摘要：只有在你需要實現代碼分隔，而且需要立即加載一些的時候才可能會用到它。

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本身預置的私有action types 
 * 對于任何未知的action, 你一定要return當前的state.
 * 如果當前的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,沒有action參數的時候用
  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`.
 *
 * 創建一個包含state tree(狀態樹)的redux store.
 * 唯一改變store中data(數據)的方法是調用`dispatch()`方法.
 * 在你的程序中應該只存在唯一一個store, 來表明state tree各部分怎樣對action做出反應
 * 你可能需要將多個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reducer是一個返回下一個state tree(狀態樹)的函數,來操作當前的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,你可以在universal(一般的,普遍的,我不知道怎么說比較合適)
 * 的程序中與服務器的state結合,或者restore一個預先連續的user session(直譯過來的,一般用不到)
 * 如果你用`combineReducers`產生一個根reducer函數,這一定是一個和`combineReducers`的key一樣的對象(根reducer是一個對象)
 *
 * @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增強器. 可選參數.用來增強第三方庫的能力集(這個詞是直譯),
 * 比如中間件,時空穿越,和持續性(也是直譯).redux的store增強器是`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redux的store,讓你可以讀取state, dispatch actions 和訂閱更改
 */

//createStore的目的只是創建一個store,這個store包含5個方法(一般只用到3個,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是個函數
    }
    // 當enhancer是函數的時候返回,然后執行,并將createStore作為參數傳入,然后createStore就在enhancer里面
    //去執行了
    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不是單個的reducer函數,而是combineReducers函數
  let currentState = preloadedState
  let currentListeners = []  //監聽函數
  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
   * 
   * @returns {any} The current state tree of your application.
   * 
   *返回你的程序的當前的state樹 
   */
  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.
   *
   * 添加一個改變事件,任何時候一個action被dispatch這個事件就會被調用,然后state樹的某一部分就
   * 會改變. 你也可以在回調函數里面調用`getState()`來查看當前的state樹
   * 
   * You may call `dispatch()` from a change listener, with the following
   * caveats:
   *
   * 以下幾種情況你也可以調用`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.
   *
   * 每次調用`dispatch`之前,訂閱都會被snapshot,
   * 當事件被觸發的時候你訂閱或者不訂閱,在當前的進程中都不會對`dispatch`有什么影響
   * 然而當下一次`dispatch`被調用時,無論嵌套與否,將會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.
   *
   * 不要期待監聽事件可以看到所有的狀態改變,因為在事件被調用前,state在嵌套的`dispatch`間
   * 可能已經更新了很多次
   * 
   * @param {Function} listener A callback to be invoked on every dispatch.
   * 每次dispatch都會被出發的回調函數
   * 
   * @returns {Function} A function to remove this change listener.
   * 返回一個移除該事件的函數
   */
  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數組

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

      isSubscribed = false

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

  /**
   * 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,被用來創建store,有當前的state樹和action就會被調用(state和action是reducer函數的參數)
   * 它的返回值會被當做下一個state樹.監聽事件會注意到state樹的改變
   * 
   * 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 為純對象的 action,如果你想要dispatch一個promise,一個Observable,
   * thunk,或是其他東西,你需要封裝store創建一個進入到相應中間件的函數.  比如,看一個`redux-thunk`
   * 的文檔,即使是中間件最終也會用這個方法dispatch  純對象的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是一個純對象,代表"什么被改變了". 保持action的連續性是個好主意,這樣你就可以記錄和
   * 重現user session,或者使用時空穿梭`redux-devtools`. 
   * action必須包含一個`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, 一個異步調用的promise)
   * 
   */
  function dispatch(action) {
    if (!isPlainObject(action)) {
      throw new Error(
        "Actions must be plain objects. " +
        "Use custom middleware for async actions."
      )//actions必須為純對象,使用特定中間件異步調用actions
    }

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

    if (isDispatching) {
      throw new Error("Reducers may not dispatch actions.")//reducer沒有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 當前用來計算 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.
   *
   * 這是一個高級 API。只有在你需要實現代碼分隔，而且需要立即加載一些 reducer 的時候才可能會用到它。
   *在實現 Redux 熱加載機制的時候也可能會用到
   * 
   * @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庫的互用性
   * observable是一個mini的 可觀察state的改變
   * 在下面這個網址查看更多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 任何對象都可以用作觀察者,這個觀察者應該有一個`next`方法
       * subscription 一個有`unsubscribe`方法的對象.可以用做退訂observable,防止進一步發出value值
       * (我也不知道什么意思,外國人說話比較隨意)
       */
      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.
  /** 
   * 當創建一個store的時候,一個初始的action就被dispatch了,所以每個reducer都會返回初始的state
   * 這個可以很高效的得到state樹
  */
  dispatch({ type: ActionTypes.INIT })

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

源碼解析請參考 https://segmentfault.com/a/11...