Koa源码学习

[rudyxu1102]

Koa

Koa代码非常简洁，主要实现了中间件的扩展的洋葱模型，其他功能可以通过中间件的形式引入。

可以带着下面的问题去阅读源码：

• 中间件是什么时候开始执行的?
• 洋葱模型是怎么实现的？
• app.use是怎么引入中间件的？

核心源码

地址

中间件是什么时候开始执行的?

当执行app.listen(3000)的时候会开始执行中间件。

  listen(...args) {
    // this.callback 会按顺序去执行中间件
    const server = http.createServer(this.callback());
    return server.listen(...args);
  }

app.use是怎么引入中间件的？

  /**
   * Use the given middleware `fn`.
   *
   * Old-style middleware will be converted.
   *
   * @param {Function} fn
   * @return {Application} self
   * @api public
   */

  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 || '-');
    // 将函数放入中间件数组
    this.middleware.push(fn);
    return this;
  }

洋葱模型是怎么实现的？

通过compose函数将中间件数组包装成一个洋葱函数fn，执行fn会执行dispatch(0)，即把dipatch(1)作为next的参数去执行第一个中间件，递归执行所有的中间件，形成一个洋葱模型。

koa-compose源码地址

/**
 * Compose `middleware` returning
 * a fully valid middleware comprised
 * of all those which are passed.
 *
 * @param {Array} middleware
 * @return {Function}
 * @api public
 */

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, dispatch.bind(null, i + 1)));
      } catch (err) {
        return Promise.reject(err)
      }
    }
  }
}

listen中执行的this.callback，先通过compose包装了一层。

 /**
   * 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;
  }