如果看不清楚,可以查看https://segmentfault.com/a/1190000018432577
理解applyMiddleware需要跟createStore结合.首先来看createStore是怎样创建store的.
再来看createStore 的源码
createStore的第三个参数enhancer就是applyMiddleware,此时createStore会返回enhancer(createStore)(reducer, preloadedState),也就是createStore在中间件里面去执行了
applyMiddleware返回的是函数A(也就是applyMiddleware(...middlewares) =函数A,然后又跑到createStore里面作为第三个参数),所以能把createStore作为参数传进去
一个小例子,测试返回函数后是什么东西
源码:
import compose from './compose'
/**
* Creates a store enhancer that applies middleware to the dispatch method
* of the Redux store. This is handy for a variety of tasks, such as expressing
* asynchronous actions in a concise manner, or logging every action payload.
*
* 创建一个redux store的dispatch方法使用中间件的store增强器. 对于不同的人任务,这将会
* 非常方便,比如可以用非常简单的方式进行异步操作,或者输出action的payload
*
* See `redux-thunk` package as an example of the Redux middleware.
*查看`redux-thunk`包作为一个redux中间件的例子
*
* Because middleware is potentially asynchronous, this should be the first
* store enhancer in the composition chain.
*
* 因为中间件默认是异步的,这应该是合成链中的第一个store增强器
*
* Note that each middleware will be given the `dispatch` and `getState` functions
* as named arguments.
*注意每个中间件都会以`dispatch` and `getState`方法作为参数
* @param {...Function} middlewares The middleware chain to be applied.提供的中间件
* @returns {Function} A store enhancer applying the middleware.store增强器应用的中间件
*/
export default function applyMiddleware(...middlewares) {
return (createStore) => (reducer, preloadedState, enhancer) => {
const store = createStore(reducer, preloadedState, enhancer)
let dispatch = store.dispatch
let chain = []
const middlewareAPI = {
getState: store.getState,
dispatch: (action) => dispatch(action)
}
chain = middlewares.map(middleware => middleware(middlewareAPI))
dispatch = compose(...chain)(store.dispatch)
return {
...store,
dispatch
}
}
}
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