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docs: React note update
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@ -3594,11 +3594,11 @@ export const AppWrapper = styled.div`
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由于本质上是方法调用，返回的内容本质上是一个组件，我们可以通过给组件传值，很方便的实现动态样式绑定：
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点击按钮动态修改样式的值
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点击按钮动态修改样式的值：
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```tsx
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// App.jsx
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import React, { PureComponent } from 'react'
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import React, { PureComponent } from 'react'··
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import { AppWrapper } from './styles/style'
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export default class App extends PureComponent {
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@ -3657,5 +3657,457 @@ export const AppWrapper = styled.div`
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`
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```
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我们之前演示过，标签模板字符串中通过`${ ... }`动态插入的变量会作为参数，当函数调用时传递给函数
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创建组件时动态传入的这些函数，当props发生变化时，就会在`div`函数中被再次调用，生成新的样式并应用到页面中
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### styled高级特性
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- 支持样式的继承
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- 支持设置主题
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```tsx
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// 样式继承
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const BasicButton = styled.button`
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padding: 8px 30px;
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border-radius: 5px
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`
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const WarnButton = styled(BasicButton)`
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background-color: orange;
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color: white;
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`
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```
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```tsx
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import { ThemeProvider } from 'styled-components'
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...
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<ThemeProvider
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theme={{
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color,
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hoverColor,
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size
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}}
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>
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...
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</ThemeProvider>
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...
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```
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### classnames
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动态绑定className，如果没有`classnames`库，我们常常写这样的代码来动态绑定className：
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```tsx
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const classList = ['item', this.state.isActive ? 'item-active' : '']
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const className = classList.join(' ')
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// or
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const className = `item ${this.state.isActive ? 'item-active' : ''}`
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// or
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const classNameMap = {
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'item': true,
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'item-active': this.state.isActive
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}
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const className = Object.entries(classNameMap).map(item => !!item[1] ? item[0] : '').join(' ')
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```
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手动编写动态绑定的className较为繁琐，可以依靠第三方库`classnames`帮我们完成（Vue已将类似的功能内置）
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```bash
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npm i classnames
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```
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Usage Sample:
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```tsx
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classNames('foo', 'bar')
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classNames('foo', { bar: true })
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classNames(...['foo', 'bar'])
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```
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## Redux
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- Redux的核心思想
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- Redux的基本使用
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- React结合Redux
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- Redux的异步操作
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- redux-devtool
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- reducer的模块拆分
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### 理解JavaScript的纯函数
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- 函数式编程中有一个非常重要的概念 **纯函数**，JavaScript符合函数式编程的范式，所以也有纯函数的概念
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- 在React开发中，纯函数被多次提及：
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- React组件被要求像一个纯函数（为什么是像，因为还有类组件）
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- Redux中有一个reducer的概念，同样是要求必须是一个纯函数
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- 掌握纯函数对于理解很多框架的设计都是有帮助的
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一个纯函数必然具备以下特征：
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- 确定的输入一定产生确定的输出
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- 函数的执行过程中，不能产生副作用
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### 为什么需要Redux
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- JS需要管理的状态越来越多，越来越复杂
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- 状态不断发生变化之间又相互依赖，这要求视图层也能同步更新
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- React提供了自动更新视图的方法，但状态仍需要手动管理
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- Redux可以帮我们管理状态，提供了**可预测的状态管理**
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- 框架无关，体积只有2KB大小
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### Redux的核心理念
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Redux的核心理念 Store
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- 定义一个统一的规范来操作数据，这样就可以做到对数据的跟踪
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- `list.push()` `list[0].age = 18`
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Redux的核心理念 Action
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- Redux要求：要修改数据，必须通过Action来修改
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- 所有数据的变化，必须通过派发（Patch）Action来更新
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- Action是一个普通的JS对象，用来描述此次更新的type与content
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- `const action = { type: 'ADD_ITEM', item: { name: 'Ziu', age: 18 } }`
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Redux的核心理念 Reducer
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- 如何将Store和Action联系在一起？
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- reducer是一个纯函数
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- 完成的工作就是：将传入的state和action结合起来，生成一个新的state
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- `patch` => `reducer` => `newState` => `Store`
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### Redux Demo
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下例中，通过`createStore`创建了一个Store（已经不推荐了）
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- initialState用于在调用`createStore`时作为默认值传入`reducer`
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- 后续每次`store.dispatch`都会调用`reducer`
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- 通过`reducer`更新state中的数据
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在React中，可以通过`store.subscribe`注册State变化的监听回调
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- 当state发生变化时，通过调用`this.forceUpdate`触发组件的更新
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- 一般情况下，我们在`componentDidMount`注册监听回调，在`componentWillUnmount`解除监听
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::: code-group
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```tsx [App.jsx]
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// App.jsx
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import React, { PureComponent } from 'react'
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import store from './store'
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export default class App extends PureComponent {
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componentDidMount() {
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// Subscribe to the store
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store.subscribe(() => {
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console.log('subscribe', store.getState())
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this.forceUpdate()
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})
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}
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componentWillUnmount() {
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store.unsubscribe()
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}
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render() {
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return (
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<div>
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<h1>App</h1>
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<p>Count: {store.getState().count}</p>
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<p>Name: {store.getState().name}</p>
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<button onClick={() => store.dispatch({ type: 'INCREMENT' })}> +1 </button>
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<button onClick={() => store.dispatch({ type: 'DECREMENT' })}> -1 </button>
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<button onClick={() => store.dispatch({ type: 'CHANGE_NAME', name: 'ZIU' })}>
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{' '}
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CHANGE_NAME{' '}
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</button>
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</div>
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)
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}
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}
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```
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```tsx [index.js]
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// store/index.js
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import { createStore } from 'redux'
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// The initial application state
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// This is the same as the state argument we passed to the createStore function
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const initialState = {
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count: 0,
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name: 'Ziu'
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}
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// Reducer: a pure function that takes the previous state and an action, and returns the next state.
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// (previousState, action) => newState
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function reducer(state = initialState, action) {
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console.log('reducer', state, action)
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switch (action.type) {
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case 'INCREMENT':
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// NOTE: Keep functions pure - do not mutate the original state.
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// Desctructure the state object and return a **new object** with the updated count
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// Instead of `return state.count++`
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return {
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...state,
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count: state.count + 1
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}
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case 'DECREMENT':
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return {
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...state,
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count: state.count - 1
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}
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case 'CHANGE_NAME':
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return {
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...state,
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name: action.name
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}
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default:
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return state
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}
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}
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const store = createStore(reducer)
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export default store
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```
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:::
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### 进一步封装
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可以将耦合在一起的代码拆分到不同文件中
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- 将`reducer`抽取出来`reducer.js`，简化`store/index.js`内容
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- 将`action.type`抽取为常量`constants.js`，使用时做导入，以保证一致性
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- 将`action`抽取出来`actionFactory.js`，用于外部dispatch时规范类型
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::: code-group
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```tsx [index.js]
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// store/index.js
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import { createStore } from 'redux'
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import reducer from './reducer'
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const store = createStore(reducer)
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export default store
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```
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```tsx [constants.js]
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// constants.js
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export const INCREMENT = 'INCREMENT'
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export const DECREMENT = 'DECREMENT'
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export const CHANGE_NAME = 'CHANGE_NAME'
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```
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```tsx [reducer.js]
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// reducer.js
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import * as actionType from './constants'
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const initialState = {
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count: 0,
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name: 'Ziu'
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}
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export default function reducer(state = initialState, action) {
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switch (action.type) {
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case actionType.INCREMENT:
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return {
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...state,
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count: state.count + 1
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}
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case actionType.DECREMENT:
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return {
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...state,
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count: state.count - 1
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}
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case actionType.CHANGE_NAME:
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return {
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...state,
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name: action.name
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}
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default:
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return state
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}
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}
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```
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```tsx [actionFactory.js]
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// actionFactory.js
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import * as actionType from './constants'
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export const increment = () => ({
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type: actionType.INCREMENT
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})
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export const decrement = () => ({
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type: actionType.DECREMENT
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})
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export const changeName = (name) => ({
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type: actionType.CHANGE_NAME,
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name
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})
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```
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:::
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```tsx
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// App.jsx
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import React, { PureComponent } from 'react'
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import store from './store'
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import { increment, decrement, changeName } from './store/actionFactory'
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export default class App extends PureComponent {
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componentDidMount() {
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store.subscribe(() => this.forceUpdate())
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}
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componentWillUnmount() {
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store.unsubscribe()
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}
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render() {
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return (
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<div>
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<h1>App</h1>
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<p>Count: {store.getState().count}</p>
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<p>Name: {store.getState().name}</p>
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<button onClick={() => store.dispatch(increment())}> +1 </button>
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<button onClick={() => store.dispatch(decrement())}> -1 </button>
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<button onClick={() => store.dispatch(changeName('ZIU'))}>CHANGE_NAME</button>
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</div>
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)
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}
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}
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```
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### Redux的三大原则
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单一数据源
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- 整个应用程序的状态都被存储在一棵Object Tree上
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- 且这个Object Tree只存储在一个Store中
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- 但Redux并不强制限制创建多Store，不利于数据维护
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- 单一数据源有利于整个应用程序的维护、追踪、修改
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State属性是只读的
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- 允许修改State的方法只有patch action，不要直接修改State
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- 确保了View或网络请求都不能修改State
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- 保证所有的修改都能被追踪、按照严格的顺序执行，不用担心竞态（race condition）的问题
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使用纯函数来执行修改
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- 通过reducer将旧State与新State联系在一起，并且返回一个**新的State**
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- 随着应用程序复杂程度增加，可以将reducer拆分为多个小的reducer，分别用于操作不同State Tree的某一部分
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- 所有的reducer都应该是纯函数，不能产生任何的副作用
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### 优化重复代码
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当编写了一些案例的时候会发现，React结合Redux时会编写很多重复的代码
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在每个需要用到Redux中状态的组件中，都需要在不同生命周期做添加订阅/解除订阅的处理，组件初始化时还要从store中取最新的状态
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针对重复代码的问题，可以使用之前学到的高阶组件来做优化
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Redux官方提供的库`react-redux`，可以让我们更方便的在React中使用Redux
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```bash
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npm i react-redux
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```
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在Profile组件中，通过高阶函数`connect`实现的
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将store中需要的状态通过`mapStoreToProps`转为props，并将需要使用store中状态的组件传入调用connect返回的函数中
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在`Profile`组件中就可以从props中获取到store中的状态
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::: code-group
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```tsx [App.jsx]
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// App.jsx
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import React, { PureComponent } from 'react'
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import { Provider } from 'react-redux'
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import store from './store'
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import Profile from './Profile'
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export default class App extends PureComponent {
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render() {
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return (
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<Provider store={store}>
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<div>
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<h2>App</h2>
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<Profile></Profile>
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</div>
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</Provider>
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)
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}
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}
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```
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```tsx [Profile.jsx]
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// Profile.jsx
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import React, { Component } from 'react'
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import { connect } from 'react-redux'
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// mapStateToProps is a function that
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// takes the state of the store as an argument
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// and returns an object with the data that the component needs from the store.
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// component will receive the data as props.
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const mapStateToProps = (state) => ({
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count: state.count
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})
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export default connect(mapStateToProps)(
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class Profile extends Component {
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render() {
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return (
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<div>
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<h2>Profile</h2>
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<p>Count: {this.props.count}</p>
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</div>
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)
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}
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}
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)
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```
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:::
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我们刚刚只是完成了对State的映射，将Store中保存的全局状态state映射到了Profile组件的props中
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connect还可以传入第二个参数，用于将action也映射到props中：
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```tsx {4,10-13,17,25,26}
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// Profile.jsx
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import React, { Component } from 'react'
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import { connect } from 'react-redux'
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import { INCREMENT, DECREMENT } from './store/constants'
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const mapStateToProps = (state) => ({
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count: state.count
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})
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const mapDispatchToProps = (dispatch) => ({
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increment: () => dispatch({ type: INCREMENT }),
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decrement: () => dispatch({ type: DECREMENT })
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})
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export default connect(
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mapStateToProps,
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mapDispatchToProps
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)(
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class Profile extends Component {
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render() {
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return (
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<div>
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<h2>Profile</h2>
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<p>Count: {this.props.count}</p>
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<button onClick={this.props.increment}> +1 </button>
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<button onClick={this.props.decrement}> -1 </button>
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</div>
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)
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}
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}
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)
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```
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本质上是`connect`内部对操作进行了封装，把逻辑隐藏起来了：
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- 调用`connect`这个**高阶函数**，返回一个**高阶组件**
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- 为高阶组件传入映射目标组件，最后高阶组件返回一个新组件
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- 新组件的props包含了来自Store中状态/dispatch的映射
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