axios 拦截器管理类链式调用手写实现及原理剖析

// 添加一个请求拦截器
axios.interceptors.request.use(function (config) {
  // 在发送请求之前可以做一些事情
  return config;
}, function (error) {
  // 处理请求错误
  return Promise.reject(error);
});
// 添加一个响应拦截器
axios.interceptors.response.use(function (response) {
  // 处理响应数据
  return response;
}, function (error) {
  // 处理响应错误
  return Promise.reject(error);
});

axios.interceptors.request.use(config => {
  config.headers.test += '1'
  return config
})
axios.interceptors.request.use(config => {
  config.headers.test += '2'
  return config
})

const myInterceptor = axios.interceptors.request.use(function () {/*...*/})
axios.interceptors.request.eject(myInterceptor)

export interface AxiosInterceptorManager<T> {
  use(resolved: ResolvedFn<T>, rejected?: RejectedFn): number
  eject(id: number): void
}
export interface ResolvedFn<T=any> {
  (val: T): T | Promise<T>
}
export interface RejectedFn {
  (error: any): any
}

import { ResolvedFn, RejectedFn } from '../types'
interface Interceptor<T> {
  resolved: ResolvedFn<T>
  rejected?: RejectedFn
}
export default class InterceptorManager<T> {
  private interceptors: Array<Interceptor<T> | null>
  constructor() {
    // 拦截器数组
    this.interceptors = []
  }
  // 收集拦截器  
  use(resolved: ResolvedFn<T>, rejected?: RejectedFn): number {
    this.interceptors.push({
      resolved,
      rejected
    })
    return this.interceptors.length - 1
  }
  // 遍历用户写的拦截器，并执行fn函数把拦截器作为参数传入
  forEach(fn: (interceptor: Interceptor<T>) => void): void {
    this.interceptors.forEach(interceptor => {
      if (interceptor !== null) {
        fn(interceptor)
      }
    })
  }
  eject(id: number): void {
    if (this.interceptors[id]) {
      // 置为null，不能直接删除
      this.interceptors[id] = null
    }
  }
}

interface Interceptors {
  request: InterceptorManager<AxiosRequestConfig>
  response: InterceptorManager<AxiosResponse>
}
export default class Axios {
  interceptors: Interceptors
  constructor() {
    this.interceptors = {
      request: new InterceptorManager<AxiosRequestConfig>(),
      response: new InterceptorManager<AxiosResponse>()
    }
  }
}

interface PromiseChain {
  resolved: ResolvedFn | ((config: AxiosRequestConfig) => AxiosPromise)
  rejected?: RejectedFn
}
request(url: any, config?: any): AxiosPromise {
  if (typeof url === 'string') {
    if (!config) {
      config = {}
    }
    config.url = url
  } else {
    config = url
  }
  // 定义一个数组，这个数组就是要执行的任务链，默认有一个真正发送请求的任务
  const chain: PromiseChain[] = [{
    resolved: dispatchRequest,
    rejected: undefined
  }]
  // 把用户定义的请求拦截器存放到任务链中，请求拦截器最后注册的最先执行，所以使用unshift方法
  this.interceptors.request.forEach(interceptor => {
    chain.unshift(interceptor)
  })
  // 把响应拦截器存放到任务链中
  this.interceptors.response.forEach(interceptor => {
    chain.push(interceptor)
  })
  // 利用config初始化一个promise
  let promise = Promise.resolve(config)
  // 遍历任务链
  while (chain.length) {
    // 取出任务链的首个任务
    const { resolved, rejected } = chain.shift()!
    // resolved的执行时机是就是上一个promise执行resolve()的时候，这样就形成了链式调用
    promise = promise.then(resolved, rejected)
  }
  return promise
}