useMemo

src/App.jsx

@@ -1,33 +1,74 @@
-import { useMutation, QueryClientProvider, QueryClient } from '@tanstack/react-query'
+import memo, { useMemo, useCallback, useState } from 'react'
-import PostAddForm from './components/PostAddForm'
 
-/*
+function MyComponent() {
-const mutation = useMutation({
+    const [count, setCount] = useState(0);
-    mutationFn: newUser => {
+    const [name, setName] = useState('John');
-        fetch('https://jsonplaceholder.typicode.com/users', {
-            method: 'GET',
-            headers: {'Content-type':'application/json'},
-            body: JSON.stringify(newUser),
-        }).then(res => res.json())
-    }
-})
-*/
     
-const createQueryClient = () => {
+    const greeting = useMemo(() => `Hello, ${name}!`,[name]);
-    new QueryClient({
+    
-        defaultOptions: {
+    return (
-            queries: {
+        <>
-                staleTime: 5 * 60*1000,
+        <h2>{greeting}</h2>
-                retry: 1,
+        <button onClick={()=>setName('Alex')}>New name</button>
-                gcTime: 10*60*1000,
+        </>
-                refetchOnWindowFocus: false,
+    )
-            },
-            mutations: {
-                retry:0,
-                onError: error => {console.error('Mutation error:',error.message)}
 }
+
+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() {
@@ -35,10 +76,7 @@ function App() {
 		<>
 			{/* Добавьте тестовый контент для проверки */}
 			<h1>Hello, React!</h1>
-        <QueryClientProvider client={createQueryClient}>
+            <ParentComponent items={[{name:'Alex'},{name:'Bob'}]} />
-            <PostAddForm />
-        </QueryClientProvider>
-            
 		</>
 	)
 }

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>
 )