React Hooks (React v16.7.0-alpha)

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什么是Hooks

“Hooks” 本意是”钩子“的意思。在 React 里，hooks 就是一系列特殊的函数，使函数组件 (functional component) 内部能够”钩住“ React 内部的 state 和 life-cycles。

为什么使用Hooks

刚刚接触react时候非常喜欢用函数式组件，因为太简洁了写起来非常快，然后然后。。写到后面发现很多自己以前写的组件需要改。。为什么呢，因为自己当时写的时候考虑的不周到，后期发现很多地方都需要生命周期和状态来进行渲染优化，然后就是大量修改函数式为classComponent。所以现在起手一般都是classComponent，只有极简单的组件用函数式比如列表item啥的。

使用Hooks的规则

• 只能在顶层调用Hooks 。不要在循环，条件或嵌套函数中调用Hook
• 只能在functional component(函数式组件)中使用

Hooks

State Hook

• 即在函数式中使用state

import { useState } from 'react';

function Example() {
  const [count, setCount] = useState(0);
  //const [age, setAge] = useState(42);
  //const [fruit, setFruit] = useState('banana');
  //const [todos, setTodos] = useState([{ text: 'Learn Hooks' }]);

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

• useState的参数就是以前state的初始值。
• useState返回的值中第一个参数是以前的state,第二个参数是setState,不过以前我们只有一个state,现在可以自由命名，更直观了，比如上面的age和setAge，fruit和setFruit

Effect Hook

• effect Hook使我们可以使用生命周期了

function FriendStatusWithCounter(props) {
  const [count, setCount] = useState(0);
  useEffect(() => {
    document.title = `You clicked ${count} times`;
  });

  const [isOnline, setIsOnline] = useState(null);
  useEffect(() => {
    ChatAPI.subscribeToFriendStatus(props.friend.id, handleStatusChange);
    return () => {
      ChatAPI.unsubscribeFromFriendStatus(props.friend.id, handleStatusChange);
    };
  });

  function handleStatusChange(status) {
    setIsOnline(status.isOnline);
  }
  // ...

• 每当 React更新之后，就会触发 useEffect（在第一次 render 和每次 update 后触发）。

Custom Hooks

• 有时，我们希望在组件之间重用一些有状态逻辑。
• 首先，我们将这个逻辑写到useFriendStatus：返回朋友在线的状态isOnline

import { useState, useEffect } from 'react';

function useFriendStatus(friendID) {
  const [isOnline, setIsOnline] = useState(null);

  function handleStatusChange(status) {
    setIsOnline(status.isOnline);
  }

  useEffect(() => {
    ChatAPI.subscribeToFriendStatus(friendID, handleStatusChange);
    return () => {
      ChatAPI.unsubscribeFromFriendStatus(friendID, handleStatusChange);
    };
  });

  return isOnline;
}

• 然后我们在其他组件中使用

function FriendListItem(props) {
  const isOnline = useFriendStatus(props.friend.id);

  return (
    <li style={{ color: isOnline ? 'green' : 'black' }}>
      {props.friend.name}
    </li>
  );
}

Other Hooks

• 你可能会发现一些不太常用的内置Hook很有用。
• useContext：

function Example() {
  const locale = useContext(LocaleContext);
  const theme = useContext(ThemeContext);
  // ...
}

• useReducer

function Todos() {
  const [todos, dispatch] = useReducer(todosReducer);
  // ...

总结

以后可以更愉快的写functional component(函数式组件)了

参考

react官方Hooks简介

Hooks例子

最后

大家好，这里是「 十年雪落 」，这个博客主要用于记录一个菜鸟程序猿的成长之路。这也是自己第一次做博客，希望和大家多多交流，一起成长！文章将会在下列地址同步更新……
博客园：www.cnblogs.com/maopixin/