# React@16.8 Hooks

import React, { useState } from 'react';

function Example() {
  // 声明一个叫 “count” 的 state 变量，
  // 声明一个叫 "setCount" 的更新函数，类似于 this.setState
  const [count, setCount] = useState(0);
  // 或者
  const [count2, setCount2] = useState({number: 0})

  return (
    <div>
      <p>You clicked {count} times</p>
      <button onClick={() => setCount(count + 1)}>
        Click me
      </button>
      {/*坑：这种写法可以防止 count 被缓存起来【一直都是初始的{number: 0}】*/}
      <button onClick={() => setCount2(count2 => count2 + 1)}>
        Click me2
      </button>
    </div>
  );
}

setCount((prevState) => ({
  ...prevState,
  left: e.pageX,
  top: e.pageY,
}))

import React, { useState, useEffect } from 'react';

function Example() {
  const [count, setCount] = useState(0);

  // 相当于 componentDidMount 和 componentDidUpdate:
  useEffect(() => {
    // 使用浏览器的 API 更新页面标题
    document.title = `You clicked ${count} times`;
  });

  return (
    <div>
      <p>You clicked {count} times</p>
      <button onClick={() => setCount(count + 1)}>
        Click me
      </button>
    </div>
  );
}

import React, { useState, useEffect } from 'react';

function FriendStatus(props) {
  const [isOnline, setIsOnline] = useState(null);

  useEffect(() => {
    function handleStatusChange(status) {
      setIsOnline(status.isOnline);
    }
    ChatAPI.subscribeToFriendStatus(props.friend.id, handleStatusChange);
    // Specify how to clean up after this effect:
    return function cleanup() {
      ChatAPI.unsubscribeFromFriendStatus(props.friend.id, handleStatusChange);
    };
  });

  if (isOnline === null) {
    return 'Loading...';
  }
  return isOnline ? 'Online' : 'Offline';
}

useEffect(() => {
  document.title = `You clicked ${count} times`;
}, [count]); // 仅在 count 更改时更新

// 只有 当函数（以及它所调用的函数）不引用 props、state 以及由它们衍生而来的值时，你才能放心地把它们从依赖列表中省略。
useEffect(() => {
  document.title = `You clicked many times`;
}, []); // 仅在组件挂载和卸载时执行 

import React, {Fragment} from 'react'
import { useState, useEffect, useCallback, useMemo } from 'react'
import { observer } from 'mobx-react'


const nameList = ['apple', 'peer', 'banana', 'lemon']
const Example = observer((props) => {
    const [price, setPrice] = useState(0)
    const [name, setName] = useState('apple')
    
    
    function getProductName() {
        console.log('getProductName触发')
        return name
    }
    // 只对name响应
    useEffect(() => {
        console.log('name effect 触发')
        getProductName()
    }, [name])
    
    // 只对price响应
    useEffect(() => {
        console.log('price effect 触发')
    }, [price])
  
    // memo化的name属性
    const memo_name = useMemo(() => {
        console.log('name memo 触发')
        return () => name  // 返回一个函数
    }, [name])

    return (
        <Fragment>
            <p>{name}</p>
            <p>{price}</p>
            <p>普通的name：{getProductName()}</p>
            <p>memo化的：{memo_name()}</p>
            <button onClick={() => setPrice(price+1)}>价钱+1</button>
            <button onClick={() => setName(nameList[Math.random() * nameList.length << 0])}>修改名字</button>
        </Fragment>
    )
})
export default Example

> name memo 触发
> getProductName触发
> name effect 触发
> getProductName触发

import React, { useState, useEffect, useMemo, useRef } from 'react';

export default function App(props){
  const [count, setCount] = useState(0);

  const doubleCount = useMemo(() => {
    return 2 * count;
  }, [count]);

  const couterRef = useRef();

  useEffect(() => {
    document.title = `The value is ${count}`;
    console.log(couterRef.current);
  }, [count]);
  
  return (
    <>
      <button ref={couterRef} onClick={() => {setCount(count + 1)}}>Count: {count}, double: {doubleCount}</button>
    </>
  );
}

export const useToggle = (defaultVisible: boolean = false) => {
  const [visible, setVisible] = useState(defaultVisible);
  const show = () => setVisible(true);
  const hide = () => setVisible(false);

  return [visible, show, hide] as [typeof visible, typeof show, typeof hide];
};

const [isOpen, open, close] = useToggle(); // 在外部可以更方便地修改名字
const [visible, show, hide] = useToggle();