useMemo

src/App.jsx

@@ -1,78 +1,82 @@
-import { useState, createContext, useContext } from 'react'
-import styles from './App.module.scss'
+import memo, { useMemo, useCallback, useState } from 'react'
 
-const UserContext = createContext('')
-
-function UserProvider({ children }) {
-    const [name,setName] = useState('');
+function MyComponent() {
+    const [count, setCount] = useState(0);
+    const [name, setName] = useState('John');
     
-    const toggleName = (newName) => {
-        setName(newName);
-    }
+    const greeting = useMemo(() => `Hello, ${name}!`,[name]);
     
     return (
-        <UserContext.Provider value={{name, toggleName}}>
-            {children}
-        </UserContext.Provider>
+        <>
+        <h2>{greeting}</h2>
+        <button onClick={()=>setName('Alex')}>New name</button>
+        </>
     )
 }
 
-function Form() {
-    const {name, toggleName} = useContext(UserContext)
-    const [newName,setNewName] = useState('');
-    const handleSubmit = (event) => {
-        event.preventDefault();
-        toggleName(event.target[0].value)
-    }
-    const handlerChange = (event) => {
-        setNewName(event.target.value);
-    }
+function MyComponent2() {
+    const [data,setData] = useState([{theme:'light'}])
+    const userPreferences = useMemo(()=>({theme: 'dark', language: 'en'}),[]);
+    
+    const processData = useMemo(()=>{
+        return data.map(item => ({ ...item,theme: userPreferences.theme }))
+    },[data, userPreferences]);
+    console.log(processData);
     return (
-        <form onSubmit={handleSubmit}>
-        <input type='text' value={newName} onChange={handlerChange} />
-        <button type='submit'>Войти</button>
-        </form>
+        <>
+            <h2 className={data.theme}>2</h2>
+            <button onClick={()=>setData(data)}>set new theme</button>
+        </>
     )
 }
 
-function ExitButton() {
-    const {name, toggleName} = useContext(UserContext)
+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('');
+    
+    const listProps = useMemo(()=>({
+            items: items.filter(item => item.name.includes(filter)),
+            onItemClick: id => console.log('Clicked',id),
+            theme: 'dark'
+        }),[items,filter]);
     
     return (
-        <button onClick={() => toggleName('')}>Выход</button>
+        <>
+        <Memoized {...listProps} />
+        <button onClick={()=>setFilter('Bob')}>Click</button>
+        </>
     )
 }
 
-function Toolbar() {
-    const {name, toggleName} = useContext(UserContext)
-    return(
-        <div className={styles.container}>
-            <h3>Панель пользователя</h3>
-            {name && `Привет ${name}`}
-            {name ? <ExitButton /> : <Form /> }
-            
-        </div>
-    )
-}
-
-function Page() {
+function Memoized(props) {
+    console.log(props);
     return (
-        <div>
-            <h1 style={{textAlign: 'center'}}>Главная страница</h1>
-            <Toolbar />
-        </div>
+        <>
+        <h3 className={props.theme}>Memoized</h3>
+        {props.items.map((item) => (
+         <>
+            <li>
+                {item.name}
+            </li>
+         </>))}
+        </>
     )
 }
 
-function App() {    
-    
+function App() {
 	return (
 		<>
 			{/* Добавьте тестовый контент для проверки */}
 			<h1>Hello, React!</h1>
-        <UserProvider>
-            <Page />
-        </UserProvider>
+            <ParentComponent items={[{name:'Alex'},{name:'Bob'}]} />
 		</>
 	)
 }

src/App.module.scss

@@ -1,13 +0,0 @@
-.container {
-    height: 500px;
-    border: 1px solid black;
-    border-radius: 10px;
-    background-color: lightblue;
-}
-.item {
-    font-size: 16pt;
-    border: 1px dotted gray;
-    /*margin: 5px 10px;
-    padding: 10px;*/
-    border-radius: 5px;
-}

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>
 )