你想要的——redux源碼分析

你們好，今天給你們帶來的是redux（v3.6.0）的源碼分析~

首先是redux的github地址 點我

接下來咱們看看redux在項目中的簡單使用，通常咱們都從最簡單的開始入手哈

備註：例子中結合的是react進行使用，固然redux不只僅能結合react，還能結合市面上其餘大多數的框架，這也是它比較流弊的地方

首先是建立一個store

import React from 'react'
import { render } from 'react-dom'
// 首先咱們必須先導入redux中的createStore方法，用於建立store
// 導入applyMiddleware方法，用於使用中間件
import { createStore, applyMiddleware } from 'redux'
import { Provider } from 'react-redux'
// 導入redux的中間件thunk
import thunk from 'redux-thunk'
// 導入redux的中間件createLogger
import { createLogger } from 'redux-logger'
// 咱們還必須本身定義reducer函數，用於根據咱們傳入的action來訪問新的state
import reducer from './reducers'
import App from './containers/App'

// 建立存放中間件數組
const middleware = [ thunk ]
if (process.env.NODE_ENV !== 'production') {
  middleware.push(createLogger())
}
// 調用createStore方法來建立store，傳入的參數分別是reducer和運用中間件的函數
const store = createStore(
  reducer,
  applyMiddleware(...middleware)
)
// 將store做爲屬性傳入，這樣在每一個子組件中就均可以獲取這個store實例，而後使用store的方法
render(
  <Provider store={store}>
    <App />
  </Provider>,
  document.getElementById('root')
)

接下來咱們看看reducer是怎麼定義的

// 首先咱們導入redux中的combineReducers方法
import { combineReducers } from 'redux'
// 導入actions，這個非必須，可是推薦這麼作
import {
  SELECT_REDDIT, INVALIDATE_REDDIT,
  REQUEST_POSTS, RECEIVE_POSTS
} from '../actions'

// 接下來這個兩個方法selectedReddit，postsByReddit就是reducer方法
// reducer方法負責根據傳入的action的類型，返回新的state，這裏能夠傳入默認的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
  }
}

// 最後咱們經過combineReducers這個方法，將全部的reducer方法合併成一個方法，也就是rootReducer方法
const rootReducer = combineReducers({
  postsByReddit,
  selectedReddit
})
// 導出這個rootReducer方法
export default rootReducer

接下來看看action的定義,其實action就是一個對象，對象中約定有一個必要的屬性type，和一個非必要的屬性payload；type表明了action的類型，指明瞭這個action對state修改的意圖，而payload則是傳入一些額外的數據供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最簡單的用法，接下來咱們就來看看redux源碼裏面具體是怎麼實現的吧

首先咱們看看整個redux項目的目錄結構，從目錄中咱們能夠看出，redux的項目源碼其實比較簡單

接下來就從入口文件index.js開始看吧，這個文件其實沒有實現什麼實質性的功能，只是導出了redux所提供的能力

// 入口文件
// 首先引入相應的模塊，具體模塊的內容後續會詳細分析
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.'
  )
}
// 導出相應的功能
export {
  createStore,
  combineReducers,
  bindActionCreators,
  applyMiddleware,
  compose
}

緊接着，咱們就來看看redux中一個重要的文件，createStore.js。這個文件用於建立store

// 建立store的文件，提供了redux中store的全部內置的功能，也是redux中比較重要的一個文件

// 首先引入相應的模塊
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.
 */

 // 定義了有個內部使用的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.
 */

// 導出建立store的方法
// 這個方法接收三個參數，分別是 reducer,預先加載的state,以及功能加強函數enhancer
export default function createStore(reducer, preloadedState, enhancer) {
  // 調整參數，若是沒有傳入預先加載的state，而且第二個參數是一個函數的話，則把第二個參數爲功能加強函數enhancer
  if (typeof preloadedState === 'function' && typeof enhancer === 'undefined') {
    enhancer = preloadedState
    preloadedState = undefined
  }
  // 判斷enhancer必須是一個函數
  if (typeof enhancer !== 'undefined') {
    if (typeof enhancer !== 'function') {
      throw new Error('Expected the enhancer to be a function.')
    }
    // 這是一個很重要的處理，它將createStore方法做爲參數傳入enhancer函數，而且執行enhancer
    // 這裏主要是提供給redux中間件的使用，以此來達到加強整個redux流程的效果
    // 經過這個函數，也給redux提供了無限多的可能性
    return enhancer(createStore)(reducer, preloadedState)
  }
  // reducer必須是一個函數，不然報錯
  if (typeof reducer !== 'function') {
    throw new Error('Expected the reducer to be a function.')
  }
  // 將傳入的reducer緩存到currentReducer變量中
  let currentReducer = reducer
  // 將傳入的preloadedState緩存到currentState變量中
  let currentState = preloadedState
  // 定義當前的監聽者隊列
  let currentListeners = []
  // 定義下一個循環的監聽者隊列
  let nextListeners = currentListeners
  // 定義一個判斷是否在dispatch的標誌位
  let isDispatching = false

  // 判斷是否能執行下一次監聽隊列
  function ensureCanMutateNextListeners() {
    if (nextListeners === currentListeners) {
      // 這裏是將當前監聽隊列經過拷貝的形式賦值給下次監聽隊列，這樣作是爲了防止在當前隊列執行的時候會影響到自身，因此拷貝了一份副本
      nextListeners = currentListeners.slice()
    }
  }

  /**
   * Reads the state tree managed by the store.
   *
   * @returns {any} The current state tree of your application.
   */
   // 獲取當前的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.
   */

  // 往監聽隊列裏面去添加監聽者
  function subscribe(listener) {
    // 監聽者必須是一個函數
    if (typeof listener !== 'function') {
      throw new Error('Expected listener to be a function.')
    }
    // 聲明一個變量來標記是否已經subscribed，經過閉包的形式被緩存
    let isSubscribed = true
    // 建立一個當前currentListeners的副本，賦值給nextListeners
    ensureCanMutateNextListeners()
    // 將監聽者函數push到nextListeners中
    nextListeners.push(listener)
    // 返回一個取消監聽的函數
    // 原理很簡單就是從將當前函數從數組中刪除，使用的是數組的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中經過dispatch一個action，來觸發對store中的state的修改
  // 參數就是一個action
  function dispatch(action) {
    // 這裏判斷一下action是不是一個純對象，若是不是則拋出錯誤
    if (!isPlainObject(action)) {
      throw new Error(
        'Actions must be plain objects. ' +
        'Use custom middleware for async actions.'
      )
    }
    // action中必需要有type屬性，不然拋出錯誤
    if (typeof action.type === 'undefined') {
      throw new Error(
        'Actions may not have an undefined "type" property. ' +
        'Have you misspelled a constant?'
      )
    }
    // 若是上一次dispatch還沒結束，則不能繼續dispatch下一次
    if (isDispatching) {
      throw new Error('Reducers may not dispatch actions.')
    }

    try {
      // 將isDispatching設置爲true，表示當次dispatch開始
      isDispatching = true
      // 利用傳入的reducer函數處理state和action，返回新的state
      // 推薦不直接修改原有的currentState
      currentState = currentReducer(currentState, action)
    } finally {
      // 當次的dispatch結束
      isDispatching = false
    }
    // 每次dispatch結束以後，就執行監聽隊列中的監聽函數
    // 將nextListeners賦值給currentListeners，保證下一次執行ensureCanMutateNextListeners方法的時候會從新拷貝一個新的副本
    // 簡單粗暴的使用for循環執行
    const listeners = currentListeners = nextListeners
    for (let i = 0; i < listeners.length; i++) {
      const listener = listeners[i]
      listener()
    }
    // 最後返回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，顧名思義就是替換當前的reducer處理函數
  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
   */
  // 這個函數通常來講用不到，他是配合其餘特色的框架或編程思想來使用的如rx.js，感興趣的朋友能夠自行學習
  // 這裏就很少作介紹
  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一個初始化的action
  dispatch({ type: ActionTypes.INIT })

  // 最後返回這個store的全部能力
  return {
    dispatch,
    subscribe,
    getState,
    replaceReducer,
    [$$observable]: observable
  }
}

接下來咱們看看combineReducers.js這個文件，一般咱們會用它來合併咱們的reducer方法

這個文件用於合併多個reducer，而後返回一個根reducer

由於store中只容許有一個reducer函數，因此當咱們須要進行模塊拆分的時候，就必需要用到這個方法

// 一開始先導入相應的函數
import { ActionTypes } from './createStore'
import isPlainObject from 'lodash/isPlainObject'
import warning from './utils/warning'

// 獲取UndefinedState的錯誤信息
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) {
  // 獲取reducers的全部key
  const reducerKeys = Object.keys(reducers)
  const argumentName = action && action.type === ActionTypes.INIT ?
    'preloadedState argument passed to createStore' :
    'previous state received by the reducer'
  // 當reducers對象是一個空對象的話，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必須是一個對象
  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('", "')}"`
    )
  }
  // 判斷state中是否有reducer沒有的key，由於redux對state分模塊的時候,是依據reducer來劃分的
  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函數，檢測當遇到位置action的時候，reducer是否會返回一個undefined,若是是的話則拋出錯誤
// 接受一個reducers對象
function assertReducerShape(reducers) {
  // 遍歷這個reducers對象
  Object.keys(reducers).forEach(key => {
    const reducer = reducers[key]
    // 獲取reducer函數在處理當state是undefined，actionType爲初始默認type的時候返回的值
    const initialState = reducer(undefined, { type: ActionTypes.INIT })
    // 若是這個值是undefined，則拋出錯誤，由於初始state不該該是undefined
    if (typeof initialState === 'undefined') {
      throw new Error(
        `Reducer "${key}" returned undefined during initialization. ` +
        `If the state passed to the reducer is undefined, you must ` +
        `explicitly return the initial state. The initial state may ` +
        `not be undefined. If you don't want to set a value for this reducer, ` +
        `you can use null instead of undefined.`
      )
    }
    // 當遇到一個不知道的action的時候，reducer也不能返回undefined，不然也會拋出報錯
    const type = '@@redux/PROBE_UNKNOWN_ACTION_' + Math.random().toString(36).substring(7).split('').join('.')
    if (typeof reducer(undefined, { type }) === 'undefined') {
      throw new Error(
        `Reducer "${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.
 */

// 導出combineReducers方法，接受一個參數reducers對象
export default function combineReducers(reducers) {
  // 獲取reducers對象的key值
  const reducerKeys = Object.keys(reducers)
  // 定義一個最終要返回的reducers對象
  const finalReducers = {}
  // 遍歷這個reducers對象的key
  for (let i = 0; i < reducerKeys.length; i++) {
    // 緩存每一個key值
    const key = reducerKeys[i]

    if (process.env.NODE_ENV !== 'production') {
      if (typeof reducers[key] === 'undefined') {
        warning(`No reducer provided for key "${key}"`)
      }
    }
    // 相應key的值是個函數，則將改函數緩存到finalReducers中
    if (typeof reducers[key] === 'function') {
      finalReducers[key] = reducers[key]
    }
  }
  // 獲取finalReducers的全部的key值，緩存到變量finalReducerKeys中
  const finalReducerKeys = Object.keys(finalReducers)

  let unexpectedKeyCache
  if (process.env.NODE_ENV !== 'production') {
    unexpectedKeyCache = {}
  }
  // 定義一個變量，用於緩存錯誤對象
  let shapeAssertionError
  try {
    // 作錯誤處理，詳情看後面assertReducerShape方法
    // 主要就是檢測，
    assertReducerShape(finalReducers)
  } catch (e) {
    shapeAssertionError = e
  }

  return function combination(state = {}, action) {
    // 若是有錯誤，則拋出錯誤
    if (shapeAssertionError) {
      throw shapeAssertionError
    }

    if (process.env.NODE_ENV !== 'production') {
      // 獲取警告提示
      const warningMessage = getUnexpectedStateShapeWarningMessage(state, finalReducers, action, unexpectedKeyCache)
      if (warningMessage) {
        warning(warningMessage)
      }
    }
    // 定義一個變量來表示state是否已經被改變
    let hasChanged = false
    // 定義一個變量，來緩存改變後的state
    const nextState = {}
    // 開始遍歷finalReducerKeys
    for (let i = 0; i < finalReducerKeys.length; i++) {
      // 獲取有效的reducer的key值
      const key = finalReducerKeys[i]
      // 根據key值獲取對應的reducer函數
      const reducer = finalReducers[key]
      // 根據key值獲取對應的state模塊
      const previousStateForKey = state[key]
      // 執行reducer函數，獲取相應模塊的state
      const nextStateForKey = reducer(previousStateForKey, action)
      // 若是獲取的state是undefined,則拋出錯誤
      if (typeof nextStateForKey === 'undefined') {
        const errorMessage = getUndefinedStateErrorMessage(key, action)
        throw new Error(errorMessage)
      }
      // 將獲取到的新的state賦值給新的state對應的模塊，key則爲當前reducer的key
      nextState[key] = nextStateForKey
      // 判讀state是否發生改變
      hasChanged = hasChanged || nextStateForKey !== previousStateForKey
    }
    // 若是state發生改變則返回新的state，不然返回原來的state
    return hasChanged ? nextState : state
  }
}

接下來咱們在看看bindActionCreators.js這個文件

首先先認識actionCreators,簡單來講就是建立action的方法，redux的action是一個對象，而咱們常用一些函數來建立這些對象，則這些函數就是actionCreators

而這個文件實現的功能，是根據綁定的actionCreator，來實現自動dispatch的功能

import warning from './utils/warning'
// 對於每一個actionCreator方法，執行以後都會獲得一個action
// 這個bindActionCreator方法，會返回一個可以自動執行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.
 */
// 對外暴露這個bindActionCreators方法
export default function bindActionCreators(actionCreators, dispatch) {
  // 若是傳入的actionCreators參數是個函數，則直接調用bindActionCreator方法
  if (typeof actionCreators === 'function') {
    return bindActionCreator(actionCreators, dispatch)
  }
  // 錯誤處理
  if (typeof actionCreators !== 'object' || actionCreators === null) {
    throw new Error(
      `bindActionCreators expected an object or a function, instead received ${actionCreators === null ? 'null' : typeof actionCreators}. ` +
      `Did you write "import ActionCreators from" instead of "import * as ActionCreators from"?`
    )
  }
  // 若是actionCreators是一個對象，則獲取對象中的key
  const keys = Object.keys(actionCreators)
  // 定義一個緩存對象
  const boundActionCreators = {}
  // 遍歷actionCreators的每一個key
  for (let i = 0; i < keys.length; i++) {
    // 獲取每一個key
    const key = keys[i]
    // 根據每一個key獲取特定的actionCreator方法
    const actionCreator = actionCreators[key]
    // 若是actionCreator是一個函數，則直接調用bindActionCreator方法，將返回的匿名函數緩存到boundActionCreators對象中
    if (typeof actionCreator === 'function') {
      boundActionCreators[key] = bindActionCreator(actionCreator, dispatch)
    } else {
      warning(`bindActionCreators expected a function actionCreator for key '${key}', instead received type '${typeof actionCreator}'.`)
    }
  }
  // 最後返回boundActionCreators對象
  // 用戶獲取到這個對象後，可拿出對象中的每一個key的對應的值，也就是各個匿名函數，執行匿名函數就能夠實現dispatch功能
  return boundActionCreators
}

接下來咱們看看applyMiddleware.js這個文件，這個文件讓redux有着無限多的可能性。爲何這麼說呢，你往下看就知道了

// 這個文件的代碼邏輯其實很簡單
// 首先導入compose函數，等一下咱們會詳細分析這個compose函數
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.
 */
 // 接下來導出applyMiddleware這個方法，這個方法也是咱們常常用來做爲createStore中enhance參數的一個方法
export default function applyMiddleware(...middlewares) {
  // 首先先返回一個匿名函數，有沒有發現這個函數跟createStore很類似啊
  // 沒錯其實他就是咱們的以前看到的createStore
  return (createStore) => (reducer, preloadedState, enhancer) => {
    // 首先用原來的createStore建立一個store，並把它緩存起來
    const store = createStore(reducer, preloadedState, enhancer)
    // 獲取store中原始的dispatch方法
    let dispatch = store.dispatch
    // 定一個執行鏈數組
    let chain = []
    // 緩存原有store中getState和dispatch方法
    const middlewareAPI = {
      getState: store.getState,
      dispatch: (action) => dispatch(action)
    }
    // 執行每一箇中間件函數，並將middlewareAPI做爲參數傳入，得到一個執行鏈數組
    chain = middlewares.map(middleware => middleware(middlewareAPI))
    // 將執行鏈數組傳入compose方法中，並當即執行返回的方法得到最後包裝事後的dispatch
    // 這個過程簡單來講就是，每一箇中間件都會接受一個store.dispatch方法，而後基於這個方法進行包裝，而後返回一個新的dispatch
    // 這個新的dispatch又做爲參數傳入下一個中間件函數，而後有進行包裝。。。一直循環這個過程，直到最後獲得一個最終的dispatch
    dispatch = compose(...chain)(store.dispatch)
    // 返回一個store對象，並將新的dispatch方法覆蓋原有的dispatch方法
    return {
      ...store,
      dispatch
    }
  }
}

看到這裏，其實你已經看完了大部分redux的內容，最後咱們看看上述文件中使用到的compose方法是如何實現的。

打開compose.js，咱們發現其實實現方式就是利用es5中數組的reduce方法來實現這種效果的

/**
 * 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) {
  // 判斷函數數組是否爲空
  if (funcs.length === 0) {
    return arg => arg
  }
  // 若是函數數組只有一個元素，則直接執行
  if (funcs.length === 1) {
    return funcs[0]
  }

  // 不然，就利用reduce方法執行每一箇中間件函數，並將上一個函數的返回做爲下一個函數的參數
  return funcs.reduce((a, b) => (...args) => a(b(...args)))
}

哈哈，以上就是今天給你們分享的redux源碼分析~但願你們可以喜歡咯