Redux 源码探索

|-- src
    |-- applyMiddleware.js  将middleware串联起来生成一个更强大的dispatch函数，就是中间件的本质作用
    |-- bindActionCreators.js  将action creators转换成拥有同名keys的action对象
    |-- combineReducers.js  将多个reducer组合起来，每一个reducer独立管理自己对应的state
    |-- compose.js  函数式编程中的组合函数，在applyMiddleware中调用，将middleware从右向左依次调用，生成一个从左向右执行middleware的dispatch增强函数
    |-- createStore.js  核心功能，创建一个store，实现了4个核心函数，dispatch、subscribe、getState、replaceReducer
    |-- index.js 对外暴露api
    |-- utils 供其他的函数调用的工具函数或变量
        |-- actionTypes.js 内置action type变量，用于初始化/重置 state
        |-- isPlainObject.js 用于校验是否是纯对象
        |-- warning.js 错误提示函数

import __DO_NOT_USE__ActionTypes from './utils/actionTypes'
 
// 仅用于判断代码是否被压缩
function isCrushed() {}
 
// 如果非生产环境压缩了代码会提示用户，使用压缩后的redux代码会降低性能
if (
  process.env.NODE_ENV !== 'production' &&
  typeof isCrushed.name === 'string' &&
  isCrushed.name !== 'isCrushed'
) {
  warning('~~~')
}
 
export {
  createStore,
  combineReducers,
  bindActionCreators,
  applyMiddleware,
  compose,
  __DO_NOT_USE__ActionTypes
}

// src/createStore.js
function createStore(reducer, preloadedState, enhancer) {
  // 这段代码用于判断传递的参数是否符合规范
  if (
    (typeof preloadedState === 'function' && typeof enhancer === 'function') ||
    (typeof enhancer === 'function' && typeof arguments[3] === 'function')
  ) {
    throw new Error(
      'It looks like you are passing several store enhancers to ' +
        'createStore(). This is not supported. Instead, compose them ' +
        'together to a single function.'
    )
  }
  // 判断第二个参数为function时，第三个参数又不存在时，把第二个参数作为enhancer
  if (typeof preloadedState === 'function' && typeof enhancer === 'undefined') {
    enhancer = preloadedState
    preloadedState = undefined
  }
 
  // 如果 enhancer 函数存在，执行 enhancer 函数，并且不再继续执行下去
  if (typeof enhancer !== 'undefined') {
    if (typeof enhancer !== 'function') {
      throw new Error('Expected the enhancer to be a function.')
    }
 
    return enhancer(createStore)(reducer, preloadedState)
  }
  // reducer是必选参数且必须是一个函数
  if (typeof reducer !== 'function') {
    throw new Error('Expected the reducer to be a function.')
  }
 
  // 声明内部使用变量
  // 存储值
  let currentReducer = reducer
  // 设置默认state
  let currentState = preloadedState
  // 存放回调函数的订阅数组
  let currentListeners = []
  // 下一次的订阅数组
  let nextListeners = currentListeners
  // 用于防止在执行reducer函数时再次触发dispatch函数
  let isDispatching = false
 
  /**
   * 浅拷贝一份currentListeners 为下一阶段监听器提供快照
   * 用于防止在触发订阅/取消订阅回调函数时出现错误
   */
  function ensureCanMutateNextListeners() {
    if (nextListeners === currentListeners) {
      nextListeners = currentListeners.slice()
    }
  }
 
  /**
   * 获取state
   * @returns {any} 返回currentState
   */
  function getState() {
    // 正在执行dispatch函数时不能获取，其实一般情况下不会出现这种情况
    if (isDispatching) {
      throw new Error(
        'You may not call store.getState() while the reducer is executing. ' +
          'The reducer has already received the state as an argument. ' +
          'Pass it down from the top reducer instead of reading it from the store.'
      )
    }
 
    return currentState
  }
 
  /**
   * 用于订阅state的更新
   *
   * @param {Function} listener 订阅函数
   * @returns {Function} 返回可以移除listener的函数
   */
  function subscribe(listener) {
    // 传入的listener必须是一个函数
    if (typeof listener !== 'function') {
      throw new Error('Expected the listener to be a function.')
    }
    // 保证纯函数不带来副作用
    if (isDispatching) {
      throw new Error(
        'You may not call store.subscribe() while the reducer is executing. ' +
          'If you would like to be notified after the store has been updated, subscribe from a ' +
          'component and invoke store.getState() in the callback to access the latest state. ' +
          'See https://redux.js.org/api-reference/store#subscribe(listener) for more details.'
      )
    }
 
    let isSubscribed = true
 
    // 只有第一次执行或者每次dispatch之后的第一次subscribe时会把 currentListeners 拷贝并覆盖 nextListeners 中
    ensureCanMutateNextListeners()
    // nextListeners 新增listener，currentListeners并不会新增
    nextListeners.push(listener)
 
    // 取消订阅
    return function unsubscribe() {
      if (!isSubscribed) {
        return
      }
 
      // 保证纯函数不带来副作用
      if (isDispatching) {
        throw new Error(
          'You may not unsubscribe from a store listener while the reducer is executing. ' +
            'See https://redux.js.org/api-reference/store#subscribe(listener) for more details.'
        )
      }
 
      isSubscribed = false
      // 取消订阅后 把 currentListeners 拷贝并覆盖 nextListeners 中
      ensureCanMutateNextListeners()
      const index = nextListeners.indexOf(listener)
      // 从nextListeners中删除unsubscribe的listener
      nextListeners.splice(index, 1)
      // 清空当前的订阅数组
      currentListeners = null
    }
  }
 
  /**
   * 用于执行reducer函数的函数
   */
  function dispatch(action) {
    // 保证action是个纯对象，即字面量对象或Object创建的对象
    if (!isPlainObject(action)) {
      throw new Error(
        'Actions must be plain objects. ' +
          'Use custom middleware for async actions.'
      )
    }
    // action必须有type属性存在
    if (typeof action.type === 'undefined') {
      throw new Error(
        'Actions may not have an undefined "type" property. ' +
          'Have you misspelled a constant?'
      )
    }
    // 正在执行reducer函数时不允许再次触发
    if (isDispatching) {
      throw new Error('Reducers may not dispatch actions.')
    }
 
    try {
      isDispatching = true
      // 执行reducer
      currentState = currentReducer(currentState, action)
    } finally {
      isDispatching = false
    }
    // 通知所有之前通过subscribe订阅state更新的回调listener
    const listeners = (currentListeners = nextListeners)
    for (let i = 0; i < listeners.length; i++) {
      const listener = listeners[i]
      listener()
    }
 
    return action
  }
 
  /**
   * 用于替换当前存储的reducer函数
   * 比如当你需要进行代码拆分或者想要动态加载一些reducer、想要为redux实现热重载时
   *
   * @param {Function} nextReducer 用于替换 store 中的 reducer
   * @returns {void}
   */
  function replaceReducer(nextReducer) {
    if (typeof nextReducer !== 'function') {
      throw new Error('Expected the nextReducer to be a function.')
    }
 
    currentReducer = nextReducer
 
    // ActionTypes.REPLACE其实就是ActionTypes.INIT
    // 重新INIT依次是为了获取新的reducer中的默认参数
    dispatch({ type: ActionTypes.REPLACE })
  }
 
  // 用于观察和响应的互通 私有属性，暂时并未不做扩充
  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' || observer === null) {
          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
      }
    }
  }
 
  // reducer要求对无法识别的action返回state，就是因为需要通过ActionTypes.INIT获取默认参数值并返回
  // 当initailState和reducer的参数默认值都存在的时候，reducer参数默认值将不起作用
  // 因为在createStore声明变量时currState就已经被赋值了initialState
  // 同时这个initialState也是服务端渲染的初始状态入口
  dispatch({ type: ActionTypes.INIT })
 
  return {
    dispatch,
    subscribe,
    getState,
    replaceReducer,
    [?observable]: observable
  }
}
 

// 判断第二个参数为function时，第三个参数又不存在时，把第二个参数作为enhancer
if (typeof preloadedState === 'function' && typeof enhancer === 'undefined') {
  enhancer = preloadedState
  preloadedState = undefined
}
 
// 如果 enhancer 函数存在，执行 enhancer 函数，并且不再继续执行下去
if (typeof enhancer !== 'undefined') {
  if (typeof enhancer !== 'function') {
    throw new Error('Expected the enhancer to be a function.')
  }
 
  return enhancer(createStore)(reducer, preloadedState)
}

// 用于
function getUndefinedStateErrorMessage(key, action) {
  const actionType = action && action.type
  const actionDescription =
    (actionType && `action "${String(actionType)}"`) || 'an action'
  // 即使没有值应该返回null，而不要返回undefined
  return (
    `Given ${actionDescription}, reducer "${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)
  // 判断actionType是来自redux内部init type还是其他的
  const argumentName =
    action && action.type === ActionTypes.INIT
      ? 'preloadedState argument passed to createStore'
      : 'previous state received by the reducer'
 
  // 监测是否没有传递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.'
    )
  }
  // state必须是个纯对象
  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combineReducer合并的reducer个数是否和finalReducers个数是否相同
  // 并获取不相同的key
  const unexpectedKeys = Object.keys(inputState).filter(
    key => !reducers.hasOwnProperty(key) && !unexpectedKeyCache[key]
  )
  // 标记不相同的key
  unexpectedKeys.forEach(key => {
    unexpectedKeyCache[key] = true
  })
  // 当是调用replaceReducers时，就不需要再判断，直接返回
  if (action && action.type === ActionTypes.REPLACE) return
  // 告诉你有什么值是多出来的，会被忽略掉
  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.`
    )
  }
}
// 用来判断初始化和随机状态下返回的是不是 undefined
function assertReducerShape(reducers) {
  Object.keys(reducers).forEach(key => {
    // 遍历 reducer
    const reducer = reducers[key]
    // 初始化 reducer，得到一个state值
    const initialState = reducer(undefined, { type: ActionTypes.INIT })
    // 初始化状态下 state 不能返回undefined
    if (typeof initialState === 'undefined') {
      throw new Error(
        `Reducer "${key}" returned undefined during initialization. ` +
          `If the state passed to the reducer is undefined, you must ` +
          `explicitly return the initial state. The initial state may ` +
          `not be undefined. If you don't want to set a value for this reducer, ` +
          `you can use null instead of undefined.`
      )
    }
    // 随机状态下 state 也能返回undefined
    if (
      typeof reducer(undefined, {
        type: ActionTypes.PROBE_UNKNOWN_ACTION()
      }) === 'undefined'
    ) {
      throw new Error(
        `Reducer "${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.`
      )
    }
  })
}

export default function combineReducers(reducers) {
  // 获取传入reducers的所有key
  const reducerKeys = Object.keys(reducers)
  // 最后真正有效的reducer存在这里
  const finalReducers = {}
  // 筛选出有效的reducer
  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}"`)
      }
    }
    // reducer必须是函数，无效的数据不会被合并进来
    if (typeof reducers[key] === 'function') {
      finalReducers[key] = reducers[key]
    }
  }
  // 所有可用reducer
  const finalReducerKeys = Object.keys(finalReducers)
 
  // This is used to make sure we don't warn about the same
  // keys multiple times.
  // 用于配合getUnexpectedStateShapeWarningMessage辅助函数过滤掉多出来的值
  let unexpectedKeyCache
  if (process.env.NODE_ENV !== 'production') {
    unexpectedKeyCache = {}
  }
 
  let shapeAssertionError
  try {
    // 校验reducers是否都是有效数据
    assertReducerShape(finalReducers)
  } catch (e) {
    shapeAssertionError = e
  }
 
  // 返回一个合并后的 reducer 函数，与普通的 reducer 一样
  // 主要逻辑：取得每个子reducer对应的state，与action一起作为参数给每个子reducer执行。
  return function combination(state = {}, action) {
    if (shapeAssertionError) {
      throw shapeAssertionError
    }
 
    if (process.env.NODE_ENV !== 'production') {
      // 开发环境下校验有哪些值是多出来的
      const warningMessage = getUnexpectedStateShapeWarningMessage(
        state,
        finalReducers,
        action,
        unexpectedKeyCache
      )
      if (warningMessage) {
        warning(warningMessage)
      }
    }
 
    let hasChanged = false // 标志state是否有变化
    const nextState = {}
    // 下面就会进行获取reducer对应的state，与action一起作为参数给每个子reducer执行。
    for (let i = 0; i < finalReducerKeys.length; i++) {
      // 获取本次循环的子reducer
      const key = finalReducerKeys[i]
      const reducer = finalReducers[key]
      // 获取该子reducer对应的旧的state
      const previousStateForKey = state[key]
      // 该子reducer执行后返回的新的state
      const nextStateForKey = reducer(previousStateForKey, action)
      // 如果新的state是undefined就抛出对应错误
      if (typeof nextStateForKey === 'undefined') {
        const errorMessage = getUndefinedStateErrorMessage(key, action)
        throw new Error(errorMessage)
      }
      // 用新的state替换旧的state
      nextState[key] = nextStateForKey
      // 判断state是否改变
      hasChanged = hasChanged || nextStateForKey !== previousStateForKey
    }
    // 改变后返回新的state，未改变返回旧的值
    return hasChanged ? nextState : state
  }
}

function applyMiddleware(...middlewares) {
  // middlewares 为传入的一系列中间件，等待调用
  // 这里返回的就是enhancer，enhancer会接收createStore并返回一个函数
  // 在createStore中，当存在enhancer时，拦截了createStore下面需要执行的代码，
  // 返回执行enhancer(createStore)(reducer, preloadedState)的结果
  return createStore =>
    (...args) => {
      // 执行enhancer(createStore)(reducer, preloadedState)是会调用下面代码
      // 继续执行createStore，返回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.'
        )
      }
      // 传递给中间件使用的api
      const middlewareAPI = {
        getState: store.getState,
        dispatch: (...args) => dispatch(...args)
      }
      // 遍历并执行中间件函数，把执行结果存放于chain
      // 每一个中间件函数的返回值是一个改造的dispatch函数
      const chain = middlewares.map(middleware => middleware(middlewareAPI))
      // 用compose去改造dispatch函数，得到一个增强的dispatch函数
      dispatch = compose(...chain)(store.dispatch)
      // compose(func1,func2,func3)
      // 返回一个函数: (...args) => func1( func2( func3(...args) ) )
      // 传入的dispatch被func3改造后得到一个新的dispatch，新的dispatch继续被func2改造.
      // 因此每个中间件内部必须执行dispatch才会继续下一个中间件的调用
 
      // 继续返回store，并用加强版的dispatch覆盖旧的dispatch
      return {
        ...store,
        dispatch
      }
    }
}

// 返回一个高阶函数
function bindActionCreator(actionCreator, dispatch) {
  return function () {
    // 执行actionCreator，获取action，再用dispatch进行派发action
    return dispatch(actionCreator.apply(this, arguments))
  }
}
 
export default function bindActionCreators(actionCreators, dispatch) {
  // actionCreators如果是函数，就认为就是一个actionCreator函数，
  // 直接调用bindActionCreator转换生成一个可以用dispatch派发的action
  if (typeof actionCreators === 'function') {
    return bindActionCreator(actionCreators, dispatch)
  }
  // 传入的只能是对象或者函数，函数在上面已经进行操作了
  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"?`
    )
  }
  // 这里处理actionCreators是一组包含actionCreator的对象时
  const boundActionCreators = {}
  for (const key in actionCreators) {
    const actionCreator = actionCreators[key]
    if (typeof actionCreator === 'function') {
      // 每一个key再次调用bindActionCreator进行转换
      boundActionCreators[key] = bindActionCreator(actionCreator, dispatch)
    }
  }
  // 返回处理后的对象
  return boundActionCreators
}

function compose(...funcs) {
  // 没有参数的话直接返回一个函数组件
  // compose()(a) -> a
  // 先返回一个返回自身参数的函数 -> 函数执行,返回a
  if (funcs.length === 0) {
    return arg => arg
  }
 
  // 只有一个函数参数时返回该函数参数
  // compose(withA)(a) -> withA(a)
  // 先返回一个withA函数 -> 函数执行,并且参数为a
  if (funcs.length === 1) {
    return funcs[0]
  }
 
  // 用reduce遍历funcs，并且形成最终需要执行的函数
  // compose(withA, withB, withC, withD)(a)
  // -> withA(withB(withC(withD(a))))
  return funcs.reduce((a, b) => {
    return (...args) => a(b(...args))
  })
  // 当a，b参数为withA，withB时， return (...args) -> withA(withB(...args))
  // 当a，b参数为上一轮返回函数，withC时，
  // return (...args2) -> (...args) => withA(withB(...args))(withC(...args2))
  // 执行结果为：
  // (...args2) => withA(withB(withC))(withC(...args2))
  // ... 持续执行,最终结果返回一个函数，函数的参数放在funcs最后一个函数：
  // (...argsN) => withA(withB(withC(...withN(...argsN))))
}