useMemo

src/App.jsx

@@ -1,108 +1,82 @@
-import React from 'react'
+import memo, { useMemo, useCallback, useState } from 'react'
 
-class MyComponent extends React.Component {
-    constructor(props) {
-        super(props);
-        this.state = { data: null };
-        this.handleClick = this.handleClick.bind(this);
-    }
+function MyComponent() {
+    const [count, setCount] = useState(0);
+    const [name, setName] = useState('John');
     
-    render() {
-        return (
-            <div>
-            {this.state.data ? (
-                    <div>Данные: {this.state.data}</div>
-                ) : (
-                    <div>Загрузка...</div>
-                )}
-                <button onClick={this.handleClick}>Обновить</button>
-            </div>
-        )
-    }
+    const greeting = useMemo(() => `Hello, ${name}!`,[name]);
     
-    componentDidMount() {
-        fetch('https://api.example.com/data')
-        .then(responce => responce.json())
-        .then(json => this.setState({data: json}));
-        
-        this.timerId = setInterval(()=> console.log('Tick'), 1000);
-    }
-    
-    componentWillUnmout() {
-        clearInterval(this.timerId);
-    }
-    
-    handleClick() {
-        this.setState({data: 'Данные'})
-    }
+    return (
+        <>
+        <h2>{greeting}</h2>
+        <button onClick={()=>setName('Alex')}>New name</button>
+        </>
+    )
 }
 
-class MyComponent2 extends React.Component {
-    controller = new AbortController()
-
-    componentDidMount() {
-        fetch('https://api.example.com/data',{ singnal: this.controller.signal })
-        .then(responce => responce.json())
-        .then(json => this.setState({data: json}))
-        .catch(error => {
-            if(error.name == 'AbortError') {
-                console.log('Aborted');
-            } else {
-                console.log('Other error',error);
-            }
-        });
-    }
-
-    componentWillUnmount() {
-        this.controller.abort()
-    }
-}
-class MyComponent3 extends React.Component {
-    componentDidMount() {
-        this.loadData();
-    }
+function MyComponent2() {
+    const [data,setData] = useState([{theme:'light'}])
+    const userPreferences = useMemo(()=>({theme: 'dark', language: 'en'}),[]);
     
-    async loadData() {
-        try {
-            const response = await fetch('https://api.example.com/data');
-            const data = await response.json();
-            this.setSate({ data });
-        } catch {
-            console.log('error');
-        }
-    }
+    const processData = useMemo(()=>{
+        return data.map(item => ({ ...item,theme: userPreferences.theme }))
+    },[data, userPreferences]);
+    console.log(processData);
+    return (
+        <>
+            <h2 className={data.theme}>2</h2>
+            <button onClick={()=>setData(data)}>set new theme</button>
+        </>
+    )
 }
 
-class Timer extends React.Component {
-    constructor(props) {
-        super(props);
-        this.state = { seconds: 0 }
-    }
+function MyComponent3() {
+    const sum = useMemo(()=> a+b,[a,b]);//Бессмысленное применение и вредное
+
+    //Применение оправдано
+    const complexResult = useMemo(()=>{
+        return heavyMethOperations(largeDataSet);
+    },[largeDataSet])
+}
+
+function ParentComponent({ items }) {
+    const [filter, setFilter] = useState('');
     
-    componentDidMount() {
-        console.log('Timer mounted');
-        this.interval = setInterval(()=>{
-            this.setState(prevState => ({seconds: prevState.seconds + 1}))
-        },1000);
-    }
+    const listProps = useMemo(()=>({
+            items: items.filter(item => item.name.includes(filter)),
+            onItemClick: id => console.log('Clicked',id),
+            theme: 'dark'
+        }),[items,filter]);
     
-    componentWillUnmount() {
-        console.log('Timer unmount');
-        clearInterval(this.interval);
-    }
-    render() {
-        return <h2>Прошло секунд: {this.state.seconds}</h2>
-    }
+    return (
+        <>
+        <Memoized {...listProps} />
+        <button onClick={()=>setFilter('Bob')}>Click</button>
+        </>
+    )
+}
+
+function Memoized(props) {
+    console.log(props);
+    return (
+        <>
+        <h3 className={props.theme}>Memoized</h3>
+        {props.items.map((item) => (
+         <>
+            <li>
+                {item.name}
+            </li>
+         </>))}
+        </>
+    )
 }
 
 function App() {
-    const [show,setShow] = React.useState(true);
-    
 	return (
 		<>
-			<h1>Таймер с классом</h1>
-            <button onClick={()=>setShow(prev => !prev)}>{show ? 'Скрыть' : 'Показать'} таймер</button>
-            {show && <Timer />}
+			{/* Добавьте тестовый контент для проверки */}
+			<h1>Hello, React!</h1>
+            <ParentComponent items={[{name:'Alex'},{name:'Bob'}]} />
 		</>
 	)
 }