양파 모형

1. app.use()

 app.use(async (ctx, next) => {}); //  

소스 코드:

  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;
  }

2. app.listen()

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

소스 코드:

  listen(...args) {
    debug('listen');
    // this.callback() connection listener,  ， 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() {
    //  koa-compose,  ， 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)  promise
    /* 
       return Promise.resolve(fn(context, function next () {
         return dispatch(i + 1)
       }))
        ， context next ，next 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  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)
      }
    }
  }
}

총결산

전체 프로세스는: app.use 함수는 중간부품을 중간부품 그룹에 넣습니다.앱이 http 요청을 받았을 때connectionlistener 함수를 실행합니다. 이 함수에서는koa-compose에서 되돌아오는 함수(return dispatch(0)를 실행합니다. 이때 첫 번째 중간부품 함수를 실행하고content와next 함수를 전송합니다. 각각next 함수는promise를 되돌려줍니다.중간부품 함수에서 await next (), 즉 다음 중간부품 함수 (return dispatch (i + 1)) 를 실행하고, 모든 중간부품이 실행될 때까지 undefined 값을 fn (i===middleware.length) fn =next) 에 부여합니다.