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js delayed loading ideas and first screen delayed loading explanation

巴扎黑Original: 2017-07-24 14:31:521878browse

Function: Ensure the speed of page opening (if the home page cannot be opened within 3 seconds, it is considered a dead page)

Principle:

　1) For the pictures in the first screen content: First, give the corresponding area a default picture position (the default picture needs to be very small, generally can be maintained within 5kb), when the first screen content is loaded After completion (or you can give a delay time), I start loading the real picture

　2) For pictures on other screens: I also give a default picture the space bit, when the scroll bar scrolls to the corresponding area, we start loading the real picture

Extension: asynchronous loading of data, starting with only loading and binding the data of the first two screens , the subsequent data will not be processed. When the page scrolls to the corresponding area, the data binding rendering data will be re-requested

. For details, please see the figure below

The delayed loading code for the first screen is as follows:

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Document</title>
    <style>
        *{
            padding:0;
            margin:0;
            font-size:14px;
        }
        #banner{
            margin:10px auto;
            width:300px;
            height:150px;
            border:1px solid green;
            background:url('img/default.gif') no-repeat center center;/*给当前的区域加一个默认图占位，告诉用户此处的图片正在加载中*/}
        #banner img{
            display:none;/*在开始的时候IMG的SRC属性没有地址，这样的话在IE浏览器中会显示一张碎图，不美观，所以我们让其默认是隐藏的，当真实的图片加载完成后在显示*/width:100%;
            height:100%;
        }</style>
</head>
<body>
    <div id=&#39;banner&#39;>
        <!--trueImg是当前标签的自定义属性，存储的是真实图片的地址-->
        <img src="" alt="" trueImg="img/jd.jpg">
    </div>
    <script>var banner = document.getElementById('banner'),imgFir = banner.getElementsByTagName('img')[0]
        window.setTimeout(function(){// imgFir.src = imgFir.getAttribute('trueImg');// imgFir.style.display = "block"//以上处理还是不完整的：如果我们获取的真实图片的地址是错误的，赋值给IMG的SRC属性的时候，不仅控制台会报错，而且页面中还会出现碎图//获取图片的地址，验证地址的有效性，是有效的才赋值，不是有效的是不进行处理的// var oImg = document.createElement('img')var oImg = new Image;//创建一个临时的IMG标签oImg.src = imgFir.getAttribute('trueImg');
            oImg.onload = function(){//当图片能够正常加载imgFir.src = this.src;
                imgFir.style.display = "block";
                oImg = nullconsole.log('加载完成')
            }
            console.log('正在加载中...')
        },500)</script>
</body>
</html>

Multi-screen single Lazy loading of a picture

The specific code is as follows:

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Document</title>
    <style>
        *{
            padding:0;
            margin:0;
            font-size:14px;
        }
        #banner{
            margin:10px auto;
            width:300px;
            height:150px;
            border:1px solid green;
            background:url('img/default.gif') no-repeat center center;/*给当前的区域加一个默认图占位，告诉用户此处的图片正在加载中*/}
        #banner img{
            display:none;/*在开始的时候IMG的SRC属性没有地址，这样的话在IE浏览器中会显示一张碎图，不美观，所以我们让其默认是隐藏的，当真实的图片加载完成后在显示*/width:100%;
            height:100%;
        }</style>
</head>
<body>
    <div id=&#39;banner&#39;>
        <!--trueImg是当前标签的自定义属性，存储的是真实图片的地址-->
        <img src="" alt="" trueImg="img/jd.jpg">
    </div>
    <script>var banner = document.getElementById('banner'),imgFir = banner.getElementsByTagName('img')[0]

        window.onscroll = function(){if(banner.isLoad){return;
            }var A = banner.offsetHeight+utils.offset(banner).top;var B = utils.win("clientHeight") + utils.win("scrollTop");if(A<B){//当条件成立，我们加载真实的图片，第一次加载完成后，我们在让页面滚动的过程中A<B一直成立，又重新执行下面的操作，导致了重复给一个容器中的图片进行加载var oImg = new Image;
                oImg.src = imgFir.getAttribute('trueImg');
                oImg.onload = function(){
                    imgFir.src = this.src;
                    imgFir.style.display = 'block';
                    oImg = null;
                    
                }
                banner.isLoad = true;//设置一个自定义属性告诉浏览器我已经把图片加载完了(不管是否正常的加载，只要处理过一次以后都不需要处理了)}
        }</script>
</body>
</html>

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