React 项目中Redux 中间件的理解

时间 2019-12-07

原文原文链接

前言

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，因此才想把中间件从新梳理一遍；在以前看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能够忽略这层概念；如今对这部分的笔记从新梳理，这里只针对这个中间件作一个理解。react

若是想学习项目的底层建设，建议先去学习官网redux案例，以后在学习react-router的使用git

Redux 中间件介绍

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 。本身在项目中使用它改变数据流，实现异步 action ；下面会对日志输出作一个开场。github

使用 Redux 中间件

Redux 中 applyMiddleware 的方法，能够应用多个中间件，这里先只写一个中间件，以日志输出中间件为例ajax

//利用中间件作打印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);
// 也能够直接这样，能够参考createStore
// createStore(
//     rootReducer,
//     applyMiddleware(logger)
// )
export default store;

logger 中间件结构分析

const logger = store => next => action => {
    let result = next(action); // 返回的也是一样的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

从applyMiddleware源码开始分析

///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
        }
    }
}

最外层store

//源码分析
chain = middlewares.map(middleware => middleware(middlewareAPI));

咱们发现store是middlewareAPI,后端

//store
var middlewareAPI = {
    getState: store.getState,
    dispatch: (action) => dispatch(action)
}

而后就剩下api

next => action => {
    let result = next(action); // 返回的也是一样的action值
    console.log('dispatch', action);
    console.log('nextState', store.getState());
    return result;
};

中间层next

//源码分析
dispatch = compose(...chain)(store.dispatch)

先来分析compose(...chain)promise

//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react-router

//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

由于咱们这里的中间件就只有一个，因此没有使用到reduceRight直接返回，直接返回func[0]（自己）;再由compose(...chain)(store.dispatch),咱们能够知道next就是store.dispatchapp

(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;
};

咱们以后调用的dispath就是触发的是上面这个函数（这里就单个中间件）；

多个中间件

经过上面的 applyMiddleware , compose 和中间件的结构，
假设应用了以下的中间件: [A, B, C]，这里咱们使用es5的结构作分析
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 /*...*/;
        }
    }
}

经过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)，咱们转化下

const last = C;
const rest = [A,B]
dispatch = rest.reduceRight(
    (composed, f) =>{
        return f(composed)
    }, 
    last(store.dispatch)
)

咱们获得的结果

dispatch = A(B(C(store.dispatch)));

进一步分析，咱们获得的结果

dispatch = A(B(C(store.dispatch)));

//执行C(next)，获得结果

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);

若是next(action)下面还有须要执行的代码，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，而后由外向内执行。原本咱们能够直接使用store.dispatch(action),可是咱们能够经过中间件对action作一些处理或转换，好比异步操做，异步回调后再执行next；这样的设计很巧妙，只有等待next，才能够继续作操做，和平时直接异步回调又有些不同

项目实践 ->异步

咱们知道redux中actions分为actionType，actionCreator，而后在由reducer进行修改数据；

官方例子中async直接在actionCreator作了ajax请求；

咱们把ajax放入中间件触发下面要讲的与官方real-world相似

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') {//从新分发
            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;

这样一来咱们能够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);
    };
}


//其余地方调用复用的方法以下：
export { request } from './request';

正常的写法，不是异步的，就是以前的写法

export function cartSelect(id) {
    return { 
        type: types.CART_MAIN_SELECT, 
        id
    };
}

而后就是下一个中间件的处理 api.js

//本身封装的ajax，能够使用别的，好比isomorphic-fetch
import net from 'net';
//项目中所有的接口，至关于一个关于异步的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) {
        //咱们约定这个没声明，就不是咱们设计的异步action，执行下一个中间件
        return next(action);
    }

    let ACTION_TYPE = action['type'];
    let { apiName, params = {} , opts = {} } = API_OPT;
    /**
     * 若是有传递localData，就不会触发ajax了，直接触发_success
     * 当前也能够传其余参数
     */
    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：项目中所有的接口，至关于一个关于异步的actionType有一个对应的后端接口，因此咱们才能够经过API_ROOT[apiName]找到这个接口

以cart为列子(下面是对应的每一个文件)：

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作一个统一的绑定，因此在这里单独引入
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'://能够不写
            /*...*/
            return newState;
        case types.HOME_MAIN_GET + '_SUCCESS':
            /*...*/
            return newState;
        case types.HOME_MAIN_GET + '_ERROR'://能够不写
            /*...*/
            return newState;
        default:
            return state;
    }
};

异步，数据验证均可以经过中间件作处理；引用Generator，Async/Await，Promise处理，能够参考社区中的一些其余方式，好比：