useMemo

src/App.jsx

@@ -1,67 +1,83 @@
-import React, { useState, useReducer, useRef, memo } from 'react'
+import memo, { useMemo, useCallback, useState } from 'react'
 
-function tasksReducer(state, action) {
+function MyComponent() {
-    switch(action.type) {
+    const [count, setCount] = useState(0);
-        case 'ADD_TASK':
+    const [name, setName] = useState('John');
-            return [...state, action.payload]
+    
-        case 'TOGGLE_TASK':
+    const greeting = useMemo(() => `Hello, ${name}!`,[name]);
-            return state.map(task => task.id === action.payload ? {...task, complite: !task.complite} : task)
+    
-        case 'REMOVE_TASK':
+    return (
-            return state.filter(task => task.id !== action.payload)
+        <>
-        default:
+        <h2>{greeting}</h2>
-            return state
+        <button onClick={()=>setName('Alex')}>New name</button>
-    }
+        </>
+    )
 }
 
-const TaskList = memo(function TaskList({tasks, onToggle, onRemove}) {
+function MyComponent2() {
-    console.log("Rendred")
+    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 (
-        <ul>
+        <>
-        {tasks.map(({id, text, complite}) => (
+            <h2 className={data.theme}>2</h2>
-                <li key={id}
+            <button onClick={()=>setData(data)}>set new theme</button>
-                    style={{textDecoration: complite ? 'line-through' : 'none' }}>
+        </>
-                    <input type="checkbox" checked={complite} onChange={()=>onToggle(id)} />
-                    {text}
-                    <button onClick={()=> onRemove(id)} style={{marginLeft: 10}}>
-                        Удалить
-                    </button>
-                </li>
-            ))}
-        </ul>
     )
-})
+}
+
+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 (
+        <>
+        <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 [tasks, dispatch] = useReducer(tasksReducer,[])
-    const [input,setInput] = useState('')
-    const [error,setError] = useState('')
-    const inputRef = useRef(null)
-    
-    const addTask = () => {
-        if (input.trim() === '') {
-            setError('Пустой текст задачи')
-            return
-        }
-        setError('')
-        dispatch({
-            type: 'ADD_TASK',
-            payload: {id: Date.now(), text: input.trim(), complite: false }
-        })
-        setInput('')
-        inputRef.current.focus()
-    }
-    
 	return (
-		<div style={{padding: 20}}>
+		<>
-			<h1>Список дел</h1>
+			{/* Добавьте тестовый контент для проверки */}
-			<input ref={inputRef} value={input} onChange={e => setInput(e.target.value)} />
+			<h1>Hello, React!</h1>
-			<button onClick={addTask}>Добавить</button>
+            <ParentComponent items={[{name:'Alex'},{name:'Bob'}]} />
-            {error && <p style={{color: 'red'}}>{error}</p>}
+		</>
-            
-            <TaskList tasks={tasks} 
-                onToggle={id => dispatch({type: 'TOGGLE_TASK', payload: id})}
-                onRemove={id => dispatch({type: 'REMOVE_TASK', payload: id})} />
-		</div>
 	)
 }
 

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>
 )