useMemo

src/App.jsx

@@ -1,50 +1,72 @@
-import { memo, useState, useMemo } from 'react'
+import memo, { useMemo, useCallback, useState } from 'react'
 
-function memoize(fn) {
+function MyComponent() {
-    const cache = new Map();
+    const [count, setCount] = useState(0);
-    return function(...args) {
+    const [name, setName] = useState('John');
-        const key = JSON.stringify(args);
-        if (cache.has(key)) {
-            return cache.get(key);
-        }
-        const result = fn.apply(this, args);
-        cache.set(key,result);
-        return result;
-    }
-}
-
-function fibonachi(n, memo={}) {
-    if (n in memo) {console.log(n);return memo[n];}
-    if (n <= 1) return 1;
-    memo[n] = fibonachi(n-1, memo) + fibonachi(n - 2, memo);
-    return memo[n];
-}
-
-const MyComponent = ({ value }) => {
-    console.log('Rendring');
-    return <div>{value}</div>
-}
-
-const MemoizedComponent = memo(MyComponent);
-
-const Child = memo(({ children }) => {
-    console.log('Child component rendred');
-    return <div>{children}</div>;
-});
-
-const Parent = () => {
-    const [count,setCount] = useState(0);
-    const [text, setText] = useState('');
     
-    const stableChild = useMemo(() => <span>Memo text</span>,[]);
+    const greeting = useMemo(() => `Hello, ${name}!`,[name]);
     
     return (
         <>
-            <Child>
+        <h2>{greeting}</h2>
-                {stableChild}
+        <button onClick={()=>setName('Alex')}>New name</button>
-            </Child>
+        </>
-            <input type="text" onChange={(e) => setText(e.target.value)} value={text} />
+    )
-            <button onClick={() => setCount(count + 1)}>Increment {count}</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>
+         </>))}
         </>
     )
 }
@@ -52,9 +74,10 @@ const Parent = () => {
 function App() {
 	return (
 		<>
+			{/* Добавьте тестовый контент для проверки */}
 			<h1>Hello, React!</h1>
-            <Parent />
+            <ParentComponent items={[{name:'Alex'},{name:'Bob'}]} />
-        </>
+		</>
 	)
 }