
width: 90%;
padding: 5px 10px 5px 26px;
}
</style>
<style type="text/css">
#box {
background-color: rgba(40, 40, 190, 255);
border: 4px solid rgb(20, 20, 120);
transition: background-color 1s, border 1s;
width: 350px;
height: 350px;
display: flex;
align-items: center;
justify-content: center;
padding: 20px;
}
.vertical {
color: white;
font: 32px "Arial";
}
.extra {
width: 350px;
height: 350px;
margin-top: 10px;
border: 4px solid rgb(20, 20, 120);
text-align: center;
padding: 20px;
}
</style>
<title>Intersection Observer API - a_simple_example - code sample</title>
</head>
<body>
<div style="height: 1000px;"> </div>
<div id="box" style="background-color: rgb(40, 40, 190);">
<div class="vertical">
Welcome to <strong>The Box!</strong>
</div>
</div>
<div style="height: 1000px;"> </div>
<script>
let boxElement;
let prevRatio = 0.0;
const increasingColor = 'rgba(40, 40, 190, ratio)';
const decreasingColor = 'rgba(190, 40, 40, ratio)';
// Set things up
window.addEventListener('load', () => {
boxElement = document.querySelector('#box');
createObserver();
}, false);
function createObserver() {
const options = {
root: null,
rootMargin: '0px',
threshold: buildThresholdList()
};
const observer = new IntersectionObserver(handleIntersect, options);
observer.observe(boxElement);
}
function buildThresholdList() {
const thresholds = [];
const numSteps = 20;
for (let i = 1.0; i <= numSteps; i++) {
const ratio = i / numSteps;
thresholds.push(ratio);
}
thresholds.push(0);
return thresholds;
}
function handleIntersect(entries) {
entries.forEach((entry) => {
if (entry.intersectionRatio > prevRatio) {
entry.target.style.backgroundColor = increasingColor.replace('ratio', entry
.intersectionRatio);
} else {
entry.target.style.backgroundColor = decreasingColor.replace('ratio', entry
.intersectionRatio);
}
prevRatio = entry.intersectionRatio;
});
}
</script>
</body>
</html>
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<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<meta name="robots" content="noindex, nofollow">
<style type="text/css">
body {
padding: 0;
margin: 0;
}
svg:not(:root) {
display: block;
}
.playable-code {
background-color: #f4f7f8;
border: none;
border-left: 6px solid #558abb;
border-width: medium medium medium 6px;
color: #4d4e53;
height: 100px;
width: 90%;
padding: 10px 10px 0;
}
.playable-canvas {
border: 1px solid #4d4e53;
border-radius: 2px;
}
.playable-buttons {
text-align: right;
width: 90%;
padding: 5px 10px 5px 26px;
}
</style>
<style type="text/css">
body {
font-family: "Open Sans", "Arial", "Helvetica", sans-serif;
background-color: aliceblue;
}
.wrapper {
display: grid;
grid-template-columns: auto minmax(min-content, 1fr);
grid-template-rows: auto minmax(min-content, 1fr);
max-width: 700px;
margin: 0 auto;
background-color: aliceblue;
}
header {
grid-column: 1 / -1;
grid-row: 1;
background-color: aliceblue;
}
aside {
grid-column: 1;
grid-row: 2;
background-color: cornsilk;
padding: 5px 10px;
}
aside ul {
padding-left: 0;
}
aside ul li {
list-style: none;
}
aside ul li a {
text-decoration: none;
}
main {
grid-column: 2;
grid-row: 2;
margin: 0;
margin-left: 16px;
font-size: 16px;
}
article {
background-color: white;
padding: 6px;
}
article:not(:last-child) {
margin-bottom: 8px;
}
article h2 {
margin-top: 0;
}
.ad {
height: 96px;
padding: 6px;
border-color: #555;
border-style: solid;
border-width: 1px;
}
.ad:not(:last-child) {
margin-bottom: 8px;
}
.ad h2 {
margin-top: 0;
}
.ad div {
position: relative;
float: right;
padding: 0 4px;
height: 20px;
width: 120px;
font-size: 14px;
bottom: 30px;
border: 1px solid black;
background-color: rgba(255, 255, 255, 0.5);
}
</style>
<title>Timing element visibility with the Intersection Observer API - building_the_site - code sample</title>
</head>
<body>
<div class="wrapper">
<header>
<h1>A Fake Blog</h1>
<h2>Showing Intersection Observer in action!</h2>
</header>
<aside>
<nav>
<ul>
<li><a href="#link1">A link</a></li>
<li><a href="#link2">Another link</a></li>
<li><a href="#link3">One more link</a></li>
</ul>
</nav>
</aside>
<main>
</main>
</div>
<script>
/* eslint-disable indent */
/* eslint-disable no-unused-vars */
/* eslint-disable prefer-const */
let contentBox;
let nextArticleID = 1;
let visibleAds = new Set();
let previouslyVisibleAds = null;
let adObserver;
let refreshIntervalID = 0;
window.addEventListener('load', startup, false);
function startup() {
contentBox = document.querySelector('main');
document.addEventListener('visibilitychange', handleVisibilityChange, false);
const observerOptions = {
root: null,
rootMargin: '0px',
threshold: [0.0, 0.75]
};
adObserver = new IntersectionObserver(intersectionCallback, observerOptions);
buildContents();
refreshIntervalID = window.setInterval(handleRefreshInterval, 1000);
}
function handleVisibilityChange() {
if (document.hidden) {
if (!previouslyVisibleAds) {
previouslyVisibleAds = visibleAds;
visibleAds = [];
previouslyVisibleAds.forEach(function(adBox) {
updateAdTimer(adBox);
adBox.dataset.lastViewStarted = 0;
});
}
} else {
previouslyVisibleAds.forEach(function(adBox) {
adBox.dataset.lastViewStarted = performance.now();
});
visibleAds = previouslyVisibleAds;
previouslyVisibleAds = null;
}
}
function intersectionCallback(entries) {
entries.forEach(function(entry) {
const adBox = entry.target;
if (entry.isIntersecting) {
debugger;
if (entry.intersectionRatio >= 0.75) {
adBox.dataset.lastViewStarted = entry.time;
visibleAds.add(adBox);
}
} else {
debugger;
visibleAds.delete(adBox);
if ((entry.intersectionRatio === 0.0) && (adBox.dataset.totalViewTime >= 60000)) {
replaceAd(adBox);
}
}
});
}
function handleRefreshInterval() {
debugger;
const redrawList = [];
visibleAds.forEach(function(adBox) {
const previousTime = adBox.dataset.totalViewTime;
updateAdTimer(adBox);
if (previousTime != adBox.dataset.totalViewTime) {
redrawList.push(adBox);
}
});
if (redrawList.length) {
window.requestAnimationFrame(function(time) {
redrawList.forEach(function(adBox) {
drawAdTimer(adBox);
});
});
}
}
function updateAdTimer(adBox) {
const lastStarted = adBox.dataset.lastViewStarted;
const currentTime = performance.now();
if (lastStarted) {
const diff = currentTime - lastStarted;
adBox.dataset.totalViewTime = parseFloat(adBox.dataset.totalViewTime) + diff;
}
adBox.dataset.lastViewStarted = currentTime;
}
function drawAdTimer(adBox) {
const timerBox = adBox.querySelector('.timer');
const totalSeconds = adBox.dataset.totalViewTime / 1000;
const sec = Math.floor(totalSeconds % 60);
const min = Math.floor(totalSeconds / 60);
timerBox.innerText = min + ':' + sec.toString().padStart(2, '0');
}
const loremIpsum = '<p>Lorem ipsum dolor sit amet, consectetur adipiscing' +
' elit. Cras at sem diam. Vestibulum venenatis massa in tincidunt' +
' egestas. Morbi eu lorem vel est sodales auctor hendrerit placerat' +
' risus. Etiam rutrum faucibus sem, vitae mattis ipsum ullamcorper' +
' eu. Donec nec imperdiet nibh, nec vehicula libero. Phasellus vel' +
' malesuada nulla. Aliquam sed magna aliquam, vestibulum nisi at,' +
' cursus nunc.</p>';
function buildContents() {
for (let i = 0; i < 5; i++) {
contentBox.appendChild(createArticle(loremIpsum));
// if (!(i % 2)) {
// loadRandomAd();
// }
if (i == 3) {
loadRandomAd();
}
}
}
function createArticle(contents) {
const articleElem = document.createElement('article');
articleElem.id = nextArticleID;
const titleElem = document.createElement('h2');
titleElem.id = nextArticleID;
titleElem.innerText = 'Article ' + nextArticleID + ' title';
articleElem.appendChild(titleElem);
articleElem.innerHTML += contents;
nextArticleID += 1;
return articleElem;
}
function loadRandomAd(replaceBox) {
const ads = [{
bgcolor: '#cec',
title: 'Eat Green Beans',
body: 'Make your mother proud—they\'re good for you!'
},
{
bgcolor: 'aquamarine',
title: 'MillionsOfFreeBooks.whatever',
body: 'Read classic literature online free!'
},
{
bgcolor: 'lightgrey',
title: '3.14 Shades of Gray: A novel',
body: 'Love really does make the world go round...'
},
{
bgcolor: '#fee',
title: 'Flexbox Florist',
body: 'When life\'s layout gets complicated, send flowers.'
}
];
let adBox, title, body, timerElem;
const ad = ads[Math.floor(Math.random() * ads.length)];
if (replaceBox) {
adObserver.unobserve(replaceBox);
adBox = replaceBox;
title = replaceBox.querySelector('.title');
body = replaceBox.querySelector('.body');
timerElem = replaceBox.querySelector('.timer');
} else {
adBox = document.createElement('div');
adBox.className = 'ad';
title = document.createElement('h2');
body = document.createElement('p');
timerElem = document.createElement('div');
adBox.appendChild(title);
adBox.appendChild(body);
adBox.appendChild(timerElem);
}
adBox.style.backgroundColor = ad.bgcolor;
title.className = 'title';
body.className = 'body';
title.innerText = ad.title;
body.innerHTML = ad.body;
adBox.dataset.totalViewTime = 0;
adBox.dataset.lastViewStarted = 0;
timerElem.className = 'timer';
timerElem.innerText = '0:00';
if (!replaceBox) {
contentBox.appendChild(adBox);
}
adObserver.observe(adBox);
}
function replaceAd(adBox) {
let visibleTime;
updateAdTimer(adBox);
visibleTime = adBox.dataset.totalViewTime;
console.log(' Replacing ad: ' + adBox.querySelector('h2').innerText + ' - visible for ' + visibleTime);
loadRandomAd(adBox);
}
</script>
</body>
</html>
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