useMemo

src/App.jsx

@@ -1,96 +1,82 @@
-import React from 'react'
+import memo, { useMemo, useCallback, useState } from 'react'
-import styles from './App.module.scss'
 
-class Clock extends React.Component {
+function MyComponent() {
-    constructor(props) {
+    const [count, setCount] = useState(0);
-        super(props);
+    const [name, setName] = useState('John');
-        this.state = { date: new Date() };
+    
-    }
+    const greeting = useMemo(() => `Hello, ${name}!`,[name]);
-    componentDidMount() {
+    
-        this.timerId = setInterval( () => this.tick(), 1000);
+    return (
-    }
+        <>
-    componentWillUnmount() {
+        <h2>{greeting}</h2>
-        clearInterval(this.timerId)
+        <button onClick={()=>setName('Alex')}>New name</button>
-    }
+        </>
-    tick() {
+    )
-        this.setState({ date: new Date() })
-    }
-    render() {
-        return <div>{this.state.date.toLocaleTimeString()}</div>
-    }
 }
 
-function Clock2() {
+function MyComponent2() {
-    const [date, setDate] = React.useState(new Date());
+    const [data,setData] = useState([{theme:'light'}])
+    const userPreferences = useMemo(()=>({theme: 'dark', language: 'en'}),[]);
     
-    React.useEffect(()=>{
+    const processData = useMemo(()=>{
-        const timerId = setInterval(()=> setDate(new Date()),1000);
+        return data.map(item => ({ ...item,theme: userPreferences.theme }))
-        return () => clearInterval(timerId);
+    },[data, userPreferences]);
-    },[])
+    console.log(processData);
-    
+    return (
-    return <div>{date.toLocaleTimeString()}</div>
+        <>
+            <h2 className={data.theme}>2</h2>
+            <button onClick={()=>setData(data)}>set new theme</button>
+        </>
+    )
 }
 
-function MouseTracker() {
+function MyComponent3() {
-    const [x,setX] = React.useState(0);
+    const sum = useMemo(()=> a+b,[a,b]);//Бессмысленное применение и вредное
-    const [y,setY] = React.useState(0);
+
-    
+    //Применение оправдано
-    const mouseMoveHandler = event => {
+    const complexResult = useMemo(()=>{
-        setX(event.clientX);
+        return heavyMethOperations(largeDataSet);
-        setY(event.clientY);
+    },[largeDataSet])
-    }
-    
-    React.useEffect(()=>{
-        document.addEventListener('mousemove', mouseMoveHandler);
-        return () => {
-            document.removeEventListener('mousemove',mouseMoveHandler);
-        }
-    },[]);
-    
-    return <div className={styles.container}>
-        <p>Координаты мыши:<br /> X:<b>{x}</b>, Y:<b>{y}</b></p>
-        </div>
 }
 
-function DataFetcher() {
+function ParentComponent({ items }) {
-    const [posts,setPosts] = React.useState([]);
+    const [filter, setFilter] = useState('');
-    const [users,setUsers] = React.useState([]);
     
-    const getPosts = async () => {
+    const listProps = useMemo(()=>({
-        const response = await fetch('https://jsonplaceholder.typicode.com/posts');
+            items: items.filter(item => item.name.includes(filter)),
-        const data = await response.json();
+            onItemClick: id => console.log('Clicked',id),
-        setUsers(data.reduce((acc, value) => {
+            theme: 'dark'
-            if (!(value.userId in acc))
+        }),[items,filter]);
-                return [...acc,value.userId]
-            return acc
-        }))
-        console.log(users);
-        setPosts(data.slice(1,10));
-        
-    }
     
-    React.useEffect(() => {
+    return (
-        console.log("Компонент создан");
+        <>
-        getPosts();
+        <Memoized {...listProps} />
-        return () => {console.log("Компонент удален");setPosts([])};
+        <button onClick={()=>setFilter('Bob')}>Click</button>
-    },[])
+        </>
-    
+    )
-    return <div><ol>{posts.map(post => <li id={post.id}>{post.title}</li>)}</ol></div>
+}
+
+function Memoized(props) {
+    console.log(props);
+    return (
+        <>
+        <h3 className={props.theme}>Memoized</h3>
+        {props.items.map((item) => (
+         <>
+            <li>
+                {item.name}
+            </li>
+         </>))}
+        </>
+    )
 }
 
 function App() {
-    const [tracker,setTracker] = React.useState(false);
-    const [dataFetcher,setDataFetcher] = React.useState(false);
-    
 	return (
 		<>
+			{/* Добавьте тестовый контент для проверки */}
 			<h1>Hello, React!</h1>
-            <Clock />
+            <ParentComponent items={[{name:'Alex'},{name:'Bob'}]} />
-            <Clock2 />
-            <button onClick={() => setTracker(!tracker)}>Показать/Скрыть трекер</button>
-            { tracker && <MouseTracker /> }
-            <button onClick={() => setDataFetcher(!dataFetcher)}>Показать/Скрыть посты</button>
-            { dataFetcher && <DataFetcher /> }
 		</>
 	)
 }

src/App.module.scss

@@ -1,11 +0,0 @@
-.container {
-    width: 300px;
-    height: 200px;
-    border: 2px dashed grey;
-    margin: 10px;
-    text-align: center;
-    p {
-        margin-top: 20%;
-    }
-}
-

src/main.jsx

@@ -4,7 +4,5 @@ import { createRoot } from 'react-dom/client'
 import App from './App.jsx'
 
 createRoot(document.getElementById('root')).render(
-	<StrictMode>
 		<App />
-	</StrictMode>
 )