图片懒加载与预加载（原生）

news2024/11/13 9:16:15

1、懒加载。

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <div>
        <ul>
            <li>
                <img data_src="./sex1.jpg" src="./sex1.jpg" alt="">
            </li>
            <li>
                <img data_src="./sex2.jpg" src="./sex1.jpg" alt="">
            </li>
            <li>
                <img data_src="./sex3.jpg" src="./sex1.jpg" alt="">
            </li>
            <li>
                <img data_src="./sex4.jpg" src="./sex1.jpg" alt="">
            </li>
            <li>
                <img data_src="./sex5.jpg" src="./sex1.jpg" alt="">
            </li>
        </ul>
    </div>
    
</body>
<script>
    
    //获取所有的img标签
    let imgs = document.querySelectorAll("img")
    // fn(imgs)
    function fn(imgs){
        // 遍历所有的img标签，获取dom
        for(let i=0;i<imgs.length;i++){
            //获取每张图片距离顶部的距离
            let imgOffsetTop = imgs[i].offsetTop
            // 获取每张图片被卷去的距离
            let imgScrollTop = imgs[i].scrollTop
            // 获取浏览器可视区高度
            var interHeight = window.innerHeight
            // 元素举例顶部的距离（offsetTop）-页面被卷去的高度（scrollTop）<=浏览器可视区的高度
            if(imgOffsetTop-imgScrollTop<=interHeight){
                // 将真正的src图片路径赋值给src属性
                imgs[i].src=imgs[i].getAttribute("data_src")
                console.log(imgs[4].getAttribute("data_src"))
            }
        }
    }
    
    window.addEventListener("scroll",()=>{
        fn(imgs)
    })
</script>
<style>
    ul li {
        width:200px;
        height:200px;
        list-style:none;
    }
    img {
        width:200px;
        /* height:200px; */
    }
</style>
</html>

2、预加载。

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    
</head>
<body>
    <div>
        <ul>
            <li>
                <img src="./sex1.jpg" alt="" onclick="preLoad(urlList)">
            </li>
        </ul>
    </div>
    
</body>
<script>
    // 预加载图片路径
    let urlList = ["./sex1.jpg","./sex2.jpg","./sex3.jpg","./sex4.jpg","./sex5.jpg"]
    let num = 1
    function preLoad(){
        let imgs = document.querySelector("img")
        imgs.src= urlList[num]
        let img = new Image()
        if(num<urlList.length-1){
            img.src = urlList[num+1]
            // onload是图片加载完成事件
            img.onload = function(){
            num++
            console.log('图片预加载成功',num);
            }
        }
    }
</script>
<style>
    ul li {
        width:200px;
        height:200px;
        list-style:none;
    }
    img {
        width:200px;
    }
</style>
</html>

效果图：