Разобрали useRef

src/App.jsx

@@ -1,78 +1,98 @@
-import { useState, createContext, useContext } from 'react'
+import { useState, useRef, useEffect } from 'react'
-import styles from './App.module.scss'
 
-const UserContext = createContext('')
+function Example() {
+    const myRef = useRef(null);
+    const counterRef = useRef(0);
+    const [text, setText] = useState('');
     
-function UserProvider({ children }) {
+    useEffect(() => {
-    const [name,setName] = useState('');
+        myRef.current?.focus();
+    },[]);
         
-    const toggleName = (newName) => {
+    const handlerFocus = () => {
-        setName(newName);
+        if(myRef.current) {
-    }
+            myRef.current.focus();
+        }
+    };
     
-    return (
-        <UserContext.Provider value={{name, toggleName}}>
-            {children}
-        </UserContext.Provider>
-    )
-}
-
-function Form() {
-    const {name, toggleName} = useContext(UserContext)
-    const [newName,setNewName] = useState('');
-    const handleSubmit = (event) => {
-        event.preventDefault();
-        toggleName(event.target[0].value)
-    }
     const handlerChange = (event) => {
-        setNewName(event.target.value);
+        setText(event.target.value);
     }
-    return (
+    counterRef.current += 1;
-        <form onSubmit={handleSubmit}>
+    return <div>
-        <input type='text' value={newName} onChange={handlerChange} />
+        <input ref={myRef} text="text" /><br />
-        <button type='submit'>Войти</button>
+        Количество рендров: {counterRef.current}<br />
-        </form>
+        <input text="text" onChange={handlerChange} value={text} />
-    )
+        <button onClick={handlerFocus}>Поставить фокус на поле</button>
+    </div>
 }
 
-function ExitButton() {
+function ClickCounter() {
-    const {name, toggleName} = useContext(UserContext)
+    const countRef = useRef(0);
     
-    return (
+    function handlerClick() {
-        <button onClick={() => toggleName('')}>Выход</button>
+        countRef.current += 1;
-    )
+        //alert(`Нажали ${countRef.current} раз.`);
+    }
+    return <button onClick={handlerClick}>Нажать</button>;
 }
 
-function Toolbar() {
+function TimerComponent() {
-    const {name, toggleName} = useContext(UserContext)
+    const timerIdRef = useRef(null);
-    return(
-        <div className={styles.container}>
-            <h3>Панель пользователя</h3>
-            {name && `Привет ${name}`}
-            {name ? <ExitButton /> : <Form /> }
     
-        </div>
+    useEffect(() => {
-    )
+        timerIdRef.current = setTimeout(() => {
+            alert("Прошло 3 секунды");
+        }, 3000);
+        return () => {
+            clearTimeout(timerIdRef.current);
+        }
+    },[]);
 }
 
-function Page() {
+function PrevValue({ value }) {
+    const prevValueRef = useRef();
+    
+    useEffect(() => {
+        prevValueRef.current = value;
+    });
     return (
         <div>
-            <h1 style={{textAlign: 'center'}}>Главная страница</h1>
+            Текущее значение: {value} <br />
-            <Toolbar />
+            Предыдущее значение: {prevValueRef.current}
         </div>
     )
 }
 
+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>
-        <UserProvider>
+            <ScrollList />
-            <Page />
-        </UserProvider>
 		</>
 	)
 }

src/App.module.scss

@@ -1,13 +0,0 @@
-.container {
-    height: 500px;
-    border: 1px solid black;
-    border-radius: 10px;
-    background-color: lightblue;
-}
-.item {
-    font-size: 16pt;
-    border: 1px dotted gray;
-    /*margin: 5px 10px;
-    padding: 10px;*/
-    border-radius: 5px;
-}

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>
 )