core/src/Localization/Middleware/LocaleMiddleware.php

<?php

namespace Modules\Core\Localization\Middleware;

use Closure;
use Illuminate\Http\Request;
use Illuminate\Support\Collection;
use Illuminate\Support\Facades\App;
use Illuminate\Support\Facades\URL;
use Illuminate\Support\Facades\View;
use Lunar\Models\Language;
use Modules\Core\Localization\Services\LanguageCache;
use Symfony\Component\HttpFoundation\Response;

class LocaleMiddleware
{
    public function __construct(private readonly LanguageCache $languages) {}

    public function handle(Request $request, Closure $next): Response
    {
        $languages = $this->languages->all();

        if ($languages->isEmpty()) {
            return $next($request);
        }

        $segment = (string) $request->segment(1);
        $language = $languages->firstWhere('code', $segment);

        if (! $language) {
            return $this->redirectToLocalizedUrl($request, $languages);
        }

        App::setLocale($language->code);
        $request->attributes->set('locale', $language->code);
        $request->attributes->set('language', $language);

        // Lets route() calls omit {locale} anywhere in the request lifecycle
        // (controllers, views) — without this, every route() call would need
        // locale passed explicitly every time.
        URL::defaults(['locale' => $language->code]);

        $this->shareLocaleViewData($request, $language, $languages);

        return $next($request);
    }

    /**
     * Shares the current locale and every OTHER available locale (each with its
     * own URL for the current page) with all views, so the header language
     * switcher and layout hreflang tags don't have to recompute it.
     *
     * `altLocales` is a collection, not a single value — firstWhere('code', '!=',
     * ...) would only ever surface one alternate, which happens to look correct
     * with exactly 2 configured languages (there's only one "other" to find) but
     * silently drops every locale past the first for a 3+ language store, with no
     * error, just fewer switcher options than actually configured. A view iterates
     * `$altLocales` to render as many links/dropdown entries as there are
     * alternates, whether that's 1 or 10.
     */
    private function shareLocaleViewData(Request $request, Language $language, Collection $languages): void
    {
        $route = $request->route();
        $routeName = $route?->getName();

        $altLocales = $languages
            ->reject(fn (Language $other) => $other->code === $language->code)
            ->map(fn (Language $other) => [
                'code' => $other->code,
                'name' => $other->name,
                'url' => $routeName
                    ? route($routeName, array_merge($route->parameters(), ['locale' => $other->code]))
                    : url('/'.$other->code),
            ])
            ->values();

        View::share('currentLocale', $language->code);
        View::share('altLocales', $altLocales);
    }

    private function redirectToLocalizedUrl(Request $request, Collection $languages): Response
    {
        $locale = $this->negotiateLocale($request, $languages);

        $path = trim($request->getPathInfo(), '/');
        $target = '/'.$locale.($path !== '' ? '/'.$path : '');

        $query = $request->getQueryString();
        if ($query) {
            $target .= '?'.$query;
        }

        return redirect($target);
    }

    private function negotiateLocale(Request $request, Collection $languages): string
    {
        $preferred = $request->getPreferredLanguage($languages->pluck('code')->all());

        if ($preferred) {
            return $preferred;
        }

        return $languages->firstWhere('default', true)?->code
            ?? $languages->first()->code;
    }
}