// 主函數(shù)createStore
// 返回一個store對象
export default function createStore(reducer, preloadedState, enhancer) {
  // 增強(qiáng)器
  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
  // 獲取最終的state
  function getState() {
    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
  }
  // dispatch
  // 參數(shù)action
  function dispatch(action) {
      // 校驗傳入的action
    // action必須是個對象，否則拋出錯誤信息
    if (!isPlainObject(action)) {
      throw new Error(
        'Actions must be plain objects. ' +
          'Use custom middleware for async actions.'
      )
    }
    // 檢驗action對象的必要屬性
    // type屬性是action對象必要的屬性
    // 如果傳入的action沒有type屬性，則拋出錯誤信息
    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
      // 執(zhí)行傳入的reducer函數(shù)
      // 返回state，給currentState賦值
      currentState = currentReducer(currentState, action)
    } finally {
        // 一個dispatch執(zhí)行完，還原狀態(tài)
      isDispatching = false
    }
    // 執(zhí)行訂閱函數(shù)隊列
    // dispatch執(zhí)行的同時會一并執(zhí)行訂閱隊列
    const listeners = (currentListeners = nextListeners)
    for (let i = 0; i < listeners.length; i++) {
      const listener = listeners[i]
      listener()
    }
    // 返回action
    return action
  }
  // 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.
  // 默認(rèn)執(zhí)行一次dispatch，做初始化
  dispatch({ type: ActionTypes.INIT })
  // 返回一個store對象
  return {
    dispatch,
    subscribe,
    getState,
    ...
  }
}

import createStore from 'redux'
// 創(chuàng)建一個reducer
function reducer(state={}, action) {
    switch(action.type) {
        case 'TEST':
        return {
            ...state,
            test: 'test success'
        }
    }
}
// 返回store
const store = createStore(reducer, initState={})
// 執(zhí)行dispatch
store.dispatch({
    type: 'TEST'
})
const state = store.getState() // 返回 {test: 'TEST'}

function createThunkMiddleware(extraArgument) {
    // 中間件規(guī)定格式
    // 閉包返回三層嵌套
  return ({ dispatch, getState }) => next => action => {
    if (typeof action === 'function') {
      return action(dispatch, getState, extraArgument);
    }
    return next(action);
  };
}
const thunk = createThunkMiddleware();
thunk.withExtraArgument = createThunkMiddleware;
export default thunk;

import createStore, {<!--{cke_protected}{C}%3C!%2D%2D%20%2D%2D%3E-->applyMiddleWare} from 'reduximport reduxThunk from 'redux-thunk'// 創(chuàng)建一個reducerfunction reducer(state={}, action) { switch(action.type) { case 'TEST': return { ...state, test: 'test success' } }}// 返回store// 中間件作為applyMiddleWare的參數(shù)傳入createStoreconst store = createStore(reducer, initState={},applyMiddleWare(reduxThunk))

export default function createStore(reducer, preloadedState, enhancer) {
  ...
  // 增強(qiáng)器
  // 第三個參數(shù)是enhancer，也就是我們傳入的applyMiddleWare
  if (typeof enhancer !== 'undefined') {
    if (typeof enhancer !== 'function') {
      throw new Error('Expected the enhancer to be a function.')
    }
    // 在這里return了enhancer結(jié)果
    // 傳入了createStore,reducer,preloadedState
    // 實際上是重寫了createStore
    return enhancer(createStore)(reducer, preloadedState)
  }
  ...
}

import compose from './compose'
// 傳入middlewares中間件
export default function applyMiddleware(...middlewares) {
  // 閉包嵌套返回2個方法
  return createStore => (...args) => {
      // 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.'
      )
    }
    // 返回一個對象
    // 包含getState方法和dispatch方法
    const middlewareAPI = {
      getState: store.getState,
      dispatch: (...args) => dispatch(...args) // 返回一個全新的dispatch方法，不污染原來的dispatch
    }
    // 執(zhí)行中間件第一層方法
    // 回顧下中間的格式：({getState, dispatch}) => next => action => next(action)
    // 這里會比較繞
    const chain = middlewares.map(middleware => middleware(middlewareAPI)) // 返回一個中間件的函數(shù)集合[next => action => next(action), next => action => next(action)]
    // 使用compose聚合chain函數(shù)集合
    // 返回新的dispatch
    dispatch = compose(...chain)(store.dispatch)
    return {
      ...store,
      dispatch
    }
  }
}

/** * 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))). */
// 傳入聚合函數(shù)集合
// 集合為：[next => action => next(action), next => action => next(action)]
// 返回一個新的函數(shù): (arg) => arg  
export default function compose(...funcs) {
  // 判斷如果沒有則返回一個新函數(shù)
  // 可以聯(lián)想一下dispatch的定義
  // function dispatch(action) {
      ...
      return action
  }
  if (funcs.length === 0) {
    return arg => arg
  }
  // 判斷如果只有一個中間件，則直接返回第一個
  if (funcs.length === 1) {
    return funcs[0]
  }
  // 這里用了reduce函數(shù)
  // 把后一個的中間件的結(jié)果當(dāng)成參數(shù)傳遞給下一個中間件
  // 函數(shù)列表的每個函數(shù)執(zhí)行后返回的還是一個函數(shù)：action => next(action)
  // 這個函數(shù)就是新的dispatch
  // 最后返回函數(shù)：(...args) => action => args(action)
  return funcs.reduce((a, b) => (...args) => a(b(...args)))
}

// 判斷值是否是對象類型
function isPlainObject(obj) {
    if(!obj) {
        reutrn false
    }
    return Object.prototype.toString.call(obj) === '[object, Object]'
}
export default createStore(reducer, enhancer) {
    // 先判斷有沒有傳入中間件
    // 有則之間返回
    if(typeof enhancer !== 'undefined') {
        // 必需是個函數(shù)
        // 因為需要傳參
        if(typeof enhancer !== 'function') {
            return
        }
        return enhancer(createStore)(reducer)
    }
    let state = {} // 初始化state
    let listeners = [] // 發(fā)布訂閱函數(shù)隊列
    // 定義getState 函數(shù)
    function getState() {
        // 直接返回state
        return state
    }
    // 定義dispatch 函數(shù)
    function dispatch(action) {
        try{
            // 執(zhí)行reducer, 返回state
            state = reducer(state, action)
        }catch(e) {
            console.log('dispatch error: 'e)
        } 
        // 訂閱
        listeners.forEach(listener => listener())
        // 返回action
        return action
    }
    // 定義subscribe 函數(shù)
    function subscribe(listener) {
        if(!listener) {
            return
        }
        // 必需是回掉函數(shù)
        // 因為需要在dispatch里執(zhí)行
        if(typeof listener !== 'function') {
            return
        }
        Listeners.push(listener)
    }
    // 返回對象：包含getState, dispatch, subscribe 三個方法
    return {
        getState,
        dispatch,
        subscribe
    }
}

    function compose(...funs) {
        if(!funs) {
            return arg => arg
        }
        if(funs.length === 1) {
            return funs[0]
        }
        // 遍歷傳入函數(shù)，返回一個新函數(shù)
        return funs.reduce((a,b) => (...args) => a(b(...args)))
    }

import compose from './compose'
function applyMiddleWare(...middlewares) {
    return createStore => reducer => {
        // 先返回一個store
        const store = createStore(reducer)
        // 創(chuàng)建middleApi
        const middleApi = {
            getState: store.getState,
            dispatch: (...args) => store.dispatch(...args) // 返回一個新的dispatch
        }
        // 注入middleApi
        // 并返回函數(shù)集合
        const chain = middlewares.map(middleWare => middleWare(middleApi))
        // 通過compose函數(shù)，執(zhí)行所有中間件，并返回一個新的dispatch
        const dispatch = compose(...chain)(store.dispatch)
        // 返回store對象
        return {
            getState: store.getState,
            dispatch
        }
    }
}

    function logger({getState, dispatch}) {
        return function(next) {
            return function(action) {
                console.log('prev')
                next(action)
                console.log('done')
            }
        }
    }

    import createStore from './myCreateStore'
    import applyMiddleWare from './myApplyMiddleWare'
    import logger from './logger'
    // 創(chuàng)建reducer
    function reducer(state={}, action) {
        switch(action.type) {
            case 'TEST':
            return {
                ...state,
                test: 'test success'
            }
        }
    }
    // 引入中間件
    const middleware = applyMiddleWare(logger)
    const store = createStore(reducer, middleware) // 返回{getState, dispatch}