中间件

redux默认只处理同步，对于API请求这样的异步任务则无能为力。

// constants/ActionTypes.js
export const ADD = 'ADD'
export const SQUARE = 'SQUARE'
export const SET = 'SET'

// action/math.js
import { ADD, SQUARE, SET } from '../constants/ActionTypes'
export const add = num => ({ type: ADD, num })
export const square = { type: SQUARE }
export const setNum = num => ({type: SET,num})

// reduce/math.js
import { ADD, SQUARE,SET } from '../constants/ActionTypes'
const math = (state = 10, action) => {
  switch (action.type) {
    case ADD:
      return state + action.num
    case SQUARE:
      return state * state
    case SET:
      return action.num
    default:
      return state
  }
}
export default math

// index.js
import store from './store'
import React from 'react'
import ReactDOM from 'react-dom'
import { add, square, setNum } from './action/math'
import axios from 'axios'
let uri = 'https://jsonplaceholder.typicode.com/posts/2'
const render = () => ReactDOM.render(
  <div store={store}>
    <p>{store.getState().math}</p>
    <input type="button" onClick={() => {axios.get(uri).then(res => {store.dispatch(store.dispatch(setNum(res.data.id)))})}} value="设置Num" />
    <input type="button" onClick={() => store.dispatch(add(1))} value="+1" />
    <input type="button" onClick={() => store.dispatch(add(2))} value="+2" />
    <input type="button" onClick={() => store.dispatch(square)} value="乘方" />
  </div>,
  document.getElementById('root')
)
render()
store.subscribe(render)

虽然API是异步操作，但store.dispatch并不是异步，而axios通过get方法请求回来数据后，store.dispatch在axios中的then方法中同步取得数据。

【redux-thunk】
　　如果把异步的请求和非常复杂的逻辑都放在组件里实现，组件会变得臃肿，应利用redux-thunk。将这些复杂的逻辑放到action中去处理。redux-thunk检测到action是一个函数会进行处理。

首先，使用npm进行安装

npm install --save redux-thunk

然后，使用applyMiddleware来使用thunk中间件。

import { createStore, applyMiddleware } from 'redux'
import thunk from 'redux-thunk'
import rootReducer from '../reducer'
export default createStore(rootReducer,applyMiddleware(thunk))

修改的代码如下：

// action/math.js
import { ADD, SQUARE, SET } from '../action/ActionTypes'
import axios from 'axios'
const uri = 'https://jsonplaceholder.typicode.com/posts/2'
export const add = num => ({ type: ADD, num })
export const square = { type: SQUARE }
export const setNum = () => (dispatch, getState) => {
  return axios.get(uri).then(res => {
    dispatch({
      type: SET,
      num: res.data.id
    })
  })
}

// index.js
import store from './store'
import React from 'react'
import ReactDOM from 'react-dom'
import { add, square, setNum } from './action/math'
const render = () => ReactDOM.render(
  <div store={store}>
    <p>{store.getState().math}</p>
    <input type="button" onClick={() => store.dispatch(setNum())} value="设置Num" />
    <input type="button" onClick={() => store.dispatch(add(1))} value="+1" />
    <input type="button" onClick={() => store.dispatch(add(2))} value="+2" />
    <input type="button" onClick={() => store.dispatch(square)} value="乘方" />
  </div>,
  document.getElementById('root')
)
render()
store.subscribe(render)

【注意】 redux的中间件，而不是react的。