useMemo

src/App.jsx

@@ -1,50 +1,82 @@
-import { useState, useReducer } from 'react'
+import memo, { useMemo, useCallback, useState } from 'react'
 
-function counterReducer(state, action) {
+function MyComponent() {
-    switch (action.type) {
+    const [count, setCount] = useState(0);
-        case 'increment':
+    const [name, setName] = useState('John');
-            return state + 1
-        case 'decrement':
-            return state - 1
-        case 'reset':
-            return 0
-        case 'set_value':
-            return action.value
-        default:
-            return state
-    }
-}
     
-function Counter() {
+    const greeting = useMemo(() => `Hello, ${name}!`,[name]);
-    const [count, dispatch] = useReducer(counterReducer,0);
     
     return (
-        <div>
+        <>
-            <span>Счетчик: {count}</span>
+        <h2>{greeting}</h2>
-            <button onClick={()=> dispatch({type: 'increment'})}>+</button>
+        <button onClick={()=>setName('Alex')}>New name</button>
-            <button onClick={()=> dispatch({type: 'decrement'})}>-</button>
+        </>
-            <button onClick={()=> dispatch({type: 'reset'})}>reset</button>
+    )
-            <button onClick={()=> dispatch({type: 'set_value', value: 5})}>set 5</button>
+}
-        </div>
+
+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 (
+        <>
+            <h2 className={data.theme}>2</h2>
+            <button onClick={()=>setData(data)}>set new theme</button>
+        </>
+    )
+}
+
+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, setTasks] = useState([])
-    const [filter, setFilter] = useState('all')
-    const [sortBy,setSortBy] = useState('date')
-    
-    const addTask = newTask => {
-        setTasks([...tasks,newTask])
-    }
-    
-    
-    
 	return (
 		<>
 			{/* Добавьте тестовый контент для проверки */}
 			<h1>Hello, React!</h1>
-            <Counter />
+            <ParentComponent items={[{name:'Alex'},{name:'Bob'}]} />
 		</>
 	)
 }

src/App.module.scss

@@ -1,15 +0,0 @@
-.container {
-/*    display: flex;
-    flex-direction: column;
-    flex-wrap: nowrap;*/
-    height: 500px;
-    overflow: auto;
-    border: 1px solid black;
-}
-.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>
 )