app.listen(3000)的源码 和callback

　　const http = require('http');

 listen(...args) {
    debug('listen');
    const server = http.createServer(this.callback());
    return server.listen(...args);
  }

const compose = require('koa-compose');

 /**
   * Return a request handler callback
   * for node's native http server.
   *
   * @return {Function}
   * @api public
   */

  callback() {
    const fn = compose(this.middleware);  //合并中间件

    if (!this.listenerCount('error')) this.on('error', this.onerror);  //有error消息， 比如app.on('error',(err)=>console.log('err',err))

    const handleRequest = (req, res) => {
      const ctx = this.createContext(req, res);
      return this.handleRequest(ctx, fn);
    };

    return handleRequest;
  }

  /**
   * Initialize a new context.
   *
   * @api private
   */

  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.originalUrl = request.originalUrl = req.url;
    context.state = {};
    return context;
  }

 /**
   * Handle request in callback.
   *
   * @api private
   */

  handleRequest(ctx, fnMiddleware) {
    const res = ctx.res;
    res.statusCode = 404;
    const onerror = err => ctx.onerror(err);
    const handleResponse = () => respond(ctx);
    onFinished(res, onerror);
    return fnMiddleware(ctx).then(handleResponse).catch(onerror);
  }

 /**

   * Return a request handler callback

   * for node's native http server.

   *

   * @return {Function}

   * @api public

   */

  callback() {

    const fn = compose(this.middleware);

    if (!this.listenerCount('error')) this.on('error', this.onerror);

    const handleRequest = (req, res) => {

      const ctx = this.createContext(req, res);

      return this.handleRequest(ctx, fn);

    };

    return handleRequest;

  }