redux的combineReducers源码,中文翻译

摘要：是这个函数名翻译为获取未定义的错误信息对于返回一定要很明确的返回之前的这样就可以忽略一个如果你想这个没有返回值你可以返回而不是获取与预期不符的的结构警告信息声明结构初始化时返回如果传给的是你一定要很明确地返回初始初始可能是如果你不想给这

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
  //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) //将reducers的key提取为数组
  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的key值复制了一份
      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
  }
}

源码解析请参考https://segmentfault.com/a/11...