compose.js

一、认识

---

koa-compose 是 Koa 实现中间件的核心逻辑。

二、源码

---

koa-compose

'use strict'

/**
 * 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
   */

  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)
      }
    }
  }
}

三、实现

---

function compose(middleware) {
  if (!Array.isArray(middleware)) {
    throw new TypeError("middleware 必须为函数数组");
  }
  for (const fn of middleware) {
    if (typeof fn !== "function") {
      throw new TypeError("middleware 元素必须为函数");
    }
  }

  return function (context, next) {
    let index = -1;
    function dispatch(i) {
      if (i <= index) {
        return Promise.reject(new Error("next() 函数不可以调用多次"));
      }
      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 (error) {
        return Promise.reject(error);
      }
    }
    return dispatch(0);
  };
}

四、调试

---

let fn1 = async function (context, next) {
  console.log("fn1 之前");
  await next();
  console.log("fn1 之后");
};
let fn2 = async function (context, next) {
  console.log("fn2 之前");
  await next();
  console.log("fn2 之后");
};
let fn3 = async function (context, next) {
  console.log("fn3 之前");
  await next(); // 最后一个中间件的 next 可有可无
  console.log("fn3 之后");
};

let middleware = [fn1, fn2, fn3];
compose(middleware)({});