根据道具或状态更新状态
当可以从现有的道具或州计算某些东西时，就不会将其放在状态下。相反，在渲染期间计算
function Form() {
const [firstName, setFirstName] = useState('Taylor');
const [lastName, setLastName] = useState('Swift');
// 🔴 Avoid: redundant state and unnecessary Effect
const [fullName, setFullName] = useState('');
useEffect(() => {
setFullName(firstName + ' ' + lastName);
}, [firstName, lastName]);
// ...
}
推荐
function Form() {
const [firstName, setFirstName] = useState('Taylor');
const [lastName, setLastName] = useState('Swift');
// ✅ Good: calculated during rendering
const fullName = firstName + ' ' + lastName;
// ...
}
缓存昂贵的计算
function TodoList({ todos, filter }) {
const [newTodo, setNewTodo] = useState('');
// 🔴 Avoid: redundant state and unnecessary Effect
const [visibleTodos, setVisibleTodos] = useState([]);
useEffect(() => {
setVisibleTodos(getFilteredTodos(todos, filter));
}, [todos, filter]);
// ...
}
推荐
function TodoList({ todos, filter }) {
const [newTodo, setNewTodo] = useState('');
// ✅ This is fine if getFilteredTodos() is not slow.
const visibleTodos = getFilteredTodos(todos, filter);
// ...
}
也建议
在许多情况下，上述代码很好！但是，也许getFilteredTodos()很慢，或者您有很多恋人。在这种情况下，您不想重新计算getFilteredTodos()，如果像Newtodo这样的某些无关状态变量已更改。
您可以通过将其包裹在useMemo钩子中来缓存（或回忆）：
import { useMemo, useState } from 'react';
function TodoList({ todos, filter }) {
const [newTodo, setNewTodo] = useState('');
// ✅ Does not re-run getFilteredTodos() unless todos or filter change
const visibleTodos = useMemo(() => getFilteredTodos(todos, filter), [todos, filter]);
// ...
}
当道具更改时重置所有状态
export default function ProfilePage({ userId }) {
const [comment, setComment] = useState('');
// 🔴 Avoid: Resetting state on prop change in an Effect
useEffect(() => {
setComment('');
}, [userId]);
// ...
}
这是低效的，因为ProfilePage及其子女将首先以陈旧的价值呈现，然后再次呈现。这也很复杂，因为您需要在每个具有某种状态的组件中执行此操作。例如，如果评论UI已嵌套，您也想清除嵌套的评论状态。
相反，您可以告诉React，通过给出明确的key，每个用户的配置文件在概念上都是一个不同的配置文件。将您的组件分为二，然后将key属性从外部组件传递到内部组件：
推荐
export default function ProfilePage({ userId }) {
return (
<Profile
userId={userId}
key={userId}
/>
);
}
function Profile({ userId }) {
// ✅ This and any other state below will reset on key change automatically
const [comment, setComment] = useState('');
// ...
}
在事件处理程序之间共享逻辑
当您不确定某些代码是否应效果或事件处理程序中，请问自己为什么需要运行此代码。仅将Effects用于应运行的代码，因为该组件已显示给用户
function ProductPage({ product, addToCart }) {
// 🔴 Avoid: Event-specific logic inside an Effect
useEffect(() => {
if (product.isInCart) {
showNotification(`Added ${product.name} to the shopping cart!`);
}
}, [product]);
function handleBuyClick() {
addToCart(product);
}
function handleCheckoutClick() {
addToCart(product);
navigateTo('/checkout');
}
// ...
}
推荐
function ProductPage({ product, addToCart }) {
// ✅ Good: Event-specific logic is called from event handlers
function buyProduct() {
addToCart(product);
showNotification(`Added ${product.name} to the shopping cart!`);
}
function handleBuyClick() {
buyProduct();
}
function handleCheckoutClick() {
buyProduct();
navigateTo('/checkout');
}
// ...
}
发送邮政请求
function Form() {
const [firstName, setFirstName] = useState('');
const [lastName, setLastName] = useState('');
// ✅ Good: This logic should run because the component was displayed
useEffect(() => {
post('/analytics/event', { eventName: 'visit_form' });
}, []);
// 🔴 Avoid: Event-specific logic inside an Effect
const [jsonToSubmit, setJsonToSubmit] = useState(null);
useEffect(() => {
if (jsonToSubmit !== null) {
post('/api/register', jsonToSubmit);
}
}, [jsonToSubmit]);
function handleSubmit(e) {
e.preventDefault();
setJsonToSubmit({ firstName, lastName });
}
// ...
}
推荐
function Form() {
const [firstName, setFirstName] = useState('');
const [lastName, setLastName] = useState('');
// ✅ Good: This logic runs because the component was displayed
useEffect(() => {
post('/analytics/event', { eventName: 'visit_form' });
}, []);
function handleSubmit(e) {
e.preventDefault();
// ✅ Good: Event-specific logic is in the event handler
post('/api/register', { firstName, lastName });
}
// ...
}
计算链
function Game() {
const [card, setCard] = useState(null);
const [goldCardCount, setGoldCardCount] = useState(0);
const [round, setRound] = useState(1);
const [isGameOver, setIsGameOver] = useState(false);
// 🔴 Avoid: Chains of Effects that adjust the state solely to trigger each other
useEffect(() => {
if (card !== null && card.gold) {
setGoldCardCount(c => c + 1);
}
}, [card]);
useEffect(() => {
if (goldCardCount > 3) {
setRound(r => r + 1)
setGoldCardCount(0);
}
}, [goldCardCount]);
useEffect(() => {
if (round > 5) {
setIsGameOver(true);
}
}, [round]);
useEffect(() => {
alert('Good game!');
}, [isGameOver]);
function handlePlaceCard(nextCard) {
if (isGameOver) {
throw Error('Game already ended.');
} else {
setCard(nextCard);
}
}
// ...
推荐
function Game() {
const [card, setCard] = useState(null);
const [goldCardCount, setGoldCardCount] = useState(0);
const [round, setRound] = useState(1);
// ✅ Calculate what you can during rendering
const isGameOver = round > 5;
function handlePlaceCard(nextCard) {
if (isGameOver) {
throw Error('Game already ended.');
}
// ✅ Calculate all the next state in the event handler
setCard(nextCard);
if (nextCard.gold) {
if (goldCardCount <= 3) {
setGoldCardCount(goldCardCount + 1);
} else {
setGoldCardCount(0);
setRound(round + 1);
if (round === 5) {
alert('Good game!');
}
}
}
}
// ...
初始化应用程序
function App() {
// 🔴 Avoid: Effects with logic that should only ever run once
useEffect(() => {
loadDataFromLocalStorage();
checkAuthToken();
}, []);
// ...
}
推荐
let didInit = false;
function App() {
useEffect(() => {
if (!didInit) {
didInit = true;
// ✅ Only runs once per app load
loadDataFromLocalStorage();
checkAuthToken();
}
}, []);
// ...
}
将数据传递给父母
function Parent() {
const [data, setData] = useState(null);
// ...
return <Child onFetched={setData} />;
}
function Child({ onFetched }) {
const data = useSomeAPI();
// 🔴 Avoid: Passing data to the parent in an Effect
useEffect(() => {
if (data) {
onFetched(data);
}
}, [onFetched, data]);
// ...
}
推荐
function Parent() {
const data = useSomeAPI();
// ...
// ✅ Good: Passing data down to the child
return <Child data={data} />;
}
function Child({ data }) {
// ...
}
订阅外部商店
function useOnlineStatus() {
// Not ideal: Manual store subscription in an Effect
const [isOnline, setIsOnline] = useState(true);
useEffect(() => {
function updateState() {
setIsOnline(navigator.onLine);
}
updateState();
window.addEventListener('online', updateState);
window.addEventListener('offline', updateState);
return () => {
window.removeEventListener('online', updateState);
window.removeEventListener('offline', updateState);
};
}, []);
return isOnline;
}
function ChatIndicator() {
const isOnline = useOnlineStatus();
// ...
}
推荐
尽管使用效果为此，但React具有专门建造的挂钩，用于订阅首选的外部存储。删除效果并用呼叫useSyncExternalStore替换：
function subscribe(callback) {
window.addEventListener('online', callback);
window.addEventListener('offline', callback);
return () => {
window.removeEventListener('online', callback);
window.removeEventListener('offline', callback);
};
}
function useOnlineStatus() {
// ✅ Good: Subscribing to an external store with a built-in Hook
return useSyncExternalStore(
subscribe, // React won't resubscribe for as long as you pass the same function
() => navigator.onLine, // How to get the value on the client
() => true // How to get the value on the server
);
}
function ChatIndicator() {
const isOnline = useOnlineStatus();
// ...
}
获取数据
function SearchResults({ query }) {
const [results, setResults] = useState([]);
const [page, setPage] = useState(1);
useEffect(() => {
// 🔴 Avoid: Fetching without cleanup logic
fetchResults(query, page).then(json => {
setResults(json);
});
}, [query, page]);
function handleNextPageClick() {
setPage(page + 1);
}
// ...
}
推荐
function SearchResults({ query }) {
const [results, setResults] = useState([]);
const [page, setPage] = useState(1);
useEffect(() => {
let ignore = false;
fetchResults(query, page).then(json => {
if (!ignore) {
setResults(json);
}
});
return () => {
ignore = true;
};
}, [query, page]);
function handleNextPageClick() {
setPage(page + 1);
}
// ...
}
**注意：示例来自https://beta.reactjs.org/learn/you-might-not-need-an-effect