Разобрали useRef

src/App.jsx

@@ -1,67 +1,99 @@
-import React, { useState, useReducer, useRef, memo } from 'react'
+import { useState, useRef, useEffect } from 'react'
 
-function tasksReducer(state, action) {
+function Example() {
-    switch(action.type) {
+    const myRef = useRef(null);
-        case 'ADD_TASK':
+    const counterRef = useRef(0);
-            return [...state, action.payload]
+    const [text, setText] = useState('');
-        case 'TOGGLE_TASK':
+    
-            return state.map(task => task.id === action.payload ? {...task, complite: !task.complite} : task)
+    useEffect(() => {
-        case 'REMOVE_TASK':
+        myRef.current?.focus();
-            return state.filter(task => task.id !== action.payload)
+    },[]);
-        default:
+        
-            return state
+    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>
 }
 
-const TaskList = memo(function TaskList({tasks, onToggle, onRemove}) {
+function ClickCounter() {
-    console.log("Rendred")
+    const countRef = useRef(0);
-    return (
-        <ul>
-        {tasks.map(({id, text, complite}) => (
-                <li key={id}
-                    style={{textDecoration: complite ? 'line-through' : 'none' }}>
-                    <input type="checkbox" checked={complite} onChange={()=>onToggle(id)} />
-                    {text}
-                    <button onClick={()=> onRemove(id)} style={{marginLeft: 10}}>
-                        Удалить
-                    </button>
-                </li>
-            ))}
-        </ul>
-    )
-})
     
-function App() {
+    function handlerClick() {
-    const [tasks, dispatch] = useReducer(tasksReducer,[])
+        countRef.current += 1;
-    const [input,setInput] = useState('')
+        //alert(`Нажали ${countRef.current} раз.`);
-    const [error,setError] = useState('')
+    }
-    const inputRef = useRef(null)
+    return <button onClick={handlerClick}>Нажать</button>;
+}
 
-    const addTask = () => {
+function TimerComponent() {
-        if (input.trim() === '') {
+    const timerIdRef = useRef(null);
-            setError('Пустой текст задачи')
+    
-            return
+    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>
+            Текущее значение: {value} <br />
+            Предыдущее значение: {prevValueRef.current}
+        </div>
+    )
+}
+
+function ScrollList() {
+    const listRef = useRef(null);
+    
+    const scrollToBottom = () => {
+        if(listRef.current) {
+            listRef.current.scrollTop = listRef.current.scrollHeight;
         }
-        setError('')
-        dispatch({
-            type: 'ADD_TASK',
-            payload: {id: Date.now(), text: input.trim(), complite: false }
-        })
-        setInput('')
-        inputRef.current.focus()
     }
         
-	return (
+    return (
-		<div style={{padding: 20}}>
+        <>
-			<h1>Список дел</h1>
+            <div ref={listRef} 
-			<input ref={inputRef} value={input} onChange={e => setInput(e.target.value)} />
+               style={{height: 200, overflowY: 'auto', border: '1px solid black'}}>
-			<button onClick={addTask}>Добавить</button>
+                {[...Array(20)].map((_,i)=>(
-            {error && <p style={{color: 'red'}}>{error}</p>}
+                    <div key={i}>Элемент № {i + 1}</div>
+                ))}
+            </div>
+            <button onClick={scrollToBottom}>Прокрутить вниз</button>
+        </>
+    )
+}
         
-            <TaskList tasks={tasks} 
+function App() {
-                onToggle={id => dispatch({type: 'TOGGLE_TASK', payload: id})}
+    
-                onRemove={id => dispatch({type: 'REMOVE_TASK', payload: id})} />
+    
-		</div>
+	return (
+		<>
+			<h1>Hello, React!</h1>
+            <ScrollList />
+		</>
 	)
 }
 

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>
 )