懒加载的3种实现方式

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的懒加载实现

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" data-src="https://ik.imagekit.io/demo/img/image2.jpeg?tr=w-400,h-300" />
    <img class="lazy" data-src="https://ik.imagekit.io/demo/img/image3.jpg?tr=w-400,h-300" /> -->
    <img class="lazy" data-src="https://ik.imagekit.io/demo/img/image4.jpeg?tr=w-400,h-300" />
    <img class="lazy" data-src="https://ik.imagekit.io/demo/img/image5.jpeg?tr=w-400,h-300" />
    <img class="lazy" data-src="https://ik.imagekit.io/demo/img/image6.jpeg?tr=w-400,h-300" />
    <img class="lazy" data-src="https://ik.imagekit.io/demo/img/image7.jpeg?tr=w-400,h-300" />
    <img class="lazy" data-src="https://ik.imagekit.io/demo/img/image8.jpeg?tr=w-400,h-300" />
    <img class="lazy" data-src="https://ik.imagekit.io/demo/img/image9.jpeg?tr=w-400,h-300" />
    <img class="lazy" data-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" data-src="https://ik.imagekit.io/demo/img/image1.jpeg?tr=w-400,h-300" />
    <img class="lazy" data-src="https://ik.imagekit.io/demo/img/image2.jpeg?tr=w-400,h-300" />
    <img class="lazy" data-src="https://ik.imagekit.io/demo/img/image3.jpg?tr=w-400,h-300" />
    <img class="lazy" data-src="https://ik.imagekit.io/demo/img/image4.jpeg?tr=w-400,h-300" />
    <img class="lazy" data-src="https://ik.imagekit.io/demo/img/image5.jpeg?tr=w-400,h-300" />
    <img class="lazy" data-src="https://ik.imagekit.io/demo/img/image6.jpeg?tr=w-400,h-300" />
    <img class="lazy" data-src="https://ik.imagekit.io/demo/img/image7.jpeg?tr=w-400,h-300" />
    <img class="lazy" data-src="https://ik.imagekit.io/demo/img/image8.jpeg?tr=w-400,h-300" />
    <img class="lazy" data-src="https://ik.imagekit.io/demo/img/image9.jpeg?tr=w-400,h-300" />
    <img class="lazy" data-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

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>
    <div id="bg-image" class="bg lazy"></div>
    <div id="bg-image" class="bg lazy"></div>
    <div id="bg-image" class="bg lazy"></div>
    <div id="bg-image" class="bg lazy"></div>
    <div id="bg-image" class="bg lazy"></div>
    <div id="bg-image" class="bg lazy"></div>
    <div id="bg-image" class="bg lazy"></div>
    <div id="bg-image" class="bg lazy"></div>
    <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

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" data-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" data-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>
    <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>

现成库

推荐下面这个库，使用非常简单：https://www.npmjs.com/package/lozad

我的博客即将同步至腾讯云+社区，邀请大家一同入驻：https://cloud.tencent.com/dev...

懒加载的3种实现方式

优势

img的懒加载实现

background-image的实现

渐进式懒加载

现成库

keller35

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