1. app.use()

 app.use(async (ctx, next) => {}); // 將中間件放入中間件數(shù)組

源碼：

  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); // 將中間件放入中間件數(shù)組
    return this;
  }

2. app.listen()

module.exports = app.listen(config.port, '0.0.0.0', () => {
  console.log('進入app');
});

源碼：

  listen(...args) {
    debug('listen');
    // this.callback()為connection listener, 當(dāng)客戶端連接上服務(wù)器時會觸發(fā)該函數(shù)，即handleRequest
    const server = http.createServer(this.callback()); 
    return server.listen(...args);
  }

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

  callback() {
    // 將中間件數(shù)組傳入koa-compose, 得到一個返回函數(shù)，將該函數(shù)傳入this.handleRequest
    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;
  }

  /**
   * 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);
    // fnMiddleware(ctx) 返回promise
    /* 
       return Promise.resolve(fn(context, function next () {
         return dispatch(i + 1)
       }))
       首先執(zhí)行第一個中間件函數(shù)，傳入context和next函數(shù)，next返回也是返回promise
   */
    return fnMiddleware(ctx).then(handleResponse).catch(onerror);
  }

3. 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!')
  }

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

  return function (context, next) {
    // next 為undefined
    // 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)
      }
    }
  }
}

總結(jié)

整個流程為：
app.use函數(shù)將中間件放入中間件數(shù)組。當(dāng)app接收到http請求時，會執(zhí)行connection listener函數(shù)，這個函數(shù)中會去執(zhí)行koa-compose返回的函數(shù)(return dispatch(0))，此時會執(zhí)行第一個中間件函數(shù)，傳入content和next函數(shù)，每個next函數(shù)也會返回一個promise。在中間件函數(shù)中await next()，即執(zhí)行下一個中間件函數(shù)(return dispatch(i + 1))，直到所有的中間件都執(zhí)行結(jié)束后，將undefined賦值給fn(if (i === middleware.length) fn = next)。