本文作者：IMWeb 陈映平 原文出处：IMWeb社区 未经同意，禁止转载
写在前面
redux的源码很简洁，除了applyMiddleware比较绕难以理解外，大部分还是
这里假设读者对redux有一定了解，就不科普redux的概念和API啥的啦，这部分建议直接看官方文档。
此外，源码解析的中文批注版已上传至github，可点击查看。本文相关示例代码，可点击查看。
源码解析概览
将redux下载下来，然后看下他的目录结构。
代码语言：javascript复制npm install redux这里我们需要关心的主要是src目录，源码解析需要关心的文件都在这里面了
index.js：redux主文件，主要对外暴露了几个核心APIcreateStore.js：createStore方法的定义utils：各种工具方法，其中applyMiddleware、combineReducers、bindActionCreators 为redux的几个核心方法，剩余的pick、mapValue、compose为普通的工具函数
➜ src git:(master) ✗ tree
.
├── createStore.js
├── index.js
└── utils
├── applyMiddleware.js
├── bindActionCreators.js
├── combineReducers.js
├── compose.js
├── isPlainObject.js
├── mapValues.js
└── pick.js源码解析：index.js
超级简单，暴露了几个核心API，没了
代码语言：javascript复制mport createStore from './createStore';
import combineReducers from './utils/combineReducers';
import bindActionCreators from './utils/bindActionCreators';
import applyMiddleware from './utils/applyMiddleware';
import compose from './utils/compose';
export {
createStore,
combineReducers,
bindActionCreators,
applyMiddleware,
compose
};源码解析：createStore.js
直接贴上源代码，并进行简单注解。看下redux.createStore(reducer, initialState)调用的文档说明，基本就能够看懂下面代码了。
特别强调：虽然在几个文件里，createStore.js的代码行数是最多的，但却是最容易读懂的。下面几点比较关键
redux.createStore(reducer, initialState)传入了reducer、initialState，并返回一个store对象。- store对象对外暴露了dispatch、getState、subscribe方法
- store对象通过getState() 获取内部状态
- initialState为 store 的初始状态，如果不传则为undefined
- store对象通过reducer来修改内部状态
- store对象创建的时候，内部会主动调用
dispatch({ type: ActionTypes.INIT });来对内部状态进行初始化。通过断点或者日志打印就可以看到，store对象创建的同时，reducer就会被调用进行初始化。
import isPlainObject from './utils/isPlainObject';
/**
* These are private action types reserved by Redux.
* For any unknown actions, you must return the current state.
* If the current state is undefined, you must return the initial state.
* Do not reference these action types directly in your code.
*/
// 初始化的时候(redux.createStore(reducer, initialState)时),传的action.type 就是这货啦
export var ActionTypes = {
INIT: '@@redux/INIT'
};
/**
* Creates a Redux store that holds the state tree.
* The only way to change the data in the store is to call `dispatch()` on it.
*
* There should only be a single store in your app. To specify how different
* parts of the state tree respond to actions, you may combine several reducers
* into a single reducer function by using `combineReducers`.
*
* @param {Function} reducer A function that returns the next state tree, given
* the current state tree and the action to handle.
*
* @param {any} [initialState] The initial state. You may optionally specify it
* to hydrate the state from the server in universal apps, or to restore a
* previously serialized user session.
* If you use `combineReducers` to produce the root reducer function, this must be
* an object with the same shape as `combineReducers` keys.
*
* @returns {Store} A Redux store that lets you read the state, dispatch actions
* and subscribe to changes.
*/
export default function createStore(reducer, initialState) {
if (typeof reducer !== 'function') {
throw new Error('Expected the reducer to be a function.');
}
var currentReducer = reducer;
var currentState = initialState;
var listeners = [];
var isDispatching = false;
/**
* Reads the state tree managed by the store.
*
* @returns {any} The current state tree of your application.
*/
// 这个方法没什么好讲的,返回当前的state
function getState() {
return currentState;
}
/**
* Adds a change listener. It will be called any time an action is dispatched,
* and some part of the state tree may potentially have changed. You may then
* call `getState()` to read the current state tree inside the callback.
*
* @param {Function} listener A callback to be invoked on every dispatch.
* @returns {Function} A function to remove this change listener.
*/
// 很常见的监听函数添加方式,当store.dispatch 的时候被调用
// store.subscribe(listener) 返回一个方法(unscribe),可以用来取消监听
function subscribe(listener) {
listeners.push(listener);
var isSubscribed = true;
return function unsubscribe() {
if (!isSubscribed) {
return;
}
isSubscribed = false;
var index = listeners.indexOf(listener);
listeners.splice(index, 1);
};
}
/**
* Dispatches an action. It is the only way to trigger a state change.
*
* The `reducer` function, used to create the store, will be called with the
* current state tree and the given `action`. Its return value will
* be considered the **next** state of the tree, and the change listeners
* will be notified.
*
* The base implementation only supports plain object actions. If you want to
* dispatch a Promise, an Observable, a thunk, or something else, you need to
* wrap your store creating function into the corresponding middleware. For
* example, see the documentation for the `redux-thunk` package. Even the
* middleware will eventually dispatch plain object actions using this method.
*
* @param {Object} action A plain object representing “what changed”. It is
* a good idea to keep actions serializable so you can record and replay user
* sessions, or use the time travelling `redux-devtools`. An action must have
* a `type` property which may not be `undefined`. It is a good idea to use
* string constants for action types.
*
* @returns {Object} For convenience, the same action object you dispatched.
*
* Note that, if you use a custom middleware, it may wrap `dispatch()` to
* return something else (for example, a Promise you can await).
*/
// 以下情况会报错
// 1. 传入的action不是一个对象
// 2. 传入的action是个对象,但是action.type 是undefined
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?'
);
}
if (isDispatching) {
throw new Error('Reducers may not dispatch actions.');
}
try {
isDispatching = true;
// 就是这一句啦, 将 currentState 设置为 reducer(currentState, action) 返回的值
currentState = currentReducer(currentState, action);
} finally {
isDispatching = false;
}
// 如果有监听函数,就顺序调用
listeners.slice().forEach(listener => listener());
// 最后,返回传入的action
return action;
}
/**
* Replaces the reducer currently used by the store to calculate the state.
*
* You might need this if your app implements code splitting and you want to
* load some of the reducers dynamically. You might also need this if you
* implement a hot reloading mechanism for Redux.
*
* @param {Function} nextReducer The reducer for the store to use instead.
* @returns {void}
*/
function replaceReducer(nextReducer) {
currentReducer = nextReducer;
dispatch({ type: ActionTypes.INIT });
}
// When a store is created, an "INIT" action is dispatched so that every
// reducer returns their initial state. This effectively populates
// the initial state tree.
//
// redux.createStore(reducer, initialState) 的时候, 内部会 自己调用 dispatch({ type: ActionTypes.INIT });
// 来完成state的初始化
dispatch({ type: ActionTypes.INIT });
// 返回的就是这个东东了,只有四个方法
return {
dispatch,
subscribe,
getState,
replaceReducer
};
}源码解析：combineReducers.js
redux.combineReducers(reducerMap) 的作用在于合并多个reducer函数，并返回一个新的reducer函数。因此可以看到，combineReducers 返回了一个函数，并且该函数的参数同样是state、reducer。
可以先看伪代码感受下，最终 store.getState() 返回的state，大概会是这么个样子{todos: xx, filter: xx}。简单的说，state被拆分成了两份，TodoReducer的返回值赋值给了state.todos，FilterReducer的返回值赋值给了state.filter。
function TodoReducer(state, action) {}
function FilterReducer(state, action) {}
var finalReducers = redux.combineReducers({
todos: TodoReducer,
filter: FilterReducer
});同样是直接上注解后的代码，记住几个关键就差不多了：
- combineReducers(reducerMap) 传入一个对象，并返回一个全新的reducer。调用方式跟跟普通的reducer一样，也是传入state、action。
- 通过combineReducers，对 store 的状态state进行拆分，
- reducerMap的key，就是 state 的key，而 调用对应的reducer返回的值，则是这个key对应的值。如上面的例子，state.todos == TodoReducer(state, action)
- redux.createStore(finalReducers, initialState) 调用时，同样会对 state 进行初始化。这个初始化跟通过普通的reducer进行初始化没多大区别。举例来说，如果 initialState.todos = undefined，那么 TodoReducer(state, action) 初始传入的state就是undefined；如果initialState.todos = []，那么 TodoReducer(state, action) 初始传入的state就是[];
- store.dispatch(action)，finalReducers 里面，会遍历整个reducerMap，依次调用每个reducer，并将每个reducer返回的子state赋给state对应的key。
import { ActionTypes } from '../createStore';
import isPlainObject from '../utils/isPlainObject';
import mapValues from '../utils/mapValues';
import pick from '../utils/pick';
/* eslint-disable no-console */
function getUndefinedStateErrorMessage(key, action) {
var actionType = action && action.type;
var actionName = actionType && `"${actionType.toString()}"` || 'an action';
return (
`Reducer "${key}" returned undefined handling ${actionName}. `
`To ignore an action, you must explicitly return the previous state.`
);
}
function getUnexpectedStateKeyWarningMessage(inputState, outputState, action) {
var reducerKeys = Object.keys(outputState);
var argumentName = action && action.type === ActionTypes.INIT ?
'initialState 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('", "')}"`
);
}
var unexpectedKeys = Object.keys(inputState).filter(
key => reducerKeys.indexOf(key) < 0
);
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.`
);
}
}
// 对reducer做合法性检测
// store = Redux.createStore(reducer, initialState) -->
// currentState = initialState
// currentState = currentReducer(currentState, action);
//
// 从调用关系,调用时机来看, store.getState() 的初始值(currentState)
// 为 currentReducer(initialState, { type: ActionTypes.INIT })
//
// 1. 在初始化阶段,reducer 传入的 state 值是 undefined,此时,需要返回初始state,且初始state不能为undefined
// 2. 当传入不认识的 actionType 时, reducer(state, {type}) 返回的不能是undefined
// 3. redux/ 这个 namespace 下的action 不应该做处理,直接返回 currentState 就行 (谁运气这么差会去用这种actionType...)
function assertReducerSanity(reducers) {
Object.keys(reducers).forEach(key => {
var reducer = reducers[key];
var initialState = reducer(undefined, { type: ActionTypes.INIT });
if (typeof initialState === 'undefined') {
throw new Error(
`Reducer "${key}" returned undefined during initialization. `
`If the state passed to the reducer is undefined, you must `
`explicitly return the initial state. The initial state may `
`not be undefined.`
);
}
var type = '@@redux/PROBE_UNKNOWN_ACTION_' Math.random().toString(36).substring(7).split('').join('.');
if (typeof reducer(undefined, { type }) === 'undefined') {
throw new Error(
`Reducer "${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.`
);
}
});
}
/**
* 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.
*
* @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.
*
* @returns {Function} A reducer function that invokes every reducer inside the
* passed object, and builds a state object with the same shape.
*/
export default function combineReducers(reducers) {
// 返回一个对象, key => value 且value是function(其实就是过滤掉非function)
var finalReducers = pick(reducers, (val) => typeof val === 'function');
var sanityError;
try {
// 对所有的子reducer 做一些合法性断言,如果没有出错再继续下面的处理
// 合法性断言的内容,见API注释
assertReducerSanity(finalReducers);
} catch (e) {
sanityError = e;
}
// 所有的 key: value,将value置成了undefined,费解...
// 总而言之, 初始state 就是 类似 {hello: undefined, world: undefined} 的东东
// TODO 确认这里的逻辑
var defaultState = mapValues(finalReducers, () => undefined);
return function combination(state = defaultState, action) {
if (sanityError) {
throw sanityError;
}
var hasChanged = false;
// 这段代码,简单的说,就是循环一遍 finalState[key] = fn(reducer, key)
var finalState = mapValues(finalReducers, (reducer, key) => {
var previousStateForKey = state[key];
var nextStateForKey = reducer(previousStateForKey, action);
if (typeof nextStateForKey === 'undefined') {
// 其他一个reducer返回的是undefined,于是挂啦...抛出错误
var errorMessage = getUndefinedStateErrorMessage(key, action);
throw new Error(errorMessage);
}
// 这段代码有些费解,从redux的设计理念上来讲,除了不认识的action type,其他情况都应该返回全新的state
// 也就是说
// 1. action type 认识,返回新的state,于是这里 hasChanged 为 true
// 2. action type 不认识,返回原来的state,于是这里 hasChanged 为 false
// 3. 不管action type 是否认识, 在原来的state上修改,但是返回的是修改后的state(没有返回拷贝),那么,hasChanged还是为false
hasChanged = hasChanged || nextStateForKey !== previousStateForKey;
return nextStateForKey;
});
// 开发环境中(于是记得在生产环境去掉)
// 后面再研究这段代码,毕竟不是主线路...
if (process.env.NODE_ENV !== 'production') {
var warningMessage = getUnexpectedStateKeyWarningMessage(state, finalState, action);
if (warningMessage) {
console.error(warningMessage);
}
}
return hasChanged ? finalState : state;
};
}源码解析：bindActionCreator.js
别看API注释一大堆，除去合法性检查，关键代码其实就只有几句。先看个简单例子可能方便理解一些。看完之后可能会觉得，不就是对store.dispatch 的调用进行了便捷处理嘛。。。
代码语言：javascript复制var addTodo = function(text){
return {
type: 'add_todo',
text: text
};
};
var addTodos = function(){
return {
type: 'add_todos',
items: Array.prototype.slice.call(arguments, 0)
};
};
var reducer = function(state, action){
switch (action.type) {
case 'add_todo':
return state.concat(action.text);
case 'add_todos':
return state.concat(action.items);
default:
return state;
}
};
var store = redux.createStore(reducer, []);
// 注意,关键代码在这里
var actions = redux.bindActionCreators({
addTodo: addTodo,
addTodos: addTodos
}, store.dispatch);
console.log('state is: ' store.getState());
store.dispatch({type: 'add_todo', text: '读书'});
store.dispatch({type: 'add_todos', items: ['阅读', '睡觉']});
console.log('state is: ' store.getState()); // state is: 读书,阅读,睡觉
actions.addTodo('看电影');
console.log('state is: ' store.getState()); // state is: 读书,阅读,睡觉,看电影
actions.addTodos(['刷牙', '洗澡']);
console.log('state is: ' store.getState()); // state is: 读书,阅读,睡觉,看电影,刷牙,洗澡所以，直接看代码吧，挺简单的。
代码语言：javascript复制import mapValues from '../utils/mapValues';
function bindActionCreator(actionCreator, dispatch) {
return (...args) => dispatch(actionCreator(...args));
}
/**
* Turns an object whose values are action creators, into an object with the
* same keys, but with every function wrapped into a `dispatch` call so they
* may be invoked directly. This is just a convenience method, as you can call
* `store.dispatch(MyActionCreators.doSomething())` yourself just fine.
*
* For convenience, you can also pass a single function as the first argument,
* and get a function in return.
*
* @param {Function|Object} actionCreators An object whose values are action
* creator functions. One handy way to obtain it is to use ES6 `import * as`
* syntax. You may also pass a single function.
*
* @param {Function} dispatch The `dispatch` function available on your Redux
* store.
*
* @returns {Function|Object} The object mimicking the original object, but with
* every action creator wrapped into the `dispatch` call. If you passed a
* function as `actionCreators`, the return value will also be a single
* function.
*/
// 假设 actionCreators === {addTodo: addTodo, removeTodo: removeTodo}
// 简单的来说 bindActionCreators(actionCreators, dispatch)
// 最后返回的是:
// {
// addTodo: function(text){
// dispatch( actionCreators.addTodo(text) );
// },
// removeTodo: function(text){
// dispatch( actionCreators.removeTodo(text) );
// }
// }
//
// 或者说 actionCreators === addTodo (addTodo 为 actionCreator)
// 最后返回的是
// function() {
// dispatch(actionCreators());
// }
export default function bindActionCreators(actionCreators, dispatch) {
if (typeof actionCreators === 'function') {
return bindActionCreator(actionCreators, dispatch);
}
if (typeof actionCreators !== 'object' || actionCreators === null || actionCreators === undefined) { // eslint-disable-line no-eq-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"?`
);
}
return mapValues(actionCreators, actionCreator =>
bindActionCreator(actionCreator, dispatch)
);
}源码解析：applyMiddleware.js
中间件应该是redux源码里面最绕的一部分，虽然看懂后，有一种“啊~原来不过如此”的感觉，但一开始还真是看的晕头转向的，API的说明、中间件的编写、applyMiddleware的源码实现，都不是那么好理解。
在继续源码解析之前，推荐看下官方文档对于middleware的说明，链接传送门：http://camsong.github.io/redux-in-chinese/docs/advanced/Middleware.html
虽然文档死长死长，但硬着头皮看完，还是有所收获的，终于知道 applyMiddleware 的实现这么绕了。。。
例子：redux-thunk
用redux处理过异步请求的同学应该用过redux-thunk，我们来看下他的源码，奇短无比，别说你的小伙伴了，我的小伙伴都惊呆了。
代码语言：javascript复制export default function thunkMiddleware({ dispatch, getState }) {
return next => action =>
typeof action === 'function' ?
action(dispatch, getState) :
next(action);
}翻译成ES5，是这样子滴，之后你再看其他中间件的实现，其实都大同小异，下面我们写个自定义中间件，基本就可以看出点门路来。
代码语言：javascript复制'use strict';
Object.defineProperty(exports, "__esModule", {
value: true
});
exports.default = thunkMiddleware;
function thunkMiddleware(store) {
var dispatch = store.dispatch;
var getState = store.getState;
return function (next) {
return function (action) {
return typeof action === 'function' ? action(dispatch, getState) : next(action);
};
};
}
module.exports = exports['default'];自定义中间件：logger
先看logger的实现
代码语言：javascript复制 function middleware(store){
return function(next){
return function(action){
return next(action);
}
}
}基本看出中间件声明的模版来了，就是下面这个样子。下面结合applyMiddleware的调用，来说明store、next、action 几个参数。
function logger(store){
return function(next){
return function(action){
console.log('logger: dispatching ' action.type);
var result = next(action);
console.log('logger: next state ' result);
return result;
}
}
}applyMiddleware调用例子
完整的示例代码见本小节最后面。可以看到：
- applyMiddleware 的调用方式为 applyMiddleware(...middlewares)(react.createStore)。其实这里直接先创建 store，然后applyMiddleware(...middlewares)(store) 也很容易实现相同的效果，不过作者是故意这样设计的，为了避免在同一个store上多次应用同一个middlerware（参考官方文档:尝试 #6: “单纯”地使用 Middleware ）
- 中间件顶层的store参数，并不是常规的store，虽然它也有 getState、dispatch 两个方法
// 上面的store参数，其实就是这个对象
// 其中，store 为内部的store，我们在外面 storeWithMiddleWare.dipatch的时候，内部实现是转成 store.dispatch
// 此外，可以看到 middlewareAPI.dispatch 方法，是最终封装后的dispatch（千万注意，如果在中间件内部 调用 store.dispatch，可能导致死循环 ）
var middlewareAPI = {
getState: store.getState,
// 最后面, dispatch 被覆盖, 变成包装后的 dispatch 方法
dispatch: (action) => dispatch(action)
};- 第二层的
next函数，其实是一个“dispatch”方法。熟悉express的同学大概可以猜到它的作用。storeWithMiddleWare.dispatch(action) 的时候，会顺序进入各个中间件（按照定义时的顺序）。从当前的例子来看，大约如下，其实就是柯里化啦~：
storeWithMiddleWare.dispatch(action) --> logger(store)(next)(action) --> timer(store)(next)(action) --> store.dispatch(action)
完整的示例代码
代码语言：javascript复制 function reducer(state, action){
if(typeof state==='undefined') state = [];
switch(action.type){
case 'add_todo':
return state.concat(action.text);
default:
return state;
}
}
function addTodo(text){
return {
type: 'add_todo',
text: text
};
}
// 这里的 store，并不是 redux.createStore(reducer, initialState) 出来的 store
// 而是 {getState: store.getState, dispatch: function() { store.dispatch(action); }}
//
function logger(store){
//
return function(next){
return function(action){
console.log('logger: dispatching ' action.type);
var result = next(action);
console.log('logger: next state ' result);
return result;
}
}
}
function timer(store){
return function(next){
return function(action){
console.log('timer: dispatching ' action.type);
var result = next(action);
console.log('timer: next state ' result);
return result;
}
}
}
var createStoreWidthMiddleware = redux.applyMiddleware(
logger,
timer
)(redux.createStore);
var storeWithMiddleWare = createStoreWidthMiddleware(reducer);
storeWithMiddleWare.subscribe(function(){
console.log('subscribe: state is : ' storeWithMiddleWare.getState());
});
console.log( storeWithMiddleWare.dispatch(addTodo('reading')) );源码解析
再次说下，建议先看下官方文档对中间件的介绍，不然可能会有点晕。
代码语言：javascript复制import compose from './compose';
/**
* Creates a store enhancer that applies middleware to the dispatch method
* of the Redux store. This is handy for a variety of tasks, such as expressing
* asynchronous actions in a concise manner, or logging every action payload.
*
* See `redux-thunk` package as an example of the Redux middleware.
*
* Because middleware is potentially asynchronous, this should be the first
* store enhancer in the composition chain.
*
* Note that each middleware will be given the `dispatch` and `getState` functions
* as named arguments.
*
* @param {...Function} middlewares The middleware chain to be applied.
* @returns {Function} A store enhancer applying the middleware.
*/
/*
从调用方法 applyMiddleware(...middlewares)(Redux.createStore) 可以看出
next 参数实际上是 Redux.createStore. 而 Redux.createStore 的调用方式为 Redux.createStore(reducer, initialState)
所以 applyMiddleware(...middlewares)
1. 参数: Redux.createStore
2. 返回值:一个function, 跟 Redux.createStore 接受的参数一样
*/
export default function applyMiddleware(...middlewares) {
return (next) => (reducer, initialState) => {
// 内部先创建一个store (相当于直接调用 Redux.createStore(reducer, initialState))
var store = next(reducer, initialState);
// 保存最初始的store.dispatch
var dispatch = store.dispatch;
var chain = [];
var middlewareAPI = {
getState: store.getState,
// 最后面, dispatch 被覆盖, 变成包装后的 dispatch 方法
dispatch: (action) => dispatch(action)
};
// 返回一个数组
// 贴个例子在这里做参考,redux-thunk
// function thunkMiddleware(store) {
// var dispatch = store.dispatch;
// var getState = store.getState;
//
// 这里的next其实就是dispatch
// return function (next) {
// return function (action) {
// return typeof action === 'function' ? action(dispatch, getState) : next(action);
// };
// };
//}
/*
chain 是个数组, 参考上面的 middlleware (redux-thunk),可以看到,chain的每个元素为如下形式的function
并且, 传入的 store.getState 为原始的 store.getState,而 dispatch则是包装后的 dispatch(不是原始的store.dispatch)
似乎是为了确保, 在每个middleware里调用 dispatch(action), 最终都是 用原始的 store.dispatch(action)
避免 store.dispatch 被覆盖, 导致middleware 顺序调用的过程中, store.dispatch的值变化 --> store.dispatch 返回的值可能会有不同
违背 redux 的设计理念
这里的 next 则为 原始的 store.dispatch (见下面 compose(...chain)(store.dispatch) )
function (next) {
return function (action) {
}
}
*/
chain = middlewares.map(middleware => middleware(middlewareAPI));
// compose(...chain)(store.dispatch) 返回了一个function
// 伪代码如下,
// function (action) {
// middleware(store)(store.dispatch);
// }
dispatch = compose(...chain)(store.dispatch); // 从右到左, middleware1( middleware2( middleware3(dispatch) ) )
// 于是,最终调用 applyMiddleware(...middlewares)(Redux.createStore)
// 返回的 store, getState,subscribe 方法都是原始的那个 store.getState, store.subscribe
// 至于dispatch是封装过的
return {
...store,
dispatch
};
};
}相关链接
官方文档：http://camsong.github.io/redux-in-chinese/docs/advanced/Middleware.html 源码解析github地址：https://github.com/chyingp/redux-source-insight 源码解析相关代码示例：https://github.com/chyingp/redux-source-insight/tree/master/examples
