wires up set application locale to web engine (flutter#177284)

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fixes flutter#98948

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Author: chunhtai

engine/src/flutter/lib/web_ui/lib/src/engine/platform_dispatcher.dart

@@ -799,6 +799,13 @@ class EnginePlatformDispatcher extends ui.PlatformDispatcher {
     implicitView?.semantics.updateSemantics(update);
   }
 
+  @override
+  void setApplicationLocale(ui.Locale locale) {
+    for (final EngineFlutterView view in views) {
+      view.setLocale(locale);
+    }
+  }
+
   /// This is equivalent to `locales.first`, except that it will provide an
   /// undefined (using the language tag "und") non-null locale if the [locales]
   /// list has not been set or is empty.

engine/src/flutter/lib/web_ui/lib/src/engine/view_embedder/embedding_strategy/custom_element_embedding_strategy.dart

@@ -3,6 +3,7 @@
 // found in the LICENSE file.
 
 import 'package:ui/src/engine/dom.dart';
+import 'package:ui/ui.dart' as ui;
 
 import '../hot_restart_cache_handler.dart' show registerElementForCleanup;
 import 'embedding_strategy.dart';
@@ -27,6 +28,11 @@ class CustomElementEmbeddingStrategy implements EmbeddingStrategy {
   /// The root element of the Flutter view.
   late final DomElement _rootElement;
 
+  @override
+  void setLocale(ui.Locale locale) {
+    hostElement.setAttribute('lang', locale.toLanguageTag());
+  }
+
   @override
   void attachViewRoot(DomElement rootElement) {
     rootElement

engine/src/flutter/lib/web_ui/lib/src/engine/view_embedder/embedding_strategy/embedding_strategy.dart

@@ -3,6 +3,7 @@
 // found in the LICENSE file.
 
 import 'package:ui/src/engine/dom.dart';
+import 'package:ui/ui.dart' as ui;
 
 import 'custom_element_embedding_strategy.dart';
 import 'full_page_embedding_strategy.dart';
@@ -26,6 +27,14 @@ abstract class EmbeddingStrategy {
     }
   }
 
+  /// Sets the locale for the embedded view.
+  ///
+  /// This method is typically called by the Flutter framework after it has
+  /// resolved the application's locale. It configures the embedded view to
+  /// reflect the given locale, which is important for accessibility and for
+  /// the browser to select appropriate fonts and other locale-specific resources.
+  void setLocale(ui.Locale locale);
+
   /// The DOM element in which the Flutter view is embedded.
   /// This element is the direct parent element of the <flutter-view> element.
   DomElement get hostElement;

engine/src/flutter/lib/web_ui/lib/src/engine/view_embedder/embedding_strategy/full_page_embedding_strategy.dart

@@ -4,6 +4,7 @@
 
 import 'package:ui/src/engine/dom.dart';
 import 'package:ui/src/engine/util.dart';
+import 'package:ui/ui.dart' as ui;
 
 import '../hot_restart_cache_handler.dart' show registerElementForCleanup;
 import 'embedding_strategy.dart';
@@ -25,6 +26,11 @@ class FullPageEmbeddingStrategy implements EmbeddingStrategy {
   @override
   DomEventTarget get globalEventTarget => domWindow;
 
+  @override
+  void setLocale(ui.Locale locale) {
+    domDocument.documentElement!.setAttribute('lang', locale.toLanguageTag());
+  }
+
   @override
   void attachViewRoot(DomElement rootElement) {
     /// Tweaks style so the rootElement works well with the hostElement.

engine/src/flutter/lib/web_ui/lib/src/engine/window.dart

@@ -131,6 +131,16 @@ class EngineFlutterView implements ui.FlutterView {
     semantics.updateSemantics(update);
   }
 
+  /// Sets the locale for this view.
+  ///
+  /// This method is typically called by the Flutter framework after it has
+  /// resolved the application's locale. It configures the view to reflect
+  /// the given locale, which is important for accessibility and for the
+  /// browser.
+  void setLocale(ui.Locale locale) {
+    embeddingStrategy.setLocale(locale);
+  }
+
   late final GlobalHtmlAttributes _globalHtmlAttributes = GlobalHtmlAttributes(
     rootElement: dom.rootElement,
     hostElement: embeddingStrategy.hostElement,

engine/src/flutter/lib/web_ui/test/engine/platform_dispatcher/platform_dispatcher_test.dart

@@ -229,6 +229,19 @@ void testMain() {
       expect(codec.decodeEnvelope(response!), true);
     });
 
+    test('can set application locale', () async {
+      final DomElement host1 = createDomHTMLDivElement();
+      final EngineFlutterView view1 = EngineFlutterView(dispatcher, host1);
+      final EngineFlutterView view2 = EngineFlutterView.implicit(dispatcher, null);
+      dispatcher.viewManager
+        ..registerView(view1)
+        ..registerView(view2);
+
+      dispatcher.setApplicationLocale(const ui.Locale('es', 'MX'));
+      expect(host1.getAttribute('lang'), 'es-MX');
+      expect(domDocument.querySelector('html')!.getAttribute('lang'), 'es-MX');
+    });
+
     test('can find text scale factor', () async {
       const double deltaTolerance = 1e-5;