kurz/web/i18n/i18ner.go

package i18n

import (
	"context"
	"errors"
	"net/http"

	"golang.org/x/text/message"
)

/*
WithPrinter() is a higher-order function injecting into the request context a
printer for the language best matching the Accept-Language header in the incoming
request.

The wrapped handler can use Printer() to get a message printer instance
configured for the best available language for the request.
*/
func WithPrinter(h http.Handler) http.Handler {
	h2 := func(w http.ResponseWriter, r *http.Request) {
		// Utilise le matcher de message.DefaultCatalog.
		tag := message.MatchLanguage(r.Header.Get("Accept-Language"))
		c := context.WithValue(r.Context(), "printer", message.NewPrinter(tag))
		h.ServeHTTP(w, r.WithContext(c))
	}
	return http.HandlerFunc(h2)
}

/*
Printer() returns a message printer configured for the language best matching the
Accept-Language in the request, or panic if the handler invoking it was not
wrapped by a WithPrinter() call.
 */
func Printer(r *http.Request) *message.Printer {
	p, ok := r.Context().Value("printer").(*message.Printer)
	if !ok {
		panic(errors.New("trying to use i18n.Printer in a handler not wrapped with i18.WithPrinter"))
	}
	return p
}