机制

2023年05月31日

柏拉文

越努力，越幸运

一、同步

---

1.1 同步拼接

function compose(req, res, middleware) {
  const next = () => {
    const fn = middleware.shift();
    if (fn) {
      fn(req, res, next);
    }
  };
  next();
}

function middleware1(req, res, next) {
  req.message = "aaa";
  next();
  console.log(req.message);
}
function middleware2(req, res, next) {
  req.message += "bbb";
  next();
}
function middleware3(req, res, next) {
  req.message += "ccc";
}

const req = {};
const res = {};
const middleware = [middleware1, middleware2, middleware3];
compose(req, res, middleware);

结果: aaabbbccc

分析: 其实express和koa同步实现没有什么区别，都是从上往下执行，当执行到next的时候，会将控制权交给下一个中间件，直到下一个中间件不再执行 next() 后，将会沿路折返，将控制权依次交还给前一个中间件，当交还到第一个中间件res.end时候，req.message里面就为aaabbbccc。

二、异步

---

2.1 异步拼接

function compose(req, res, middleware) {
  const next = () => {
    const fn = middleware.shift();
    if (fn) {
      fn(req, res, next);
    }
  };
  next();
}

async function middleware1(req, res, next) {
  req.message = "aaa";
  await next();
  console.log(req.message);
}
async function middleware2(req, res, next) {
  req.message += "bbb";
  await next();
}

function promiseFun() {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve("ccc");
    }, 3000);
  });
}
async function middleware3(req, res, next) {
  const result = await promiseFun();
  req.message += result;
}

const req = {};
const res = {};
const middleware = [middleware1, middleware2, middleware3];
compose(req, res, middleware);

结果: aaabbb

分析: 在express中next内部的操作都是同步的，不管下一个中间件是否有异步操作，所以加上await也没有意义