如果看不清楚,可以查看https://segmentfault.com/a/1190000018394259
import { ActionTypes } from './createStore'
import isPlainObject from 'lodash/isPlainObject'
import warning from './utils/warning'
/**
* ActionTypes 是这个
* export const ActionTypes = {
* INIT: '@@redux/INIT'
* }
*/
function getUndefinedStateErrorMessage(key, action) { //函数名翻译为获取未定义的state错误信息
const actionType = action && action.type
const 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.`
)
//对于action xxx ,reducer yyy 返回undefined
//你一定要很明确的返回之前的state,这样就可以忽略一个action
//如果你想这个reducer没有返回值,你可以返回null而不是undefined
}
//获取与预期不符的state的结构警告信息
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.`
)
}
}
//声明reducer结构
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 "${key}" returned undefined during initialization. ` +
`If the state passed to the reducer is undefined, you must ` +
`explicitly return the initial state. The initial state may ` +
`not be undefined. If you don't want to set a value for this reducer, ` +
`you can use null instead of undefined.`
)
//reducer xxx 初始化时返回undefined, 如果传给reducer的state是undefined,你一定要
//很明确地返回初始state, 初始state可能是undefined, 如果你不想给这个reducer
//设置value值,你可以用null代替undefined
}
const type = '@@redux/PROBE_UNKNOWN_ACTION_' + Math.random().toString(36).substring(7).split('').join('.')
if (typeof reducer(undefined, { type }) === 'undefined') {
throw new Error(
`Reducer "${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.`
)
//当probed(探索)随机的type时,reducer xxx 返回undefined. 不要在"redux/*"命名空间操作
// ${ActionTypes.INIT},也就是'@@redux/INIT', 或任意的action.他们是私有的.
//相反,对于未知的action,你应该返回当前的state,除非它是undefined.不管action的type是什么,
//你都应该返回初始的state,出示的state可能不是undefined,但可以是null
}
})
}
/**
* Turns an object whose values are different reducer functions, into a single
* reducer function. It will call every child reducer, and gather their results
* into a single state object, whose keys correspond to the keys of the passed
* reducer functions.
*
* 将一个value值是不同reducer函数的对象变成一个单一的reducer函数,它将会调用每一个子reducer
* 将它们的结果组合成一个单一的state对象,这个对象的key对应传进来的reducer的key
*
* @param {Object} reducers An object whose values correspond to different
* reducer functions that need to be combined into one. One handy way to obtain
* it is to use ES6 `import * as reducers` syntax. The reducers may never return
* undefined for any action. Instead, they should return their initial state
* if the state passed to them was undefined, and the current state for any
* unrecognized action.
*
* reducers是一个对应不同reducer函数的对象,这些reducer函数需要组合成一个reducer.
* 一个很方便获取到它的方法就是使用ES6 的`import * as reducers`语法,reducer可能不会
* 返回undefined.相反,它们应该返回初始的state. 如果传给它们的state是undefined,任何
* 不被识别的action都会返回当前的state
*
* @returns {Function} A reducer function that invokes every reducer inside the
* passed object, and builds a state object with the same shape.
*
* 返回一个reducer函数,会触发传进来的对象中的每一个reducer,建立一个有相同结构的state对象
*/
export default function combineReducers(reducers) {
const reducerKeys = Object.keys(reducers)
const finalReducers = {}
for (let i = 0; i < reducerKeys.length; i++) {
const key = reducerKeys[i]
if (process.env.NODE_ENV !== 'production') {
if (typeof reducers[key] === 'undefined') {
warning(`No reducer provided for key "${key}"`)
}
}
if (typeof reducers[key] === 'function') {
finalReducers[key] = reducers[key]
}
}
const finalReducerKeys = Object.keys(finalReducers)
let unexpectedKeyCache
if (process.env.NODE_ENV !== 'production') {
unexpectedKeyCache = {}
}
let shapeAssertionError
try {
assertReducerShape(finalReducers)
} catch (e) {
shapeAssertionError = e
}
return function combination(state = {}, action) {
if (shapeAssertionError) {
throw shapeAssertionError
}
if (process.env.NODE_ENV !== 'production') {
const warningMessage = getUnexpectedStateShapeWarningMessage(state, finalReducers, action, unexpectedKeyCache)
if (warningMessage) {
warning(warningMessage)
}
}
let hasChanged = false
const nextState = {}
for (let i = 0; i < finalReducerKeys.length; i++) { //finalReducerKeys就是reducers复制了一份
const key = finalReducerKeys[i] //第 i 个key
const reducer = finalReducers[key] //key所对应的reducer
const previousStateForKey = state[key] //把key作为属性赋给state
const nextStateForKey = reducer(previousStateForKey, action) //返回新的state
if (typeof nextStateForKey === 'undefined') {
const errorMessage = getUndefinedStateErrorMessage(key, action)
throw new Error(errorMessage)
}
nextState[key] = nextStateForKey //给nextState添加key属性,并赋值,key与reducer名字相同
hasChanged = hasChanged || nextStateForKey !== previousStateForKey
}
return hasChanged ? nextState : state
}
}