Сделали таймер

src/App.jsx

@@ -1,98 +1,108 @@
-import { useState, useRef, useEffect } from 'react'
+import React from 'react'
 
-function Example() {
-    const myRef = useRef(null);
-    const counterRef = useRef(0);
-    const [text, setText] = useState('');
-    
-    useEffect(() => {
-        myRef.current?.focus();
-    },[]);
-        
-    const handlerFocus = () => {
-        if(myRef.current) {
-            myRef.current.focus();
-        }
-    };
-    
-    const handlerChange = (event) => {
-        setText(event.target.value);
+class MyComponent extends React.Component {
+    constructor(props) {
+        super(props);
+        this.state = { data: null };
+        this.handleClick = this.handleClick.bind(this);
     }
-    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>
-            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>
-                ))}
+    render() {
+        return (
+            <div>
+            {this.state.data ? (
+                    <div>Данные: {this.state.data}</div>
+                ) : (
+                    <div>Загрузка...</div>
+                )}
+                <button onClick={this.handleClick}>Обновить</button>
             </div>
-            <button onClick={scrollToBottom}>Прокрутить вниз</button>
-        </>
-    )
-}
-        
-function App() {
+        )
+    }
     
+    componentDidMount() {
+        fetch('https://api.example.com/data')
+        .then(responce => responce.json())
+        .then(json => this.setState({data: json}));
+        
+        this.timerId = setInterval(()=> console.log('Tick'), 1000);
+    }
+    
+    componentWillUnmout() {
+        clearInterval(this.timerId);
+    }
+    
+    handleClick() {
+        this.setState({data: 'Данные'})
+    }
+}
+
+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();
+    }
+    
+    async loadData() {
+        try {
+            const response = await fetch('https://api.example.com/data');
+            const data = await response.json();
+            this.setSate({ data });
+        } catch {
+            console.log('error');
+        }
+    }
+}
+
+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>Hello, React!</h1>
-            <ScrollList />
+			<h1>Таймер с классом</h1>
+            <button onClick={()=>setShow(prev => !prev)}>{show ? 'Скрыть' : 'Показать'} таймер</button>
+            {show && <Timer />}
 		</>
 	)
 }