useMemo

src/App.jsx

@@ -1,58 +1,82 @@
-import React from 'react'
+import memo, { useMemo, useCallback, useState } from 'react'
 
-class RandomUser extends React.Component {
+function MyComponent() {
-    constructor(props) {
+    const [count, setCount] = useState(0);
-        super(props)
+    const [name, setName] = useState('John');
-        this.state = { user: {} }
-    }
     
-    componentDidMount() {
+    const greeting = useMemo(() => `Hello, ${name}!`,[name]);
-        fetch('https://randomuser.me/api')
-        .then(responce => responce.json())
-        .then(json => this.setState({ user: json.results[0] }));
-        console.log('смонтирован')
-    }
     
-    componentDidUpdate(prevProps,prevState) {
+    return (
-        console.log(this.state);
+        <>
-        console.log('Компонент обнвлен')
+        <h2>{greeting}</h2>
-    }
+        <button onClick={()=>setName('Alex')}>New name</button>
+        </>
+    )
+}
 
-    componentWillUnmount() {
+function MyComponent2() {
-        console.log('Компонент таймера размонтирован')
+    const [data,setData] = useState([{theme:'light'}])
-    }
+    const userPreferences = useMemo(()=>({theme: 'dark', language: 'en'}),[]);
     
-    render() {
+    const processData = useMemo(()=>{
-        return <div>
+        return data.map(item => ({ ...item,theme: userPreferences.theme }))
-            {this.state.user && 
+    },[data, userPreferences]);
-                this.state.user.name &&
+    console.log(processData);
-                (
+    return (
-                <>
+        <>
-                    <h2>{this.state.user.name.first} {this.state.user.name.last}</h2>
+            <h2 className={data.theme}>2</h2>
-                    <img src={this.state.user.picture.medium} />
+            <button onClick={()=>setData(data)}>set new theme</button>
-                    <button onClick={() => {
+        </>
-                        console.log('Загружаем ... ')
+    )
-                        fetch('https://randomuser.me/api')
+}
-                        .then(responce => responce.json())
+
-                        .then(json => this.setState({ user: json.results[0] }));
+function MyComponent3() {
-                    }}>Загрузить нового</button>
+    const sum = useMemo(()=> a+b,[a,b]);//Бессмысленное применение и вредное
-                </>
+
-                )
+    //Применение оправдано
-            }
+    const complexResult = useMemo(()=>{
-            </div>
+        return heavyMethOperations(largeDataSet);
-    }
+    },[largeDataSet])
+}
+
+function ParentComponent({ items }) {
+    const [filter, setFilter] = useState('');
+    
+    const listProps = useMemo(()=>({
+            items: items.filter(item => item.name.includes(filter)),
+            onItemClick: id => console.log('Clicked',id),
+            theme: 'dark'
+        }),[items,filter]);
+    
+    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 [showClock, setShowClock] = React.useState(true);
-    
 	return (
 		<>
-			<h1>Домашнее задание: жизненый цикл</h1>
+			{/* Добавьте тестовый контент для проверки */}
-            <button onClick={()=> setShowClock(!showClock)}>
+			<h1>Hello, React!</h1>
-                {showClock ? 'Скрыть часы' : 'Показать часы' }
+            <ParentComponent items={[{name:'Alex'},{name:'Bob'}]} />
-            </button>
-            {showClock && <RandomUser />}
 		</>
 	)
 }