傻瓜式解读koa中间件处理模块koa-compose的使用

Z时代
2024-01-10
分类：综合

最近需要单独使用到koa-compose这个模块，虽然使用koa的时候大致知道中间件的执行流程，但是没仔细研究过源码用起来还是不放心(主要是这个模块代码少，多的话也没兴趣去研究了)。

koa-compose看起来代码少，但是确实绕。闭包，递归，Promise。。。看了一遍脑子里绕不清楚。看了网上几篇解读文章，都是针对单行代码做解释，还是绕不清楚。最后只好采取一种傻瓜的方式：

koa-compose去掉一些注释，类型校验后，源码如下：


function compose (middleware) {
 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)
   }
  }
 }
}

写出如下代码：


var index = -1;
function compose() {
  return dispatch(0)
}
function dispatch (i) {
   if (i <= index) return Promise.reject(new Error('next() called multiple times'))
   index = i
   var fn = middleware[i]
   if (i === middleware.length) fn = next
   if (!fn) return Promise.resolve('fn is undefined')
   try {
    return Promise.resolve(fn(context, dispatch.bind(null, i + 1)));
   } catch (err) {
    return Promise.reject(err)
   }
 }
 function f1(context,next){
  console.log('middleware 1');
  next().then(data=>console.log(data));
  console.log('middleware 1');
  return 'middleware 1 return';
 }
 function f2(context,next){
  console.log('middleware 2');
  next().then(data=>console.log(data));
  console.log('middleware 2');
  return 'middleware 2 return';
 }
 function f3(context,next){
  console.log('middleware 3');
  next().then(data=>console.log(data));
  console.log('middleware 3');
  return 'middleware 3 return';
 }
var middleware=[
 f1,f2,f3
]
var context={};
var next=function(context,next){
  console.log('middleware 4');
  next().then(data=>console.log(data));
  console.log('middleware 4');
  return 'middleware 4 return';
};
compose().then(data=>console.log(data));

直接运行结果如下：

"middleware 1"
"middleware 2"
"middleware 3"
"middleware 4"
"middleware 4"
"middleware 3"
"middleware 2"
"middleware 1"
"fn is undefined"
"middleware 4 return"
"middleware 3 return"
"middleware 2 return"
"middleware 1 return"

按着代码运行流程一步步分析：


dispatch(0)

i==0,index==-1 i>index 往下


index=0
fn=f1
Promise.resolve(f1(context, dispatch.bind(null, 0 + 1)))

这就会执行


f1(context, dispatch.bind(null, 0 + 1))

进入到f1执行上下文


console.log('middleware 1');

输出middleware 1


next()

其实就是调用dispatch(1) bind的功劳

递归开始


dispatch(1)

i==1,index==0 i>index 往下


index=1
fn=f2
Promise.resolve(f2(context, dispatch.bind(null, 1 + 1)))

这就会执行


f2(context, dispatch.bind(null, 1 + 1))

进入到f2执行上下文


console.log('middleware 2');

输出middleware 2


next()

其实就是调用dispatch(2)

接着递归


dispatch(2)

i==2,index==1 i>index 往下


index=2
fn=f3
Promise.resolve(f3(context, dispatch.bind(null, 2 + 1)))

这就会执行


f3(context, dispatch.bind(null, 2 + 1))

进入到f3执行上下文


console.log('middleware 3');

输出middleware 3


next()

其实就是调用dispatch(3)

接着递归


dispatch(3)

i==3,index==2 i>index 往下


index=3
i === middleware.length
fn=next
Promise.resolve(next(context, dispatch.bind(null, 3 + 1)))

这就会执行


next(context, dispatch.bind(null, 3 + 1))

进入到next执行上下文


console.log('middleware 4');

输出middleware 4


next()

其实就是调用dispatch(4)

接着递归


dispatch(4)

i==4,index==3 i>index 往下


index=4
fn=middleware[4]
fn=undefined
reuturn Promise.resolve('fn is undefined')

回到next执行上下文


console.log('middleware 4');

输出middleware 4


return 'middleware 4 return'
Promise.resolve('middleware 4 return')

回到f3执行上下文


console.log('middleware 3');

输出middleware 3


return 'middleware 3 return'
Promise.resolve('middleware 3 return')

回到f2执行上下文


console.log('middleware 2');

输出middleware 2


return 'middleware 2 return'
Promise.resolve('middleware 2 return')

回到f1执行上下文


console.log('middleware 1');

输出middleware 1


return 'middleware 1 return'
Promise.resolve('middleware 1 return')

回到全局上下文

至此已经输出

"middleware 1"
"middleware 2"
"middleware 3"
"middleware 4"
"middleware 4"
"middleware 3"
"middleware 2"
"middleware 1"

那么

"fn is undefined"
"middleware 4 return"
"middleware 3 return"
"middleware 2 return"
"middleware 1 return"

怎么来的呢

回头看一下，每个中间件里都有

next().then(data=>console.log(data));

按照之前的分析，then里最先拿到结果的应该是next中间件的，而且结果就是Promise.resolve('fn is undefined')的结果，然后分别是f4,f3,f2,f1。那么为什么都是最后才输出呢？


Promise.resolve('fn is undefined').then(data=>console.log(data));
console.log('middleware 4');

运行一下就清楚了

或者


setTimeout(()=>console.log('fn is undefined'),0);
console.log('middleware 4');

整个调用过程还可以看成是这样的：


function composeDetail(){
 return Promise.resolve(
  f1(context,function(){
   return Promise.resolve(
    f2(context,function(){
     return Promise.resolve(
      f3(context,function(){
       return Promise.resolve(
        next(context,function(){
         return Promise.resolve('fn is undefined')
        })
       )
      })
     )
    })
   )
  })
 )
}
composeDetail().then(data=>console.log(data));