无限滚动&懒加载&元素位置 etc....

最近遇到了无限滚动的问题,用到了IntersectionObserver API,这里正好就将相关知识梳理整理一下。

前置概念知识

  1. scrollHeight:容器元素被撑起后的实际大小高度
  2. scrollTop:容器元素的滚动距离,也是当前视口相对于容器实际高度定点的距离。
  3. offsetTop:元素的上外边框至包含元素的上内边框之间的像素距离。
  4. el.getBoundingClientRect().top:方法返回元素的大小及其相对于视口的位置。(top相对于视口的高度距离)
  5. clientHeight:元素内容区高度加上上下内边距高度,clientHeight = content + padding。
  6. IntersectionObserver API:
    (阮老师的这篇文章介绍的很清晰:连接,下面是部分引用)
var io = new IntersectionObserver(callback, option);
// 开始观察
io.observe(document.getElementById('example'));

// 停止观察
io.unobserve(element);

// 关闭观察器
io.disconnect();

由浏览器原生提供的构造函数,异步观察目标元素与其祖先元素或顶级文档视窗(viewport)交叉状态的方法,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)。

构造函数的返回值是一个观察器实例。实例的observe方法可以指定观察哪个 DOM 节点。可观察多个实例,

目标元素的可见性变化时,就会调用观察器的回调函数callback。
callback一般会触发两次。一次是目标元素刚刚进入视口(开始可见),另一次是完全离开视口(开始不可见)。

var io = new IntersectionObserver(
  entries => {
    console.log(entries);
  }
);

回调函数的参数是一个IntersectionObserverEntry对象数组,监听了多少,数组就有多少IntersectionObserverEntry对象。
对象包含了以下数据:
无限滚动&懒加载&元素位置 etc...._第1张图片
参数意思为:

time:可见性发生变化的时间,是一个高精度时间戳,单位为毫秒
target:被观察的目标元素,是一个 DOM 节点对象
rootBounds:根元素的矩形区域的信息,getBoundingClientRect()方法的返回值,如果没有根元素(即直接相对于视口滚动),则返回null
boundingClientRect:目标元素的矩形区域的信息
intersectionRect:目标元素与视口(或根元素)的交叉区域的信息
intersectionRatio:目标元素的可见比例,即intersectionRect占boundingClientRect的比例,完全可见时为1,完全不可见时小于等于0

无限滚动实现方案

1. 判断容器是否滚动到“底”

一般底部有个提示性的footer,暂定20px的话,核心代码即为:

 container.scrollHeight -
        container.scrollTop -
        container.clientHeight <
        20

注意一般会加上节流防抖,不同的浏览器的onscroll触发次数可能差别很大。判断到底后,就发请求append。

完整代码实例:


<html lang="en">

<head>
  <meta charset="UTF-8" />
  <meta name="viewport" content="width=device-width, initial-scale=1.0" />
  <title>Documenttitle>
  <style>
    body {
      margin: 0;
    }

    .container {
      height: 500px;
      overflow-y: scroll;
    }

    .item {
      width: 100%;
      height: 200px;
      margin-bottom: 20px;
      background-color: antiquewhite;
      display: flex;
      align-items: center;
      justify-content: center;
    }
  style>
head>

<body>
  <div class="container">
    <div class="item">1div>
    <div class="item">2div>
    <div class="item cur">3div>
    <div class="item">4div>
  div>
  <script type="text/javascript">
    let container = document.getElementsByClassName("container")[0];

    function debounce(fn, delay) {
      let timer = null;
      return function (args) {
        if (timer) {
          clearTimeout(timer);
        }
        timer = setTimeout(function () {
          fn.apply(this, args);
        }, delay);
      };
    }
    let k = 5;
    const bottomAppend4item = () => {
      console.log('滚动函数执行了');
      // 20一般为滚动到底部前的一个底部标签大小值,这里以刚好的margin为例
      if (
        container.scrollHeight -
        container.scrollTop -
        container.clientHeight <
        20
      ) {
        console.log('true');

        let cnt = 4;
        while (cnt--) {
          console.log(1);
          let newItem = document.createElement("div");
          newItem.setAttribute("class", "item");
          newItem.innerText = `${k++}`
          container.appendChild(newItem)
        }
        cnt = 4;

      }
    }
    const debounceAJAX = debounce(bottomAppend4item, 50)
    container.onscroll = debounceAJAX;
  script>
body>

html>

2. IntersectionObserver检测最后一个元素的intersectionRatio显示比


<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <title>Titletitle>
    <style>
      .img-area {
        width: 500px;
        height: 500px;
        margin: 0 auto;
      }

      .my-photo {
        width: 500px;
        height: 300px;
      }
    style>
  head>

  <body>
    <div id="container">
      <div class="img-area">
        <img class="my-photo" alt="loading" src="./img/1.png" />
      div>
      <div class="img-area">
        <img class="my-photo" alt="loading" src="./img/2.png" />
      div>
      <div class="img-area">
        <img class="my-photo" alt="loading" src="./img/3.png" />
      div>
      <div class="img-area">
        <img class="my-photo" alt="loading" src="./img/4.png" />
      div>
    div>
    <div class="scrollerFooter1">
      没有内容了
    div>
    <script>
      function infinityScroll(footerNode, callback) {
        var observer = new IntersectionObserver(function (changes) {
          // 注意intersectionRatio这个属性值的判断
          if (changes[0].intersectionRatio <= 0) return;

          callback();
        });
        observer.observe(document.querySelector(footerNode));
      }
      infinityScroll(".scrollerFooter1", function () {
        for (var i = 0; i < 3; i++) {
          document
            .getElementById("container")
            .appendChild(document.getElementById("container").firstChild);
        }
      });
    script>
  body>
html>

用IntersectionObserver实现懒加载方案

事先把img元素的链接放在data-*的data-src里,判断元素的intersectionRatio占比出现后,取出data-src动态赋值给src加载图片。


<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <title>Titletitle>
    <style>
      .img-area {
        width: 500px;
        height: 500px;
        margin: 0 auto;
      }

      .my-photo {
        width: 500px;
        height: 300px;
      }
    style>
  head>

  <body>
    <div class="container">
      <div class="img-area">
        <img class="my-photo" alt="loading" data-src="./img/1.png" />
      div>
      <div class="img-area">
        <img class="my-photo" alt="loading" data-src="./img/2.png" />
      div>
      <div class="img-area">
        <img class="my-photo" alt="loading" data-src="./img/3.png" />
      div>
      <div class="img-area">
        <img class="my-photo" alt="loading" data-src="./img/4.png" />
      div>
      <div class="img-area">
        <img class="my-photo" alt="loading" data-src="./img/5.png" />
      div>
      <div class="img-area">
        <img class="my-photo" alt="loading" data-src="./img/1.png" />
      div>
      <div class="img-area">
        <img class="my-photo" alt="loading" data-src="./img/2.png" />
      div>
      <div class="img-area">
        <img class="my-photo" alt="loading" data-src="./img/3.png" />
      div>
      <div class="img-area">
        <img class="my-photo" alt="loading" data-src="./img/4.png" />
      div>
      <div class="img-area">
        <img class="my-photo" alt="loading" data-src="./img/5.png" />
      div>
    div>
    <script>
      function lazyLoad(imgClassName) {
        const imgList = Array.from(document.querySelectorAll(imgClassName));
        var io = new IntersectionObserver(function (ioes) {
          ioes.forEach(function (ioe) {
            var el = ioe.target;
            var intersectionRatio = ioe.intersectionRatio;
            if (intersectionRatio > 0 && intersectionRatio <= 1) {
              if (!el.src) {
                el.src = el.dataset.src;
              }
            }
          });
        });
        imgList.forEach(function (item) {
          io.observe(item);
        });
      }
      lazyLoad(".my-photo");
    script>
  body>
html>

判断元素是否在视口中

上面两个场景问题其实都可以引申出一个方法,如何判断元素出现在视口中。

1. 滚动属性判断

公式: el.offsetTop - document.documentElement.scrollTop <= viewPortHeight

function isInViewPortOfOne (el) {
    // viewPortHeight 兼容所有浏览器写法
    const viewPortHeight = window.innerHeight || document.documentElement.clientHeight || document.body.clientHeight 
    const offsetTop = el.offsetTop
    const scrollTop = document.documentElement.scrollTop
    const top = offsetTop - scrollTop
    console.log('top', top)
     // 这里有个+100是为了提前加载+ 100
    return top <= viewPortHeight + 100
}
2. 直接getBoundingClientRect

上面在前置中就介绍了这个方法就是相对于视口的距离,el.getBoundingClientReact().top <= viewPortHeight
跟上面公式连在一起,可以得出:el.offsetTop - document.documentElement.scrollTop = el.getBoundingClientRect().top

例子:

function isInViewPortOfTwo (el) {
    const viewPortHeight = window.innerHeight || document.documentElement.clientHeight || document.body.clientHeight 
    const top = el.getBoundingClientRect() && el.getBoundingClientRect().top
    console.log('top', top)
    return top  <= viewPortHeight + 100
}
3. IntersectionObserver的intersectionRatio比

同上无限滚动和懒加载的代码和介绍

看到有人问如果上面公式的结果小于0的怎么办不判断吗,小于0其实就是滚动离开了视口区域。我们一般利用这些方法对元素做初次的渲染,这个问题不在这个场景的考虑范畴。

一些polyfill

对于getBoundingClientRect的polyfill支持,可以用更“原始”的offsetXX属性实现。

function getElementTop(element){
    var actualTop = element.offsetTop;
    var current = element.offsetParent;
    while (current !== null){
        actualTop += current. offsetTop;
        current = current.offsetParent;
    }
    return actualTop;
}

function getBoundingClientRect(element) {
    var scrollTop = document.documentElement.scrollTop;
    var scrollLeft = document.documentElement.scrollLeft;
    if (element.getBoundingClientRect) {
        if (typeof arguments.callee.offset != "number") {
            var temp = document.createElement("div");
            temp.style.cssText = "position:absolute;left:0;top:0;"; document.body.appendChild(temp);
            arguments.callee.offset = -temp.getBoundingClientRect().top - scrollTop; document.body.removeChild(temp);
            temp = null;
        }
        var rect = element.getBoundingClientRect();
        var offset = arguments.callee.offset;
        return {
            left: rect.left + offset,
            right: rect.right + offset,
            top: rect.top + offset,
            bottom: rect.bottom + offset
        };
    } else {
        var actualLeft = getElementLeft(element);
        var actualTop = getElementTop(element);
        return {
            left: actualLeft - scrollLeft,
            right: actualLeft + element.offsetWidth - scrollLeft,
            top: actualTop - scrollTop,
            bottom: actualTop + element.offsetHeight - scrollTop
        }
    }
}

封装一个动态获取高度的组件

之前一些中台项目,我们也封装了动态获取高度的React组件,用此高阶组件封装的组件,可以动态给子组件设置当前浏览器文档流剩余空间的高度,使整体刚好占满屏幕(当然超过有滚动条)。原理就是差不多也是用了上面的一些元素位置属性。

import React, { useState, useRef, useLayoutEffect } from 'react';

import { getElementTop, triggerEvent } from 'utils/tools';

type ObtainHeightProps = {
  bgColor?: string; // 背景色
  children: React.ReactNode;
  overflow?: string;
  className?: string;
};

//  计算滚动内容高度的组件
const ObtainHeight = (props: ObtainHeightProps) => {
  const { className, bgColor, overflow } = props;
  const obtainRef = useRef<HTMLDivElement>(null);
  const [height, setHeight] = useState(
    document.documentElement.clientHeight - 200
  );

  const setObtainHeight = () => {
    const contentDom: HTMLDivElement | null = obtainRef.current;
    const top = getElementTop(contentDom as HTMLDivElement);
    const clientHeight = document.documentElement.clientHeight - top - 20;
    setHeight(clientHeight);
  };
  useLayoutEffect(() => {
    setObtainHeight();
    window.onresize = () => {
      setObtainHeight();
    };
    // 主动触发一次resize,解决计算不准确的bug
    const evt = window.document.createEvent('UIEvents');
    evt.initEvent('resize', true, false);
    window.dispatchEvent(evt);
    setTimeout(() => {
      triggerEvent(window, 'resize');
    }, 200);
    return () => {
      window.onresize = null;
    };
  }, []);

  return (
    <div
      className={`${className} obtain-height`}
      ref={obtainRef}
      style={{ height, background: bgColor, overflow }}
    >
      {props.children}
    </div>
  );
};
export default ObtainHeight;

/**
 * @description: 获取元素在页面距离顶部的偏移量
 * @param {element} 元素节点
 * @return:  Number
 */
export const getElementTop: (element: HTMLDivElement) => number = (
  element: HTMLDivElement
) => {
  // 获取 element 元素距离父元素的 offsetTop 像素;
  // console.log(element);
  if (!element) return 200;
  let actualTop = element.offsetTop;
  let current = element.offsetParent;
  // console.log(element, actualTop, current);
  // 判断当前元素是都循环到 HTML 根元素了
  while (current !== null) {
    // offsetTop 循环相加
    actualTop += (current as HTMLDivElement).offsetTop;
    // 当 current 为 HTML 根元素是, current.offsetParent 的值为 null
    current = (current as HTMLDivElement).offsetParent;
    // console.log(element, actualTop, current);
  }
  // console.log(actualTop);
  return actualTop;
};
// 事件触发函数
export function triggerEvent(el: Element | Window, type: string) {
  if ('createEvent' in document) {
    // modern browsers, IE9+
    const e = document.createEvent('HTMLEvents');
    e.initEvent(type, false, true);
    el.dispatchEvent(e);
  }
}

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