Help with intercepting middleware, and writing responses

[gfffrtt]

Hello everyone, I'm just building a project with templ and go, and need help with understanding something.

func isHtml(w http.ResponseWriter) bool {
	return strings.HasPrefix(w.Header().Get("content-type"), "text/html")
}

type App struct {
	Router *http.ServeMux
}

func NewApp() *App {
	return &App{
		Router: http.NewServeMux(),
	}
}

func (a *App) Middleware(next http.HandlerFunc) http.HandlerFunc {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		if !isHtml(w) {
			next.ServeHTTP(w, r)
		}

		w.Write([]byte("<p>Loading...</p>"))
		next.ServeHTTP(w, r)
	})
}

func (a *App) Component(route string, handler http.HandlerFunc) {
	a.Router.HandleFunc(route, a.Middleware(handler))
}

func (a *App) Listen(port string) {
	http.ListenAndServe(port, a.Router)
}

func main() {
	app := NewApp()

	app.Component("/", func(w http.ResponseWriter, r *http.Request) {
		views.Component().Render(context.Background(), w)
	})

	app.Listen(":3000")
}

Here is the code I've simplified it a bit but this is the essence of it, I'm adding some middleware that intercepts "text/html" responses and writes some extra html on the, but the result is unexpected, instead of adding the extra html to the bottom of the page, I'm getting a duplication of the html.

Am I doing something really wrong here, or is this expected?

[joerdav]

I believe you might need to return here:

if !isHtml(w) {
			next.ServeHTTP(w, r)
			return
}

[gfffrtt]

I believe you might need to return here:

if !isHtml(w) {
			next.ServeHTTP(w, r)
			return
}

Yes it was that, but now I cant get the content type from the header, every time I print it print []map

func isHtml(w http.ResponseWriter) bool {
	return strings.HasPrefix(w.Header().Get("Content-Type"), "text/html")
}

type App struct {
	Router *chi.Mux
}

func NewApp() *App {
	router := chi.NewRouter()
	router.Use(middleware.Logger)
	return &App{
		Router: chi.NewRouter(),
	}
}

func (a *App) Middleware(next http.HandlerFunc) http.HandlerFunc {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		if !isHtml(w) {
			next.ServeHTTP(w, r)
			return
		}

		w.Write([]byte("<p>Loading...</p>"))
		next.ServeHTTP(w, r)
	})
}

func (a *App) Component(route string, handler http.HandlerFunc) {
	a.Router.Get(route, a.Middleware(handler))
}

func (a *App) Listen(port string) {
	http.ListenAndServe(port, a.Router)
}

func main() {
	app := NewApp()

	app.Component("/", func(w http.ResponseWriter, r *http.Request) {
		w.Header().Add("Content-Type", "text/html")
		views.Component().Render(context.Background(), w)
	})

	app.Listen(":3000")
}

[joerdav]

This may not be the only issue, but I can see that you are writing to the response before setting headers, you should set the header in the middleware.

[gfffrtt]

@joerdav Thank you for your help, but I figured out what I actually had to do, what I wanted is to intercept the reponse, so I had to first run the next handler than modify the response if it is html, but thank you man.