function a() {
    console.log(1);
    b(); // 函数b进栈，函数a将执行权交给函数b
    console.log(2); // 函数a执行完毕出栈
}
function b() {
    console.log(3);
    c(); // 函数c进栈，函数b将执行权交给函数c
    console.log(4); // 函数b执行完毕出栈，将执行权返还给函数a
}
function c() {
    console.log(5); // 函数c执行完毕出栈，将执行权返还给函数b
}
a(); // 函数a进栈

const Koa = require('koa');
const app = new Koa();

// 中间件1
app.use(async (ctx, next) => {
    console.log(1);
    next(); // 中间件1暂停执行，并将执行权交给中间件2
    console.log(2); // 中间件1继续执行
});
// 中间件2
app.use(async (ctx, next) => {
    console.log(3);
    next(); // 中间件2暂停执行，并将执行权交给中间件3
    console.log(4); // 中间件2继续执行，并将执行权返还给中间件1
});
// 中间件3
app.use(async (ctx, next) => {
    console.log(5);
    next(); // 中间件3暂停执行
    console.log(6); // 因为没有后续中间件，中间件3继续执行，并将执行权返还给中间件2
});

app.listen(8088, () => {
    console.log('server start at port 8088');
});

const app = new Koa();
app.use(logger);

use(fn) {
 if (typeof fn !== 'function') throw new TypeError('middleware must be a function!');
    if (isGeneratorFunction(fn)) {
 deprecate('Support for generators will been removed in v3. ' +
                'See the documentation for examples of how to convert old middleware ' +
                'https://github.com/koajs/koa/tree/v2.x#old-signature-middleware-v1x');
         fn = convert(fn);
 }
 debug('use %s', fn._name || fn.name || '-');
 this.middleware.push(fn); return this;
}

http.createServer(app.callback()).listen(...)

// koa-compose
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!')
  }
  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 //执行后fn为undefined,所以下一步将resolve()
      if (!fn) return Promise.resolve()
      try {
        return Promise.resolve(fn(context, function next ( ) {
          return dispatch(i + 1)
        }))
      } catch (err) {
        return Promise.reject(err)
      }
    }
  }
}
callback() {
    const fn = compose(this.middleware);

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

    return (req, res) => {
      res.statusCode = 404;
      const ctx = this.createContext(req, res);
      onFinished(res, ctx.onerror);
      fn(ctx).then(() => respond(ctx)).catch(ctx.onerror);
    };
  }

const fn = compose(this.middleware);

return (req, res) => {
      res.statusCode = 404;
      const ctx = this.createContext(req, res);//对req,res,cookies封装
      onFinished(res, ctx.onerror); //当连接结束报错时执行ctx.onerror
      fn(ctx).then(() => respond(ctx)).catch(ctx.onerror);
    };