时间:2021-07-01 10:21:17 帮助过:11人阅读
优势
性能收益:浏览器加载图片、decode、渲染都需要耗费资源,懒加载能节约性能消耗,缩短onload事件时间。
节约带宽:这个不需要解释。
通常,我们在html中展示图片,会有两种方式:
img 标签
css background-image
img有两种方式实现懒加载:
事件监听(scroll、resize、orientationChange)
<!DOCTYPE html> <html lang="en"> <head> <meta charset="UTF-8"> <meta name="viewport" content="width=device-width, initial-scale=1.0"> <meta http-equiv="X-UA-Compatible" content="ie=edge"> <title>event</title> <style> img { background: #F1F1FA; width: 400px; height: 300px; display: block; margin: 10px auto; border: 0; } </style> </head> <body> <img src="https://ik.imagekit.io/demo/img/image1.jpeg?tr=w-400,h-300" /> <img src="https://ik.imagekit.io/demo/img/image2.jpeg?tr=w-400,h-300" /> <img src="https://ik.imagekit.io/demo/img/image3.jpg?tr=w-400,h-300" /> <img class="lazy" src="https://ik.imagekit.io/demo/img/image2.jpeg?tr=w-400,h-300" /> <img class="lazy" src="https://ik.imagekit.io/demo/img/image3.jpg?tr=w-400,h-300" /> --> <img class="lazy" src="https://ik.imagekit.io/demo/img/image4.jpeg?tr=w-400,h-300" /> <img class="lazy" src="https://ik.imagekit.io/demo/img/image5.jpeg?tr=w-400,h-300" /> <img class="lazy" src="https://ik.imagekit.io/demo/img/image6.jpeg?tr=w-400,h-300" /> <img class="lazy" src="https://ik.imagekit.io/demo/img/image7.jpeg?tr=w-400,h-300" /> <img class="lazy" src="https://ik.imagekit.io/demo/img/image8.jpeg?tr=w-400,h-300" /> <img class="lazy" src="https://ik.imagekit.io/demo/img/image9.jpeg?tr=w-400,h-300" /> <img class="lazy" src="https://ik.imagekit.io/demo/img/image10.jpeg?tr=w-400,h-300" /> <script> document.addEventListener("DOMContentLoaded", function() { var lazyloadImages = document.querySelectorAll("img.lazy"); var lazyloadThrottleTimeout; function lazyload () { if(lazyloadThrottleTimeout) { clearTimeout(lazyloadThrottleTimeout); } lazyloadThrottleTimeout = setTimeout(function() { var scrollTop = window.pageYOffset; lazyloadImages.forEach(function(img) { if(img.offsetTop < (window.innerHeight + scrollTop)) { img.src = img.dataset.src; img.classList.remove('lazy'); } }); if(lazyloadImages.length == 0) { document.removeEventListener("scroll", lazyload); window.removeEventListener("resize", lazyload); window.removeEventListener("orientationChange", lazyload); } }, 20); } document.addEventListener("scroll", lazyload); window.addEventListener("resize", lazyload); window.addEventListener("orientationChange", lazyload); }); </script> </body> </html>
Intersection Observer(兼容性问题)
<!DOCTYPE html> <html lang="en"> <head> <meta charset="UTF-8"> <meta name="viewport" content="width=device-width, initial-scale=1.0"> <meta http-equiv="X-UA-Compatible" content="ie=edge"> <title>observer</title> <style> img { background: #F1F1FA; width: 400px; height: 300px; display: block; margin: 10px auto; border: 0; } </style> </head> <body> <img src="https://ik.imagekit.io/demo/img/image2.jpeg?tr=w-400,h-300" /> <img src="https://ik.imagekit.io/demo/img/image3.jpg?tr=w-400,h-300" /> --> <img class="lazy" src="https://ik.imagekit.io/demo/img/image1.jpeg?tr=w-400,h-300" /> <img class="lazy" src="https://ik.imagekit.io/demo/img/image2.jpeg?tr=w-400,h-300" /> <img class="lazy" src="https://ik.imagekit.io/demo/img/image3.jpg?tr=w-400,h-300" /> <img class="lazy" src="https://ik.imagekit.io/demo/img/image4.jpeg?tr=w-400,h-300" /> <img class="lazy" src="https://ik.imagekit.io/demo/img/image5.jpeg?tr=w-400,h-300" /> <img class="lazy" src="https://ik.imagekit.io/demo/img/image6.jpeg?tr=w-400,h-300" /> <img class="lazy" src="https://ik.imagekit.io/demo/img/image7.jpeg?tr=w-400,h-300" /> <img class="lazy" src="https://ik.imagekit.io/demo/img/image8.jpeg?tr=w-400,h-300" /> <img class="lazy" src="https://ik.imagekit.io/demo/img/image9.jpeg?tr=w-400,h-300" /> <img class="lazy" src="https://ik.imagekit.io/demo/img/image10.jpeg?tr=w-400,h-300" /> <script> document.addEventListener("DOMContentLoaded", function() { var lazyloadImages = document.querySelectorAll(".lazy"); var imageObserver = new IntersectionObserver(function(entries, observer) { entries.forEach(function(entry) { if (entry.isIntersecting) { var image = entry.target; image.src = image.dataset.src; image.classList.remove("lazy"); imageObserver.unobserve(image); } }); }); lazyloadImages.forEach(function(image) { imageObserver.observe(image); }); }); </script> </body> </html>
background-image的实现跟img的原理基本是一样的,区别是在对class的处理上:
<!DOCTYPE html> <html lang="en"> <head> <meta charset="UTF-8"> <meta name="viewport" content="width=device-width, initial-scale=1.0"> <meta http-equiv="X-UA-Compatible" content="ie=edge"> <title>background</title> <style> body { margin: 0; } .bg { height: 200px; } #bg-image.lazy { background-image: none; background-color: #F1F1FA; } #bg-image { background-image: url("https://ik.imagekit.io/demo/img/image1.jpeg?tr=w-400,h-300"); background-size: 100%; } </style> </head> <body> <p id="bg-image" class="bg lazy"></p> <p id="bg-image" class="bg lazy"></p> <p id="bg-image" class="bg lazy"></p> <p id="bg-image" class="bg lazy"></p> <p id="bg-image" class="bg lazy"></p> <p id="bg-image" class="bg lazy"></p> <p id="bg-image" class="bg lazy"></p> <p id="bg-image" class="bg lazy"></p> <script> document.addEventListener("DOMContentLoaded", function() { var lazyloadImages = document.querySelectorAll(".lazy"); var imageObserver = new IntersectionObserver(function(entries, observer) { entries.forEach(function(entry) { if (entry.isIntersecting) { var image = entry.target; image.classList.remove("lazy"); imageObserver.unobserve(image); } }); }); lazyloadImages.forEach(function(image) { imageObserver.observe(image); }); }); </script> </body> </html>
渐进式懒加载,指的是存在降级处理,通常html形式如下:
<a href="full.jpg" class="progressive replace"> <img src="tiny.jpg" class="preview" alt="image" /> </a>
这样的代码会有2个好处:
如果js执行失败,可以点击预览
大小与实际图一致的占位data URI,避免reflow
最终的代码如下:
<!DOCTYPE html> <html lang="en"> <head> <meta charset="UTF-8"> <meta name="viewport" content="width=device-width, initial-scale=1.0"> <meta http-equiv="X-UA-Compatible" content="ie=edge"> <title>progressive</title> <style> a.progressive { position: relative; display: block; overflow: hidden; outline: none; } a.progressive:not(.replace) { cursor: default; } a.progressive img { display: block; width: 100%; max-width: none; height: auto; border: 0 none; } a.progressive img.preview { filter: blur(2vw); transform: scale(1.05); } a.progressive img.reveal { position: absolute; left: 0; top: 0; will-change: transform, opacity; animation: reveal 1s ease-out; } @keyframes reveal { 0% {transform: scale(1.05); opacity: 0;} 100% {transform: scale(1); opacity: 1;} } </style> </head> <body> <a href="https://s3-us-west-2.amazonaws.com/s.cdpn.io/123941/nature5.jpg" srcset="https://s3-us-west-2.amazonaws.com/s.cdpn.io/123941/nature5.jpg 800w, https://s3-us-west-2.amazonaws.com/s.cdpn.io/123941/nature5big.jpg 1600w" class="progressive replace"> <img src="data:image/jpeg;base64,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" class="preview" alt="palm trees" /> </a> <a href="https://s3-us-west-2.amazonaws.com/s.cdpn.io/123941/nature2.jpg" class="progressive replace"> <img src="http://lorempixel.com/20/15/nature/2/" class="preview" alt="sunset" /> </a> <a href="https://s3-us-west-2.amazonaws.com/s.cdpn.io/123941/nature3.jpg" class="progressive replace"> <img src="http://lorempixel.com/20/15/nature/3/" class="preview" alt="tide" /> </a> <a href="https://s3-us-west-2.amazonaws.com/s.cdpn.io/123941/nature5.jpg" srcset="https://s3-us-west-2.amazonaws.com/s.cdpn.io/123941/nature5.jpg 800w, https://s3-us-west-2.amazonaws.com/s.cdpn.io/123941/nature5big.jpg 1600w" class="progressive replace"> <img src="data:image/jpeg;base64,/9j/4AAQSkZJRgABAQEAYABgAAD//gA7Q1JFQVRPUjogZ2QtanBlZyB2MS4wICh1c2luZyBJSkcgSlBFRyB2NjIpLCBxdWFsaXR5ID0gNzAK/9sAQwAKBwcIBwYKCAgICwoKCw4YEA4NDQ4dFRYRGCMfJSQiHyIhJis3LyYpNCkhIjBBMTQ5Oz4+PiUuRElDPEg3PT47/9sAQwEKCwsODQ4cEBAcOygiKDs7Ozs7Ozs7Ozs7Ozs7Ozs7Ozs7Ozs7Ozs7Ozs7Ozs7Ozs7Ozs7Ozs7Ozs7Ozs7Ozs7/8AAEQgABQAUAwEiAAIRAQMRAf/EAB8AAAEFAQEBAQEBAAAAAAAAAAABAgMEBQYHCAkKC//EALUQAAIBAwMCBAMFBQQEAAABfQECAwAEEQUSITFBBhNRYQcicRQygZGhCCNCscEVUtHwJDNicoIJChYXGBkaJSYnKCkqNDU2Nzg5OkNERUZHSElKU1RVVldYWVpjZGVmZ2hpanN0dXZ3eHl6g4SFhoeIiYqSk5SVlpeYmZqio6Slpqeoqaqys7S1tre4ubrCw8TFxsfIycrS09TV1tfY2drh4uPk5ebn6Onq8fLz9PX29/j5+v/EAB8BAAMBAQEBAQEBAQEAAAAAAAABAgMEBQYHCAkKC//EALURAAIBAgQEAwQHBQQEAAECdwABAgMRBAUhMQYSQVEHYXETIjKBCBRCkaGxwQkjM1LwFWJy0QoWJDThJfEXGBkaJicoKSo1Njc4OTpDREVGR0hJSlNUVVZXWFlaY2RlZmdoaWpzdHV2d3h5eoKDhIWGh4iJipKTlJWWl5iZmqKjpKWmp6ipqrKztLW2t7i5usLDxMXGx8jJytLT1NXW19jZ2uLj5OXm5+jp6vLz9PX29/j5+v/aAAwDAQACEQMRAD8A5yC3e2S3gM7OEQHdgA57VTuLAPqUoaUk7yM7R2BPSiivS5VN8stjmk2ldGVM3lyshG7bxk0UUV4skk2jdN2P/9k=" class="preview" alt="palm trees" /> </a> <a href="https://s3-us-west-2.amazonaws.com/s.cdpn.io/123941/nature2.jpg" class="progressive replace"> <img src="http://lorempixel.com/20/15/nature/2/" class="preview" alt="sunset" /> </a> <a href="https://s3-us-west-2.amazonaws.com/s.cdpn.io/123941/nature3.jpg" class="progressive replace"> <img src="http://lorempixel.com/20/15/nature/3/" class="preview" alt="tide" /> </a> <script> window.addEventListener('load', function() { var pItem = document.getElementsByClassName('progressive replace'), timer; window.addEventListener('scroll', scroller, false); window.addEventListener('resize', scroller, false); inView(); function scroller(e) { timer = timer || setTimeout(function() { timer = null; requestAnimationFrame(inView); }, 300); } function inView() { var scrollTop = window.pageYOffset; var innerHeight = window.innerHeight; var p = 0; while (p < pItem.length) { var offsetTop = pItem[p].offsetTop; if (offsetTop < (scrollTop + innerHeight)) { loadFullImage(pItem[p]); pItem[p].classList.remove('replace'); } else p++; } } function loadFullImage(item) { var img = new Image(); if (item.dataset) { img.srcset = item.dataset.srcset || ''; img.sizes = item.dataset.sizes || ''; } img.src = item.href; img.className = 'reveal'; if (img.complete) addImg(); else img.onload = addImg; function addImg() { item.addEventListener('click', function(e) { e.preventDefault(); }, false); item.appendChild(img).addEventListener('animationend', function(e) { var pImg = item.querySelector('img.preview'); if (pImg) { e.target.alt = pImg.alt || ''; item.removeChild(pImg); e.target.classList.remove('reveal'); } }); } } }, false); </script> </body> </html>
推荐下面这个库,使用非常简单:https://www.npmjs.com/package/lozad
以上就是懒加载实现的方法有哪些?(三种方法介绍)的详细内容,更多请关注Gxl网其它相关文章!