上一章，我们讲解了createStore。下面，我们来看一下combineReducer。 在redux中，我们禁止在应用中创建多个store(我们这里默认讨论的都是客户端应用，同构应用不适用这条规则)。

然而，随着应用变得越来越复杂，需要对 reducer 函数 进行拆分，拆分后的每一块独立负责管理 state 的一部分。

combineReducer就是将一个由多个reducer函数作为value的object，合并成一个rootReducer。然后就可以对这个reducer调用createStore。

下面，我们来具体看一下源码：

import { ActionTypes } from './createStore'
import isPlainObject from 'lodash/isPlainObject'
import warning from './utils/warning'function getUndefinedStateErrorMessage(key, action) {const actionType = action && action.typeconst 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. ` +`If you want this reducer to hold no value, you can return null instead of undefined.`)
}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.`)}
}
/*** 遍历reducers中的reducer，检查reducer是否是符合redux规范的reducer。* 使用ActionTypes.INIT和随机type生成的action作为第二个参数，分别进行检验* 如果返回的state为undefined，则不符合redux规范。* @param {Object} reducers */
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 ${ActionTypes.INIT} or other actions in "redux/*" ` +`namespace. They are considered private. Instead, you must return the ` +`current state for any unknown actions, unless it is undefined, ` +`in which case you must return the initial state, regardless of the ` +`action type. The initial state may not be undefined, but can be null.`)}})
}export default function combineReducers(reducers) {//第一次筛选，将reducers中不是function 的键值对给筛选掉。const reducerKeys = Object.keys(reducers)const finalReducers = {}for (let i = 0; i < reducerKeys.length; i++) {const key = reducerKeys[i]if (process.env.NODE_ENV !== 'production') {if (typeof reducers[key] === 'undefined') {warning(`No reducer provided for key "${key}"`)}}if (typeof reducers[key] === 'function') {finalReducers[key] = reducers[key]}}//第二次筛选，检测`finalReducers`中是否有不符合`redux`规范的`reducer`const finalReducerKeys = Object.keys(finalReducers)let unexpectedKeyCacheif (process.env.NODE_ENV !== 'production') {unexpectedKeyCache = {}}let shapeAssertionErrortry {assertReducerShape(finalReducers)} catch (e) {shapeAssertionError = e}return function combination(state = {}, action) {//如果刚才检测 finalReducers 发现了错误，则抛出错误。if (shapeAssertionError) {throw shapeAssertionError}//如果不是production环境则抛出warningif (process.env.NODE_ENV !== 'production') {const warningMessage = getUnexpectedStateShapeWarningMessage(state, finalReducers, action, unexpectedKeyCache)if (warningMessage) {warning(warningMessage)}}let hasChanged = falseconst nextState = {}//遍历所有的reducer，分别执行，将其计算出的state组合起来生成一个大的state.// 所以，任何action，redux都会遍历所有的reducer.for (let i = 0; i < finalReducerKeys.length; i++) {const key = finalReducerKeys[i]const reducer = finalReducers[key]//为每一个reducer计算一个state.const previousStateForKey = state[key]const nextStateForKey = reducer(previousStateForKey, action)//如果计算出来的state有undefined，抛出错误.if (typeof nextStateForKey === 'undefined') {const errorMessage = getUndefinedStateErrorMessage(key, action)throw new Error(errorMessage)}//将每一个reducer计算出来的state合并成一个大的state.nextState[key] = nextStateForKey//只要有一个reducer计算出来的state和之前的不一样，就表明状态树改变了。hasChanged = hasChanged || nextStateForKey !== previousStateForKey}return hasChanged ? nextState : state}
}
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combineReducer的代码其实还是很简单。首先它会经过两次筛选，第一次筛选将reducers中value值不是function的键值对都剔除掉，第二次筛选将reducers中不符合redux规范的reducer给筛选掉。

那么，什么是符合redux规范的reducer呢？ 我们来看一下assertReducerShape函数： 遍历reducers中的reducer，检查reducer是否是符合redux规范的reducer。 使用ActionTypes.INIT和随机type生成的action作为第二个参数，分别进行检验 如果返回的state为undefined，则不符合redux规范。

接下来设置一个flag叫做hasChanged，默认是false。然后遍历reducers中的所有reducer，分别计算这些子reducer，并将其返回的子state结合成一个大的state。比较计算出来的子state与计算之前的子state是否相同，如果不同，则将hasChanged设为true。 hasChanged = hasChanged || nextStateForKey !== previousStateForKey 只要有一个reducer计算出来的state和之前的不一样，就表明状态树改变了

最后，通过判断hasChanged是否变化，返回nextState或state。