Разобрали useRef

src/App.jsx

@@ -1,108 +1,98 @@
-import React from 'react'
+import { useState, useRef, useEffect } from 'react'
 
-class MyComponent extends React.Component {
-    constructor(props) {
-        super(props);
-        this.state = { data: null };
-        this.handleClick = this.handleClick.bind(this);
-    }
+function Example() {
+    const myRef = useRef(null);
+    const counterRef = useRef(0);
+    const [text, setText] = useState('');
     
-    render() {
-        return (
-            <div>
-            {this.state.data ? (
-                    <div>Данные: {this.state.data}</div>
-                ) : (
-                    <div>Загрузка...</div>
-                )}
-                <button onClick={this.handleClick}>Обновить</button>
-            </div>
-        )
-    }
-    
-    componentDidMount() {
-        fetch('https://api.example.com/data')
-        .then(responce => responce.json())
-        .then(json => this.setState({data: json}));
+    useEffect(() => {
+        myRef.current?.focus();
+    },[]);
         
-        this.timerId = setInterval(()=> console.log('Tick'), 1000);
-    }
+    const handlerFocus = () => {
+        if(myRef.current) {
+            myRef.current.focus();
+        }
+    };
     
-    componentWillUnmout() {
-        clearInterval(this.timerId);
-    }
-    
-    handleClick() {
-        this.setState({data: 'Данные'})
+    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>
 }
 
-class MyComponent2 extends React.Component {
-    controller = new AbortController()
-
-    componentDidMount() {
-        fetch('https://api.example.com/data',{ singnal: this.controller.signal })
-        .then(responce => responce.json())
-        .then(json => this.setState({data: json}))
-        .catch(error => {
-            if(error.name == 'AbortError') {
-                console.log('Aborted');
-            } else {
-                console.log('Other error',error);
-            }
-        });
-    }
-
-    componentWillUnmount() {
-        this.controller.abort()
-    }
-}
-class MyComponent3 extends React.Component {
-    componentDidMount() {
-        this.loadData();
-    }
+function ClickCounter() {
+    const countRef = useRef(0);
     
-    async loadData() {
-        try {
-            const response = await fetch('https://api.example.com/data');
-            const data = await response.json();
-            this.setSate({ data });
-        } catch {
-            console.log('error');
+    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>
+            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;
         }
     }
+        
+    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>
+        </>
+    )
 }
-
-class Timer extends React.Component {
-    constructor(props) {
-        super(props);
-        this.state = { seconds: 0 }
-    }
-    
-    componentDidMount() {
-        console.log('Timer mounted');
-        this.interval = setInterval(()=>{
-            this.setState(prevState => ({seconds: prevState.seconds + 1}))
-        },1000);
-    }
-    
-    componentWillUnmount() {
-        console.log('Timer unmount');
-        clearInterval(this.interval);
-    }
-    render() {
-        return <h2>Прошло секунд: {this.state.seconds}</h2>
-    }
-}
-
+        
 function App() {
-    const [show,setShow] = React.useState(true);
+    
     
 	return (
 		<>
-			<h1>Таймер с классом</h1>
-            <button onClick={()=>setShow(prev => !prev)}>{show ? 'Скрыть' : 'Показать'} таймер</button>
-            {show && <Timer />}
+			<h1>Hello, React!</h1>
+            <ScrollList />
 		</>
 	)
 }