import {createRoot} from 'react-dom/client'
import App from './App'

const elm = document.getElementById('app')
const root = createRoot(elm);
root.render(<App />);

npx create-react-app my-app

import React, {Component} from 'react'
import ReactDOM from 'react-dom'

export class Hello extends Component {
  ...
}
export default function World() {
  /* ... */
}

import World, { Hello } from './hello.js';

import React from "react";
import "./Student.css";

export const Student = (
  <div className="Student"></div>
);

const divStyle = {
  backgroundImage: 'url(' + imgUrl + ')',
};
export const Student = (
  <div style={divStyle}></div>
);

<Student name="Julie" age={23}
  pro={true} />

function Student(props) {
  return <h1>Hello, {props.name}</h1>;
}

class Student extends React.Component {
  render() {
    return (
      <h1>Hello, {this.props.name}</h1>
    );
  }
}

function Example() {
  return (
    <AlertBox>
      <h1>您有待处理的通知</h1>
    </AlertBox>
  )
}

function AlertBox(props) {
  return (
    <div className="alert-box">
      {props.children}
    </div>
  );
}

{props.children}

class AlertBox extends React.Component {
  render () {
    return (
      <div className="alert-box">
        {this.props.children}
      </div>
    );
  }
}

{this.props.children}

import { useState } from 'react';

function Student() {
  const [count, setCount] = useState(0);
  const click = () => setCount(count + 1);
  return (
    <div>
      <p>您点击了 {count} 次</p>
      <button onClick={click}>
        点击我
      </button>
    </div>
  );
}

<p>您点击了 {count} 次</p>

import React from 'react';

class Student extends React.Component {
  constructor(props) {
    super(props);
    this.state = {count: 1};
    // 确保函数可以访问组件属性（ES2015）
    this.click = this.click.bind(this);
  }
  click() {
    const count = this.state.count;
    this.setState({ count: count + 1})
  }
  render() {
    return (
      <div>
        <button onClick={this.click}>
          点击我
        </button>
        <p>您点击了{this.state.count}次</p>
      </div>
    );
  }
}

<p>您点击了{this.state.count}次</p>

const elm = ['one', 'two', 'three'];
function Student() {
  return (
    <ul>
      {elm.map((value, index) => (
        <li key={index}>{value}</li>
      ))}
    </ul>
  );
}

export default function Hello() {
  function handleClick(event) {
    event.preventDefault();
    alert("Hello World");
  }

  return (
    <a href="/" onClick={handleClick}>
      Say Hi
    </a>
  );
}

function addNumbers(x1, x2) {
  return x1 + x2;
}

const element = (
  <div>
    {addNumbers(2, 5)}
  </div>
);

import { useState } from 'react'
import Avatar from './Avatar';
import Profile from './Profile';

function Student() {
  const [count, setCount] = useState(0);
  return (
    <div>
      <Avatar src={count} />
      <Profile username={count} />
    </div>
  );
}

render() {
  return ReactDOM.createPortal(
    this.props.children,
    domNode
  );
}

import { Fragment } from 'react'
import Avatar from './Avatar';
import Profile from './Profile';

const Student = () => (
  <Fragment>
    <Avatar src="./demo.jpg" />
    <Profile username="name" />
  </Fragment>
);

const Student = () => (
  <>
    <Avatar src="./demo.jpg" />
    <Profile username="name" />
  </>
);

render() {
  return 'Look ma, no spans!';
}

const Student = () => [
  <li key="A">First item</li>,
  <li key="B">Second item</li>
];

const FancyButton = React.forwardRef(
  (props, ref) => (
    <button ref={ref} className="btn">
      {props.children}
    </button>
  )
);

// 你可以直接获取 DOM button 的 ref：
const ref = React.createRef();

<FancyButton ref={ref}>
  点击我
</FancyButton>;

import {Component,createRef} from 'react'

class MyComponent extends Component {
  constructor(props) {
    super(props);
    this.myRef = createRef();
  }

  render() {
    return <div ref={this.myRef} />;
  }
}

function CustomTextInput(props) {
  // 这里必须声明 $input，这样 ref 才可以引用它
  const $input = useRef(null);
  function handleClick() {
    $input.current.focus();
  }
  return (
    <div>
      <input type="text" ref={$input} />
      <input
        type="button" value="聚焦文本输入"
        onClick={handleClick}
      />
    </div>
  );
}

<div>
  <Header />
  <React.StrictMode>
    <div>
      <ComponentOne />
      <ComponentTwo />
    </div>
  </React.StrictMode>
  <Footer />
</div>

<Profiler id="Navigation" onRender={callback}>
  <Navigation {...props} />
</Profiler>

class CustomButton extends React.Component {
  // ...
}
CustomButton.defaultProps = {
  color: 'blue'
};

<CustomButton /> ;

class Hello extends Component {
  constructor (props) {
    super(props)
    this.state = { visible: true }
  }
}

class Hello extends Component {
  state = { visible: true }
}

function CustomButton(props) {
  const { color = 'blue' } = props;
  return <div>{color}</div>
}

function CustomButton() {
  const [color, setColor]=useState('blue')
  return <div>{color}</div>
}

<MyButton color="blue" shadowSize={2}>
  点击我
</MyButton>

React.createElement(
  MyButton,
  {color: 'blue', shadowSize: 2},
  '点击我'
);

<div className="sidebar" />

React.createElement(
  'div',
  {className: 'sidebar'}
)

const Menu = ({ children }) => (
  <div className="menu">{children}<div>
);

Menu.Item = ({ children }) => (
  <div>{children}<div>
);

<Menu>
  <Menu.Item>菜单一</Menu.Item>
  <Menu.Item>菜单二</Menu.Item>
<Menu>

let element = <h1>Hello, world!</h1>;
let emptyHeading = <h1 />;

const root = ReactDOM.createRoot(
  document.getElementById('root')
);

const element = <h1>Hello, world</h1>;
root.render(element);

const avatarUrl = "img/picture.jpg"
const element = <img src={avatarUrl} />;

const element = (
  <button className="btn">
    点击我
  </button>
);

let name = '张三';
let element = <h1>Hello, {name}</h1>;

function fullName(firstName, lastName) {
  return firstName + ' ' + lastName;
}
let element = (
  <h1>
    Hello, {fullName('三', '张')}
  </h1>
);

const divStyle = {
  color: 'blue',
  backgroundImage: 'url(' + imgUrl + ')',
};
function MyComponent() {
  return <div style={divStyle}>组件</div>;
}

const markup = {__html: '我 · 你' };

const MyComponent = () => (
  <div dangerouslySetInnerHTML={markup} />
);

const MyComponent = () => (
  <div>
    <input type="radio" id="ab" name="v">
    <label for="ab">HTML</label>
  </div>
);

<textarea defaultValue="Hello" />

<input type="radio" defaultChecked />

<div className="warp">...</div>

import React from "react";

function formatName(user) {
  return user.firstName 
    + ' ' 
    + user.lastName;
}

export function Greeting(user) {
  if (user) {
    return (
      <h1>你好, {formatName(user)}!</h1>
    );
  }
  return (
    <h1>你好, 先生。</h1>
  );
}

<Greeting firstName="三" lastName="张" />

export default function Weather(props) {
  const isLoggedIn = props.isLoggedIn;
  return (
    <div>
      <b>{isLoggedIn ? '已' : '未'}</b>登录。
    </div>
  );
}

{isShow && <div>内容</div>}

<Dropdown>
  下拉列表
  <Menu>
    <Menu.Item>菜单一</Menu.Item>
    <Menu.Item>菜单二</Menu.Item>
    <Menu.Item>菜单三</Menu.Item>
  </Menu>
</Dropdown>

function Greeting(props) {
  let button;
  if (props.isLoggedIn) {
    button = <UserGreeting />;
  } else {
    button = <GuestGreeting />;
  }
  return <div>{button}</div>;
}

function Student() {
  const [count, setCount] = useState(0);
  return (
    <Fragment>
      {/* 这里写注释 */}
    </Fragment>
  );
}

import React from 'react';

const UserName = () => <h1>Kenny</h1>;

export default function UserProfile() {
  return (
    <div className="UserProfile">
      <div>Hello</div>  
      <UserName />
    </div>
  );
}

class Welcome extends React.Component {
  render() {
    return <h1>{this.props.name}</h1>;
  }
}

import React, {PureComponent} from 'react'

class MessageBox extends PureComponent {
  ···
}

import React, { Component } from 'react';
// 高阶组件 with
const with = data => WrappedComponent => {
  return class extends Component {
    constructor(props) {
      super(props);
    }
    render() {
      return (
        <WrappedComponent data={data} />
      )
    }
  }
}

const LowComponent = (props) => (
  <div>{props.data}</div>
);

const MyComp = with('Hello')(LowComponent)

function FancyBorder(props) {
  return (
    <div className={'Fancy'+props.color}>
      {props.children}
    </div>
  );
}

function WelcomeDialog() {
  return (
    <FancyBorder color="blue">
      <h1 className="title">欢迎</h1>
      <p className="message">
        感谢您访问我们的宇宙飞船
      </p>
    </FancyBorder>
  );
}

function SplitPane(props) {
  return (
    <div className="SplitPane">
      <div className="left">
        {props.left}
      </div>
      <div className="right">
        {props.right}
      </div>
    </div>
  );
}

function App() {
  return (
    <SplitPane
      left={<Contacts />}
      right={<Chat />}
    />
  );
}

import React from 'react';
import UserAvatar from "./UserAvatar";

export default function UserProfile() {
  return (
    <div className="UserProfile">
      <UserAvatar />
      <UserAvatar />
    </div>
  );
}

import React from 'react';
import {Button} from 'uiw';
export default function UserProfile() {
  return (
    <div className="UserProfile">
      <Button type="primary">
        主要按钮
      </Button>
    </div>
  );
}

const Menu = ({ children }) => (
  <div className="menu">{children}<div>
);

Menu.Item = ({ children }) => (
  <div>{children}<div>
);

<Menu>
  <Menu.Item>菜单一</Menu.Item>
  <Menu.Item>菜单二</Menu.Item>
<Menu>

function Counter({ initialCount }) {
  const [count, setCount] = useState(initialCount);
  return (
    <>
      Count: {count} <button onClick={() => setCount(initialCount)}>Reset</button>
      <button onClick={() => setCount(prevCount => prevCount - 1)}>-</button>
      <button onClick={() => setCount(prevCount => prevCount + 1)}>+</button>
    </>
  );
}

function TextInputWithFocusButton() {
  const $input = useRef(null);
  const onButtonClick = () => {
    $input.current.focus();
  };
  return (
    <>
      <input ref={$input} type="text" />
      <button onClick={onButtonClick}>
        聚焦输入
      </button>
    </>
  );
}

function FancyInput(props, ref) {
  const inputRef = useRef();
  useImperativeHandle(ref, () => ({
    focus: () => {
      inputRef.current.focus();
    }
  }));
  return <input ref={inputRef} />;
}
FancyInput = forwardRef(FancyInput);

<FancyInput ref={inputRef} />
inputRef.current.focus()

useEffect(() => {
  const subs = props.source.subscribe();
  return () => {
    subs.unsubscribe();
  };
}, [props.source]);

const memoizedCallback = useCallback(
  () => {
    doSomething(a, b);
  },
  [a, b],
);

const memoizedValue = useMemo(
  () => {
    return computeExpensiveValue(a, b)
  },
  [a, b]
);

function Checkbox() {
  const id = useId();
  return (
    <>
      <label htmlFor={id}>
        你喜欢React吗？
      </label>
      <input id={id} type="checkbox" />
    </>
  );
};

function useFriendStatus(friendID) {
  const [
    isOnline, setIsOnline
  ] = useState(null);
  // ...
  // 在开发者工具中的这个 Hook 旁边显示标签
  // e.g. "FriendStatus: Online"
  useDebugValue(
    isOnline ? 'Online' : 'Offline'
  );
  return isOnline;
}

useEffect(
  () => {
    // componentDidMount
    // 组件挂载时，可以在这里完成你的任务
    return () => {
      // componentWillUnmount
      // 卸载时执行，清除 effect
    };
  },
  [ ]
);

class Welcome extends React.Component {
  render() {
    return <h1>Hello, {this.props.name}</h1>;
  }
}

constructor(props) {
  super(props);
  // 不要在这里调用 this.setState()
  this.state = { counter: 0 };
  this.handleClick = this.handleClick.bind(this);
}

class ErrorBoundary extends React.Component {
  constructor(props) {
    super(props);
    this.state = { hasError: false };
  }

  static getDerivedStateFromError(error) {
    // 更新 state 使下一次渲染可以显降级 UI
    return { hasError: true };
  }

  render() {
    if (this.state.hasError) {
      // 你可以渲染任何自定义的降级  UI
      return <h1>Something went wrong.</h1>;
    }

    return this.props.children;
  }
}

componentDidUpdate(prevProps) {
  // 典型用法（不要忘记比较 props）：
  if (this.props.uid !== prevProps.uid) {
    this.fetchData(this.props.uid);
  }
}

getSnapshotBeforeUpdate(prevProps, prevState) {
  // 我们是否在 list 中添加新的 items ？
  // 捕获滚动​​位置以便我们稍后调整滚动位置。
  if (prevProps.list.length < this.props.list.length) {
    const list = this.listRef.current;
    return list.scrollHeight - list.scrollTop;
  }
  return null;
}

import PropTypes from 'prop-types'

MyComponent.propTypes = {
  email:      PropTypes.string,
  seats:      PropTypes.number,
  callback:   PropTypes.func,
  isClosed:   PropTypes.bool,
  any:        PropTypes.any
  symbol:     PropTypes.symbol,
}

MyComponent.propTypes = {
  // 确保这个 prop 没有被提供时，会打印警告信息
  requiredFunc: PropTypes.func.isRequired,

  // 任意类型的必需数据
  requiredAny: PropTypes.any.isRequired,
}

MyComponent.propTypes = {
  // 只能是特定的值，枚举类型。
  optionalEnum: PropTypes.oneOf([
    'News', 'Photos'
  ]),
  // 一个对象可以是几种类型中的任意一个类型
  optionalUnion: PropTypes.oneOfType([
    PropTypes.string,
    PropTypes.number,
    PropTypes.instanceOf(Message)
  ]),
}

MyComponent.propTypes = {
  // 任何可被渲染的元素
  // (包括数字、字符串、元素或数组)
  // (或 Fragment) 也包含这些类型。
  node: PropTypes.node,

  // 一个 React 元素。
  element: PropTypes.element,

  // 一个 React 元素类型（即，MyComponent）
  elementType: PropTypes.elementType,
}

MyComponent.propTypes = {
  // 可以指定一个对象由某一类型的值组成
  objectOf: PropTypes.objectOf(
    PropTypes.number
  ),
  // 可以指定一个对象由特定的类型值组成
  objectWithShape: PropTypes.shape({
    color: PropTypes.string,
    fontSize: PropTypes.number
  }),
  // 带有额外属性警告的对象
  objectWithStrictShape: PropTypes.exact({
    name: PropTypes.string,
    quantity: PropTypes.number
  }),
}

MyComponent.propTypes = {
  custom: (props, propName, compName) => {
    if (!/matchm/.test(props[propName])) {
      // 它在验证失败时应返回一个 Error 对象
      return new Error(
        '无效的prop `'
        ` \`${propName}\` 提供给` + 
        ` \`${compName}\`。验证失败。`
      );

    }
  },
}

MyComponent.propTypes = {
  arrayProp: PropTypes.arrayOf((propValue, key, componentName, location, propFullName) => {
    if (!/matchme/.test(propValue[key])) {
      // 它应该在验证失败时返回一个 Error 对象。
      return new Error(
        'Invalid prop `' + propFullName + '` supplied to' +
        ' `' + componentName + '`. Validation failed.'
      );
    }
  })
}

MyComponent.propTypes = {
  arr: PropTypes.arrayOf(PropTypes.number),
};

MyComponent.propTypes = {
  message: PropTypes.instanceOf(Message),
};