redux教程之源码解析3applyMiddleware（分析在注释中） _middleware

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applyMiddleware是另一个核心函数首先我们需要知道如何使用中间件 eg: import { createStore, applyMiddleware } from ‘redux‘ import todos from ‘./reducers‘function logger({ getState }) { return (next) => (action) => { console.log(‘will dispatch‘, action)// 调用 middleware 链中下一个 middleware 的 dispatch。 let returnValue = https://www.songbingjia.com/android/next(action)console.log(‘state after dispatch‘, getState())// 一般会是 action 本身，除非 // 后面的 middleware 修改了它。 return returnValue } }let store = createStore( todos, [ ‘Use Redux‘ ], applyMiddleware(logger) )store.dispatch({ type: ‘ADD_TODO‘, text: ‘Understand the middleware‘ })

【redux教程之源码解析3applyMiddleware（分析在注释中）】

中间件其实就是对reducer就行功能扩展，例如上面的例子，logger即在执行reducer时，输出正在执行的action名字 const a = [()=> {console.log(1)},()=> {console.log(2)},()=> {console.log(3)},()=> {console.log(4)}] undefined const b = a.reduce((a, b) => (...args) => a(b(...args))) undefined b(1) VM8485:1 4 VM8485:1 3 VM8485:1 2 VM8485:1 1 undefined

源码 export default function applyMiddleware(...middlewares) { /**返回一个新的createStore函数 */ return createStore => (...args) => { /**通过传入的reducer和action生成store */ const store = createStore(...args) let dispatch = () => { throw new Error( `Dispatching while constructing your middleware is not allowed. ` + `Other middleware would not be applied to this dispatch.` ) } /**对store和disptch改变成动态的，这样每次传入一个middleware的Store和disptch都是上一个middleware处理过的 */ const middlewareAPI = { getState: store.getState, dispatch: (...args) => dispatch(...args) } const chain = middlewares.map(middleware => middleware(middlewareAPI)) /**链式处理函数，即依次执行middleware数组中的各个中间件 */ dispatch = compose(...chain)(store.dispatch)/**将所有中间件对reducer的扩展加入后，返回store和dispatch */ return { ...store, dispatch } } }