如何进行Redux的源码解析

如何进行Redux的源码解析，很多新手对此不是很清楚，为了帮助大家解决这个难题，下面小编将为大家详细讲解，有这方面需求的人可以来学习下，希望你能有所收获。

预热

redux 函数内部包含了大量柯里化函数以及代码组合思想

柯里化函数(curry)

通俗的来讲，可以用一句话概括柯里化函数：返回函数的函数

// example const funcA = (a) => {   return const funcB = (b) => {     return a + b   } };

上述的funcA函数接收一个参数，并返回同样接收一个参数的funcB函数。

柯里化函数有什么好处呢?

• 避免了给一个函数传入大量的参数--我们可以通过柯里化来构建类似上例的函数嵌套，将参数的代入分离开，更有利于调试

• 降低耦合度和代码冗余，便于复用

举个栗子：

// 已知listA, listB两个Array，都由int组成，需要筛选出两个Array的交集 const listA = [1, 2, 3, 4, 5]; const listB = [2, 3, 4];  const checkIfDataExist = (list) => {   return (target) => {     return list.some(value => value === target)   }; }; // 调用一次checkIfDataExist函数，并将listA作为参数传入，来构建一个新的函数。 // 而新函数的作用则是：检查传入的参数是否存在于listA里 const ifDataExist = checkIfDataExist(listA);  // 使用新函数来对listB里的每一个元素进行筛选 const intersectionList = listB.filter(value => ifDataExist(value)); console.log(intersectionList); // [2, 3, 4]

代码组合(compose)

代码组合就像是数学中的结合律：

const compose = (f, g) => {   return (x) => {     return f(g(x));   }; }; // 还可以再简洁点 const compose = (f, g) => (x) => f(g(x));

通过这样函数之间的组合，可以大大增加可读性，效果远大于嵌套一大堆的函数调用，并且我们可以随意更改函数的调用顺序

Redux

combineReducers

// 回顾一下combineReducers的使用格式  // 两个reducer const todos = (state = INIT.todos, action) => {   // .... }; const filterStatus = (state = INIT.filterStatus, action) => {   // ... };  const appReducer = combineReducers({   todos,   filterStatus });

还记得combineReducers的黑魔法吗?即：

1. 传入的Object参数中，对象的key与value所代表的reducer function同名

2. 各个reducer function的名称和需要传入该reducer的state参数同名

源码标注解读(省略部分)：

export default function combineReducers(reducers) {   // ***次筛选，参数reducers为Object   // 筛选掉reducers中不是function的键值对   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)    // 二次筛选，判断reducer中传入的值是否合法（!== undefined）   // 获取筛选完之后的所有key   var sanityError   try {     // assertReducerSanity函数用于遍历finalReducers中的reducer，检查传入reducer的state是否合法     assertReducerSanity(finalReducers)   } catch (e) {     sanityError = e   }      // 返回一个function。该方法接收state和action作为参数   return function combination(state = {}, action) {     // 如果之前的判断reducers中有不法值，则抛出错误     if (sanityError) {       throw sanityError     }     // 如果不是production环境则抛出warning     if (process.env.NODE_ENV !== 'production') {       var warningMessage = getUnexpectedStateShapeWarningMessage(state, finalReducers, action)       if (warningMessage) {         warning(warningMessage)       }     }      var hasChanged = false     var nextState = {}     // 遍历所有的key和reducer，分别将reducer对应的key所代表的state，代入到reducer中进行函数调用     for (var i = 0; i < finalReducerKeys.length; i++) {       var key = finalReducerKeys[i]       var reducer = finalReducers[key]       // 这也就是为什么说combineReducers黑魔法--要求传入的Object参数中，reducer function的名称和要和state同名的原因       var previousStateForKey = state[key]       var nextStateForKey = reducer(previousStateForKey, action)       // 如果reducer返回undefined则抛出错误       if (typeof nextStateForKey === 'undefined') {         var errorMessage = getUndefinedStateErrorMessage(key, action)         throw new Error(errorMessage)       }       // 将reducer返回的值填入nextState       nextState[key] = nextStateForKey       // 如果任一state有更新则hasChanged为true       hasChanged = hasChanged || nextStateForKey !== previousStateForKey     }     return hasChanged ? nextState : state   } }  // 检查传入reducer的state是否合法 function assertReducerSanity(reducers) {   Object.keys(reducers).forEach(key => {     var reducer = reducers[key]     // 遍历全部reducer，并给它传入(undefined, action)     // 当***个参数传入undefined时，则为各个reducer定义的默认参数     var initialState = reducer(undefined, { type: ActionTypes.INIT })          // ActionTypes.INIT几乎不会被定义，所以会通过switch的default返回reducer的默认参数。如果没有指定默认参数，则返回undefined，抛出错误     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.`       )     }   }) }

createStore

// 回顾下使用方法 const store = createStore(reducers, state, enhance);

源码标注解读(省略部分)：

// 对于未知的action.type，reducer必须返回默认的参数state。这个ActionTypes.INIT就可以用来监测当reducer传入未知type的action时，返回的state是否合法 export var ActionTypes = {   INIT: '@@redux/INIT' }  export default function createStore(reducer, initialState, enhancer) {   // 检查你的state和enhance参数有没有传反   if (typeof initialState === 'function' && typeof enhancer === 'undefined') {     enhancer = initialState     initialState = undefined   }   // 如果有传入合法的enhance，则通过enhancer再调用一次createStore   if (typeof enhancer !== 'undefined') {     if (typeof enhancer !== 'function') {       throw new Error('Expected the enhancer to be a function.')     }     return enhancer(createStore)(reducer, initialState)   }    if (typeof reducer !== 'function') {     throw new Error('Expected the reducer to be a function.')   }    var currentReducer = reducer   var currentState = initialState   var currentListeners = []   var nextListeners = currentListeners   var isDispatching = false // 是否正在分发事件    function ensureCanMutateNextListeners() {     if (nextListeners === currentListeners) {       nextListeners = currentListeners.slice()     }   }    // 我们在action middleware中经常使用的getState()方法，返回当前state   function getState() {     return currentState   }    // 注册listener，同时返回一个取消事件注册的方法。当调用store.dispatch的时候调用listener   function subscribe(listener) {     if (typeof listener !== 'function') {       throw new Error('Expected listener to be a function.')     }      var isSubscribed = true      ensureCanMutateNextListeners()     nextListeners.push(listener)      return function unsubscribe() {       if (!isSubscribed) {         return       }        isSubscribed = false       // 从nextListeners中去除掉当前listener       ensureCanMutateNextListeners()       var index = nextListeners.indexOf(listener)       nextListeners.splice(index, 1)     }   }    // dispatch方法接收的action是个对象，而不是方法。   // 这个对象实际上就是我们自定义action的返回值，因为dispatch的时候，已经调用过我们的自定义action了，比如 dispatch(addTodo())   function dispatch(action) {     if (!isPlainObject(action)) {       throw new Error(         'Actions must be plain objects. ' +         'Use custom middleware for async actions.'       )     }      if (typeof action.type === 'undefined') {       throw new Error(         'Actions may not have an undefined "type" property. ' +         'Have you misspelled a constant?'       )     }     // 调用dispatch的时候只能一个个调用，通过dispatch判断调用的状态     if (isDispatching) {       throw new Error('Reducers may not dispatch actions.')     }      try {       isDispatching = true       currentState = currentReducer(currentState, action)     } finally {       isDispatching = false     }     // 遍历调用各个linster     var listeners = currentListeners = nextListeners     for (var i = 0; i < listeners.length; i++) {       listeners[i]()     }      return action   }   // Replaces the reducer currently used by the store to calculate the state.   function replaceReducer(nextReducer) {     if (typeof nextReducer !== 'function') {       throw new Error('Expected the nextReducer to be a function.')     }      currentReducer = nextReducer     dispatch({ type: ActionTypes.INIT })   }   // 当create store的时候，reducer会接受一个type为ActionTypes.INIT的action，使reducer返回他们默认的state，这样可以快速的形成默认的state的结构   dispatch({ type: ActionTypes.INIT })    return {     dispatch,     subscribe,     getState,     replaceReducer   } }

thunkMiddleware

源码及其简单简直给跪...

// 返回以 dispatch 和 getState 作为参数的action export default function thunkMiddleware({ dispatch, getState }) {   return next => action => {     if (typeof action === 'function') {       return action(dispatch, getState);     }      return next(action);   }; }

applyMiddleware

先复习下用法：

// usage import {createStore, applyMiddleware} from 'redux'; import thunkMiddleware from 'redux-thunk';  const store = createStore(       reducers,       state,       applyMiddleware(thunkMiddleware) );

applyMiddleware首先接收thunkMiddleware作为参数，两者组合成为一个新的函数(enhance)，之后在createStore内部，因为enhance的存在，将会变成返回enhancer(createStore)(reducer, initialState)

源码标注解读(省略部分)：

// 定义一个代码组合的方法 // 传入一些function作为参数，返回其链式调用的形态。例如， // compose(f, g, h) 最终返回 (...args) => f(g(h(...args))) export default function compose(...funcs) {   if (funcs.length === 0) {     return arg => arg   } else {     const last = funcs[funcs.length - 1]     const rest = funcs.slice(0, -1)     return (...args) => rest.reduceRight((composed, f) => f(composed), last(...args))   } }  export default function applyMiddleware(...middlewares) {   // 最终返回一个以createStore为参数的匿名函数   // 这个函数返回另一个以reducer, initialState, enhancer为参数的匿名函数   return (createStore) => (reducer, initialState, enhancer) => {     var store = createStore(reducer, initialState, enhancer)     var dispatch     var chain = []      var middlewareAPI = {       getState: store.getState,       dispatch: (action) => dispatch(action)     }     // 每个 middleware 都以 middlewareAPI 作为参数进行注入，返回一个新的链。此时的返回值相当于调用 thunkMiddleware 返回的函数： (next) => (action) => {} ，接收一个next作为其参数     chain = middlewares.map(middleware => middleware(middlewareAPI))     // 并将链代入进 compose 组成一个函数的调用链     // compose(...chain) 返回形如(...args) => f(g(h(...args)))，f/g/h都是chain中的函数对象。     // 在目前只有 thunkMiddleware 作为 middlewares 参数的情况下，将返回 (next) => (action) => {}     // 之后以 store.dispatch 作为参数进行注入     dispatch = compose(...chain)(store.dispatch)      return {       ...store,       dispatch     }   } }

一脸懵逼?没关系，来结合实际使用总结一下：

当我们搭配redux-thunk这个库的时候，在redux配合components时，通常这么写

import thunkMiddleware from 'redux-thunk'; import { createStore, applyMiddleware, combineReducer } from 'redux'; import * as reducers from './reducers.js';  const appReducer = combineReducer(reducers); const store = createStore(appReducer, initialState, applyMiddleware(thunkMiddleware));

还记得当createStore收到的参数中有enhance时会怎么做吗?

// createStore.js if (typeof enhancer !== 'undefined') {   if (typeof enhancer !== 'function') {     throw new Error('Expected the enhancer to be a function.')   }   return enhancer(createStore)(reducer, initialState) }

也就是说，会变成下面的情况

applyMiddleware(thunkMiddleware)(createStore)(reducer, initialState)

• applyMiddleware(thunkMiddleware)

applyMiddleware接收thunkMiddleware作为参数，返回形如(createStore) => (reducer, initialState, enhancer) => {}的函数。

• applyMiddleware(thunkMiddleware)(createStore)

以 createStore 作为参数，调用上一步返回的函数(reducer, initialState, enhancer) => {}

• applyMiddleware(thunkMiddleware)(createStore)(reducer, initialState)

以(reducer, initialState)为参数进行调用。

在这个函数内部，thunkMiddleware被调用，其作用是监测type是function的action

因此，如果dispatch的action返回的是一个function，则证明是中间件，则将(dispatch, getState)作为参数代入其中，进行action 内部下一步的操作。否则的话，认为只是一个普通的action，将通过next(也就是dispatch)进一步分发。

也就是说，applyMiddleware(thunkMiddleware)作为enhance，最终起了这样的作用：

对dispatch调用的action(例如，dispatch(addNewTodo(todo)))进行检查，如果action在***次调用之后返回的是function，则将(dispatch, getState)作为参数注入到action返回的方法中，否则就正常对action进行分发，这样一来我们的中间件就完成喽~

因此，当action内部需要获取state，或者需要进行异步操作，在操作完成之后进行事件调用分发的话，我们就可以让action 返回一个以(dispatch, getState)为参数的function而不是通常的Object，enhance就会对其进行检测以便正确的处理。

bindActionCreator

这个方法感觉比较少见，我个人也很少用到

在传统写法下，当我们要把 state 和 action 注入到子组件中时，一般会这么做：

import { connect } from 'react-redux'; import {addTodo, deleteTodo} from './action.js';  class TodoComponect extends Component {   render() {     return (            )   } }  function mapStateToProps(state) {   return {     state   } } function mapDispatchToProps(dispatch) {   return {     deleteTodo: (id) => {       dispatch(deleteTodo(id));     },     addTodo: (todo) => {       dispatch(addTodo(todo));     }   } } export default connect(mapStateToProps, mapDispatchToProps)(TodoComponect);

使用bindActionCreators可以把 action 转为同名 key 的对象，但使用 dispatch 把每个 action 包围起来调用

惟一使用 bindActionCreators 的场景是当你需要把 action creator 往下传到一个组件上，却不想让这个组件觉察到 Redux 的存在，而且不希望把 Redux store 或 dispatch 传给它。

// 在本组件内的应用   addTodo(todo) {     let action = TodoActions.addTodo(todo);     this.props.dispatch(action);   }      deleteTodo(id) {     let action = TodoActions.deleteTodo(id);     this.props.dispatch(action);   }      render() {     let dispatch = this.props.dispatch;     // 传递给子组件     let boundActionCreators = bindActionCreators(TodoActions, dispatch);     return (            )   } }  function mapStateToProps(state) {   return {     state   } } export default connect(mapStateToProps)(TodoComponect)

bindActionCreator源码解析

function bindActionCreator(actionCreator, dispatch) {   return (...args) => dispatch(actionCreator(...args)) }  // bindActionCreators期待一个Object作为actionCreators传入，里面是 key: action export default function bindActionCreators(actionCreators, dispatch) {   // 如果只是传入一个action，则通过bindActionCreator返回被绑定到dispatch的函数   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"?`     )   }    // 遍历并通过bindActionCreator分发绑定至dispatch   var keys = Object.keys(actionCreators)   var boundActionCreators = {}   for (var i = 0; i < keys.length; i++) {     var key = keys[i]     var actionCreator = actionCreators[key]     if (typeof actionCreator === 'function') {       boundActionCreators[key] = bindActionCreator(actionCreator, dispatch)     }   }   return boundActionCreators }

react-redux

Provider

export default class Provider extends Component {   getChildContext() {     // 将其声明为 context 的属性之一     return { store: this.store }   }    constructor(props, context) {     super(props, context)     // 接收 redux 的 store 作为 props     this.store = props.store   }    render() {     return Children.only(this.props.children)   } }  if (process.env.NODE_ENV !== 'production') {   Provider.prototype.componentWillReceiveProps = function (nextProps) {     const { store } = this     const { store: nextStore } = nextProps      if (store !== nextStore) {       warnAboutReceivingStore()     }   } }  Provider.propTypes = {   store: storeShape.isRequired,   children: PropTypes.element.isRequired } Provider.childContextTypes = {   store: storeShape.isRequired }

connect

传入mapStateToProps,mapDispatchToProps,mergeProps,options。

首先获取传入的参数，如果没有则以默认值代替

const defaultMapStateToProps = state => ({}) // eslint-disable-line no-unused-vars const defaultMapDispatchToProps = dispatch => ({ dispatch }) const { pure = true, withRef = false } = options

之后，通过

const finalMergeProps = mergeProps || defaultMergeProps

选择合并stateProps,dispatchProps,parentProps的方式，默认的合并方式 defaultMergeProps 为：

const defaultMergeProps = (stateProps, dispatchProps, parentProps) => ({   ...parentProps,   ...stateProps,   ...dispatchProps })

返回一个以 Component 作为参数的函数。在这个函数内部，生成了一个叫做Connect的 Component

// ...   return function wrapWithConnect(WrappedComponent) {     const connectDisplayName = `Connect(${getDisplayName(WrappedComponent)})`     // 检查参数合法性     function checkStateShape(props, methodName) {}     // 合并props     function computeMergedProps(stateProps, dispatchProps, parentProps) {       const mergedProps = finalMergeProps(stateProps, dispatchProps, parentProps)       if (process.env.NODE_ENV !== 'production') {         checkStateShape(mergedProps, 'mergeProps')       }       return mergedProps     }          // start of Connect     class Connect extends Component {       constructor(props, context) {         super(props, context);         this.store = props.store || context.store                  const storeState = this.store.getState()         this.state = { storeState }         this.clearCache()       }              computeStateProps(store, props) {         // 调用configureFinalMapState，使用传入的mapStateToProps方法（或默认方法），将state map进props       }       configureFinalMapState(store, props) {}              computeDispatchProps(store, props) {         // 调用configureFinalMapDispatch，使用传入的mapDispatchToProps方法（或默认方法），将action使用dispatch封装map进props       }       configureFinalMapDispatch(store, props) {}              // 判断是否更新props       updateStatePropsIfNeeded() {}       updateDispatchPropsIfNeeded() {}       updateMergedPropsIfNeeded() {}              componentDidMount() {         // 内部调用this.store.subscribe(this.handleChange.bind(this))         this.trySubscribe()       }       handleChange() {         const storeState = this.store.getState()         const prevStoreState = this.state.storeState         // 对数据进行监听，发送改变时调用         this.setState({ storeState })       }              // 取消监听，清除缓存       componentWillUnmount() {         this.tryUnsubscribe()         this.clearCache()       }              render() {         this.renderedElement = createElement(WrappedComponent,             this.mergedProps         )         return this.renderedElement       }     }     // end of Connect          Connect.displayName = connectDisplayName     Connect.WrappedComponent = WrappedComponent     Connect.contextTypes = {       store: storeShape     }     Connect.propTypes = {       store: storeShape     }          return hoistStatics(Connect, WrappedComponent)   } // ...

我们看见，在connect的***，返回了使用hoistStatics包装的Connect和WrappedComponent

hoistStatics是什么鬼?为什么使用它?

Copies non-react specific statics from a child component to a parent component. Similar to Object.assign, but with React static keywords blacklisted from being overridden.

也就是说，它类似于Object.assign，作用是将子组件中的 static 方法复制进父组件，但不会覆盖组件中的关键字方法(如 componentDidMount)

import hoistNonReactStatic from 'hoist-non-react-statics';  hoistNonReactStatic(targetComponent, sourceComponent);

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