Initial commit.

Packages/Infrastructure/Sources/Public/Middlewares/LocalizationMiddleware.swift

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+import Foundation
+import HTTPTypes
+import Hummingbird
+import Localization
+
+/// Resolves the visitor's preferred language and records it on the request context.
+///
+/// Placed ahead of the localized responders in the middleware chain, it reads the request's `Accept-Language` header, negotiates the best supported
+/// match (falling back to the default language), and stores it on the context's ``LocalizedRequestContext/language``.
+///
+/// The request is otherwise passed through untouched — the URL and routing are not affected — so each page is served at its existing path and varies its
+/// content by header.
+public struct LocalizationMiddleware<Context: LocalizedRequestContext> {
+
+    // MARK: Properties
+
+    /// Negotiates the request's language from its `Accept-Language` header.
+    private let negotiate: Negotiate
+
+    // MARK: Initializers
+
+    /// Creates a localization middleware that negotiates against the given bundle's String Catalog languages.
+    /// - Parameter bundle: the bundle whose String Catalog names the supported languages.
+    public init(
+        bundle: Bundle
+    ) {
+        self.negotiate = .init(bundle: bundle)
+    }
+
+}
+
+// MARK: - RouterMiddleware
+
+extension LocalizationMiddleware: RouterMiddleware {
+
+    // MARK: Functions
+
+    /// Negotiates the request's language and records it on the context before passing it down.
+    /// - Parameters:
+    ///   - request: the incoming request.
+    ///   - context: the context the request is resolved against.
+    ///   - next: the next responder in the middleware chain.
+    /// - Returns: the downstream response.
+    /// - Throws: any error thrown downstream.
+    public func handle(
+        _ request: Request,
+        context: Context,
+        next: (Request, Context) async throws -> Response
+    ) async throws -> Response {
+        var context = context
+
+        context.language = negotiate(
+            acceptLanguage: request.headers[.acceptLanguage]
+        )
+
+        return try await next(request, context)
+    }
+
+}