多张图片懒加载

多种图片懒加载 并实现回到顶部

html + css

<style type="text/css">
    ul { margin: 0; padding: 0;	list-style: none; }
    ul li { height: 900px; margin: 100px auto; text-align: center; }
    ul li img { width: 200px; height: 200px; }
    div {
        display: none;
	position: fixed;
	right: 20px;
	bottom: 10px;
	width: 40px;
	height: 40px;
	background-color: deeppink;
	text-align: center;
	line-height: 40px;
    }
</style>

<ul></ul>
<div>Top</div>

js代码

<script>
	/* 模拟后台获取的数据 */
	const imgData = [
		{
			src: ‘./images/1.jpg‘,
			txt: ‘1‘
		},
		{
			src: ‘./images/2.jpg‘,
			txt: ‘2‘
		},
		{
			src: ‘./images/3.jpg‘,
			txt: ‘3‘
		},
		{
			src: ‘./images/4.jpg‘,
			txt: ‘4‘
		},
		{
			src: ‘./images/5.jpg‘,
			txt: ‘5‘
		},
		{
			src: ‘./images/6.jpg‘,
			txt: ‘6‘
		},
		{
			src: ‘./images/7.jpg‘,
			txt: ‘7‘
		},
		{
			src: ‘./images/8.jpg‘,
			txt: ‘8‘
		}
	]
	
	const oUl = document.querySelector(‘ul‘)
	const oDiv = document.querySelector(‘div‘)
	
	/* 创建文档碎片 */
	const frg = document.createDocumentFragment()
	for (let val of imgData) {
		let oLi = document.createElement(‘li‘)
		oLi.innerHTML = `
			<img data-src="${val.src}" src="./images/loading.gif" alt="${val.txt}">
			<p>${val.txt}</p>
		`
		frg.appendChild(oLi)
	}
	/* 插入到ul中 */
	oUl.appendChild(frg)
	
	/* 第一眼快速加载 */
	window.setTimeout(imgLoad, 100)
	
	/* 滚轮滑动加载 */
	window.addEventListener(‘scroll‘, throttle(imgLoad, 200))
	
	/* 点击返回顶部 */
	oDiv.addEventListener(‘click‘, toTop)
	
	/* 图片加载 */
	function imgLoad() {
		// console.log(1)
		let oImg = document.querySelectorAll(‘img‘)
		//当前页面可视高度
		let ch = document.documentElement.clientHeight
		//滚轮滑动距离
		let st = document.documentElement.scrollTop
		//到原始顶部距离
		let ct = ch + st
		for (let i = 0; i < oImg.length; i++) {
			//到原始页面顶部距离	此时相对于body
			let imgTop = oImg[i].offsetTop
			if (imgTop <= ct) {
				//换回真正的src
				oImg[i].src = oImg[i].dataset.src
			}
		}
		/* 返回顶部样式设置 */
		st >= ch * 0.5 ? oDiv.style.display = ‘block‘ : oDiv.style.display = ‘none‘
	}
	
	/* 节流 */
	function throttle(fn, delay) {
		let preTime = Date.now()
		let timer = null
		return function () {
			clearTimeout(timer)
			let args = arguments
			let curTime = Date.now()
			let remainTime = delay - (curTime - preTime)
			if (remainTime <= 0) {
				fn.apply(this, args)
				preTime = Date.now()
			} else {
				timer = setTimeout(() => {
					fn.apply(this, args)
				}, remainTime)
			}
		}
	}
	
	/* 返回顶部 */
	function toTop() {
		//滑动速度
		const speed = 200
		let timer = null
		move()
		function move() {
			//开局一个清除定时器
			clearTimeout(timer)
			//过渡
			document.documentElement.scrollTop -= speed
			//最后加速效果
			if (document.documentElement.scrollTop -500 <= 0) {
				document.documentElement.scrollTop = 0
				//退出
				return
			}
			timer = setTimeout(move, 10)
		}
	}

</script>