Capturing the HTTP status code from http.ResponseWriter

main.go

package main

import (
    "fmt"
    "log"
    "net/http"
)

func main() {
    // We need to cast handleRoot to a http.HandlerFunc since wrapHandlerWithLogging
    // takes a http.Handler, not an arbitrary function.
    handler := wrapHandlerWithLogging(http.HandlerFunc(handleRoot))
    http.Handle("/", handler)

    log.Fatal(http.ListenAndServe(":8080", nil))
}

// ---- Handlers

func handleRoot(w http.ResponseWriter, req *http.Request) {
    if req.URL.Path != "/" {
        http.NotFound(w, req)
        return
    }

    fmt.Fprintf(w, "Hello, World!")
}

// ---- Logging

func wrapHandlerWithLogging(wrappedHandler http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
        log.Printf("--> %s %s", req.Method, req.URL.Path)

        lrw := NewLoggingResponseWriter(w)
        wrappedHandler.ServeHTTP(lrw, req)

        statusCode := lrw.statusCode
        log.Printf("<-- %d %s", statusCode, http.StatusText(statusCode))
    })
}

type loggingResponseWriter struct {
    http.ResponseWriter
    statusCode int
}

func NewLoggingResponseWriter(w http.ResponseWriter) *loggingResponseWriter {
    // WriteHeader(int) is not called if our response implicitly returns 200 OK, so
    // we default to that status code.
    return &loggingResponseWriter{w, http.StatusOK}
}

func (lrw *loggingResponseWriter) WriteHeader(code int) {
    lrw.statusCode = code
    lrw.ResponseWriter.WriteHeader(code)
}

the solution works ! But can you explain when and where did WriteHeader is being called / invoke?