Разобрали useRef

src/App.jsx

@@ -1,45 +1,98 @@
-import React, { useMemo, useCallback, useState } from 'react'
+import { useState, useRef, useEffect } from 'react'
 
-const TodoItem = React.memo(({ item, onToggle, onDelete }) => {
+function Example() {
-    console.log(`rendred ${item.id}`);
+    const myRef = useRef(null);
-    return <li>
+    const counterRef = useRef(0);
-            {item.name}
+    const [text, setText] = useState('');
-            <button onClick={() => onToggle(item.id)}>Done</button>
-            <button onClick={() => onDelete(item.id)}>remove</button>
-        </li>
-})
-
-function ItemList({ items }) {
-
-    const handleToggle = useCallback(id => {
-        items.done = true;
-        console.log(`Item ${id} done`)
-        console.log(id)
-    },[]);
     
-    const handleDelete = useCallback(id => {
+    useEffect(() => {
-        items.pop(id);
+        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 (
-        <>
+        <div>
-            <h3>List items</h3>
+            Текущее значение: {value} <br />
-            {items.map(item => (
+            Предыдущее значение: {prevValueRef.current}
-             <TodoItem key={item.id} item={item}
+        </div>
-              onToggle={handleToggle} onDelete={handleDelete} />
-             ))}
-        </>
     )
 }
 
+function ScrollList() {
+    const listRef = useRef(null);
+    
+    const scrollToBottom = () => {
+        if(listRef.current) {
+            listRef.current.scrollTop = listRef.current.scrollHeight;
+        }
+    }
+        
+    return (
+        <>
+            <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 App() {
+    
+    
 	return (
 		<>
-			{/* Добавьте тестовый контент для проверки */}
 			<h1>Hello, React!</h1>
-            <ItemList items={[
+            <ScrollList />
-                {id:1,name:'Первый',done:false},
-                {id:2,name:'Второй',done:false}
-            ]} />
 		</>
 	)
 }