From b9e2630da3dc5cbb2fab33f52fe9c097ac2d1e57 Mon Sep 17 00:00:00 2001
From: Virgil <virgil@lethean.io>
Date: Tue, 31 Mar 2026 20:44:15 +0000
Subject: [PATCH] feat(html): allow swapping context translators

Co-Authored-By: Virgil <virgil@lethean.io>
---
 context.go          | 43 ++++++++++++++++++++++++++++++-------------
 context_test.go     | 21 +++++++++++++++++++++
 docs/development.md |  2 +-
 docs/history.md     |  2 +-
 4 files changed, 53 insertions(+), 15 deletions(-)

diff --git a/context.go b/context.go
index 110757a..1710b2f 100644
--- a/context.go
+++ b/context.go
@@ -19,6 +19,23 @@ type Context struct {
 	service      Translator
 }
 
+func applyLocaleToService(svc Translator, locale string) {
+	if svc == nil || locale == "" {
+		return
+	}
+
+	if setter, ok := svc.(interface{ SetLanguage(string) error }); ok {
+		base := locale
+		for i := 0; i < len(base); i++ {
+			if base[i] == '-' || base[i] == '_' {
+				base = base[:i]
+				break
+			}
+		}
+		_ = setter.SetLanguage(base)
+	}
+}
+
 // NewContext creates a new rendering context with sensible defaults.
 // Usage example: html := Render(Text("welcome"), NewContext("en-GB"))
 func NewContext(locale ...string) *Context {
@@ -35,18 +52,18 @@ func NewContext(locale ...string) *Context {
 // Usage example: ctx := NewContextWithService(myTranslator, "en-GB")
 func NewContextWithService(svc Translator, locale ...string) *Context {
 	ctx := NewContext(locale...)
-	ctx.service = svc
-	if len(locale) > 0 {
-		if setter, ok := svc.(interface{ SetLanguage(string) error }); ok {
-			base := locale[0]
-			for i := 0; i < len(base); i++ {
-				if base[i] == '-' || base[i] == '_' {
-					base = base[:i]
-					break
-				}
-			}
-			_ = setter.SetLanguage(base)
-		}
-	}
+	ctx.SetService(svc)
+	return ctx
+}
+
+// SetService swaps the translator used by the context.
+// Usage example: ctx.SetService(myTranslator)
+func (ctx *Context) SetService(svc Translator) *Context {
+	if ctx == nil {
+		return nil
+	}
+
+	ctx.service = svc
+	applyLocaleToService(svc, ctx.Locale)
 	return ctx
 }
diff --git a/context_test.go b/context_test.go
index 091785c..7c86db7 100644
--- a/context_test.go
+++ b/context_test.go
@@ -46,3 +46,24 @@ func TestNewContextWithService_AppliesLocaleToService_Good(t *testing.T) {
 		t.Fatalf("NewContextWithService locale translation = %q, want %q", got, "o")
 	}
 }
+
+func TestContext_SetService_AppliesLocale_Good(t *testing.T) {
+	svc, _ := i18n.New()
+	ctx := NewContext("fr-FR")
+
+	if got := ctx.SetService(svc); got != ctx {
+		t.Fatal("SetService should return the same context for chaining")
+	}
+
+	got := Text("prompt.yes").Render(ctx)
+	if got != "o" {
+		t.Fatalf("SetService locale translation = %q, want %q", got, "o")
+	}
+}
+
+func TestContext_SetService_NilContext_Ugly(t *testing.T) {
+	var ctx *Context
+	if got := ctx.SetService(nil); got != nil {
+		t.Fatal("SetService on nil context should return nil")
+	}
+}
diff --git a/docs/development.md b/docs/development.md
index ba2761d..46cbcc1 100644
--- a/docs/development.md
+++ b/docs/development.md
@@ -309,6 +309,6 @@ func TestGenerateClass_ValidTag(t *testing.T) {
 
 - `NewLayout("XYZ")` silently produces empty output for unrecognised slot letters. Valid letters are `H`, `L`, `C`, `R`, `F`. There is no error or warning.
 - `Responsive.Variant()` accepts only `*Layout`, not arbitrary `Node` values. Arbitrary subtrees must be wrapped in a single-slot layout first.
-- `Context.service` is unexported. Custom translation injection requires `NewContextWithService()`. There is no way to swap the translator after construction.
+- `Context.service` is unexported. Custom translation injection uses `NewContextWithService()`, and `Context.SetService()` can swap the translator later while preserving locale-aware services.
 - The WASM module has no integration test for the JavaScript exports. `size_test.go` tests binary size only; it does not exercise `renderToString` behaviour from JavaScript.
 - `codegen.GenerateBundle()` now renders output classes in sorted slot-key order so generated bundles are stable between runs.
diff --git a/docs/history.md b/docs/history.md
index 24e284a..fc6ff0b 100644
--- a/docs/history.md
+++ b/docs/history.md
@@ -101,7 +101,7 @@ These are not regressions; they are design choices or deferred work recorded for
 
 3. **Responsive accepts only Layout.** `Responsive.Variant()` takes `*Layout` rather than `Node`. The rationale is that `CompareVariants` in the pipeline needs access to the slot structure. Accepting `Node` would require a different approach to variant analysis.
 
-4. **Context.service is private.** The i18n service cannot be set after construction or swapped. This is a conservative choice; relaxing it requires deciding whether mutation should be safe for concurrent use.
+4. **Context.service is private.** The i18n service is still unexported, but callers can now swap it explicitly with `Context.SetService()`. This keeps the field encapsulated while allowing controlled mutation.
 
 5. **TypeScript definitions not generated.** `codegen.GenerateBundle()` produces JS only. A `.d.ts` companion would benefit TypeScript consumers of the generated Web Components.