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文章目录󰁋
  1. 一、index.js
  2. 二、createStore.js
  3. 三、combineReducers.js
  4. 四、bindActionCreator.js
  5. 五、applyMiddleware.js

Redux之源码分析(九)

一、index.js

https://github.com/reactjs/redux/blob/master/src/index.js

  • 暴露了几个核心API
import 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
};

二、createStore.js

https://github.com/reactjs/redux/blob/master/src/createStore.js

  • redux.createStore(reducer, initialState) 传入了reducerinitialState,并返回一个store对象
  • store对象对外暴露了dispatchgetState、subscribe方法
  • store对象通过getState() 获取内部状态
  • initialStatestore 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
};
}

三、combineReducers.js

https://github.com/reactjs/redux/blob/master/src/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statereducer

  • 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.todosFilterReducer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
});
  • 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stateaction
  • 通过combineReducers,对 store 的状态state进行拆分
  • reducerMap的key,就是 statekey,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;
};
}

四、bindActionCreator.js

https://github.com/reactjs/redux/blob/master/src/bindActionCreators.js

  • 先看个简单例子可能方便理解一些
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
  • 直接看代码
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)
);
}

五、applyMiddleware.js

https://github.com/reactjs/redux/blob/master/src/applyMiddleware.js

  • 中间件应该是redux源码里面最绕的一部分

例子:redux-thunk

export default function thunkMiddleware({ dispatch, getState }) {
return next => action =>
typeof action === 'function' ?
action(dispatch, getState) :
next(action);
}

//es5
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);
};
};
}

自定义中间件:logger

  • 先看logger的实现
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storenextaction 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

  • 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
  • 中间件顶层的store参数,并不是常规的store,虽然它也有 getStatedispatch 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
  • 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

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

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
};
};
}
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