恐慌：最后一个参数必须为http.HandlerFunc类型

Question

I have this helper function, which is compiling fine:

func Middleware(adapters ...interface{}) http.HandlerFunc {

    log.Info("length of adapters:", len(adapters))

    if len(adapters) < 1 {
        panic("Adapters need to have length > 0.");
    }

    h, ok := (adapters[len(adapters)-1]).(http.HandlerFunc)

    if ok == false {
        panic("Last argument needs to be of type http.HandlerFunc") // ERROR HERE
    }

    adapters = adapters[:len(adapters)-1]

    for _, adapt := range adapters {
        h = (adapt.(AdapterFunc))(h)
    }

    return h

}

I am calling it like so:

router.HandleFunc("/share", h.makeGetMany(v)).Methods("GET")

func (h Handler) makeGetMany(v Injection) http.HandlerFunc {
    return mw.Middleware(
        mw.Allow("admin"),
        func(w http.ResponseWriter, r *http.Request) {
            log.Println("now we are sending response.");
            json.NewEncoder(w).Encode(v.Share)
        },
    )
}

the problem is that I am getting this error and I cannot figure out why:

    panic: Last argument needs to be of type http.HandlerFunc

    goroutine 1 [running]:
    huru/mw.Middleware(0xc420083d40, 0x2, 0x2, 0xc42011f3c0)
            /home/oleg/codes/huru/api/src/huru/mw/middleware.go:301 +0x187
    huru/routes/share.Handler.makeGetMany(0xc4200ae1e0, 0x10)
            /home/oleg/codes/huru/api/src/huru/routes/share/share.go:62 +0x108

it does confirm that the length of the adapters slice is 2:

 length of adapters:2

anyone know why that type assertion would fail in this case? Makes no sense. Maybe I am not actually retrieving the last argument of the slice or something? Is there a better way to pop the last argument off the slice?

Answer 1

You need to wrap the 2nd argument of mw.Middleware() statement into http.Handler type by using http.HandlerFunc().

func (h Handler) makeGetMany(v Injection) http.HandlerFunc {
    return mw.Middleware(
        mw.Allow("admin"),
        http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
            log.Println("now we are sending response.");
            json.NewEncoder(w).Encode(v.Share)
        }),
    )
}