Разобрали useRef

src/App.jsx

@@ -1,82 +1,98 @@
-import memo, { useMemo, useCallback, useState } from 'react'
+import { useState, useRef, useEffect } from 'react'
 
-function MyComponent() {
-    const [count, setCount] = useState(0);
-    const [name, setName] = useState('John');
+function Example() {
+    const myRef = useRef(null);
+    const counterRef = useRef(0);
+    const [text, setText] = useState('');
     
-    const greeting = useMemo(() => `Hello, ${name}!`,[name]);
+    useEffect(() => {
+        myRef.current?.focus();
+    },[]);
+        
+    const handlerFocus = () => {
+        if(myRef.current) {
+            myRef.current.focus();
+        }
+    };
     
+    const handlerChange = (event) => {
+        setText(event.target.value);
+    }
+    counterRef.current += 1;
+    return <div>
+        <input ref={myRef} text="text" /><br />
+        Количество рендров: {counterRef.current}<br />
+        <input text="text" onChange={handlerChange} value={text} />
+        <button onClick={handlerFocus}>Поставить фокус на поле</button>
+    </div>
+}
+
+function ClickCounter() {
+    const countRef = useRef(0);
+    
+    function handlerClick() {
+        countRef.current += 1;
+        //alert(`Нажали ${countRef.current} раз.`);
+    }
+    return <button onClick={handlerClick}>Нажать</button>;
+}
+
+function TimerComponent() {
+    const timerIdRef = useRef(null);
+    
+    useEffect(() => {
+        timerIdRef.current = setTimeout(() => {
+            alert("Прошло 3 секунды");
+        }, 3000);
+        return () => {
+            clearTimeout(timerIdRef.current);
+        }
+    },[]);
+}
+
+function PrevValue({ value }) {
+    const prevValueRef = useRef();
+    
+    useEffect(() => {
+        prevValueRef.current = value;
+    });
     return (
-        <>
-        <h2>{greeting}</h2>
-        <button onClick={()=>setName('Alex')}>New name</button>
-        </>
+        <div>
+            Текущее значение: {value} <br />
+            Предыдущее значение: {prevValueRef.current}
+        </div>
     )
 }
 
-function MyComponent2() {
-    const [data,setData] = useState([{theme:'light'}])
-    const userPreferences = useMemo(()=>({theme: 'dark', language: 'en'}),[]);
+function ScrollList() {
+    const listRef = useRef(null);
     
-    const processData = useMemo(()=>{
-        return data.map(item => ({ ...item,theme: userPreferences.theme }))
-    },[data, userPreferences]);
-    console.log(processData);
+    const scrollToBottom = () => {
+        if(listRef.current) {
+            listRef.current.scrollTop = listRef.current.scrollHeight;
+        }
+    }
+        
     return (
         <>
-            <h2 className={data.theme}>2</h2>
-            <button onClick={()=>setData(data)}>set new theme</button>
+            <div ref={listRef} 
+               style={{height: 200, overflowY: 'auto', border: '1px solid black'}}>
+                {[...Array(20)].map((_,i)=>(
+                    <div key={i}>Элемент № {i + 1}</div>
+                ))}
+            </div>
+            <button onClick={scrollToBottom}>Прокрутить вниз</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() {
+    
+    
 	return (
 		<>
-			{/* Добавьте тестовый контент для проверки */}
 			<h1>Hello, React!</h1>
-            <ParentComponent items={[{name:'Alex'},{name:'Bob'}]} />
+            <ScrollList />
 		</>
 	)
 }