React/Redux项目结束后,当我在研究react-router源码的时候发现当中有一部分含中间件的思想,所以才想把中间件重新梳理一遍;在之前看redux了解到中间件,redux层面中间件的理解对项目前期比较有帮助,虽然项目中后期基本可以忽略这层概念;现在对这部分的笔记重新梳理,这里只针对这个中间件做一个理解。
如果想学习项目的底层建设,建议先去学习官网redux案例,之后在学习react-router的使用
Redux
目的是提供第三方插件的模式,改变action -> reducer
的过程。变为 action -> middlewares -> reducer
。自己在项目中使用它改变数据流,实现异步 action
;下面会对日志输出做一个开场。
Redux 中 applyMiddleware
的方法,可以应用多个中间件,这里先只写一个中间件,以日志输出中间件为例
//利用中间件做打印log
import {createStore,applyMiddleware} from 'redux';
import logger from '../api/logger';
import rootReducer from '../reducer/rootReducer';
let createStoreWithMiddleware = applyMiddleware(logger)(createStore);
let store = createStoreWithMiddleware(rootReducer);
// 也可以直接这样,可以参考createStore
// createStore(
// rootReducer,
// applyMiddleware(logger)
// )
export default store;
const logger = store => next => action => {
let result = next(action); // 返回的也是同样的action值
console.log('dispatch', action);
console.log('nextState', store.getState());
return result;
};
export default logger;
store => next => action =>{}
实现了三层函数嵌套,最后返回 next
,给下一个中间件使用,接下来把三层函数拆解;
///redux/src/applyMiddleware.js
export default function applyMiddleware(...middlewares) {
return (createStore) => (reducer, initialState, enhancer) => {
var store = createStore(reducer, initialState, enhancer)
var dispatch = store.dispatch
var chain = []
var middlewareAPI = {
getState: store.getState,
dispatch: (action) => dispatch(action)
}
chain = middlewares.map(middleware => middleware(middlewareAPI))
dispatch = compose(...chain)(store.dispatch)
return {
...store,
dispatch
}
}
}
//源码分析
chain = middlewares.map(middleware => middleware(middlewareAPI));
我们发现store是middlewareAPI,
//store
var middlewareAPI = {
getState: store.getState,
dispatch: (action) => dispatch(action)
}
然后就剩下
next => action => {
let result = next(action); // 返回的也是同样的action值
console.log('dispatch', action);
console.log('nextState', store.getState());
return result;
};
//源码分析
dispatch = compose(...chain)(store.dispatch)
先来分析compose(…chain)
//compose源码
export default function compose(...funcs) {
if (funcs.length === 0) {
return arg => arg
}
if (funcs.length === 1) {
return funcs[0]
}
const last = funcs[funcs.length - 1]
const rest = funcs.slice(0, -1)
return (...args) => rest.reduceRight((composed, f) => f(composed), last(...args))
}
compose利用Array.prototype.reduceRight的方法
//reduceRight遍历介绍
[0, 1, 2, 3, 4].reduceRight(function(previousValue, currentValue, index, array) {
return previousValue + currentValue;
}, 10);
//结果 10+4+3+2+1+0 = 20
因为我们这里的中间件就只有一个,所以没有使用到reduceRight直接返回,直接返回func[0]
(本身);再由compose(...chain)(store.dispatch)
,我们可以知道next就是store.dispatch
(action) => {
let result = store.dispatch(action); // 这里的next就是store.dispatch
console.log('dispatch', action);
console.log('nextState', store.getState());
return result;
};
我们之后调用的dispath
就是触发的是上面这个函数(这里就单个中间件);
通过上面的 applyMiddleware
, compose
和中间件的结构,
假设应用了如下的中间件: [A, B, C],这里我们使用es5的结构做分析
分析action触发的完整流程
三个中间件
//A
function A(store) {
return function A(next) {
return function A(action) {
/*...*/;
next(action);
/*...*/;
return /*...*/;
}
}
}
//B
function B(store) {
return function B(next) {
return function B(action) {
/*...*/;
next(action);
/*...*/;
return /*...*/;
}
}
}
//C
function C(store) {
return function C(next) {
return function C(action) {
/*...*/;
next(action);
/*...*/;
return /*...*/;
}
}
}
通过chain = middlewares.map(middleware => middleware(middlewareAPI))
,三个中间件的状态变化
//A
function A(next) {
return function A(action) {
/*...*/;
next(action);
/*...*/;
return /*...*/;
}
}
//B
function B(next) {
return function B(action) {
/*...*/;
next(action);
/*...*/;
return /*...*/;
}
}
//C
function C(next) {
return function C(action) {
/*...*/;
next(action);
/*...*/;
return /*...*/;
}
}
再由dispatch = compose(...chain)(store.dispatch)
,我们转化下
const last = C;
const rest = [A,B]
dispatch = rest.reduceRight(
(composed, f) =>{
return f(composed)
},
last(store.dispatch)
)
我们得到的结果
dispatch = A(B(C(store.dispatch)));
进一步分析,我们得到的结果
dispatch = A(B(C(store.dispatch)));
//执行C(next),得到结果
A(B(function C(action) {/*...*/;next(action);/*...*/;return /*...*/;}));
//此时的next = store.dispatch
//继续执行B(next)
A(function B(action) {/*...*/;next(action);/*...*/;return /*...*/;});
//此时的next = function C(action) {/*...*/;next(action);/*...*/;return /*...*/;}
//继续执行A(next)
function A(action) {/*...*/;next(action);/*...*/;return /*...*/;};
//此时的next = function B(action) {/*...*/;next(action);/*...*/;return /*...*/;}
一个action触发执行顺序,A(action) -> B(action) -> C(action) -> store.dispatch(action)
(生产最新的 store 数据);
如果next(action)
下面还有需要执行的代码,继续执行 C(next 后的代码)->B(next 后的代码)->A(next 后的代码)
总结:先从内到外生成新的func,然后由外向内执行。本来我们可以直接使用store.dispatch(action)
,但是我们可以通过中间件对action做一些处理或转换,比如异步操作,异步回调后再执行next;这样的设计很巧妙,只有等待next,才可以继续做操作,和平时直接异步回调又有些不一样
我们知道redux中actions分为actionType,actionCreator,然后在由reducer进行修改数据;
官方例子中async直接在actionCreator做了ajax请求;
我们把ajax放入中间件触发下面要讲的与官方real-world类似
我这边使用redux-thunk
applyMiddleware(reduxThunk, api)
先来看看redux-thunk的源码
function createThunkMiddleware(extraArgument) {
return ({ dispatch, getState }) => next => action => {
if (typeof action === 'function') {//重新分发
return action(dispatch, getState, extraArgument);
}
return next(action);//传递给下一个中间件
};
}
const thunk = createThunkMiddleware();
thunk.withExtraArgument = createThunkMiddleware;
export default thunk;
这样一来我们可以把异步写成一个复用的actionCreator;
import * as types from '../../constants/actions/common';
export function request(apiName, params, opts = {}) {
return (dispatch, getState) => {
let action = {
'API': {
apiName: apiName,
params: params,
opts: opts
},
type: types.API_REQUEST
};
return dispatch(action);
};
}
//其他地方调用复用的方法如下:
export { request } from './request';
正常的写法,不是异步的,就是之前的写法
export function cartSelect(id) {
return {
type: types.CART_MAIN_SELECT,
id
};
}
然后就是下一个中间件的处理 api.js
//自己封装的ajax,可以使用别的,比如isomorphic-fetch
import net from 'net';
//项目中全部的接口,相当于一个关于异步的actionType有一个对应的后端接口
import API_ROOT from 'apiRoot';
export default store => next => action => {
let API_OPT = action['API'];
if (!API_OPT) {
//我们约定这个没声明,就不是我们设计的异步action,执行下一个中间件
return next(action);
}
let ACTION_TYPE = action['type'];
let { apiName, params = {} , opts = {} } = API_OPT;
/**
* 如果有传递localData,就不会触发ajax了,直接触发_success
* 当前也可以传其他参数
*/
let { localData } = opts;
let {
onSuccess,
onError,
onProgress,
ajaxType = 'GET',
param
} = params;
// 触发下一个action
let nextAction = function(type, param, opts) {
action['type'] = type;
action['opts'] = opts;
delete param['onSuccess'];
delete param['onError'];
const nextRequestAction = {...action,...param}
return nextRequestAction;
};
params={
...params,
data: null
};
// 触发正在请求的action
let result = next(nextAction(apiName + '_ON', params, opts));
net.ajax({
url: API_ROOT[apiName],
type: ajaxType,
param,
localData,
success: data => {
onSuccess && onSuccess(data);
params={
...params,
data
};
//触发请求成功的action
return next(nextAction(apiName + '_SUCCESS', params, opts));
},
error: data => {
onError && onError(data);
//触发请求失败的action
return next(nextAction(apiName + '_ERROR', params, opts));
}
});
return result;
};
强调一点:项目中全部的接口,相当于一个关于异步的actionType有一个对应的后端接口,所以我们才可以通过API_ROOT[apiName]找到这个接口
以cart为列子(下面是对应的每个文件):
actionType:
//异步
export const CART_MAIN_GET = 'CART_MAIN_GET';
//非异步
export const CART_MAIN_SELECT = 'CART_MAIN_SELECT';
api:
const api = {
'CART_MAIN_GET':'/shopping-cart/show-shopping-cart'
};
export default api;
APIROOT修改:
import cart from './api/cart';
const APIROOT = {
...cart
};
export default API;
actionCreator:
//项目中使用redux的bindActionCreators做一个统一的绑定,所以在这里单独引入
export { request } from './request';
//下面是非异步的方法
export function cartSelect(id) {
return {
type: types.CART_MAIN_SELECT,
id
};
}
项目中发起结构是这样的:
let url = types.CART_MAIN_GET;
let param = {};
let params = {
param: param,
ajaxType: 'GET',
onSuccess: (res) => {
/*...*/
},
onError: (res) => {
/*...*/
}
};
request(url, params, {});
其对应的reducers就是下面
import * as types from '../constants/actions/cart';
const initialState = {
main:{
isFetching: 0,//是否已经获取
didInvalidate:1,//是否失效
itemArr:[],//自定义模版
itemObj:{},//自定义模版数据
header:{}//头部导航
}
};
export default function(state = initialState, action) {
let newState;
switch (action.type) {
case types.HOME_MAIN_GET + '_ON'://可以不写
/*...*/
return newState;
case types.HOME_MAIN_GET + '_SUCCESS':
/*...*/
return newState;
case types.HOME_MAIN_GET + '_ERROR'://可以不写
/*...*/
return newState;
default:
return state;
}
};
异步,数据验证都可以通过中间件做处理;引用Generator,Async/Await,Promise处理,可以参考社区中的一些其他方式,比如:
redux-promise
redux-saga