koa2中间件原理

源码：koa-compose.js

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)
      }
    }
  }
}

koa2的中间件流向是先按顺序执行next以前内容，然后对next以后内容进行回溯。
middleware是使用use添加的中间件数组，
核心代码只有一句Promise.resolve(fn(context, dispatch.bind(null, i + 1))); dispatch.bind(null, i + 1)替换掉了next，也就是说将所有的中间件以next占位符进行替换，类似于react setState的事务抽取机制