Redux官方代码库提供了以下几个模块文件：
applyMiddleware.js
bindActionCreators.js
combineReducers.js
compose.js
createStore.js
compose.js
/**
* 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))).
*/
export default function compose(...funcs) {
if (funcs.length ===0) {
return arg => arg
}
if (funcs.length ===1) {
return funcs[0]
}
return funcs.reduce((a, b) => (...args) => a(b(...args)))
}
以上代码很好理解，当compose无参数时，返回一个空函数，参数为唯一函数时，直接将这个函数作为返回值，重点在于最后一部分：
return funcs.reduce((a, b) => (...args) => a(b(...args)))
对多个参数组合成的函数数组进行reduce操作，其实以上代码等同于：
return funcs.reduceRight((composed, f) => f(composed));
相当于对数组内的所有函数，从右至左，将前一个函数作为后一个函数的入口参数依次返回，比如compose(fn1,fn2,fn3)最后返回的结果应该是这样子的：
fn1(fn2(fn3))
bindActionCreators.js
import warning from'./utils/warning'
function bindActionCreator (actionCreator, dispatch) {
return (...args) => dispatch(actionCreator(...args))
}
export default function bindActionCreators (actionCreators, dispatch) {
if (typeof actionCreators ==='function') {
return bindActionCreator(actionCreators, dispatch)
}
if (typeof actionCreators !=='object'|| actionCreators ===null) {
throw new Error(`bindActionCreators expected an object or a function, instead received ${actionCreators === null ? 'null' : typeof actionCreators}. ` + `Did you write "import ActionCreators from" instead of "import * as ActionCreators from"?`)
}
const keys = Object.keys(actionCreators)
const boundActionCreators ={}
for (let i =0; i < keys.length; i++) {
const key = keys[i]
const actionCreator = actionCreators[key]
if (typeof actionCreator ==='function') {
boundActionCreators[key] = bindActionCreator(actionCreator, dispatch)
} else {
warning(`bindActionCreators expected a function actionCreator for key '${key}', instead received type '${typeof actionCreator}'.`)
}
}
return boundActionCreators
}
对于单个actionCreator，代码很简单，直接返回一个被dispatch包裹过的action而已，对于多个actionCreators，如果入口参数是一个function，说明只提供了一个actionCreator，直接调用bindActionCreator(actionCreators,dispatch)，对于以对象形式输入的多个actionCreators，对其遍历输出每一个bindActionCreator(actionCreators,dispatch)并封装在具有同名键值的boundActionCreators对象中，这样在我们需要调用action的地方直接boundActionCreators[actionCreate定义名]就可以了。
createStore.js
//用于校验是否是纯对象
import isPlainObject from'lodash/isPlainObject'
//内部私有属性，暂时不做扩展
import $$observable from'symbol-observable'
//内部action,用于调用所有reducers生成初始state
export const ActionTypes ={INIT:'@@redux/INIT'}
export default function createStore(reducer, preloadedState, enhancer) {
if (typeof preloadedState ==='function' && typeof enhancer ==='undefined') {
enhancer = preloadedState
preloadedState = undefined
}
if (typeof enhancer !=='undefined') {
if (typeof enhancer !=='function') {
throw new Error('Expected the enhancer to be a function.')
}
//函数柯里化，enhancer提供增强版(中间件扩展)的store
return enhancer(createStore)(reducer, preloadedState)
}
//reducer必须是一个function
if (typeof reducer !=='function') {
throw new Error('Expected the reducer to be a function.')
}
//store内部私有变量(外部无法直接访问)
let currentReducer = reducer
let currentState = preloadedState
let currentListeners = []
let nextListeners = currentListeners
let isDispatching = false
//为下一阶段监听器快照提供备份
function ensureCanMutateNextListeners () {
if (nextListeners === currentListeners) {
nextListeners = currentListeners.slice()
}
}
//获取最新state
function getState() {
return currentState
}
//用于订阅state的更新
function subscribe(listener) {
if (typeof listener !=='function') {
throw new Error('Expected listener to be a function.')
}
//保证只有第一次执行unsubscribe()才是有效的，只取消注册当前listener
let isSubscribed =true
//为每次订阅提供快照备份nextListeners，主要防止在遍历执行currentListeners回调
//过程中触发了订阅/取消订阅功能，若直接更新currentListeners将造成当前循环体逻辑混乱
//因此所有订阅/取消订阅的listeners都是在nextListeners中存储的，并不会影响当前的dispatch(action)
ensureCanMutateNextListeners()
nextListeners.push(listener)
//返回一个取消订阅的函数
return function unsubscribe() {
//保证当前listener只被取消注册一次
if (!isSubscribed) { return }
isSubscribed =false
ensureCanMutateNextListeners()
const index = nextListeners.indexOf(listener)
nextListeners.splice(index,1)
}
}
function dispatch(action) {
//保证dispatch是个纯对象，即字面量对象或Object创建的对象
//这是因为原始版的dispatch只支持同步action，约定的格式是纯对象
//可以使用中间件来dispatch扩展功能，增加action的类型种类
if (!isPlainObject(action)) {
throw new Error('Actions must be plain objects. '+'Use custom middleware for async actions.')
}
//action必须要有key为type的动作类型
if (typeof action.type ==='undefined') {
throw new Error('Actions may not have an undefined "type" property. '+'Have you misspelled a constant?')
}
//判断在执行dispatch的过程中是否已存在dispatch的执行流
//保证dispatch中对应的reducer不允许有其他dispatch操作
if (isDispatching) {
throw new Error('Reducers may not dispatch actions.')
}
try {
//根据提供的action，执行根reducer从而更新整颗状态树
isDispatching = true
currentState = currentReducer(currentState, action)
} finally {
isDispatching = false
}
//通知所有之前通过subscribe订阅state更新的回调listener
const listeners = currentListeners = nextListeners
for(let i =0; i < listeners.length; i++) {
const listener = listeners[i]listener()
}
return action
}
//替换当前reducers，如从其他文件引入了新的reducers进行热加载
function replaceReducer (nextReducer) {
if (typeof nextReducer !=='function') {
throw new Error('Expected the nextReducer to be a function.')
}
}
function observable () {
const outerSubscribe = subscribe
return {
subscribe (observer) {
if (typeof observer !=='object') {
throw new TypeError('Expected the observer to be an object.')
}
function observeState() {
if (observer.next) {
observer.next(getState())
}
}
observeState()
const unsubscribe = outerSubscribe(observeState)
return { unsubscribe }
},
[$$observable] () {
return this
}
}
}
dispatch({ type: ActionTypes.INIT })
return {
dispatch,
subscribe,
getState,
replaceReducer,
[$$observable]: observable
}
}
一眼望去，还是有些懵逼的，但如果我们把它划分为以下三个部分分别理解或许就简单多了。
- 入口参数：reducer、preloadState、enhancer
- 内部变量：currentReducer、currentState、currentListeners、nextListeners、isDispatching
- 输出：dispatch、subscribe、getState、replaceReducer
