From d2ad4419bb07ef4ee7bd401c1c3e0d6fd6187127 Mon Sep 17 00:00:00 2001
From: Greg Spencer <gspencergoog@users.noreply.github.com>
Date: Tue, 1 Dec 2020 09:54:56 -0800
Subject: [PATCH] Fix PlatformDispatcher.locale to return something meaningful
 when there are no locales. (#22608)

Returns an "undefined" locale (language code "und") from PlatformDispatcher.locale when no locales are defined.
---
 lib/ui/platform_dispatcher.dart                    | 11 ++++++++---
 lib/web_ui/lib/src/engine/platform_dispatcher.dart |  6 +++++-
 lib/web_ui/test/engine/window_test.dart            |  1 +
 testing/dart/window_hooks_integration_test.dart    | 13 +++++++++++++
 4 files changed, 27 insertions(+), 4 deletions(-)

diff --git a/lib/ui/platform_dispatcher.dart b/lib/ui/platform_dispatcher.dart
index a218b283e..dc7223b79 100644
--- a/lib/ui/platform_dispatcher.dart
+++ b/lib/ui/platform_dispatcher.dart
@@ -549,9 +549,10 @@ class PlatformDispatcher {
   /// This is the first locale selected by the user and is the user's primary
   /// locale (the locale the device UI is displayed in)
   ///
-  /// This is equivalent to `locales.first` and will provide an empty non-null
-  /// locale if the [locales] list has not been set or is empty.
-  Locale get locale => locales.first;
+  /// 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.
+  Locale get locale => locales.isEmpty ? const Locale.fromSubtags() : locales.first;
 
   /// The full system-reported supported locales of the device.
   ///
@@ -1318,6 +1319,10 @@ class Locale {
   /// [region](https://github.com/unicode-org/cldr/blob/master/common/validity/region.xml) for
   /// each of languageCode, scriptCode and countryCode respectively.
   ///
+  /// The [languageCode] subtag is optional. When there is no language subtag,
+  /// the parameter should be omitted or set to "und". When not supplied, the
+  /// [languageCode] defaults to "und", an undefined language code.
+  ///
   /// The [countryCode] subtag is optional. When there is no country subtag,
   /// the parameter should be omitted or passed `null` instead of an empty-string.
   ///
diff --git a/lib/web_ui/lib/src/engine/platform_dispatcher.dart b/lib/web_ui/lib/src/engine/platform_dispatcher.dart
index 78e912436..7f6ce039c 100644
--- a/lib/web_ui/lib/src/engine/platform_dispatcher.dart
+++ b/lib/web_ui/lib/src/engine/platform_dispatcher.dart
@@ -546,6 +546,10 @@ class EnginePlatformDispatcher extends ui.PlatformDispatcher {
     EngineSemanticsOwner.instance.updateSemantics(update);
   }
 
+  /// 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.
+  ///
   /// We use the first locale in the [locales] list instead of the browser's
   /// built-in `navigator.language` because browsers do not agree on the
   /// implementation.
@@ -554,7 +558,7 @@ class EnginePlatformDispatcher extends ui.PlatformDispatcher {
   ///
   /// * https://developer.mozilla.org/en-US/docs/Web/API/NavigatorLanguage/languages,
   ///   which explains browser quirks in the implementation notes.
-  ui.Locale get locale => locales.first;
+  ui.Locale get locale => locales.isEmpty ? const ui.Locale.fromSubtags() : locales.first;
 
   /// The full system-reported supported locales of the device.
   ///
diff --git a/lib/web_ui/test/engine/window_test.dart b/lib/web_ui/test/engine/window_test.dart
index 04ae42243..6dc9497c0 100644
--- a/lib/web_ui/test/engine/window_test.dart
+++ b/lib/web_ui/test/engine/window_test.dart
@@ -292,6 +292,7 @@ void testMain() {
     // that the list is populated again).
     EnginePlatformDispatcher.instance.debugResetLocales();
     expect(window.locales, isEmpty);
+    expect(window.locale, equals(const ui.Locale.fromSubtags()));
     expect(localeChangedCount, 0);
     html.window.dispatchEvent(html.Event('languagechange'));
     expect(window.locales, isNotEmpty);
diff --git a/testing/dart/window_hooks_integration_test.dart b/testing/dart/window_hooks_integration_test.dart
index 27b2da80d..91c2036db 100644
--- a/testing/dart/window_hooks_integration_test.dart
+++ b/testing/dart/window_hooks_integration_test.dart
@@ -426,4 +426,17 @@ void main() {
     expectEquals(window.padding.bottom, 0.0);
     expectEquals(window.systemGestureInsets.bottom, 44.0);
   });
+
+  test('PlatformDispatcher.locale returns unknown locale when locales is set to empty list', () {
+    late Locale locale;
+    runZoned(() {
+      window.onLocaleChanged = () {
+        locale = PlatformDispatcher.instance.locale;
+      };
+    });
+
+    _updateLocales(<String>[]);
+    expectEquals(locale, const Locale.fromSubtags());
+    expectEquals(locale.languageCode, 'und');
+  });
 }
-- 
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