Koa 中间件源代码分析

首先是通过 app.use 收集中间件函数

/**
 * 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 || '-');
  // 收集中间件函数到数组中
  this.middleware.push(fn);
  return this;
}

返回一个用于原生 http server 的回调

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

callback() {
    // 构建中间件机制模型
  const fn = compose(this.middleware);

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

  const handleRequest = (req, res) => {
    const ctx = this.createContext(req, res);
    if (!this.ctxStorage) {
      return this.handleRequest(ctx, fn);
    }
      // 执行中间件相关函数
    return this.ctxStorage.run(ctx, async() => {
      return await this.handleRequest(ctx, fn);
    });
  };

  return handleRequest;
}

中间件机制模型 koa-compose

/**
 * Expose compositor.
 */

module.exports = compose

/**
 * 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
   */

  // 中间件机制核心代码
  return function (context, next) {
    // last called middleware #
    let index = -1
    return dispatch(0)
    function dispatch(i) {
      // i 始终大于 index, 否则是 next 被调用多次
      if (i <= index) return Promise.reject(new Error('next() called multiple times'))
      index = i
      let fn = middleware[i] // 获取到当前执行的中间件函数
      if (i === middleware.length) fn = next // 如果 i-arr.length 那么则执行完毕，fn === undefined ，
      if (!fn) return Promise.resolve() // 当 fn 处于 undefined 时，结束执行，
      try {
        return Promise.resolve(fn(context, dispatch.bind(null, i + 1))); // 执行 中间件函数，并且把dispath(i+1) 当作是 next（中间件函数内部的 next ） 函数执行
      } catch (err) {
        return Promise.reject(err)
      }
    }
  }
}