Сделали таймер

src/App.jsx

@@ -1,82 +1,108 @@
-import memo, { useMemo, useCallback, useState } from 'react'
+import React from 'react'
 
-function MyComponent() {
+class MyComponent extends React.Component {
-    const [count, setCount] = useState(0);
+    constructor(props) {
-    const [name, setName] = useState('John');
+        super(props);
+        this.state = { data: null };
+        this.handleClick = this.handleClick.bind(this);
+    }
     
-    const greeting = useMemo(() => `Hello, ${name}!`,[name]);
+    render() {
+        return (
+            <div>
+            {this.state.data ? (
+                    <div>Данные: {this.state.data}</div>
+                ) : (
+                    <div>Загрузка...</div>
+                )}
+                <button onClick={this.handleClick}>Обновить</button>
+            </div>
+        )
+    }
     
-    return (
+    componentDidMount() {
-        <>
+        fetch('https://api.example.com/data')
-        <h2>{greeting}</h2>
+        .then(responce => responce.json())
-        <button onClick={()=>setName('Alex')}>New name</button>
+        .then(json => this.setState({data: json}));
-        </>
+        
-    )
+        this.timerId = setInterval(()=> console.log('Tick'), 1000);
+    }
+    
+    componentWillUnmout() {
+        clearInterval(this.timerId);
+    }
+    
+    handleClick() {
+        this.setState({data: 'Данные'})
+    }
 }
 
-function MyComponent2() {
+class MyComponent2 extends React.Component {
-    const [data,setData] = useState([{theme:'light'}])
+    controller = new AbortController()
-    const userPreferences = useMemo(()=>({theme: 'dark', language: 'en'}),[]);
 
-    const processData = useMemo(()=>{
+    componentDidMount() {
-        return data.map(item => ({ ...item,theme: userPreferences.theme }))
+        fetch('https://api.example.com/data',{ singnal: this.controller.signal })
-    },[data, userPreferences]);
+        .then(responce => responce.json())
-    console.log(processData);
+        .then(json => this.setState({data: json}))
-    return (
+        .catch(error => {
-        <>
+            if(error.name == 'AbortError') {
-            <h2 className={data.theme}>2</h2>
+                console.log('Aborted');
-            <button onClick={()=>setData(data)}>set new theme</button>
+            } 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');
+        }
+    }
 }
 
-function MyComponent3() {
+class Timer extends React.Component {
-    const sum = useMemo(()=> a+b,[a,b]);//Бессмысленное применение и вредное
+    constructor(props) {
+        super(props);
+        this.state = { seconds: 0 }
+    }
     
-    //Применение оправдано
+    componentDidMount() {
-    const complexResult = useMemo(()=>{
+        console.log('Timer mounted');
-        return heavyMethOperations(largeDataSet);
+        this.interval = setInterval(()=>{
-    },[largeDataSet])
+            this.setState(prevState => ({seconds: prevState.seconds + 1}))
-}
+        },1000);
+    }
     
-function ParentComponent({ items }) {
+    componentWillUnmount() {
-    const [filter, setFilter] = useState('');
+        console.log('Timer unmount');
-    
+        clearInterval(this.interval);
-    const listProps = useMemo(()=>({
+    }
-            items: items.filter(item => item.name.includes(filter)),
+    render() {
-            onItemClick: id => console.log('Clicked',id),
+        return <h2>Прошло секунд: {this.state.seconds}</h2>
-            theme: 'dark'
+    }
-        }),[items,filter]);
-    
-    return (
-        <>
-        <Memoized {...listProps} />
-        <button onClick={()=>setFilter('Bob')}>Click</button>
-        </>
-    )
-}
-
-function Memoized(props) {
-    console.log(props);
-    return (
-        <>
-        <h3 className={props.theme}>Memoized</h3>
-        {props.items.map((item) => (
-         <>
-            <li>
-                {item.name}
-            </li>
-         </>))}
-        </>
-    )
 }
 
 function App() {
+    const [show,setShow] = React.useState(true);
+    
 	return (
 		<>
-			{/* Добавьте тестовый контент для проверки */}
+			<h1>Таймер с классом</h1>
-			<h1>Hello, React!</h1>
+            <button onClick={()=>setShow(prev => !prev)}>{show ? 'Скрыть' : 'Показать'} таймер</button>
-            <ParentComponent items={[{name:'Alex'},{name:'Bob'}]} />
+            {show && <Timer />}
 		</>
 	)
 }