Redux:百行代码千行文档

　　接触Redux不过短短半年，从开始看官方文档的一头雾水，到渐渐已经理解了Redux到底是在做什么，但是绝大数场景下Redux都是配合React一同使用的，因而会引入了React-Redux库，但是正是因为React-Redux库封装了大量方法，使得我们对Redux的理解变的开始模糊。这篇文章将会在Redux源码的角度分析Redux，希望你在阅读之前有部分Redux的基础。

　　上图是Redux的流程图，具体的不做介绍，不了解的同学可以查阅一下Redux的官方文档。写的非常详细。下面的代码结构为Redux的master分支:

├── applyMiddleware.js
├── bindActionCreators.js
├── combineReducers.js
├── compose.js
├── createStore.js
├── index.js
└── utils

└── warning.js

Redux中src文件夹下目录如上所示,文件名基本就是对应我们所熟悉的Redux的API，首先看一下index.js中的代码:

/*
* 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.'
  )
}

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

　　上面的代码非常的简单了，只不过是把所有的方法对外导出。其中isCrushed是用来检查函数名是否已经被压缩(minification)。如果函数当前不是在生产环境中并且函数名被压缩了，就提示用户。process是Node 应用自带的一个全局变量，可以获取当前进程的若干信息。在许多前端库中，经常会使用 process.env.NODE_ENV这个环境变量来判断当前是在开发环境还是生产环境中。这个小例子我们可以get到一个hack的方法，如果判断一个js函数名时候被压缩呢？我们可以先预定义一个虚函数(虽然JavaScript中没有虚函数一说，这里的虚函数(dummy function)指代的是没有函数体的函数),然后判断执行时的函数名是否和预定义的一样，就像上面的代码：

function isCrushed() {}
if(typeof isCrushed.name === 'string' && isCrushed.name !== 'isCrushed'){
    //has minified
}

compose

 　　从易到难，我们在看一个稍微简单的对外方法compose

/**
 * 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) {
  if (funcs.length === 0) {
    return arg => arg
  }

  if (funcs.length === 1) {
    return funcs[0]
  }

  return funcs.reduce((a, b) => (...args) => a(b(...args)))
}

 　　理解这个函数之前我们首先看一下reduce方法，这个方法我是看了好多遍现在仍然是印象模糊，虽然之前介绍过reduce,但是还是再次回忆一下Array.prototye.reduce:

The reduce() method applies a function against an accumulator and each element in the array (from left to right) to reduce it to a single value.

 　　reduce()函数对一个累加值和数组中的每一个元素(从左到右)应用一个函数，将其reduce成一个单值，例如:

var sum = [0, 1, 2, 3].reduce(function(acc, val) {
  return acc + val;
}, 0);
// sum is 6

 　　reduce()函数接受两个参数:一个回调函数和初始值，回调函数会被从左到右应用到数组的每一个元素，其中回调函数的定义是

/**
 * accumulator: 累加器累加回调的值，它是上一次调用回调时返回的累积值或者是初始值
 * currentValue: 当前数组遍历的值
 * currenIndex: 当前元素的索引值
 * array: 整个数组
 */
function (accumulator,currentValue,currentIndex,array){
    
}

　　现在回头看看compose函数都在做什么，compose函数从左到右组合(compose)多个单参函数。最右边的函数可以按照定义接受多个参数，如果compose的参数为空，则返回一个空函数。如果参数长度为1，则返回函数本身。如果函数的参数为数组，这时候我们返回

  return funcs.reduce((a, b) => (...args) => a(b(...args)))

　　我们知道reduce函数返回是一个值。上面函数传入的回调函数是(a, b) => (...args) => a(b(...args))其中a是当前的累积值，b是数组中当前遍历的值。假设调用函数的方式是compose(f,g,h),首先第一次执行回调函数时，a的实参是函数f,b的实参是g,第二次调用的是，a的实参是(...args) => f(g(...args)),b的实参是h，最后函数返回的是(...args) =>x(h(...args)),其中x为(...args) => f(g(...args))，所以我们最后可以推导出运行compose(f,g,h)的结果是(...args) => f(g(h(...args)))。发现了没有，这里其实通过reduce实现了reduceRight的从右到左遍历的功能，但是却使得代码相对较难理解。在Redux 1.0.1版本中compose的实现如下:

export default function compose(...funcs) {
     return funcs.reduceRight((composed, f) => f(composed));
}

　　这样看起来是不是更容易理解compose函数的功能。

bindActionCreators

　　bindActionCreators也是Redux中非常常见的API，主要实现的就是将ActionCreator与dispatch进行绑定，看一下官方的解释:

Turns an object whose values are action creators, into an object with the same keys, but with every action creator wrapped into a dispatch call so they may be invoked directly.

　　翻译过来就是bindActionCreators将值为actionCreator的对象转化成具有相同键值的对象，但是每一个actionCreator都会被dispatch所包裹调用，因此可以直接使用。话不多说，来看看它是怎么实现的：

import warning from './utils/warning'

function bindActionCreator(actionCreator, dispatch) {
  return (...args) => dispatch(actionCreator(...args))
}

export default function bindActionCreators(actionCreators, dispatch) {
  if (typeof actionCreators === 'function') {
    return bindActionCreator(actionCreators, dispatch)
  }

  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"?`
    )
  }

  const keys = Object.keys(actionCreators)
  const boundActionCreators = {}
  for (let i = 0; i < keys.length; i++) {
    const key = keys[i]
    const actionCreator = actionCreators[key]
    if (typeof actionCreator === 'function') {
      boundActionCreators[key] = bindActionCreator(actionCreator, dispatch)
    } else {
      warning(`bindActionCreators expected a function actionCreator for key '${key}', instead received type '${typeof actionCreator}'.`)
    }
  }
  return boundActionCreators
}

　　对于处理单个actionCreator的方法是

function bindActionCreator(actionCreator, dispatch) {
  return (...args) => dispatch(actionCreator(...args))
}

　　代码也是非常的简单，无非是返回一个新的函数，该函数调用时会将actionCreator返回的纯对象进行dispatch。而对于函数bindActionCreators首先会判断actionCreators是不是函数,如果是函数就直接调用bindActionCreator。当actionCreators不是对象时会抛出错误。接下来:

  const keys = Object.keys(actionCreators)
  const boundActionCreators = {}
  for (let i = 0; i < keys.length; i++) {
    const key = keys[i]
    const actionCreator = actionCreators[key]
    if (typeof actionCreator === 'function') {
      boundActionCreators[key] = bindActionCreator(actionCreator, dispatch)
    } else {
      warning(`bindActionCreators expected a function actionCreator for key '${key}', instead received type '${typeof actionCreator}'.`)
    }
  }
  return boundActionCreators

　　这段代码也是非常简单，甚至我觉得我都能写出来，无非就是对对象actionCreators中的所有值调用bindActionCreator,然后返回新的对象。恭喜你，又解锁了一个文件~

applyMiddleware

　　applyMiddleware是Redux Middleware的一个重要API,这个部分代码已经不需要再次解释了，没有看过的同学戳这里Redux:Middleware你咋就这么难，里面有详细的介绍。

createStore

　　createStore作为Redux的核心API,其作用就是生成一个应用唯一的store。其函数的签名为:

function createStore(reducer, preloadedState, enhancer) {}

　　前两个参数非常熟悉，reducer是处理的reducer纯函数，preloadedState是初始状态，而enhancer使用相对较少，enhancer是一个高阶函数，用来对原始的createStore的功能进行增强。具体我们可以看一下源码:

具体代码如下:

import isPlainObject from 'lodash/isPlainObject'
import $$observable from 'symbol-observable'

export const ActionTypes = {
  INIT: '@@redux/INIT'
}

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()
    }
  }


  function getState() {
    return currentState
  }

  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
  }

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

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

  function observable() {
    const outerSubscribe = subscribe
    return {
      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
      }
    }
  }

  dispatch({ type: ActionTypes.INIT })

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

我们来逐步解读一下:

  if (typeof preloadedState === 'function' && typeof enhancer === 'undefined') {
    enhancer = preloadedState
    preloadedState = undefined
  }

　　我们发现如果没有传入参数enhancer，并且preloadedState的值又是一个函数的话，createStore会认为你省略了preloadedState，因此第二个参数就是enhancer。

  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.')
  }

　　如果你传入了enhancer但是却又不是函数类型。会抛出错误。如果传入的reducer也不是函数，抛出相关错误。接下来才是createStore重点,初始化:

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

　　currentReducer是用来存储当前的reducer函数。currentState用来存储当前store中的数据，初始化为默认的preloadedState,currentListeners用来存储当前的监听者。而isDispatching用来当前是否属于正在处理dispatch的阶段。然后函数声明了一系列函数，最后返回了:

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

　　显然可以看出来返回来的函数就是store。比如我们可以调用store.dispatch。让我们依次看看各个函数在做什么。

dispatch

  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
  }

　　我们看看dispath做了什么，首先检查传入的action是不是纯对象，如果不是则抛出异常。然后检测，action中是否存在type，不存在也给出相应的错误提示。然后判断isDispatching是否为true，主要是预防的是在reducer中做dispatch操作，如果在reduder中做了dispatch,而dispatch又必然会导致reducer的调用，就会造成死循环。然后我们将isDispatching置为true，调用当前的reducer函数，并且返回新的state存入currentState,并将isDispatching置回去。最后依次调用监听者store已经发生了变化，但是我们并没有将新的store作为参数传递给监听者，因为我们知道监听者函数内部可以通过调用唯一获取store的函数store.getState()获取最新的store。

getState

  function getState() {
    return currentState
  }

　　实在太简单了，自行体会。

replaceReducer

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

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

　　replaceReducer的使用相对也是非常少的，主要用户热更新reducer。

subscribe

  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)
    }
  }

　　subscribe用来订阅store变化的函数。首先判断传入的listener是否是函数。然后又调用了ensureCanMutateNextListeners,

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

　　可以看到ensureCanMutateNextListeners用来判断nextListeners和currentListeners是否是完全相同，如果相同(===)，将nextListeners赋值为currentListeners的拷贝(值相同，但不是同一个数组)，然后将当前的监听函数传入nextListeners。最后返回一个unsubscribe函数用来移除当前监听者函数。需要注意的是，isSubscribed是以闭包的形式判断当前监听者函数是否在监听，从而保证只有第一次调用unsubscribe才是有效的。但是为什么会存在nextListeners呢？

　　首先可以在任何时间点添加listener。无论是dispatchaction时，还是state值正在发生改变的时候。但是需要注意的，在每一次调用dispatch之前，订阅者仅仅只是一份快照(snapshot),如果是在listeners被调用期间发生订阅(subscribe)或者解除订阅(unsubscribe),在本次通知中并不会立即生效，而是在下次中生效。因此添加的过程是在nextListeners中添加的订阅者，而不是直接添加到currentListeners。然后在每一次调用dispatch的时候都会做:

const listeners = currentListeners = nextListeners

来同步currentListeners和nextListeners。

### observable

　　该部分不属于本次文章讲解到的内容，主要涉及到RxJS和响应异步Action。以后有机会(主要是我自己搞明白了)，会单独讲解。

## combineReducers

　　combineReducers的主要作用就是将大的reducer函数拆分成一个个小的reducer分别处理，看一下它是如何实现的:

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
  }
}

　　首先，通过一个for循环去遍历参数reducers,将对应值为函数的属性赋值到finalReducers。然后声明变量unexpectedKeyCache,如果在非生产环境，会将其初始化为{}。然后执行assertReducerShape(finalReducers)，如果抛出异常会将错误信息存储在shapeAssertionError。我们看一下shapeAssertionError在做什么？

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.`
      )
    }
  })
}

可以看出assertReducerShape函数的主要作用就是判断reducers中的每一个reducer在action为{ type: ActionTypes.INIT }时是否有初始值，如果没有则会抛出异常。并且会对reduer执行一次随机的action,如果没有返回，则抛出错误，告知你不要处理redux中的私有的action,对于未知的action应当返回当前的stat。并且初始值不能为undefined但是可以是null。

　　接着我们看到combineReducers返回了一个combineReducers函数:

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
}

在combination函数中我们首先对shapeAssertionError中可能存在的异常进行处理。接着，如果是在开发环境下，会执行getUnexpectedStateShapeWarningMessage,看看getUnexpectedStateShapeWarningMessage是如何定义的：

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 ${argumentName} has unexpected type of "` +
      ({}).toString.call(inputState).match(/\s([a-z|A-Z]+)/)[1] +
      `". Expected argument to be an object with the following ` +
      `keys: "${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.`
    )
  }
}

　　我们简要地看看getUnexpectedStateShapeWarningMessage处理了哪几种问题:

1. reducer中是不是存在reducer

2. state是否是纯Object对象

3. state中存在reducer没有处理的项，但是仅会在第一次提醒，之后就忽略了。

然后combination执行其核心部分代码:

    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

　　使用变量nextState记录本次执行reducer返回的state。hasChanged用来记录前后state是否发生改变。循环遍历reducers，将对应的store的部分交给相关的reducer处理，当然对应各个reducer返回的新的state仍然不可以是undefined。最后根据hasChanged是否改变来决定返回nextState还是state,这样就保证了在不变的情况下仍然返回的是同一个对象。

　　最后，其实我们发现Redux的源码非常的精炼，也并不复杂，但是Dan Abramov能从Flux的思想演变到现在的Redux思想也是非常不易，希望此篇文章使得你对Redux有更深的理解。