Redux 源码解读（长文慎入）

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() {}
// 就是根据 isCrushed 是否被压缩了，来警告开发者正在非生产环境使用一个压缩过的代码。
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 looseenvify (https://github.com/zertosh/looseenvify) for browserify ' +
    'or DefinePlugin for webpack (http://stackoverflow.com/questions/30030031) ' +
    'to ensure you have the correct code for your production build.'
  )
}

export {
  createStore,
  combineReducers,
  bindActionCreators,
  applyMiddleware,
  compose
}

export default function createStore(reducer, preloadedState, enhancer) {
  if (typeof preloadedState === 'function' && typeof enhancer === 'undefined') {
    enhancer = preloadedState
    preloadedState = undefined
  }

  if (typeof enhancer !== 'undefined') {
    if (typeof enhancer !== 'function') {
      throw new Error('Expected the enhancer to be a function.')
    }
    return enhancer(createStore)(reducer, preloadedState)
  }
  
  if (typeof reducer !== 'function') {
    throw new Error('Expected the reducer to be a function.')
  }

let currentReducer = reducer
let currentState = preloadedState
let currentListeners = []
let nextListeners = currentListeners
let isDispatching = false

function ensureCanMutateNextListeners() {
  if (nextListeners === currentListeners) {
    nextListeners = currentListeners.slice()
  }
}

/**
  * Reads the state tree managed by the store.
  *
  * @returns {any} The current state tree of your application.
  */
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.
    */  
function subscribe(listener) {
    if (typeof listener !== 'function') {
      throw new Error('Expected listener to be a function.')
    }

    let isSubscribed = true

    ensureCanMutateNextListeners()
    nextListeners.push(listener)

    return function unsubscribe() {
      if (!isSubscribed) {
        return
      }

      isSubscribed = false

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

function dispatch(action) {
  if (!isPlainObject(action)) {
    throw new Error(
      'Actions must be plain objects. ' +
      'Use custom middleware for async actions.'
    )
  }

  if (typeof action.type === 'undefined') {
    throw new Error(
      'Actions may not have an undefined "type" property. ' +
      'Have you misspelled a constant?'
    )
  }

  if (isDispatching) {
    throw new Error('Reducers may not dispatch actions.')
  }

  try {
    isDispatching = true
    currentState = currentReducer(currentState, action)
  } finally {
    isDispatching = false
  }

  const listeners = currentListeners = nextListeners
  for (let i = 0; i < listeners.length; i++) {
    const listener = listeners[i]
    listener()
  }

  return action
}

if (isDispatching) {
  throw new Error('Reducers may not dispatch actions.')
}

/**
  * 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}
  */
function replaceReducer(nextReducer) {
  if (typeof nextReducer !== 'function') {
    throw new Error('Expected the nextReducer to be a function.')
  }

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

export const ActionTypes = {
  INIT: '@@redux/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/proposalobservable
  */
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({ type: ActionTypes.INIT })

return {
  dispatch,
  subscribe,
  getState,
  replaceReducer,
  [$$observable]: observable
}

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) {
  const reducerKeys = Object.keys(reducers)
  const argumentName = action && action.type === ActionTypes.INIT ?
    'preloadedState argument passed to createStore' :
    'previous state received by the reducer'

  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.'
    )
  }

  if (!isPlainObject(inputState)) {
    return (
      `The ${argumentName} has unexpected type of "` +
      ({}).toString.call(inputState).match(/\s([az|AZ]+)/)[1] +
      `". Expected argument to be an object with the following ` +
      `keys: "${reducerKeys.join('", "')}"`
    )
  }

  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.`
    )
  }
}

function assertReducerShape(reducers) {
  Object.keys(reducers).forEach(key => {
    const reducer = reducers[key]
    const initialState = reducer(undefined, { type: ActionTypes.INIT })

    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.`
      )
    }

    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.
  */
export default function combineReducers(reducers) {
  const reducerKeys = Object.keys(reducers)
  const finalReducers = {}
  for (let i = 0; i < reducerKeys.length; i++) {
    const key = reducerKeys[i]

    if (process.env.NODE_ENV !== 'production') {
      if (typeof reducers[key] === 'undefined') {
        warning(`No reducer provided for key "${key}"`)
      }
    }

    if (typeof reducers[key] === 'function') {
      finalReducers[key] = reducers[key]
    }
  }
  const finalReducerKeys = Object.keys(finalReducers)

  let unexpectedKeyCache
  if (process.env.NODE_ENV !== 'production') {
    unexpectedKeyCache = {}
  }

  let shapeAssertionError
  try {
    assertReducerShape(finalReducers)
  } catch (e) {
    shapeAssertionError = e
  }

  return function combination(state = {}, action) {
    if (shapeAssertionError) {
      throw shapeAssertionError
    }

    if (process.env.NODE_ENV !== 'production') {
      const warningMessage = getUnexpectedStateShapeWarningMessage(state, finalReducers, action, unexpectedKeyCache)
      if (warningMessage) {
        warning(warningMessage)
      }
    }

    let hasChanged = false
    const nextState = {}
    for (let i = 0; i < finalReducerKeys.length; i++) {
      const key = finalReducerKeys[i]
      const reducer = finalReducers[key]
      const previousStateForKey = state[key]
      const nextStateForKey = reducer(previousStateForKey, action)
      if (typeof nextStateForKey === 'undefined') {
        const errorMessage = getUndefinedStateErrorMessage(key, action)
        throw new Error(errorMessage)
      }
      nextState[key] = nextStateForKey
      hasChanged = hasChanged || nextStateForKey !== previousStateForKey
    }
    return hasChanged ? nextState : state
  }
}

let hasChanged = false
const nextState = {}
// 遍历 finalReducerKeys
for (let i = 0; i < finalReducerKeys.length; i++) {
// 拿到当前的 reducer key
    const key = finalReducerKeys[i]
// 根据 reducer key 拿到具体的 reducer 函数
      const reducer = finalReducers[key]
// 获取之前的 key 对应的 state
      const previousStateForKey = state[key]
// 计算下一个当前 key 对应的 state
      const nextStateForKey = reducer(previousStateForKey, action)
// 如果计算出来的 state 为 undefined 那么报错
      if (typeof nextStateForKey === 'undefined') {
        const errorMessage = getUndefinedStateErrorMessage(key, action)
        throw new Error(errorMessage)
      }
// 把当前 key 对应的 state 赋值到下一个全局 state
      nextState[key] = nextStateForKey
// 只要有一个 key 对应的 state 发生了变化，那么就认为整个 state 发生了变化
      hasChanged = hasChanged || nextStateForKey !== previousStateForKey
}
// 根据 state 是否发生变化，返回下一个 state 或者上一个 state
    return hasChanged ? nextState : state
  }

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 `reduxthunk` 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.
  */
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
    }
  }
}

export default function createStore(reducer, preloadedState, enhancer) {
  if (typeof preloadedState === 'function' && typeof enhancer === 'undefined') {
    enhancer = preloadedState
    preloadedState = undefined
  }

  if (typeof enhancer !== 'undefined') {
    if (typeof enhancer !== 'function') {
      throw new Error('Expected the enhancer to be a function.')
    }

    return enhancer(createStore)(reducer, preloadedState)
  }

const store = createStore(
  reducer,
  applyMiddleware(...middleware)
)

return enhancer(createStore)(reducer, preloadedState)

// applyMiddleware
const store = createStore(reducer, preloadedState, enhancer)

//applyMiddleware
dispatch = compose(...chain)(store.dispatch)

const middlewareAPI = {
      getState: store.getState,
      dispatch: (action) => dispatch(action)
}
// 把 middlewareAPI 传入到每个中间件中
chain = middlewares.map(middleware => middleware(middlewareAPI))

return {
  ...store,
  dispatch
}

export default ({dispatch, getState}) => next => action => {
  const {error, payload} = action
  if (error) {
    dispatch(showToast(payload.message || payload.toString()))
  }
  return next(action)
}

const mid1 = () => next => action => {
  console.log('mid1 before')
  next(action)
  console.log('mid1 after')
}
const mid2 = () => next => action => {
  console.log('mid2 before')
  next(action)
  console.log('mid2 after')
}
const mid3 = () => next => action => {
  console.log('mid3 before')
  next(action)
  console.log('mid3 after')
}

dispatch = compose(...chain)(store.dispatch)

dispatch = (store.dispatch) => mid1(mid2(mid3(store.dispatch)))

mid1 before
mid2 before
mid3 before
mid3 after
mid2 after
mid1 after

const mid2 = ({ dispatch, getStore }) => next => action => {
  console.log('mid2 before')
  dispatch(action)
  console.log('mid2 after')
}

const mid2 = ({ dispatch, getStore }) => next => action => {
  console.log('mid2 before')
  if(action.isApi) {
    dispatch({
      isApi: false,
      ...
    })
  }
  dispatch(action)
  console.log('mid2 after')
}

export default ({dispatch, getState}) => {
return next => action => {
  const {
    types,
    api,
    callType,
    meta,
    body,
    shouldCallAPI
  } = action
  const state = getState()
  const callTypeList = ['get', 'post']
  if (!api) {
    return next(action)
  }
  if (!(types.start && types.success && types.failure)) {
    throw new Error('Expected types has start && success && failure keys.')
  }
  if (callTypeList.indexOf(callType) === 1) {
    throw new Error(`API callType Must be one of ${callTypeList}`)
  }

  const {start, success, failure} = types
  if (!shouldCallAPI(state)) {
    return false
  }

  dispatch({
    type: start,
    payload: {
      ...body
    },
    meta
  })
  const mapCallTypeToFetch = {
      post: () => fetch(api, {
        method: 'post',
        // credentials 设置为每次请求都带上 cookie
        credentials: 'include',
        headers: {
          'Accept': 'application/json',
          'Content-Type': 'application/json'
        },
        body: JSON.stringify(bodyWithSource)
      }),
      get: () => {
        const toString = Object.keys(bodyWithSource).map(function (key, index) {
          return encodeURIComponent(key) + '=' + encodeURIComponent(bodyWithSource[key])
        }).join('&')
        return fetch(`${api}?${toString}`, {
          method: 'get',
          credentials: 'include',
          headers: {
            'Accept': 'application/json'
          }
        })
      }
    }
    const fetching = mapCallTypeToFetch[callType]()
... 省略一堆业务逻辑
  return fetching.then(res => {
    clearTimeout(loadingTimer)
    dispatch(hideLoading())
    if (res.ok) {
      try {
        return res.json()
      } catch (err) {
        throw new Error(err)
      }
    } else {
      dispatch(showToast('请求出错'))
      return Promise.reject(res.text())
    }
  })
    .then(res => resBehaviour(res))
    .then(res => {
      dispatch({
        type: success,
        meta,
        payload: {
          ...res.data
        }
      })
      return Promise.resolve(res)
    })
    .catch(err => {
      console.error(`接口请求出错,${err}`)
      return Promise.reject(err)
    })
}