import { ActionTypes } from './createStore'
import isPlainObject from 'lodash/isPlainObject'
import warning from './utils/warning'

function getUndefinedStateErrorMessage(key, action) {
  var actionType = action && action.type
  var actionName = actionType && `"${actionType.toString()}"` || 'an action'

  return (
    `Given action ${actionName}, reducer "${key}" returned undefined. ` +
    `To ignore an action, you must explicitly return the previous state.`
  )
}

function getUnexpectedStateShapeWarningMessage(inputState, reducers, action) {
  var reducerKeys = Object.keys(reducers)
  var argumentName = action && action.type === ActionTypes.INIT ?
    'initialState 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('", "')}"`
    )
  }

  var unexpectedKeys = Object.keys(inputState).filter(key => !reducers.hasOwnProperty(key))

  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.`
    )
  }
}

function assertReducerSanity(reducers) {
  Object.keys(reducers).forEach(key => {
    var reducer = reducers[key]
    var 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.`
      )
    }

    var 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.`
      )
    }
  })
}

/**
 * Turns an object whose values are different reducer functions, into a single
 * reducer function. It will call every child reducer, and gather their results
 * into a single state object, whose keys correspond to the keys of the passed
 * reducer functions.
 *
 * @param {Object} reducers An object whose values correspond to different
 * reducer functions that need to be combined into one. One handy way to obtain
 * it is to use ES6 `import * as reducers` syntax. The reducers may never return
 * undefined for any action. Instead, they should return their initial state
 * if the state passed to them was undefined, and the current state for any
 * unrecognized action.
 *
 * @returns {Function} A reducer function that invokes every reducer inside the
 * passed object, and builds a state object with the same shape.
 */
export default function combineReducers(reducers) {
  var reducerKeys = Object.keys(reducers)
  var finalReducers = {}
  for (var i = 0; i < reducerKeys.length; i++) {
    var key = reducerKeys[i]
    if (typeof reducers[key] === 'function') {
      finalReducers[key] = reducers[key]
    }
  }
  var finalReducerKeys = Object.keys(finalReducers)

  var sanityError
  try {
    assertReducerSanity(finalReducers)
  } catch (e) {
    sanityError = e
  }

  return function combination(state = {}, action) {
    if (sanityError) {
      throw sanityError
    }

    if (process.env.NODE_ENV !== 'production') {
      var warningMessage = getUnexpectedStateShapeWarningMessage(state, finalReducers, action)
      if (warningMessage) {
        warning(warningMessage)
      }
    }

    var hasChanged = false
    var nextState = {}
    for (var i = 0; i < finalReducerKeys.length; i++) {
      var key = finalReducerKeys[i]
      var reducer = finalReducers[key]
      var previousStateForKey = state[key]
      var nextStateForKey = reducer(previousStateForKey, action)
      if (typeof nextStateForKey === 'undefined') {
        var errorMessage = getUndefinedStateErrorMessage(key, action)
        throw new Error(errorMessage)
      }
      nextState[key] = nextStateForKey
      hasChanged = hasChanged || nextStateForKey !== previousStateForKey
    }
    return hasChanged ? nextState : state
  }
}
 

一片段一片段的分析：

var reducerKeys = Object.keys(reducers)
  var finalReducers = {}
  for (var i = 0; i < reducerKeys.length; i++) {
    var key = reducerKeys[i]
    if (typeof reducers[key] === 'function') {
      finalReducers[key] = reducers[key]
    }
  }
  var finalReducerKeys = Object.keys(finalReducers)
 

該片段是將reducers物件的所有key取出並獲取其對應的reducer

 var sanityError
  try {
    assertReducerSanity(finalReducers)
  } catch (e) {
    sanityError = e
  }

assertReducerSanity函式原始碼：

Object.keys(reducers).forEach(key => {
    var reducer = reducers[key]
    var 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.`
      )
    }

    var 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.`
      )
    }
  })
 

該片段是對reducer做合法性檢驗，檢驗的形式是①reducer(undefined, { type: ActionTypes.INIT })，先檢驗initialState返回的是不是undefined；②reducer(undefined, { type })，檢驗隨機的一個type是否會返回undefined。

之後的片段是返回一個函式，該函式會逐個呼叫每一個reducer，並返回一個單獨的物件state，物件的形式是key與Reducers的key保持一致。這個返回的函式才是最終的reducer，因此dispatch的時候是呼叫了這個函式，這樣的話每次都會呼叫每一個小的reducer（可以自己斷點來驗證）,驗證結果如圖所示：

可以看到每次dispatch的時候兩個reducer都會被呼叫到，其中ADD_TODO在todos reducer中有case匹配到，會根據傳進來的action得到新的state，而ADD_TODO在visibility reducer中沒有case匹配到，會到default語句中，然後返回原來的結果。這樣就又合成新的state了，這就是combineReducer的機理。

前面4句是combineReducers函式內部呼叫的，主要是呼叫assertReducerSanity產生的結果，後面兩句是createStore函式內部呼叫dispatch({type:ActionTypes.INIT })產生的結果

 for (var i = 0; i < finalReducerKeys.length; i++) {
      var key = finalReducerKeys[i]
      var reducer = finalReducers[key]
      var previousStateForKey = state[key]
      var nextStateForKey = reducer(previousStateForKey, action)
      if (typeof nextStateForKey === 'undefined') {
        var errorMessage = getUndefinedStateErrorMessage(key, action)
        throw new Error(errorMessage)
      }
      nextState[key] = nextStateForKey
      hasChanged = hasChanged || nextStateForKey !== previousStateForKey
}

這部分的程式碼的意思是每一個reducer(previousStateForKey, action)返回的值就是新的state[key]，比如todoReducer返回的值就是state[todoReducer]的值，最終每一個reducer給state一個key,最後組成一個完整的state.

state的形式會是這樣的：

{
    todods:......
    visible:......
}

注意1：state只有一個，所以這個state就是createStore創建出來的state。react-redux的connect中mapStateToProps的state引數就是這個state。state的屬性有幾個是根據combineReducers({reducer1,reducer2})中的引數有幾個reducer決定的。

注意2：var reducer = finalReducers[key]   var previousStateForKey = state[key]   var nextStateForKey = reducer(previousStateForKey, action) 這三句話表明到具體某一個reducer執行時，傳進去的是state對應key的值。比如todos Reducer傳進去的是state.todos，visibility Reducer傳進去的是state.visibility

最後在createStore(combineReducer({reducer1,reducer2},initialState)的時候內部還會呼叫一個dispatch({ type: ActionTypes.INIT })，具體可參考createStore原始碼：https://github.com/reactjs/redux/blob/master/src/createStore.js。

附此部分原始碼：

// 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.
  dispatch({ type: ActionTypes.INIT })

  return {
    dispatch,
    subscribe,
    getState,
    replaceReducer
  }