ServeHTTP方法从何而来

Question

This has confused me for the last month of learning Go:

func Auth(next http.HandlerFunc) http.HandlerFunc {

    return func(w http.ResponseWriter, r *http.Request) {  // hmmmm

        // ...
        next.ServeHTTP(w, r)
    }
}

here we can see that the Auth func returns type http.HandlerFunc. That type is just a func. So when you call next.ServeHTTP, when/where is that method defined?

Answer 1

https://golang.org/src/net/http/server.go?s=59707:59754#L1950

// The HandlerFunc type is an adapter to allow the use of
// ordinary functions as HTTP handlers. If f is a function
// with the appropriate signature, HandlerFunc(f) is a
// Handler that calls f.
type HandlerFunc func(ResponseWriter, *Request)

// ServeHTTP calls f(w, r).
func (f HandlerFunc) ServeHTTP(w ResponseWriter, r *Request) {
    f(w, r)
}

Literally any function with the signature func(ResponseWriter, *Request) can be cast to a HandlerFunc, which gives it the method ServeHTTP - which then simply calls the function.