axios、koa、redux、vuex中间件梳理

• 2020.12.04

本文将从koa、axios、vuex和redux的实现来阐述中间件相关的知识。

axios(v0.19.2)

我们先来看一下基本使用:

import Axios from "axios";

const instance = Axios.create({
  baseURL: 'app/api',
  timeout: 10000,
  withCredentials: true,
});

我们需要通过调用Axios.create来创建一个axios实例,先看看Axios中是什么样的:

function Axios(instanceConfig) {
  this.defaults = instanceConfig;
  this.interceptors = {
    request: new InterceptorManager(),
    response: new InterceptorManager()
  };
}

创建一个Axios的实例的过程中,默认配置了一个初始化的默认配置项,interceptors中的request和response对象对应两个InterceptorManager实例。

InterceptorManager

function InterceptorManager() {
  this.handlers = [];
}

/**
 * Add a new interceptor to the stack
 *
 * @param {Function} fulfilled The function to handle `then` for a `Promise`
 * @param {Function} rejected The function to handle `reject` for a `Promise`
 *
 * @return {Number} An ID used to remove interceptor later
 */
InterceptorManager.prototype.use = function use(fulfilled, rejected) {
  this.handlers.push({
    fulfilled: fulfilled,
    rejected: rejected
  });
  return this.handlers.length - 1;
};

/**
 * Remove an interceptor from the stack
 *
 * @param {Number} id The ID that was returned by `use`
 */
InterceptorManager.prototype.eject = function eject(id) {
  if (this.handlers[id]) {
    this.handlers[id] = null;
  }
};

/**
 * Iterate over all the registered interceptors
 *
 * This method is particularly useful for skipping over any
 * interceptors that may have become `null` calling `eject`.
 *
 * @param {Function} fn The function to call for each interceptor
 */
InterceptorManager.prototype.forEach = function forEach(fn) {
  utils.forEach(this.handlers, function forEachHandler(h) {
    if (h !== null) {
      fn(h);
    }
  });
};

InterceptorManager中主要提供了3个方法use、eject、forEach分别用于处理中间件、移除事件、触发所有的监听事件,use方法中处理结果返回的是this.handlers.length - 1,这也是实现链接调用的关键处。

create流程

回归到create流程上来:

// bind工具函数
function bind(fn, thisArg) {
  return function wrap() {
    var args = new Array(arguments.length);
    for (var i = 0; i < args.length; i++) {
      args[i] = arguments[i];
    }
    return fn.apply(thisArg, args);
  };
};

/**
 * Extends object a by mutably adding to it the properties of object b.
 *
 * @param {Object} a The object to be extended
 * @param {Object} b The object to copy properties from
 * @param {Object} thisArg The object to bind function to
 * @return {Object} The resulting value of object a
 */
function extend(a, b, thisArg) {
  forEach(b, function assignValue(val, key) {
    if (thisArg && typeof val === 'function') {
      a[key] = bind(val, thisArg);
    } else {
      a[key] = val;
    }
  });
  return a;
}

/**
 * Create an instance of Axios
 *
 * @param {Object} defaultConfig The default config for the instance
 * @return {Axios} A new instance of Axios
 */
function createInstance(defaultConfig) {
  var context = new Axios(defaultConfig);
  var instance = bind(Axios.prototype.request, context);

  // Copy axios.prototype to instance
  utils.extend(instance, Axios.prototype, context);

  // Copy context to instance
  utils.extend(instance, context);

  return instance;
}

// 源码: axios/lib/defaults.js
function getDefaultAdapter() {
  var adapter;
  if (typeof XMLHttpRequest !== 'undefined') {
    // For browsers use XHR adapter
    adapter = require('./adapters/xhr');
  } else if (typeof process !== 'undefined' && Object.prototype.toString.call(process) === '[object process]') {
    // For node use HTTP adapter
    adapter = require('./adapters/http');
  }
  return adapter;
}

/**
 * 源码: axios/lib/core/mergeConfig.js
 * 
 * Config-specific merge-function which creates a new config-object
 * by merging two configuration objects together.
 *
 * @param {Object} config1
 * @param {Object} config2
 * @returns {Object} New object resulting from merging config2 to config1
 */
function mergeConfig(config1, config2) {
  // eslint-disable-next-line no-param-reassign
  config2 = config2 || {};
  var config = {};

  var valueFromConfig2Keys = ['url', 'method', 'params', 'data'];
  var mergeDeepPropertiesKeys = ['headers', 'auth', 'proxy'];
  var defaultToConfig2Keys = [
    'baseURL', 'url', 'transformRequest', 'transformResponse', 'paramsSerializer',
    'timeout', 'withCredentials', 'adapter', 'responseType', 'xsrfCookieName',
    'xsrfHeaderName', 'onUploadProgress', 'onDownloadProgress',
    'maxContentLength', 'validateStatus', 'maxRedirects', 'httpAgent',
    'httpsAgent', 'cancelToken', 'socketPath'
  ];

  utils.forEach(valueFromConfig2Keys, function valueFromConfig2(prop) {
    if (typeof config2[prop] !== 'undefined') {
      config[prop] = config2[prop];
    }
  });

  utils.forEach(mergeDeepPropertiesKeys, function mergeDeepProperties(prop) {
    if (utils.isObject(config2[prop])) {
      config[prop] = utils.deepMerge(config1[prop], config2[prop]);
    } else if (typeof config2[prop] !== 'undefined') {
      config[prop] = config2[prop];
    } else if (utils.isObject(config1[prop])) {
      config[prop] = utils.deepMerge(config1[prop]);
    } else if (typeof config1[prop] !== 'undefined') {
      config[prop] = config1[prop];
    }
  });

  utils.forEach(defaultToConfig2Keys, function defaultToConfig2(prop) {
    if (typeof config2[prop] !== 'undefined') {
      config[prop] = config2[prop];
    } else if (typeof config1[prop] !== 'undefined') {
      config[prop] = config1[prop];
    }
  });

  var axiosKeys = valueFromConfig2Keys
    .concat(mergeDeepPropertiesKeys)
    .concat(defaultToConfig2Keys);

  var otherKeys = Object
    .keys(config2)
    .filter(function filterAxiosKeys(key) {
      return axiosKeys.indexOf(key) === -1;
    });

  utils.forEach(otherKeys, function otherKeysDefaultToConfig2(prop) {
    if (typeof config2[prop] !== 'undefined') {
      config[prop] = config2[prop];
    } else if (typeof config1[prop] !== 'undefined') {
      config[prop] = config1[prop];
    }
  });

  return config;
};

// defaults配置
const defaults = {
    adapter: getDefaultAdapter(), 
};

var axios = createInstance(defaults);

axios.Axios = Axios;

axios.create = function create(instanceConfig) {
  return createInstance(mergeConfig(axios.defaults, instanceConfig));
};

Request请求

/**
 * Dispatch a request
 *
 * @param {Object} config The config specific for this request (merged with this.defaults)
 */
Axios.prototype.request = function request(config) {
  /*eslint no-param-reassign:0*/
  // Allow for axios('example/url'[, config]) a la fetch API
  if (typeof config === 'string') {
    config = arguments[1] || {};
    config.url = arguments[0];
  } else {
    config = config || {};
  }

  config = mergeConfig(this.defaults, config);

  // Set config.method
  if (config.method) {
    config.method = config.method.toLowerCase();
  } else if (this.defaults.method) {
    config.method = this.defaults.method.toLowerCase();
  } else {
    config.method = 'get';
  }

  // Hook up interceptors middleware
  var chain = [dispatchRequest, undefined];
  var promise = Promise.resolve(config);

  this.interceptors.request.forEach(function unshiftRequestInterceptors(interceptor) {
    chain.unshift(interceptor.fulfilled, interceptor.rejected);
  });

  this.interceptors.response.forEach(function pushResponseInterceptors(interceptor) {
    chain.push(interceptor.fulfilled, interceptor.rejected);
  });

  while (chain.length) {
    promise = promise.then(chain.shift(), chain.shift());
  }

  return promise;
};