使用IntersectionObserver 实现下拉加载更多

IntersectionObserver是浏览器原生提供的构造函数，接受两个参数：callback是可见性变化时的回调函数，option是配置对象（该参数可选）。
<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport"
        content="width=device-width, user-scalable=no, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <!-- <script src="https://cdn.polyfill.io/v2/polyfill.min.js?features=es6"></script> -->
    <title>intersectionObserver</title>
    <style>
        * {
            margin: 0;
            padding: 0;
            box-sizing: border-box;
        }

        div {
            margin: 0 auto;
            max- 100%;
             600px;
        }

        #top {
            height: 600px;
            background-color: #aaaaaa;
        }

        #middle {
            margin-top: 200px;
            opacity: 0;
            height: 800px;
            background-color: #000000;
        }

        #bottom {
            height: 200px;
            background-color: #333;
        }
        div.move,
        #top.move,
        #middle.move,
        #bottom.move {
            animation: movefromleft 2s;
            animation-fill-mode: forwards;
        }

        @keyframes movefromleft {
            from {
                opacity: 0;
                transform: translateX(-300px);
            }

            to {
                opacity: 1;
                transform: translateX(0px);
            }
        }
    </style>
</head>

<body>
    <div id="top"></div>
    <div id="middle"></div>
    <div id="bottom"></div>
    <div id="b_B"></div>
</body>
<script>
    window.onload = (event) => {
        const middle = document.getElementById('middle'),
            top = document.getElementById('top'),
            bottom = document.getElementById('bottom');
        var last = document.getElementById('b_B')
        const animationClass = 'move';
        // 创建监听
        const intersectionObserver = new IntersectionObserver((entries) => {
            for (entry of entries) {
                if (entry.target.id == 'b_B' && entry.intersectionRatio > 0) {
                    
                    var c = document.createElement('div');
                    c.style.cssText = "border:1px solid black; background-color: pink; height: 100px;";
                    document.querySelector('body').appendChild(c)
                    bottom.parentNode.insertBefore(c, bottom)
                    intersectionObserver.observe(c);
                    addAnimationClass(entry.target, animationClass)
                }
                if (entry.intersectionRatio > 0) {
                    console.log('====================================');
                    console.log(entry.target);
                    console.log('====================================');
                    addAnimationClass(entry.target, animationClass);

                } else {
                    removeAnimationClass(entry.target, animationClass);
                }
            }
        });




        // let attributesChangeEl = document.querySelector("body")
        // let config = { childList: true }
        // let observer1 = new MutationObserver(mutationsList => {

        //     last = mutationsList[0].addedNodes[0]
        // })
        // observer1.observe(attributesChangeEl, config)



        // 添加动画class的操作
        function addAnimationClass(elem, animationClass) {
            elem.className.includes(animationClass) ? 1 : elem.className = elem.className  + animationClass;
        }
        // 移除动画class的操作
        function removeAnimationClass(elem, animationClass) {
            elem.className.includes(animationClass) ? elem.className = elem.className.replace(animationClass, '') : 1;
        }
        // 开启监听
        intersectionObserver.observe(top);

        intersectionObserver.observe(middle);
       
        
        intersectionObserver.observe(last);
    }

</script>

</html>