Lang Negotiation Middleware

middleware/lang is the HTTP language negotiation middleware. It does not load message catalogs; lang.Provider() does that. The middleware only reads request language preferences and stores the current request *lang.Translator in ctx.Context().

Install

go get github.com/duxweb/runa/lang
go get github.com/duxweb/runa/middleware/lang

Connect to an application

package main

import (
    "context"

    "github.com/duxweb/runa"
    "github.com/duxweb/runa/lang"
    langmw "github.com/duxweb/runa/middleware/lang"
    "github.com/duxweb/runa/route"
)

func main() {
    app := runa.New()
    app.Install(
        route.Provider(route.Addr(":8080")),
        lang.Provider(),
    )

    route.Default().Use(langmw.New(
        langmw.Query("lang"),
        langmw.Cookie("lang"),
        langmw.Header("Accept-Language"),
    ))

    route.Default().Get("/", func(ctx *route.Context) error {
        tr := lang.From(ctx.Context())
        return ctx.Text(tr.T("user_welcome", "Name", "Runa"))
    })

    if err := app.Run(context.Background()); err != nil {
        panic(err)
    }
}

Default sources

When no sources are passed, langmw.New() uses:

langmw.Query("lang")
langmw.Cookie("lang")
langmw.Header("Accept-Language")

So ?lang=zh-CN wins first, then cookie, then browser Accept-Language.

Relationship with route.Context

The middleware does two things:

• ctx.SetContext(lang.WithTranslator(ctx.Context(), translator))
• ctx.SetLang(translator.Locale())

Application code can then use:

tr := lang.From(ctx.Context())
current := ctx.Lang()

Use lang.From(ctx.Context()) for translation and ctx.Lang() when you only need the current locale string.

Use with template t

When lang/view.Provider() is installed, {{ t "key" }} automatically uses the translator stored in request context by this middleware:

app.Install(
    lang.Provider(),
    view.Provider(),
    langview.Provider(),
)
route.Default().Use(langmw.New())

Template:

<h1>{{ t "user_welcome" "Name" .Name }}</h1>

Common mistakes

Mounting middleware without lang.Provider

The middleware needs lang.Default() to create request translators. Install lang.Provider() first.

Using the wrong header name

Browser language preferences are usually in Accept-Language, not Language.

Negotiating language inside handlers

Language negotiation is cross-cutting HTTP behavior. Put it in global or group middleware, then read lang.From(ctx.Context()) in handlers.

API quick reference

• langmw.New(sources...) creates language negotiation middleware.
• langmw.Query(name) reads language from query.
• langmw.Cookie(name) reads language from cookie.
• langmw.Header(name) reads language from header.
• lang.From(ctx.Context()) reads the current request translator.