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;
总结
以上为个人经验，希望能给大家一个参考，也希望大家多多支持编程网。
