ki18n: derive language from request context via strategy chain

Replace Tr(lang, ...) with Tr(ctx, ...) — language is now resolved once
per request by LangMiddleware and stored in context. Callers no longer
need to thread a lang string through to templates.

Adds Strategy interface with CookieStrategy and AcceptLanguageStrategy
built-ins; middleware chains them in order, falling back to en-GB.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

1 files changed, 69 insertions, 4 deletions

diff --git a/ki18n/ki18n.go b/ki18n/ki18n.go
index f2bed8d..8f20ad4 100644
--- a/ki18n/ki18n.go
+++ b/ki18n/ki18n.go
@@ -1,12 +1,81 @@
 package ki18n
 
 import (
+	"context"
 	"io/fs"
+	"net/http"
 
 	"github.com/a-h/templ"
 	"github.com/kataras/i18n"
 )
 
+type contextKey struct{}
+
+const defaultLang = "en-GB"
+
+type Strategy interface {
+	Detect(r *http.Request) string
+}
+
+type cookieStrategy struct{ name string }
+
+func (s cookieStrategy) Detect(r *http.Request) string {
+	c, err := r.Cookie(s.name)
+	if err != nil {
+		return ""
+	}
+	return c.Value
+}
+
+func CookieStrategy(name string) Strategy {
+	return cookieStrategy{name: name}
+}
+
+type acceptLanguageStrategy struct{}
+
+func (acceptLanguageStrategy) Detect(r *http.Request) string {
+	lang := i18n.Default.GetLocale(r).Language()
+	if lang == defaultLang {
+		if r.Header.Get("Accept-Language") == "" {
+			return ""
+		}
+	}
+	return lang
+}
+
+var AcceptLanguageStrategy Strategy = acceptLanguageStrategy{}
+
+func LangMiddleware(strategies ...Strategy) func(http.Handler) http.Handler {
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			lang := ""
+			for _, s := range strategies {
+				if l := s.Detect(r); l != "" {
+					lang = l
+					break
+				}
+			}
+			if lang == "" {
+				lang = defaultLang
+			}
+			ctx := context.WithValue(r.Context(), contextKey{}, lang)
+			next.ServeHTTP(w, r.WithContext(ctx))
+		})
+	}
+}
+
+func Tr(ctx context.Context, format string, args ...any) templ.Component {
+	lang, _ := ctx.Value(contextKey{}).(string)
+	if lang == "" {
+		lang = defaultLang
+	}
+	msg := i18n.Tr(lang, format, args...)
+	if msg == "" {
+		msg = format
+	}
+	return templ.Raw(msg)
+}
+
 func Init(fs fs.FS) error {
 	loader, err := i18n.FS(fs, "*/*.yml")
 	if err != nil {
@@ -19,7 +88,3 @@ func Init(fs fs.FS) error {
 	i18n.Default = inst
 	return nil
 }
-
-func Tr(lang string, format string, args ...any) templ.Component {
-	return templ.Raw(i18n.Tr(lang, format, args...))
-}