gin 框架中middleware 怎么嵌套调用的

//通过Next函数实现嵌套,举例如下
package main

import "fmt"

type A struct{
       index int
       handles HandlesFunc
}

func (a *A) Handle(f HandleFunc){
       a.handles=append(a.handles,f)
}

func (a* A) Next(){
       a.index++    //这个是关键
       for i:=len(a.handles);a.index<i;a.index++ {
              a.handles[a.index](a)
       }
}

type HandleFunc func(a *A)
type HandlesFunc []HandleFunc

func main(){
       a:=&A{
              index:-1,
              handles:make(HandlesFunc,0),
       }
       a.Handle(func(a* A){fmt.Println("in function 1");a.Next();fmt.Println("exit function 1")})
       a.Handle(func(a* A){fmt.Println("in function 2");a.Next();fmt.Println("exit function 2")})
       a.Next()
}


//结果如下

in function 1
in function 2
exit function 2
exit function 1