前端性能优化--懒加载和预加载

ssddgkke

2019-07-01

懒加载

代码实现

html

<!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>Document</title>
    <link rel="stylesheet" href="lazyload.css">
</head>

<body>
    <div class="image-list">
        <img src="" class="image-item" lazyload="true" data-original="https://img.alicdn.com/imgextra/i2/252339290/TB29PYUXnIlyKJjSZFrXXXn2VXa_!!252339290-0-beehive-scenes.jpg_180x180xzq90.jpg_.webp" alt="">
        <img src="" class="image-item" lazyload="true" data-original="https://img.alicdn.com/imgextra/i4/2260152888/O1CN01vw2e251XCkJr0VPZU_!!2260152888-0-beehive-scenes.jpg_250x250xz.jpg" alt="">
        <img src="" class="image-item" lazyload="true" data-original="https://img.alicdn.com/imgextra/i3/24687421/TB2Xg1izsyYBuNkSnfoXXcWgVXa_!!24687421-0-beehive-scenes.jpg_250x250xz.jpg" alt="">
        <img src="" class="image-item" lazyload="true" data-original="https://img.alicdn.com/imgextra/i4/890151842/TB2II1mnZbI8KJjy1zdXXbe1VXa_!!890151842-0-beehive-scenes.jpg_250x250xz.jpg" alt="">
        <img src="" class="image-item" lazyload="true" data-original="https://img.alicdn.com/imgextra/i2/2096513830/TB2l1W0kRnTBKNjSZPfXXbf1XXa_!!2096513830-0-beehive-scenes.jpg_250x250xz.jpg" alt="">
        <img src="" class="image-item" lazyload="true" data-original="https://img.alicdn.com/imgextra/i3/2586222636/TB2RDGrpqAoBKNjSZSyXXaHAVXa_!!2586222636-0-daren.jpg_250x250xz.jpg" alt="">
        <img src="" class="image-item" lazyload="true" data-original="https://img.alicdn.com/imgextra/i2/1870112525/TB2Ae.xbOwIL1JjSZFsXXcXFFXa_!!1870112525-2-beehive-scenes.png_250x250xz.jpg" alt="">
        <img src="" class="image-item" lazyload="true" data-original="https://img.alicdn.com/imgextra/i2/2194952831/TB2PwWty7qvpuFjSZFhXXaOgXXa_!!2194952831-0-beehive-scenes.jpg_250x250xz.jpg" alt="">
    </div>
    <script src="./lazyload.js"></script>
</body>

</html>

css

.image-item {
    display: inline-block;
    width: 100%;
    height: 300px;
    background: gray;
}

js

var viewHeight = document.documentElement.clientHeight;

        function lazyload() {
            var imgs = document.querySelectorAll('img[data-original][lazyload]');
            imgs.forEach(item => {

                if (item.dataset.original == '') {
                    return;
                }
                var rect = item.getBoundingClientRect();
                if (rect.bottom >= 0 && rect.top < viewHeight) {
                    var img = new Image();
                    img.src = item.dataset.original;
                    console.log(img.src)
                    img.onload = function() {
                        item.src = img.src;
                    }
                    item.removeAttribute('data-original');
                    item.removeAttribute('lazyload')
                }
            })
        }
        lazyload();
        document.addEventListener('scroll', lazyload)

效果

蓝色为已经请求加载的图片
红色为还没有请求加载的图片

默认打开，没有操作前

前端性能优化--懒加载和预加载

向下滚动前

前端性能优化--懒加载和预加载

向下滚动后

前端性能优化--懒加载和预加载

冷门属性

ocument.addEventListener（事件监听）

document.addEventListener('scroll', function() {
    console.log('aa')
})

前端性能优化--懒加载和预加载

dataset

用`dataset.original`获取`data-original` 的属性

img.onload

事件在图片加载完成后立即执行

getBoundingClientRect

用于获取某个元素相对于视窗的位置集合。集合中有top, right, bottom, left等属性

文档各种高度

document.documentElement.clientHeight:视口的大小
document.documentElement.scrollHeight:文档的大小
document.documentElement.offsetHeight:文档的大小

前端性能优化--懒加载和预加载

预加载

new Image()

var image = new Image();
image.src = 'https://img.alicdn.com/imgextra/i4/2260152888/O1CN01vw2e251XCkJr0VPZU_!!2260152888-0-beehive-scenes.jpg_250x250xz.jpg';

<script src='./myPreload.js'></script>

xml

<script src="https://cdn.bootcss.com/PreloadJS/1.0.1/preloadjs.min.js"></script>
    <script>
        var queue = new createjs.LoadQueue();
        queue.installPlugin(createjs.Sound);
        queue.on("complete", handleComplete, this);
        queue.loadFile({
            id: "sound",
            src: "http://path/to/sound.mp3"
        });
        queue.loadManifest([{
            id: "myImage",
            src: "path/to/myImage.jpg"
        }]);

        function handleComplete() {
            createjs.Sound.play("sound");
            var image = queue.getResult("myImage");
            document.body.appendChild(image);
        }
    </script>

性能优化 lazyload