koa中next()的原理是什么？

Answer 1

next是函数，返回的是promise。

app.use(async (ctx, next) => {
 console.log(1);
 await next();
 console.log(2);
});

use把函数放进this.middleware中，
listen时，compose是koa-compose，转换。

class Application extends Emitter {
  // 代码有简化组合
  listen(){
    const  fnMiddleware = compose(this.middleware);
    fnMiddleware(ctx).then(handleResponse).catch(onerror);
  },
 use(fn){
  this.middleware.push(fn);
  return this;
 }
}

koa-compose源码

function compose (middleware) {
  // 校验middleware 是数组和数组每一项都是函数的校验

  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!')
  }


  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)
      }
    }
  }
}

移除一些校验和报错代码翻译下就是这样，也就是常说的洋葱模型。

// simpleKoaCompose
const [fn1, fn2, fn3] = this.middleware;
const fnMiddleware = function(context){
    return Promise.resolve(
      fn1(context, function next(){
        return Promise.resolve(
          fn2(context, function next(){
              return Promise.resolve(
                  fn3(context, function next(){
                    return Promise.resolve();
                  })
              )
          })
        )
    })
  );
};
fnMiddleware(ctx).then(handleResponse).catch(onerror);

具体源码可以看我这篇文章。
若川：学习 koa 源码的整体架构，浅析koa洋葱模型原理和co原理