你想要的——redux源码分析

摘要：大家好，今天给大家带来的是的源码分析首先是的地址点我接下来我们看看在项目中的简单使用，一般我们都从最简单的开始入手哈备注例子中结合的是进行使用，当然不仅仅能结合，还能结合市面上其他大多数的框架，这也是它比较流弊的地方首先是创建一个首先我们

大家好，今天给大家带来的是redux（v3.6.0）的源码分析~

首先是redux的github地址 点我

接下来我们看看redux在项目中的简单使用，一般我们都从最简单的开始入手哈

备注：例子中结合的是react进行使用，当然redux不仅仅能结合react，还能结合市面上其他大多数的框架，这也是它比较流弊的地方

首先是创建一个store

import React from "react"
import { render } from "react-dom"
// 首先我们必须先导入redux中的createStore方法，用于创建store
// 导入applyMiddleware方法，用于使用中间件
import { createStore, applyMiddleware } from "redux"
import { Provider } from "react-redux"
// 导入redux的中间件thunk
import thunk from "redux-thunk"
// 导入redux的中间件createLogger
import { createLogger } from "redux-logger"
// 我们还必须自己定义reducer函数，用于根据我们传入的action来访问新的state
import reducer from "./reducers"
import App from "./containers/App"

// 创建存放中间件数组
const middleware = [ thunk ]
if (process.env.NODE_ENV !== "production") {
  middleware.push(createLogger())
}
// 调用createStore方法来创建store，传入的参数分别是reducer和运用中间件的函数
const store = createStore(
  reducer,
  applyMiddleware(...middleware)
)
// 将store作为属性传入，这样在每个子组件中就都可以获取这个store实例，然后使用store的方法
render(
  
    
  ,
  document.getElementById("root")
)

接下来我们看看reducer是怎么定义的

// 首先我们导入redux中的combineReducers方法
import { combineReducers } from "redux"
// 导入actions，这个非必须，但是推荐这么做
import {
  SELECT_REDDIT, INVALIDATE_REDDIT,
  REQUEST_POSTS, RECEIVE_POSTS
} from "../actions"

// 接下来这个两个方法selectedReddit，postsByReddit就是reducer方法
// reducer方法负责根据传入的action的类型，返回新的state，这里可以传入默认的state
const selectedReddit = (state = "reactjs", action) => {
  switch (action.type) {
    case SELECT_REDDIT:
      return action.reddit
    default:
      return state
  }
}

const posts = (state = {
  isFetching: false,
  didInvalidate: false,
  items: []
}, action) => {
  switch (action.type) {
    case INVALIDATE_REDDIT:
      return {
        ...state,
        didInvalidate: true
      }
    case REQUEST_POSTS:
      return {
        ...state,
        isFetching: true,
        didInvalidate: false
      }
    case RECEIVE_POSTS:
      return {
        ...state,
        isFetching: false,
        didInvalidate: false,
        items: action.posts,
        lastUpdated: action.receivedAt
      }
    default:
      return state
  }
}

const postsByReddit = (state = { }, action) => {
  switch (action.type) {
    case INVALIDATE_REDDIT:
    case RECEIVE_POSTS:
    case REQUEST_POSTS:
      return {
        ...state,
        [action.reddit]: posts(state[action.reddit], action)
      }
    default:
      return state
  }
}

// 最后我们通过combineReducers这个方法，将所有的reducer方法合并成一个方法，也就是rootReducer方法
const rootReducer = combineReducers({
  postsByReddit,
  selectedReddit
})
// 导出这个rootReducer方法
export default rootReducer

接下来看看action的定义,其实action就是一个对象，对象中约定有一个必要的属性type，和一个非必要的属性payload；type代表了action的类型，指明了这个action对state修改的意图，而payload则是传入一些额外的数据供reducer使用

export const REQUEST_POSTS = "REQUEST_POSTS"
export const RECEIVE_POSTS = "RECEIVE_POSTS"
export const SELECT_REDDIT = "SELECT_REDDIT"
export const INVALIDATE_REDDIT = "INVALIDATE_REDDIT"

export const selectReddit = reddit => ({
  type: SELECT_REDDIT,
  reddit
})
export const invalidateReddit = reddit => ({
  type: INVALIDATE_REDDIT,
  reddit
})

export const requestPosts = reddit => ({
  type: REQUEST_POSTS,
  reddit
})

export const receivePosts = (reddit, json) => ({
  type: RECEIVE_POSTS,
  reddit,
  posts: json.data.children.map(child => child.data),
  receivedAt: Date.now()
})

const fetchPosts = reddit => dispatch => {
  dispatch(requestPosts(reddit))
  return fetch(`https://www.reddit.com/r/${reddit}.json`)
    .then(response => response.json())
    .then(json => dispatch(receivePosts(reddit, json)))
}

const shouldFetchPosts = (state, reddit) => {
  const posts = state.postsByReddit[reddit]
  if (!posts) {
    return true
  }
  if (posts.isFetching) {
    return false
  }
  return posts.didInvalidate
}

export const fetchPostsIfNeeded = reddit => (dispatch, getState) => {
  if (shouldFetchPosts(getState(), reddit)) {
    return dispatch(fetchPosts(reddit))
  }
}

以上就是redux最简单的用法，接下来我们就来看看redux源码里面具体是怎么实现的吧

首先我们看看整个redux项目的目录结构，从目录中我们可以看出，redux的项目源码其实比较简单

接下来就从入口文件index.js开始看吧，这个文件其实没有实现什么实质性的功能，只是导出了redux所提供的能力

// 入口文件
// 首先引入相应的模块，具体模块的内容后续会详细分析
import createStore from "./createStore"
import combineReducers from "./combineReducers"
import bindActionCreators from "./bindActionCreators"
import applyMiddleware from "./applyMiddleware"
import compose from "./compose"
import warning from "./utils/warning"

/*
* This is a dummy function to check if the function name has been altered by minification.
* If the function has been minified and NODE_ENV !== "production", warn the user.
*/
function isCrushed() {}

if (
  process.env.NODE_ENV !== "production" &&
  typeof isCrushed.name === "string" &&
  isCrushed.name !== "isCrushed"
) {
  warning(
    "You are currently using minified code outside of NODE_ENV === "production". " +
    "This means that you are running a slower development build of Redux. " +
    "You can use loose-envify (https://github.com/zertosh/loose-envify) for browserify " +
    "or DefinePlugin for webpack (http://stackoverflow.com/questions/30030031) " +
    "to ensure you have the correct code for your production build."
  )
}
// 导出相应的功能
export {
  createStore,
  combineReducers,
  bindActionCreators,
  applyMiddleware,
  compose
}

紧接着，我们就来看看redux中一个重要的文件，createStore.js。这个文件用于创建store

// 创建store的文件，提供了redux中store的所有内置的功能，也是redux中比较重要的一个文件

// 首先引入相应的模块
import isPlainObject from "lodash/isPlainObject"
import $$observable from "symbol-observable"

/**
 * These are private action types reserved by Redux.
 * For any unknown actions, you must return the current state.
 * If the current state is undefined, you must return the initial state.
 * Do not reference these action types directly in your code.
 */

 // 定义了有个内部使用的ActionType
export const ActionTypes = {
  INIT: "@@redux/INIT"
}

/**
 * Creates a Redux store that holds the state tree.
 * The only way to change the data in the store is to call `dispatch()` on it.
 *
 * There should only be a single store in your app. To specify how different
 * parts of the state tree respond to actions, you may combine several reducers
 * into a single reducer function by using `combineReducers`.
 *
 * @param {Function} reducer A function that returns the next state tree, given
 * the current state tree and the action to handle.
 *
 * @param {any} [preloadedState] The initial state. You may optionally specify it
 * to hydrate the state from the server in universal apps, or to restore a
 * previously serialized user session.
 * If you use `combineReducers` to produce the root reducer function, this must be
 * an object with the same shape as `combineReducers` keys.
 *
 * @param {Function} [enhancer] The store enhancer. You may optionally specify it
 * to enhance the store with third-party capabilities such as middleware,
 * time travel, persistence, etc. The only store enhancer that ships with Redux
 * is `applyMiddleware()`.
 *
 * @returns {Store} A Redux store that lets you read the state, dispatch actions
 * and subscribe to changes.
 */

// 导出创建store的方法
// 这个方法接收三个参数，分别是 reducer,预先加载的state,以及功能增强函数enhancer
export default function createStore(reducer, preloadedState, enhancer) {
  // 调整参数，如果没有传入预先加载的state，并且第二个参数是一个函数的话，则把第二个参数为功能增强函数enhancer
  if (typeof preloadedState === "function" && typeof enhancer === "undefined") {
    enhancer = preloadedState
    preloadedState = undefined
  }
  // 判断enhancer必须是一个函数
  if (typeof enhancer !== "undefined") {
    if (typeof enhancer !== "function") {
      throw new Error("Expected the enhancer to be a function.")
    }
    // 这是一个很重要的处理，它将createStore方法作为参数传入enhancer函数，并且执行enhancer
    // 这里主要是提供给redux中间件的使用，以此来达到增强整个redux流程的效果
    // 通过这个函数，也给redux提供了无限多的可能性
    return enhancer(createStore)(reducer, preloadedState)
  }
  // reducer必须是一个函数，否则报错
  if (typeof reducer !== "function") {
    throw new Error("Expected the reducer to be a function.")
  }
  // 将传入的reducer缓存到currentReducer变量中
  let currentReducer = reducer
  // 将传入的preloadedState缓存到currentState变量中
  let currentState = preloadedState
  // 定义当前的监听者队列
  let currentListeners = []
  // 定义下一个循环的监听者队列
  let nextListeners = currentListeners
  // 定义一个判断是否在dispatch的标志位
  let isDispatching = false

  // 判断是否能执行下一次监听队列
  function ensureCanMutateNextListeners() {
    if (nextListeners === currentListeners) {
      // 这里是将当前监听队列通过拷贝的形式赋值给下次监听队列，这样做是为了防止在当前队列执行的时候会影响到自身，所以拷贝了一份副本
      nextListeners = currentListeners.slice()
    }
  }

  /**
   * Reads the state tree managed by the store.
   *
   * @returns {any} The current state tree of your application.
   */
   // 获取当前的state
  function getState() {
    return currentState
  }

  /**
   * Adds a change listener. It will be called any time an action is dispatched,
   * and some part of the state tree may potentially have changed. You may then
   * call `getState()` to read the current state tree inside the callback.
   *
   * You may call `dispatch()` from a change listener, with the following
   * caveats:
   *
   * 1. The subscriptions are snapshotted just before every `dispatch()` call.
   * If you subscribe or unsubscribe while the listeners are being invoked, this
   * will not have any effect on the `dispatch()` that is currently in progress.
   * However, the next `dispatch()` call, whether nested or not, will use a more
   * recent snapshot of the subscription list.
   *
   * 2. The listener should not expect to see all state changes, as the state
   * might have been updated multiple times during a nested `dispatch()` before
   * the listener is called. It is, however, guaranteed that all subscribers
   * registered before the `dispatch()` started will be called with the latest
   * state by the time it exits.
   *
   * @param {Function} listener A callback to be invoked on every dispatch.
   * @returns {Function} A function to remove this change listener.
   */

  // 往监听队列里面去添加监听者
  function subscribe(listener) {
    // 监听者必须是一个函数
    if (typeof listener !== "function") {
      throw new Error("Expected listener to be a function.")
    }
    // 声明一个变量来标记是否已经subscribed，通过闭包的形式被缓存
    let isSubscribed = true
    // 创建一个当前currentListeners的副本，赋值给nextListeners
    ensureCanMutateNextListeners()
    // 将监听者函数push到nextListeners中
    nextListeners.push(listener)
    // 返回一个取消监听的函数
    // 原理很简单就是从将当前函数从数组中删除，使用的是数组的splice方法
    return function unsubscribe() {
      if (!isSubscribed) {
        return
      }

      isSubscribed = false

      ensureCanMutateNextListeners()
      const index = nextListeners.indexOf(listener)
      nextListeners.splice(index, 1)
    }
  }

  /**
   * Dispatches an action. It is the only way to trigger a state change.
   *
   * The `reducer` function, used to create the store, will be called with the
   * current state tree and the given `action`. Its return value will
   * be considered the **next** state of the tree, and the change listeners
   * will be notified.
   *
   * The base implementation only supports plain object actions. If you want to
   * dispatch a Promise, an Observable, a thunk, or something else, you need to
   * wrap your store creating function into the corresponding middleware. For
   * example, see the documentation for the `redux-thunk` package. Even the
   * middleware will eventually dispatch plain object actions using this method.
   *
   * @param {Object} action A plain object representing “what changed”. It is
   * a good idea to keep actions serializable so you can record and replay user
   * sessions, or use the time travelling `redux-devtools`. An action must have
   * a `type` property which may not be `undefined`. It is a good idea to use
   * string constants for action types.
   *
   * @returns {Object} For convenience, the same action object you dispatched.
   *
   * Note that, if you use a custom middleware, it may wrap `dispatch()` to
   * return something else (for example, a Promise you can await).
   */

  // redux中通过dispatch一个action，来触发对store中的state的修改
  // 参数就是一个action
  function dispatch(action) {
    // 这里判断一下action是否是一个纯对象，如果不是则抛出错误
    if (!isPlainObject(action)) {
      throw new Error(
        "Actions must be plain objects. " +
        "Use custom middleware for async actions."
      )
    }
    // action中必须要有type属性，否则抛出错误
    if (typeof action.type === "undefined") {
      throw new Error(
        "Actions may not have an undefined "type" property. " +
        "Have you misspelled a constant?"
      )
    }
    // 如果上一次dispatch还没结束，则不能继续dispatch下一次
    if (isDispatching) {
      throw new Error("Reducers may not dispatch actions.")
    }

    try {
      // 将isDispatching设置为true，表示当次dispatch开始
      isDispatching = true
      // 利用传入的reducer函数处理state和action，返回新的state
      // 推荐不直接修改原有的currentState
      currentState = currentReducer(currentState, action)
    } finally {
      // 当次的dispatch结束
      isDispatching = false
    }
    // 每次dispatch结束之后，就执行监听队列中的监听函数
    // 将nextListeners赋值给currentListeners，保证下一次执行ensureCanMutateNextListeners方法的时候会重新拷贝一个新的副本
    // 简单粗暴的使用for循环执行
    const listeners = currentListeners = nextListeners
    for (let i = 0; i < listeners.length; i++) {
      const listener = listeners[i]
      listener()
    }
    // 最后返回action
    return action
  }

  /**
   * Replaces the reducer currently used by the store to calculate the state.
   *
   * You might need this if your app implements code splitting and you want to
   * load some of the reducers dynamically. You might also need this if you
   * implement a hot reloading mechanism for Redux.
   *
   * @param {Function} nextReducer The reducer for the store to use instead.
   * @returns {void}
   */
  // replaceReducer方法，顾名思义就是替换当前的reducer处理函数
  function replaceReducer(nextReducer) {
    if (typeof nextReducer !== "function") {
      throw new Error("Expected the nextReducer to be a function.")
    }

    currentReducer = nextReducer
    dispatch({ type: ActionTypes.INIT })
  }

  /**
   * Interoperability point for observable/reactive libraries.
   * @returns {observable} A minimal observable of state changes.
   * For more information, see the observable proposal:
   * https://github.com/tc39/proposal-observable
   */
  // 这个函数一般来说用不到，他是配合其他特点的框架或编程思想来使用的如rx.js，感兴趣的朋友可以自行学习
  // 这里就不多做介绍
  function observable() {
    const outerSubscribe = subscribe
    return {
      /**
       * The minimal observable subscription method.
       * @param {Object} observer Any object that can be used as an observer.
       * The observer object should have a `next` method.
       * @returns {subscription} An object with an `unsubscribe` method that can
       * be used to unsubscribe the observable from the store, and prevent further
       * emission of values from the observable.
       */
      subscribe(observer) {
        if (typeof observer !== "object") {
          throw new TypeError("Expected the observer to be an object.")
        }

        function observeState() {
          if (observer.next) {
            observer.next(getState())
          }
        }

        observeState()
        const unsubscribe = outerSubscribe(observeState)
        return { unsubscribe }
      },

      [$$observable]() {
        return this
      }
    }
  }

  // When a store is created, an "INIT" action is dispatched so that every
  // reducer returns their initial state. This effectively populates
  // the initial state tree.

  // dispatch一个初始化的action
  dispatch({ type: ActionTypes.INIT })

  // 最后返回这个store的所有能力
  return {
    dispatch,
    subscribe,
    getState,
    replaceReducer,
    [$$observable]: observable
  }
}

接下来我们看看combineReducers.js这个文件，通常我们会用它来合并我们的reducer方法

这个文件用于合并多个reducer，然后返回一个根reducer

因为store中只允许有一个reducer函数，所以当我们需要进行模块拆分的时候，就必须要用到这个方法

// 一开始先导入相应的函数
import { ActionTypes } from "./createStore"
import isPlainObject from "lodash/isPlainObject"
import warning from "./utils/warning"

// 获取UndefinedState的错误信息
function getUndefinedStateErrorMessage(key, action) {
  const actionType = action && action.type
  const actionName = (actionType && `"${actionType.toString()}"`) || "an action"

  return (
    `Given action ${actionName}, reducer "${key}" returned undefined. ` +
    `To ignore an action, you must explicitly return the previous state. ` +
    `If you want this reducer to hold no value, you can return null instead of undefined.`
  )
}

function getUnexpectedStateShapeWarningMessage(inputState, reducers, action, unexpectedKeyCache) {
  // 获取reducers的所有key
  const reducerKeys = Object.keys(reducers)
  const argumentName = action && action.type === ActionTypes.INIT ?
    "preloadedState argument passed to createStore" :
    "previous state received by the reducer"
  // 当reducers对象是一个空对象的话，返回警告文案
  if (reducerKeys.length === 0) {
    return (
      "Store does not have a valid reducer. Make sure the argument passed " +
      "to combineReducers is an object whose values are reducers."
    )
  }
  // state必须是一个对象
  if (!isPlainObject(inputState)) {
    return (
      `The ${argumentName} has unexpected type of "` +
      ({}).toString.call(inputState).match(/s([a-z|A-Z]+)/)[1] +
      `". Expected argument to be an object with the following ` +
      `keys: "${reducerKeys.join("", "")}"`
    )
  }
  // 判断state中是否有reducer没有的key，因为redux对state分模块的时候,是依据reducer来划分的
  const unexpectedKeys = Object.keys(inputState).filter(key =>
    !reducers.hasOwnProperty(key) &&
    !unexpectedKeyCache[key]
  )

  unexpectedKeys.forEach(key => {
    unexpectedKeyCache[key] = true
  })

  if (unexpectedKeys.length > 0) {
    return (
      `Unexpected ${unexpectedKeys.length > 1 ? "keys" : "key"} ` +
      `"${unexpectedKeys.join("", "")}" found in ${argumentName}. ` +
      `Expected to find one of the known reducer keys instead: ` +
      `"${reducerKeys.join("", "")}". Unexpected keys will be ignored.`
    )
  }
}
// assertReducerShape函数，检测当遇到位置action的时候，reducer是否会返回一个undefined,如果是的话则抛出错误
// 接受一个reducers对象
function assertReducerShape(reducers) {
  // 遍历这个reducers对象
  Object.keys(reducers).forEach(key => {
    const reducer = reducers[key]
    // 获取reducer函数在处理当state是undefined，actionType为初始默认type的时候返回的值
    const initialState = reducer(undefined, { type: ActionTypes.INIT })
    // 如果这个值是undefined，则抛出错误，因为初始state不应该是undefined
    if (typeof initialState === "undefined") {
      throw new Error(
        `Reducer "${key}" returned undefined during initialization. ` +
        `If the state passed to the reducer is undefined, you must ` +
        `explicitly return the initial state. The initial state may ` +
        `not be undefined. If you don"t want to set a value for this reducer, ` +
        `you can use null instead of undefined.`
      )
    }
    // 当遇到一个不知道的action的时候，reducer也不能返回undefined，否则也会抛出报错
    const type = "@@redux/PROBE_UNKNOWN_ACTION_" + Math.random().toString(36).substring(7).split("").join(".")
    if (typeof reducer(undefined, { type }) === "undefined") {
      throw new Error(
        `Reducer "${key}" returned undefined when probed with a random type. ` +
        `Don"t try to handle ${ActionTypes.INIT} or other actions in "redux/*" ` +
        `namespace. They are considered private. Instead, you must return the ` +
        `current state for any unknown actions, unless it is undefined, ` +
        `in which case you must return the initial state, regardless of the ` +
        `action type. The initial state may not be undefined, but can be null.`
      )
    }
  })
}

/**
 * Turns an object whose values are different reducer functions, into a single
 * reducer function. It will call every child reducer, and gather their results
 * into a single state object, whose keys correspond to the keys of the passed
 * reducer functions.
 *
 * @param {Object} reducers An object whose values correspond to different
 * reducer functions that need to be combined into one. One handy way to obtain
 * it is to use ES6 `import * as reducers` syntax. The reducers may never return
 * undefined for any action. Instead, they should return their initial state
 * if the state passed to them was undefined, and the current state for any
 * unrecognized action.
 *
 * @returns {Function} A reducer function that invokes every reducer inside the
 * passed object, and builds a state object with the same shape.
 */

// 导出combineReducers方法，接受一个参数reducers对象
export default function combineReducers(reducers) {
  // 获取reducers对象的key值
  const reducerKeys = Object.keys(reducers)
  // 定义一个最终要返回的reducers对象
  const finalReducers = {}
  // 遍历这个reducers对象的key
  for (let i = 0; i < reducerKeys.length; i++) {
    // 缓存每个key值
    const key = reducerKeys[i]

    if (process.env.NODE_ENV !== "production") {
      if (typeof reducers[key] === "undefined") {
        warning(`No reducer provided for key "${key}"`)
      }
    }
    // 相应key的值是个函数，则将改函数缓存到finalReducers中
    if (typeof reducers[key] === "function") {
      finalReducers[key] = reducers[key]
    }
  }
  // 获取finalReducers的所有的key值，缓存到变量finalReducerKeys中
  const finalReducerKeys = Object.keys(finalReducers)

  let unexpectedKeyCache
  if (process.env.NODE_ENV !== "production") {
    unexpectedKeyCache = {}
  }
  // 定义一个变量，用于缓存错误对象
  let shapeAssertionError
  try {
    // 做错误处理，详情看后面assertReducerShape方法
    // 主要就是检测，
    assertReducerShape(finalReducers)
  } catch (e) {
    shapeAssertionError = e
  }

  return function combination(state = {}, action) {
    // 如果有错误，则抛出错误
    if (shapeAssertionError) {
      throw shapeAssertionError
    }

    if (process.env.NODE_ENV !== "production") {
      // 获取警告提示
      const warningMessage = getUnexpectedStateShapeWarningMessage(state, finalReducers, action, unexpectedKeyCache)
      if (warningMessage) {
        warning(warningMessage)
      }
    }
    // 定义一个变量来表示state是否已经被改变
    let hasChanged = false
    // 定义一个变量，来缓存改变后的state
    const nextState = {}
    // 开始遍历finalReducerKeys
    for (let i = 0; i < finalReducerKeys.length; i++) {
      // 获取有效的reducer的key值
      const key = finalReducerKeys[i]
      // 根据key值获取对应的reducer函数
      const reducer = finalReducers[key]
      // 根据key值获取对应的state模块
      const previousStateForKey = state[key]
      // 执行reducer函数，获取相应模块的state
      const nextStateForKey = reducer(previousStateForKey, action)
      // 如果获取的state是undefined,则抛出错误
      if (typeof nextStateForKey === "undefined") {
        const errorMessage = getUndefinedStateErrorMessage(key, action)
        throw new Error(errorMessage)
      }
      // 将获取到的新的state赋值给新的state对应的模块，key则为当前reducer的key
      nextState[key] = nextStateForKey
      // 判读state是否发生改变
      hasChanged = hasChanged || nextStateForKey !== previousStateForKey
    }
    // 如果state发生改变则返回新的state，否则返回原来的state
    return hasChanged ? nextState : state
  }
}

接下来我们在看看bindActionCreators.js这个文件

首先先认识actionCreators,简单来说就是创建action的方法，redux的action是一个对象，而我们经常使用一些函数来创建这些对象，则这些函数就是actionCreators

而这个文件实现的功能，是根据绑定的actionCreator，来实现自动dispatch的功能

import warning from "./utils/warning"
// 对于每个actionCreator方法，执行之后都会得到一个action
// 这个bindActionCreator方法，会返回一个能够自动执行dispatch的方法
function bindActionCreator(actionCreator, dispatch) {
  return (...args) => dispatch(actionCreator(...args))
}

/**
 * Turns an object whose values are action creators, into an object with the
 * same keys, but with every function wrapped into a `dispatch` call so they
 * may be invoked directly. This is just a convenience method, as you can call
 * `store.dispatch(MyActionCreators.doSomething())` yourself just fine.
 *
 * For convenience, you can also pass a single function as the first argument,
 * and get a function in return.
 *
 * @param {Function|Object} actionCreators An object whose values are action
 * creator functions. One handy way to obtain it is to use ES6 `import * as`
 * syntax. You may also pass a single function.
 *
 * @param {Function} dispatch The `dispatch` function available on your Redux
 * store.
 *
 * @returns {Function|Object} The object mimicking the original object, but with
 * every action creator wrapped into the `dispatch` call. If you passed a
 * function as `actionCreators`, the return value will also be a single
 * function.
 */
// 对外暴露这个bindActionCreators方法
export default function bindActionCreators(actionCreators, dispatch) {
  // 如果传入的actionCreators参数是个函数，则直接调用bindActionCreator方法
  if (typeof actionCreators === "function") {
    return bindActionCreator(actionCreators, dispatch)
  }
  // 错误处理
  if (typeof actionCreators !== "object" || actionCreators === null) {
    throw new Error(
      `bindActionCreators expected an object or a function, instead received ${actionCreators === null ? "null" : typeof actionCreators}. ` +
      `Did you write "import ActionCreators from" instead of "import * as ActionCreators from"?`
    )
  }
  // 如果actionCreators是一个对象，则获取对象中的key
  const keys = Object.keys(actionCreators)
  // 定义一个缓存对象
  const boundActionCreators = {}
  // 遍历actionCreators的每个key
  for (let i = 0; i < keys.length; i++) {
    // 获取每个key
    const key = keys[i]
    // 根据每个key获取特定的actionCreator方法
    const actionCreator = actionCreators[key]
    // 如果actionCreator是一个函数，则直接调用bindActionCreator方法，将返回的匿名函数缓存到boundActionCreators对象中
    if (typeof actionCreator === "function") {
      boundActionCreators[key] = bindActionCreator(actionCreator, dispatch)
    } else {
      warning(`bindActionCreators expected a function actionCreator for key "${key}", instead received type "${typeof actionCreator}".`)
    }
  }
  // 最后返回boundActionCreators对象
  // 用户获取到这个对象后，可拿出对象中的每个key的对应的值，也就是各个匿名函数，执行匿名函数就可以实现dispatch功能
  return boundActionCreators
}

接下来我们看看applyMiddleware.js这个文件，这个文件让redux有着无限多的可能性。为什么这么说呢，你往下看就知道了

// 这个文件的代码逻辑其实很简单
// 首先导入compose函数，等一下我们会详细分析这个compose函数
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.
 *
 * See `redux-thunk` package as an example of the Redux middleware.
 *
 * Because middleware is potentially asynchronous, this should be the first
 * store enhancer in the composition chain.
 *
 * Note that each middleware will be given the `dispatch` and `getState` functions
 * as named arguments.
 *
 * @param {...Function} middlewares The middleware chain to be applied.
 * @returns {Function} A store enhancer applying the middleware.
 */
 // 接下来导出applyMiddleware这个方法，这个方法也是我们经常用来作为createStore中enhance参数的一个方法
export default function applyMiddleware(...middlewares) {
  // 首先先返回一个匿名函数，有没有发现这个函数跟createStore很相似啊
  // 没错其实他就是我们的之前看到的createStore
  return (createStore) => (reducer, preloadedState, enhancer) => {
    // 首先用原来的createStore创建一个store，并把它缓存起来
    const store = createStore(reducer, preloadedState, enhancer)
    // 获取store中原始的dispatch方法
    let dispatch = store.dispatch
    // 定一个执行链数组
    let chain = []
    // 缓存原有store中getState和dispatch方法
    const middlewareAPI = {
      getState: store.getState,
      dispatch: (action) => dispatch(action)
    }
    // 执行每个中间件函数，并将middlewareAPI作为参数传入，获得一个执行链数组
    chain = middlewares.map(middleware => middleware(middlewareAPI))
    // 将执行链数组传入compose方法中，并立即执行返回的方法获得最后包装过后的dispatch
    // 这个过程简单来说就是，每个中间件都会接受一个store.dispatch方法，然后基于这个方法进行包装，然后返回一个新的dispatch
    // 这个新的dispatch又作为参数传入下一个中间件函数，然后有进行包装。。。一直循环这个过程，直到最后得到一个最终的dispatch
    dispatch = compose(...chain)(store.dispatch)
    // 返回一个store对象，并将新的dispatch方法覆盖原有的dispatch方法
    return {
      ...store,
      dispatch
    }
  }
}

看到这里，其实你已经看完了大部分redux的内容，最后我们看看上述文件中使用到的compose方法是如何实现的。

打开compose.js，我们发现其实实现方式就是利用es5中数组的reduce方法来实现这种效果的

/**
 * Composes single-argument functions from right to left. The rightmost
 * function can take multiple arguments as it provides the signature for
 * the resulting composite function.
 *
 * @param {...Function} funcs The functions to compose.
 * @returns {Function} A function obtained by composing the argument functions
 * from right to left. For example, compose(f, g, h) is identical to doing
 * (...args) => f(g(h(...args))).
 */

export default function compose(...funcs) {
  // 判断函数数组是否为空
  if (funcs.length === 0) {
    return arg => arg
  }
  // 如果函数数组只有一个元素，则直接执行
  if (funcs.length === 1) {
    return funcs[0]
  }

  // 否则，就利用reduce方法执行每个中间件函数，并将上一个函数的返回作为下一个函数的参数
  return funcs.reduce((a, b) => (...args) => a(b(...args)))
}

哈哈，以上就是今天给大家分享的redux源码分析~希望大家能够喜欢咯