浅析Koa2源码中洋葱模型的实现

const Koa = require('koa')

 const app = new Koa();

 app.use(async (ctx, next)=>{
     console.log(1)
     await next();
     console.log('1-1')
 });
 
 app.use(async (ctx, next) => {
     console.log(2)
     await next();
     console.log("2-1")
 })

 app.use(async (ctx, next) => {
     console.log(3)
 })

 app.listen(3000,function(){
     console.log("3000")
 })

//端口号3000
1 2 3 2-1 1-1

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

callback() {
    const fn = compose(this.middleware);

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

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

    return handleRequest;
 }

this.middleware = [];//中间件集合

use(fn) {
   if (typeof fn !== 'function') throw new TypeError('middleware must be a function!');
   if (isGeneratorFunction(fn)) {
     deprecate('Support for generators will be removed in v3. ' +
               'See the documentation for examples of how to convert old middleware ' +
               'https://github.com/koajs/koa/blob/master/docs/migration.md');
     fn = convert(fn);
   }
   debug('use %s', fn._name || fn.name || '-');
  
   this.middleware.push(fn);
   return this;
 }

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!')
  }

  /**
   * @param {Object} context
   * @return {Promise}
   * @api public
   */

  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'))
      index = i
      let fn = middleware[i]
      if (i === middleware.length) fn = next
      if (!fn) return Promise.resolve()
      try {
        return Promise.resolve(fn(context, function next () {
          return dispatch(i + 1)
        }))
      } catch (err) {
        return Promise.reject(err)
      }
    }
  }
}

return Promise.resolve(fn(context, function next () {
          return dispatch(i + 1)
  }))

/**
   * 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);
  }

  /**
   * 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.cookies = new Cookies(req, res, {
      keys: this.keys,
      secure: request.secure
    });
    request.ip = request.ips[0] || req.socket.remoteAddress || '';
    context.accept = request.accept = accepts(req);
    context.state = {};
    return context;
  }