koa:当创建一个服务时，做了什么

创建一个简单的应用

// 1、实例化appvar Koa = require('koa');const app = new Koa();// 2、声明中间件函数async function responseTime(ctx, next) {  const start = new Date();  await next();  const ms = new Date() - start + 10;  ctx.set('X-Response-Time', `${ms}ms`);}async function responseContent(ctx, next) {  if ('/' != this.url) return;  ctx.body = 'Hello World!';}// 3、使用中间件app.use(responseTime);app.use(responseContent);// 4、应用监听端口app.listen(3000);

app对象

const Emitter = require('events');class Application extends Emitter {    constructor() {        super();        this.proxy = false;        // 用于存储中间件        this.middleware = [];        this.subdomainOffset = 2;        this.env = process.env.NODE_ENV || 'development';        // 上下文        this.context = Object.create(context);        this.request = Object.create(request);        this.response = Object.create(response);     }  }

app.use()添加中间件时做了什么

  use(fn) {    // 省略fn判断    this.middleware.push(fn);    return this;  }

app.listen(3000)监听端口

  listen() {    const server = http.createServer(this.callback());    return server.listen.apply(server, arguments);  }  callback() {    // 返回一个用于dispatch中间件的函数，默认从index = 0开始    const fn = compose(this.middleware);    if (!this.listeners('error').length) this.on('error', this.onerror);    return (req, res) => {      res.statusCode = 404;      // 使用请求和响应对象创建上下文对象      const ctx = this.createContext(req, res);      onFinished(res, ctx.onerror);      // 执行中间件函数并响应      fn(ctx).then(() => respond(ctx)).catch(ctx.onerror);    };  }function compose (middleware) {  if (!Array.isArray(middleware)) throw new TypeError('Middleware stack must be an array!')  for (const fn of middleware) {    if (typeof fn !== 'function') throw new TypeError('Middleware must be composed of functions!')  }  // 提供公开api用于调度中间件函数  return function (context, next) {    // last called middleware #    let index = -1    return dispatch(0)    function dispatch (i) {      if (i <= index) return Promise.reject(new Error('next() called multiple times'))      // 改变变量index      index = i      // 取出当前中间件      let fn = middleware[i]      if (i === middleware.length) fn = next      if (!fn) return Promise.resolve()      try {        return Promise.resolve(fn(context, function next () {          //触发下一中间件执行          return dispatch(i + 1)        }))      } catch (err) {        return Promise.reject(err)      }    }  }}  // 创建上下文对象  createContext(req, res) {    const context = Object.create(this.context);    const request = context.request = Object.create(this.request);    const response = context.response = Object.create(this.response);    context.app = request.app = response.app = this;    context.req = request.req = response.req = req;    context.res = request.res = response.res = res;    request.ctx = response.ctx = context;    request.response = response;    response.request = request;    context.onerror = context.onerror.bind(context);    context.originalUrl = request.originalUrl = req.url;    context.cookies = new Cookies(req, res, {      keys: this.keys,      secure: request.secure    });    context.accept = request.accept = accepts(req);    context.state = {};    return context;  }

思维导图