摘要：只有在你需要實(shí)現(xiàn)代碼分隔，而且需要立即加載一些的時(shí)候才可能會(huì)用到它。

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.
 * 
 * 這些都是redux本身預(yù)置的私有action types 
 * 對(duì)于任何未知的action, 你一定要return當(dāng)前的state.
 * 如果當(dāng)前的state是undefined, 你一定要return最初始的state.
 * 一定,一定,一定不要在代碼中直接引用action types .
 */
export const ActionTypes = {  //初始化action的type,沒(méi)有action參數(shù)的時(shí)候用
  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`.
 *
 * 創(chuàng)建一個(gè)包含state tree(狀態(tài)樹(shù))的redux store.
 * 唯一改變store中data(數(shù)據(jù))的方法是調(diào)用`dispatch()`方法.
 * 在你的程序中應(yīng)該只存在唯一一個(gè)store, 來(lái)表明state tree各部分怎樣對(duì)action做出反應(yīng)
 * 你可能需要將多個(gè)reducer用`combineReducers`組合在一起
 * 
 * @param {Function} reducer A function that returns the next state tree, given
 * the current state tree and the action to handle.
 * 
 * @param {Function} reducer 參數(shù)reducer是一個(gè)返回下一個(gè)state tree(狀態(tài)樹(shù))的函數(shù),來(lái)操作當(dāng)前的state和action
 *
 * @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 {any} [preloadedState] 初始化的state,可選參數(shù),你可以在universal(一般的,普遍的,我不知道怎么說(shuō)比較合適)
 * 的程序中與服務(wù)器的state結(jié)合,或者restore一個(gè)預(yù)先連續(xù)的user session(直譯過(guò)來(lái)的,一般用不到)
 * 如果你用`combineReducers`產(chǎn)生一個(gè)根reducer函數(shù),這一定是一個(gè)和`combineReducers`的key一樣的對(duì)象(根reducer是一個(gè)對(duì)象)
 *
 * @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()`.
 * 
 * @param {Function} [enhancer] store增強(qiáng)器. 可選參數(shù).用來(lái)增強(qiáng)第三方庫(kù)的能力集(這個(gè)詞是直譯),
 * 比如中間件,時(shí)空穿越,和持續(xù)性(也是直譯).redux的store增強(qiáng)器是`applyMiddleware()`
 *
 * @returns {Store} A Redux store that lets you read the state, dispatch actions
 * and subscribe to changes.
 * 
 * @returns {Store} 返回值 一個(gè)redux的store,讓你可以讀取state, dispatch actions 和訂閱更改
 */

//createStore的目的只是創(chuàng)建一個(gè)store,這個(gè)store包含5個(gè)方法(一般只用到3個(gè),最常用的是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.")//期望enhancer是個(gè)函數(shù)
    }
    // 當(dāng)enhancer是函數(shù)的時(shí)候返回,然后執(zhí)行,并將createStore作為參數(shù)傳入,然后createStore就在enhancer里面
    //去執(zhí)行了
    return enhancer(createStore)(reducer, preloadedState)
  }

  if (typeof reducer !== "function") {
    throw new Error("Expected the reducer to be a function.")
  }

  let currentReducer = reducer  //一般此reducer不是單個(gè)的reducer函數(shù),而是combineReducers函數(shù)
  let currentState = preloadedState
  let currentListeners = []  //監(jiān)聽(tīng)函數(shù)
  let nextListeners = currentListeners
  let isDispatching = false

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

  /**
   * Reads the state tree managed by the store.
   *
   * 讀取被store管理的state樹(shù)
   * 
   * @returns {any} The current state tree of your application.
   * 
   *返回你的程序的當(dāng)前的state樹(shù) 
   */
  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.
   *
   * 添加一個(gè)改變事件,任何時(shí)候一個(gè)action被dispatch這個(gè)事件就會(huì)被調(diào)用,然后state樹(shù)的某一部分就
   * 會(huì)改變. 你也可以在回調(diào)函數(shù)里面調(diào)用`getState()`來(lái)查看當(dāng)前的state樹(shù)
   * 
   * You may call `dispatch()` from a change listener, with the following
   * caveats:
   *
   * 以下幾種情況你也可以調(diào)用`dispatch()`
   * 
   * 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.
   *
   * 每次調(diào)用`dispatch`之前,訂閱都會(huì)被snapshot,
   * 當(dāng)事件被觸發(fā)的時(shí)候你訂閱或者不訂閱,在當(dāng)前的進(jìn)程中都不會(huì)對(duì)`dispatch`有什么影響
   * 然而當(dāng)下一次`dispatch`被調(diào)用時(shí),無(wú)論嵌套與否,將會(huì)使用最近的訂閱列表的snapshot
   * 
   * 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.
   *
   * 不要期待監(jiān)聽(tīng)事件可以看到所有的狀態(tài)改變,因?yàn)樵谑录徽{(diào)用前,state在嵌套的`dispatch`間
   * 可能已經(jīng)更新了很多次
   * 
   * @param {Function} listener A callback to be invoked on every dispatch.
   * 每次dispatch都會(huì)被出發(fā)的回調(diào)函數(shù)
   * 
   * @returns {Function} A function to remove this change listener.
   * 返回一個(gè)移除該事件的函數(shù)
   */
  function subscribe(listener) {
    if (typeof listener !== "function") {
      throw new Error("Expected listener to be a function.")
    }

    let isSubscribed = true

    ensureCanMutateNextListeners()
    nextListeners.push(listener) //添加事件到nextListeners數(shù)組

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

      isSubscribed = false

      ensureCanMutateNextListeners()
      const index = nextListeners.indexOf(listener)
      nextListeners.splice(index, 1) //從nextListeners數(shù)組中移除事件
    }
  }

  /**
   * Dispatches an action. It is the only way to trigger a state change.
   *
   * dispatch  action是唯一觸發(fā)state改變的途徑
   * 
   * 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.
   *
   * `reducer`函數(shù),被用來(lái)創(chuàng)建store,有當(dāng)前的state樹(shù)和action就會(huì)被調(diào)用(state和action是reducer函數(shù)的參數(shù))
   * 它的返回值會(huì)被當(dāng)做下一個(gè)state樹(shù).監(jiān)聽(tīng)事件會(huì)注意到state樹(shù)的改變
   * 
   * 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.
   *
   * 最基本的用法是僅支持 為純對(duì)象的 action,如果你想要dispatch一個(gè)promise,一個(gè)Observable,
   * thunk,或是其他東西,你需要封裝store創(chuàng)建一個(gè)進(jìn)入到相應(yīng)中間件的函數(shù).  比如,看一個(gè)`redux-thunk`
   * 的文檔,即使是中間件最終也會(huì)用這個(gè)方法dispatch  純對(duì)象的action
   * 
   * @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.
   *
   * action是一個(gè)純對(duì)象,代表"什么被改變了". 保持action的連續(xù)性是個(gè)好主意,這樣你就可以記錄和
   * 重現(xiàn)user session,或者使用時(shí)空穿梭`redux-devtools`. 
   * action必須包含一個(gè)`type`屬性,即使是`undefined`. 通常使用字符串常量表示
   * 
   * @returns {Object} For convenience, the same action object you dispatched.
   *
   * 為了方便,返回你dispatch的action
   * 
   * Note that, if you use a custom middleware, it may wrap `dispatch()` to
   * return something else (for example, a Promise you can await).
   * 注意 如果你想使用特定的中間件,可封裝`dispatch`返回其他東西(比如, 一個(gè)異步調(diào)用的promise)
   * 
   */
  function dispatch(action) {
    if (!isPlainObject(action)) {
      throw new Error(
        "Actions must be plain objects. " +
        "Use custom middleware for async actions."
      )//actions必須為純對(duì)象,使用特定中間件異步調(diào)用actions
    }

    if (typeof action.type === "undefined") {
      throw new Error(
        "Actions may not have an undefined "type" property. " +
        "Have you misspelled a constant?"
      )//actions可能有一個(gè)未定義的type屬性,你可能拼錯(cuò)了這個(gè)常量
    }

    if (isDispatching) {
      throw new Error("Reducers may not dispatch actions.")//reducer沒(méi)有dispatch action
    }

    try {
      isDispatching = true
      //dispatch的目的就是改變currentState
      currentState = currentReducer(currentState, action) //currentReducer = reducer
    } finally {
      isDispatching = false
    }

    const listeners = currentListeners = nextListeners  //訂閱函數(shù)的事件
    for (let i = 0; i < listeners.length; i++) {
      const listener = listeners[i]
      listener()
    }

    return action
  }

  /**
   * Replaces the reducer currently used by the store to calculate the state.
   *
   * 替換 store 當(dāng)前用來(lái)計(jì)算 state 的 reducer。
   * 
   * 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.
   *
   * 這是一個(gè)高級(jí) API。只有在你需要實(shí)現(xiàn)代碼分隔，而且需要立即加載一些 reducer 的時(shí)候才可能會(huì)用到它。
   *在實(shí)現(xiàn) Redux 熱加載機(jī)制的時(shí)候也可能會(huì)用到
   * 
   * @param {Function} nextReducer The reducer for the store to use instead.
   * store所替換的reducer
   * @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 })
  }

  /**
   * 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
   * 
   * observable/reactive庫(kù)的互用性
   * observable是一個(gè)mini的 可觀察state的改變
   * 在下面這個(gè)網(wǎng)址查看更多observable的信息
   */
  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.
       * 
       * mini的可觀察訂閱的方法
       * observer是 任何對(duì)象都可以用作觀察者,這個(gè)觀察者應(yīng)該有一個(gè)`next`方法
       * subscription 一個(gè)有`unsubscribe`方法的對(duì)象.可以用做退訂observable,防止進(jìn)一步發(fā)出value值
       * (我也不知道什么意思,外國(guó)人說(shuō)話(huà)比較隨意)
       */
      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.
  /** 
   * 當(dāng)創(chuàng)建一個(gè)store的時(shí)候,一個(gè)初始的action就被dispatch了,所以每個(gè)reducer都會(huì)返回初始的state
   * 這個(gè)可以很高效的得到state樹(shù)
  */
  dispatch({ type: ActionTypes.INIT })

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

源碼解析請(qǐng)參考 https://segmentfault.com/a/11...