useMemo

src/App.jsx

@@ -1,32 +1,72 @@
-import React, { useMemo, useCallback, useState } from 'react'
+import memo, { useMemo, useCallback, useState } from 'react'
 
-const TodoItem = React.memo(({ item, onToggle, onDelete }) => {
-    console.log(`rendred ${item.id}`);
-    return <li>
-            {item.name}
-            <button onClick={() => onToggle(item.id)}>Done</button>
-            <button onClick={() => onDelete(item.id)}>remove</button>
-        </li>
-})
+function MyComponent() {
+    const [count, setCount] = useState(0);
+    const [name, setName] = useState('John');
     
-function ItemList({ items }) {
+    const greeting = useMemo(() => `Hello, ${name}!`,[name]);
     
-    const handleToggle = useCallback(id => {
-        items.done = true;
-        console.log(`Item ${id} done`)
-        console.log(id)
-    },[]);
-    
-    const handleDelete = useCallback(id => {
-        items.pop(id);
-    },[]);
     return (
         <>
-            <h3>List items</h3>
-            {items.map(item => (
-             <TodoItem key={item.id} item={item}
-              onToggle={handleToggle} onDelete={handleDelete} />
-             ))}
+        <h2>{greeting}</h2>
+        <button onClick={()=>setName('Alex')}>New name</button>
+        </>
+    )
+}
+
+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>
+         </>))}
         </>
     )
 }
@@ -36,10 +76,7 @@ function App() {
 		<>
 			{/* Добавьте тестовый контент для проверки */}
 			<h1>Hello, React!</h1>
-            <ItemList items={[
-                {id:1,name:'Первый',done:false},
-                {id:2,name:'Второй',done:false}
-            ]} />
+            <ParentComponent items={[{name:'Alex'},{name:'Bob'}]} />
 		</>
 	)
 }