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应用场景
懒加载列表或叫做无限滚动列表,也是一种性能优化的方式,其可疑不必一次性请求所有数据,可以看做是分页的另一种实现形式,较多适用于移动端提升用户体验,新闻、资讯浏览等。
效果预览
思路剖析
设置临界元素,当临界元素进入可视范围时请求并追加新数据。
根据可视窗口和滚动元素组建的关系确定数据加载时机。
container.clientHeight - wrapper.scrollTop <= wrapper.clientHeight
原生代码实现
index.html
<body> <div id="wrapper" onscroll="handleScroll()"> <ul id="container"></ul> </div> <script type="text/javascript" src="./index.js"></script></body>
index.css
* { margin: 0; padding: 0;}#wrapper { margin: 100px auto; width: 300px; height: 300px; border: 1px solid rgba(100, 100, 100, 0.2); overflow-y: scroll;}ul#container { list-style: none; padding: 0; width: 100%;}ul#container > li { height: 30px; width: 100%;}ul#container > li.green-item { background-color: #c5e3ff;}ul#container > li.red-item { background-color: #fff5d5;}
index.js
// 模拟数据构造const arr = [];const nameArr = ['Alice', 'July', 'Roman', 'David', 'Sara', 'Lisa', 'Mike'];let curPage = 1;let noData = false;const curPageSize = 20;const getPageData = (page, pageSize) => { if (page > 5) return []; const arr = []; // const nameArr = ['Alice', 'July', 'Roman', 'David', 'Sara', 'Lisa', 'Mike']; for (let i = 0; i < pageSize; i++) { arr.push({ number: i + (page - 1) * pageSize, name: `${nameArr[i % nameArr.length]}`, }); } return arr;};const wrapper = document.getElementById('wrapper');const container = document.getElementById('container');let plainWrapper = null;const handleScroll = () => { // 当临界元素进入可视范围时,加载下一页数据 if ( !noData && container.clientHeight - wrapper.scrollTop <= wrapper.clientHeight ) { curPage++; console.log(curPage); const newData = getPageData(curPage, curPageSize); renderList(newData); }};const renderList = (data) => { // 没有更多数据时 if (!data.length) { noData = true; plainWrapper.innerText = 'no more data...'; return; } plainWrapper && container.removeChild(plainWrapper); //移除上一个临界元素 const fragment = document.createDocumentFragment(); data.forEach((item) => { const li = document.createElement('li'); li.className = item.number % 2 === 0 ? 'green-item' : 'red-item'; //奇偶行元素不同色 const text = document.createTextNode( `${`${item.number}`.padStart(7, '0')}-${item.name}` ); li.appendChild(text); fragment.appendChild(li); }); const plainNode = document.createElement('li'); const text = document.createTextNode('scroll to load more...'); plainNode.appendChild(text); plainWrapper = plainNode; fragment.appendChild(plainNode); //添加新的临界元素 container.appendChild(fragment);};// 初始渲染renderList(getPageData(curPage, curPageSize));
迁移到React
在 React
中实现时可以省去复杂的手动渲染逻辑部分,更关注数据。
store/data.ts
import { IDataItem } from '../interface';const nameArr = ['Alice', 'July', 'Roman', 'David', 'Sara', 'Lisa', 'Mike'];export const getPageData = ( page: number = 1, pageSize: number = 10): Array<IDataItem> => { if (page > 5) return []; const arr = []; // const nameArr = ['Alice', 'July', 'Roman', 'David', 'Sara', 'Lisa', 'Mike']; for (let i = 0; i < pageSize; i++) { arr.push({ number: i + (page - 1) * pageSize, name: `${nameArr[i % nameArr.length]}`, }); } return arr;};
LazyList.tsx
import React, { FC, useCallback, useEffect, useReducer, useRef } from 'react';import { getPageData } from './store/data';import { IDataItem } from './interface';import styles from './index.module.css';export interface IProps { curPageSize?: number;}export interface IState { curPage: number; noData: boolean; listData: Array<IDataItem>;}const LazyList: FC<IProps> = ({ curPageSize = 10 }: IProps) => { const clientRef: any = useRef(null); const scrollRef: any = useRef(null); const [state, dispatch] = useReducer( (state: IState, action: any): IState => { switch (action.type) { case 'APPEND': return { ...state, listData: [...state.listData, ...action.payload.listData], }; default: return { ...state, ...action.payload }; } }, { curPage: 1, noData: false, listData: [], } ); const handleScroll = useCallback(() => { const { clientHeight: wrapperHeight } = scrollRef.current; const { scrollTop, clientHeight } = clientRef.current; // 当临界元素进入可视范围时,加载下一页数据 if (!state.noData && wrapperHeight - scrollTop <= clientHeight) { console.log(state.curPage); const newData = getPageData(state.curPage, curPageSize); dispatch({ type: 'APPEND', payload: { listData: newData }, }); dispatch({ payload: { curPage: state.curPage + 1, noData: !(newData.length > 0), }, }); } }, [state.curPage, state.noData]); useEffect(() => { const newData = getPageData(1, curPageSize); dispatch({ type: 'APPEND', payload: { listData: newData }, }); dispatch({ payload: { curPage: 2, noData: !(newData.length > 0), }, }); }, []); return ( <div className={styles[`wrapper`]} ref={clientRef} onScroll={handleScroll}> <ul className={styles[`container`]} ref={scrollRef}> {state.listData.map(({ number, name }) => ( <li key={number} className={ number % 2 === 0 ? styles[`green-item`] : styles[`red-item`] } > {number}-{name} </li> ))} {<li>{state.noData ? 'no more' : 'scroll'}</li>} </ul> </div> );};export default LazyList;
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