Android native locale resolution algorithm

shell/platform/android/BUILD.gn

@@ -450,6 +450,7 @@ action("robolectric_tests") {
     "test/io/flutter/plugin/common/StandardMethodCodecTest.java",
     "test/io/flutter/plugin/editing/InputConnectionAdaptorTest.java",
     "test/io/flutter/plugin/editing/TextInputPluginTest.java",
+    "test/io/flutter/plugin/localization/LocalizationPluginTest.java",
     "test/io/flutter/plugin/mouse/MouseCursorPluginTest.java",
     "test/io/flutter/plugin/platform/PlatformPluginTest.java",
     "test/io/flutter/plugin/platform/SingleViewPresentationTest.java",

shell/platform/android/io/flutter/plugin/localization/LocalizationPlugin.java

@@ -26,9 +26,25 @@ public LocalizationPlugin(
     this.localizationChannel = localizationChannel;
   }
 
+  /**
+   * Computes the {@link Locale} in supportedLocales that best matches the user's preferred locales.
+   *
+   * <p>FlutterEngine must be non-null when this method is invoked.
+   */
+  @SuppressWarnings("deprecation")
   public Locale resolveNativeLocale(List<Locale> supportedLocales) {
-    Locale platformResolvedLocale = null;
+    if (supportedLocales == null || supportedLocales.isEmpty()) {
+      return null;
+    }
+
+    // Android improved the localization resolution algorithms after API 24 (7.0, Nougat).
+    // See https://developer.android.com/guide/topics/resources/multilingual-support
+    //
+    // LanguageRange and Locale.lookup was added in API 26 and is the preferred way to
+    // select a locale. Pre-API 26, we implement a manual locale resolution.
     if (Build.VERSION.SDK_INT >= android.os.Build.VERSION_CODES.O) {
+      // Modern locale resolution using LanguageRange
+      // https://developer.android.com/guide/topics/resources/multilingual-support#postN
       List<Locale.LanguageRange> languageRanges = new ArrayList<>();
       LocaleList localeList = context.getResources().getConfiguration().getLocales();
       int localeCount = localeList.size();
@@ -37,14 +53,60 @@ public Locale resolveNativeLocale(List<Locale> supportedLocales) {
         String localeString = locale.toString();
         // This string replacement converts the locale string into the ranges format.
         languageRanges.add(new Locale.LanguageRange(localeString.replace("_", "-")));
+        languageRanges.add(new Locale.LanguageRange(locale.getLanguage()));
+        languageRanges.add(new Locale.LanguageRange(locale.getLanguage() + "-*"));
       }
+      Locale platformResolvedLocale = Locale.lookup(languageRanges, supportedLocales);
+      if (platformResolvedLocale != null) {
+        return platformResolvedLocale;
+      }
+      return supportedLocales.get(0);
+    } else if (Build.VERSION.SDK_INT >= android.os.Build.VERSION_CODES.N) {
+      // Modern locale resolution without languageRange
+      // https://developer.android.com/guide/topics/resources/multilingual-support#postN
+      LocaleList localeList = context.getResources().getConfiguration().getLocales();
+      for (int index = 0; index < localeList.size(); ++index) {
+        Locale preferredLocale = localeList.get(index);
+        // Look for exact match.
+        for (Locale locale : supportedLocales) {
+          if (preferredLocale.equals(locale)) {
+            return locale;
+          }
+        }
+        // Look for exact language only match.
+        for (Locale locale : supportedLocales) {
+          if (preferredLocale.getLanguage().equals(locale.toLanguageTag())) {
+            return locale;
+          }
+        }
+        // Look for any locale with matching language.
+        for (Locale locale : supportedLocales) {
+          if (preferredLocale.getLanguage().equals(locale.getLanguage())) {
+            return locale;
+          }
+        }
+      }
+      return supportedLocales.get(0);
+    }
 
-      // TODO(garyq): This should be modified to achieve Android's full
-      // locale resolution:
-      // https://developer.android.com/guide/topics/resources/multilingual-support
-      platformResolvedLocale = Locale.lookup(languageRanges, supportedLocales);
+    // Legacy locale resolution
+    // https://developer.android.com/guide/topics/resources/multilingual-support#preN
+    Locale preferredLocale = context.getResources().getConfiguration().locale;
+    if (preferredLocale != null) {
+      // Look for exact match.
+      for (Locale locale : supportedLocales) {
+        if (preferredLocale.equals(locale)) {
+          return locale;
+        }
+      }
+      // Look for exact language only match.
+      for (Locale locale : supportedLocales) {
+        if (preferredLocale.getLanguage().equals(locale.toString())) {
+          return locale;
+        }
+      }
     }
-    return platformResolvedLocale;
+    return supportedLocales.get(0);
   }
 
   /**

shell/platform/android/test/io/flutter/FlutterTestSuite.java

@@ -13,6 +13,7 @@
 import io.flutter.embedding.engine.FlutterEngineCacheTest;
 import io.flutter.embedding.engine.FlutterEnginePluginRegistryTest;
 import io.flutter.embedding.engine.FlutterJNITest;
+import io.flutter.embedding.engine.LocalizationPluginTest;
 import io.flutter.embedding.engine.RenderingComponentTest;
 import io.flutter.embedding.engine.plugins.shim.ShimPluginRegistryTest;
 import io.flutter.embedding.engine.renderer.FlutterRendererTest;
@@ -52,6 +53,7 @@
   FlutterRendererTest.class,
   FlutterViewTest.class,
   InputConnectionAdaptorTest.class,
+  LocalizationPluginTest.class,
   PlatformPluginTest.class,
   PluginComponentTest.class,
   PreconditionsTest.class,

shell/platform/android/test/io/flutter/embedding/engine/FlutterJNITest.java

@@ -96,7 +96,10 @@ public void computePlatformResolvedLocaleCallsLocalizationPluginProperly() {
           "en", "CA", ""
         };
     result = flutterJNI.computePlatformResolvedLocale(supportedLocales);
-    assertEquals(result.length, 0); // This should change when full algo is implemented.
+    assertEquals(result.length, 3);
+    assertEquals(result[0], "en");
+    assertEquals(result[1], "CA");
+    assertEquals(result[2], "");
 
     supportedLocales =
         new String[] {
@@ -137,7 +140,11 @@ public void computePlatformResolvedLocaleCallsLocalizationPluginProperly() {
     localeList = new LocaleList();
     when(config.getLocales()).thenReturn(localeList);
     result = flutterJNI.computePlatformResolvedLocale(supportedLocales);
-    assertEquals(result.length, 0);
+    // The first locale is default.
+    assertEquals(result.length, 3);
+    assertEquals(result[0], "fr");
+    assertEquals(result[1], "FR");
+    assertEquals(result[2], "");
   }
 
   public void onDisplayPlatformView__callsPlatformViewsController() {