Use `FlutterError.reportError` instead of `debugPrint` for l10n warning (#93076)

packages/flutter/lib/src/foundation/assertions.dart

@@ -248,6 +248,7 @@ abstract class _ErrorDiagnostic extends DiagnosticsProperty<List<Object>> {
 ///    problem that was detected.
 ///  * [ErrorHint], which provides specific, non-obvious advice that may be
 ///    applicable.
+///  * [ErrorSpacer], which renders as a blank line.
 ///  * [FlutterError], which is the most common place to use an
 ///    [ErrorDescription].
 class ErrorDescription extends _ErrorDiagnostic {
@@ -323,6 +324,7 @@ class ErrorSummary extends _ErrorDiagnostic {
 ///  * [ErrorDescription], which provides an explanation of the problem and its
 ///    cause, any information that may help track down the problem, background
 ///    information, etc.
+///  * [ErrorSpacer], which renders as a blank line.
 ///  * [FlutterError], which is the most common place to use an [ErrorHint].
 class ErrorHint extends _ErrorDiagnostic {
   /// A lint enforces that this constructor can only be called with a string
@@ -516,14 +518,52 @@ class FlutterErrorDetails with Diagnosticable {
   /// This won't be called if [stack] is null.
   final IterableFilter<String>? stackFilter;
 
-  /// A callback which, when called with a [StringBuffer] will write to that buffer
-  /// information that could help with debugging the problem.
+  /// A callback which will provide information that could help with debugging
+  /// the problem.
   ///
-  /// Information collector callbacks can be expensive, so the generated information
-  /// should be cached, rather than the callback being called multiple times.
+  /// Information collector callbacks can be expensive, so the generated
+  /// information should be cached by the caller, rather than the callback being
+  /// called multiple times.
   ///
-  /// The text written to the information argument may contain newlines but should
-  /// not end with a newline.
+  /// The callback is expected to return an iterable of [DiagnosticsNode] objects,
+  /// typically implemented using `sync*` and `yield`.
+  ///
+  /// {@tool snippet}
+  /// In this example, the information collector returns two pieces of information,
+  /// one broadly-applicable statement regarding how the error happened, and one
+  /// giving a specific piece of information that may be useful in some cases but
+  /// may also be irrelevant most of the time (an argument to the method).
+  ///
+  /// ```dart
+  /// void climbElevator(int pid) {
+  ///   try {
+  ///     // ...
+  ///   } catch (error, stack) {
+  ///     FlutterError.reportError(FlutterErrorDetails(
+  ///       exception: error,
+  ///       stack: stack,
+  ///       informationCollector: () sync* {
+  ///         yield ErrorDescription('This happened while climbing the space elevator.');
+  ///         yield ErrorHint('The process ID is: $pid');
+  ///       },
+  ///     ));
+  ///   }
+  /// }
+  /// ```
+  /// {@end-tool}
+  ///
+  /// The following classes may be of particular use:
+  ///
+  ///  * [ErrorDescription], for information that is broadly applicable to the
+  ///    situation being described.
+  ///  * [ErrorHint], for specific information that may not always be applicable
+  ///    but can be helpful in certain situations.
+  ///  * [DiagnosticsStackTrace], for reporting stack traces.
+  ///  * [ErrorSpacer], for adding spaces (a blank line) between other items.
+  ///
+  /// For objects that implement [Diagnosticable] one may consider providing
+  /// additional information by yielding the output of the object's
+  /// [Diagnosticable.toDiagnosticsNode] method.
   final InformationCollector? informationCollector;
 
   /// Whether this error should be ignored by the default error reporting

packages/flutter/lib/src/widgets/app.dart

@@ -111,11 +111,11 @@ typedef LocaleResolutionCallback = Locale? Function(Locale? locale, Iterable<Loc
 /// To summarize, the main matching priority is:
 ///
 ///  1. [Locale.languageCode], [Locale.scriptCode], and [Locale.countryCode]
-///  1. [Locale.languageCode] and [Locale.scriptCode] only
-///  1. [Locale.languageCode] and [Locale.countryCode] only
-///  1. [Locale.languageCode] only (with caveats, see above)
-///  1. [Locale.countryCode] only when all [preferredLocales] fail to match
-///  1. Returns the first element of [supportedLocales] as a fallback
+///  2. [Locale.languageCode] and [Locale.scriptCode] only
+///  3. [Locale.languageCode] and [Locale.countryCode] only
+///  4. [Locale.languageCode] only (with caveats, see above)
+///  5. [Locale.countryCode] only when all [preferredLocales] fail to match
+///  6. Returns the first element of [supportedLocales] as a fallback
 ///
 /// This algorithm does not take language distance (how similar languages are to each other)
 /// into account, and will not handle edge cases such as resolving `de` to `fr` rather than `zh`
@@ -885,7 +885,7 @@ class WidgetsApp extends StatefulWidget {
   /// `[const Locale('en', 'US')]`.
   ///
   /// The order of the list matters. The default locale resolution algorithm,
-  /// `basicLocaleListResolution`, attempts to match by the following priority:
+  /// [basicLocaleListResolution], attempts to match by the following priority:
   ///
   ///  1. [Locale.languageCode], [Locale.scriptCode], and [Locale.countryCode]
   ///  2. [Locale.languageCode] and [Locale.scriptCode] only
@@ -899,7 +899,7 @@ class WidgetsApp extends StatefulWidget {
   ///
   /// The default locale resolution algorithm can be overridden by providing a
   /// value for [localeListResolutionCallback]. The provided
-  /// `basicLocaleListResolution` is optimized for speed and does not implement
+  /// [basicLocaleListResolution] is optimized for speed and does not implement
   /// a full algorithm (such as the one defined in
   /// [Unicode TR35](https://unicode.org/reports/tr35/#LanguageMatching)) that
   /// takes distances between languages into account.
@@ -1493,35 +1493,37 @@ class _WidgetsAppState extends State<WidgetsApp> with WidgetsBindingObserver {
       if (unsupportedTypes.isEmpty)
         return true;
 
-      // Currently the Cupertino library only provides english localizations.
-      // Remove this when https://github.com/flutter/flutter/issues/23847
-      // is fixed.
-      if (listEquals(unsupportedTypes.map((Type type) => type.toString()).toList(), <String>['CupertinoLocalizations']))
-        return true;
-
-      final StringBuffer message = StringBuffer();
-      message.writeln('\u2550' * 8);
-      message.writeln(
-        "Warning: This application's locale, $appLocale, is not supported by all of its\n"
-        'localization delegates.',
-      );
-      for (final Type unsupportedType in unsupportedTypes) {
-        // Currently the Cupertino library only provides english localizations.
-        // Remove this when https://github.com/flutter/flutter/issues/23847
-        // is fixed.
-        if (unsupportedType.toString() == 'CupertinoLocalizations')
-          continue;
-        message.writeln(
-          '> A $unsupportedType delegate that supports the $appLocale locale was not found.',
-        );
-      }
-      message.writeln(
-        'See https://flutter.dev/tutorials/internationalization/ for more\n'
-        "information about configuring an app's locale, supportedLocales,\n"
-        'and localizationsDelegates parameters.',
-      );
-      message.writeln('\u2550' * 8);
-      debugPrint(message.toString());
+      FlutterError.reportError(FlutterErrorDetails(
+        exception: "Warning: This application's locale, $appLocale, is not supported by all of its localization delegates.",
+        library: 'widgets',
+        informationCollector: () sync* {
+          for (final Type unsupportedType in unsupportedTypes) {
+            yield ErrorDescription(
+              '• A $unsupportedType delegate that supports the $appLocale locale was not found.',
+            );
+          }
+          yield ErrorSpacer();
+          if (unsupportedTypes.length == 1 && unsupportedTypes.single.toString() == 'CupertinoLocalizations') {
+            // We previously explicitly avoided checking for this class so it's not uncommon for applications
+            // to have omitted importing the required delegate.
+            yield ErrorHint(
+              'If the application is built using GlobalMaterialLocalizations.delegate, consider using '
+              'GlobalMaterialLocalizations.delegates (plural) instead, as that will automatically declare '
+              'the appropriate Cupertino localizations.'
+            );
+            yield ErrorSpacer();
+          }
+          yield ErrorHint(
+            'The declared supported locales for this app are: ${widget.supportedLocales.join(", ")}'
+          );
+          yield ErrorSpacer();
+          yield ErrorDescription(
+            'See https://flutter.dev/tutorials/internationalization/ for more '
+            "information about configuring an app's locale, supportedLocales, "
+            'and localizationsDelegates parameters.',
+          );
+        },
+      ));
       return true;
     }());
     return true;

packages/flutter/test/widgets/app_test.dart

@@ -461,6 +461,30 @@ void main() {
     );
   });
 
+  testWidgets("WidgetsApp reports an exception if the selected locale isn't supported", (WidgetTester tester) async {
+    late final List<Locale>? localesArg;
+    late final Iterable<Locale> supportedLocalesArg;
+    await tester.pumpWidget(
+      MaterialApp( // This uses a MaterialApp because it introduces some actual localizations.
+        localeListResolutionCallback: (List<Locale>? locales, Iterable<Locale> supportedLocales) {
+          localesArg = locales;
+          supportedLocalesArg = supportedLocales;
+          return const Locale('C_UTF-8');
+        },
+        builder: (BuildContext context, Widget? child) => const Placeholder(),
+        color: const Color(0xFF000000),
+      ),
+    );
+    if (!kIsWeb) {
+      // On web, `flutter test` does not guarantee a particular locale, but
+      // when using `flutter_tester`, we guarantee that it's en-US, zh-CN.
+      // https://github.com/flutter/flutter/issues/93290
+      expect(localesArg, const <Locale>[Locale('en', 'US'), Locale('zh', 'CN')]);
+    }
+    expect(supportedLocalesArg, const <Locale>[Locale('en', 'US')]);
+    expect(tester.takeException(), "Warning: This application's locale, C_UTF-8, is not supported by all of its localization delegates.");
+  });
+
   testWidgets('WidgetsApp creates a MediaQuery if `useInheritedMediaQuery` is set to false', (WidgetTester tester) async {
     late BuildContext capturedContext;
     await tester.pumpWidget(

packages/flutter_localizations/test/basics_test.dart

@@ -37,7 +37,7 @@ void main() {
         ],
         localizationsDelegates: <LocalizationsDelegate<dynamic>>[
           _DummyLocalizationsDelegate(),
-          GlobalMaterialLocalizations.delegate,
+          ...GlobalMaterialLocalizations.delegates,
         ],
         home: PageView(),
       )
@@ -52,9 +52,7 @@ void main() {
     // Regression test for https://github.com/flutter/flutter/pull/16782
     await tester.pumpWidget(
       MaterialApp(
-        localizationsDelegates: const <LocalizationsDelegate<dynamic>>[
-          GlobalMaterialLocalizations.delegate,
-        ],
+        localizationsDelegates: GlobalMaterialLocalizations.delegates,
         supportedLocales: const <Locale>[
           Locale('es', 'ES'),
           Locale('zh'),

packages/flutter_localizations/test/material/date_time_test.dart

@@ -35,9 +35,7 @@ void main() {
         await tester.pumpWidget(MaterialApp(
           supportedLocales: <Locale>[locale],
           locale: locale,
-          localizationsDelegates: const <LocalizationsDelegate<dynamic>>[
-            GlobalMaterialLocalizations.delegate,
-          ],
+          localizationsDelegates: GlobalMaterialLocalizations.delegates,
           home: Builder(builder: (BuildContext context) {
             completer.complete(MaterialLocalizations.of(context).formatHour(timeOfDay));
             return Container();
@@ -82,9 +80,7 @@ void main() {
         await tester.pumpWidget(MaterialApp(
           supportedLocales: <Locale>[locale],
           locale: locale,
-          localizationsDelegates: const <LocalizationsDelegate<dynamic>>[
-            GlobalMaterialLocalizations.delegate,
-          ],
+          localizationsDelegates: GlobalMaterialLocalizations.delegates,
           home: Builder(builder: (BuildContext context) {
             completer.complete(MaterialLocalizations.of(context).formatTimeOfDay(timeOfDay));
             return Container();
@@ -126,9 +122,7 @@ void main() {
         await tester.pumpWidget(MaterialApp(
           supportedLocales: <Locale>[locale],
           locale: locale,
-          localizationsDelegates: const <LocalizationsDelegate<dynamic>>[
-            GlobalMaterialLocalizations.delegate,
-          ],
+          localizationsDelegates: GlobalMaterialLocalizations.delegates,
           home: Builder(builder: (BuildContext context) {
             final MaterialLocalizations localizations = MaterialLocalizations.of(context);
             completer.complete(<DateType, String>{
@@ -184,12 +178,9 @@ void main() {
 
     await tester.pumpWidget(MaterialApp(
       locale: const Locale('en', 'US'),
-      localizationsDelegates: const <LocalizationsDelegate<dynamic>>[
-        GlobalMaterialLocalizations.delegate,
-      ],
+      localizationsDelegates: GlobalMaterialLocalizations.delegates,
       home: Builder(builder: (BuildContext context) {
         dateFormat = DateFormat('EEE, d MMM yyyy HH:mm:ss', 'en_US');
-
         return Container();
       }),
     ));

packages/flutter_localizations/test/override_test.dart

@@ -175,9 +175,10 @@ void main() {
 
     await tester.pumpWidget(
       buildFrame(
-        delegates: <FooMaterialLocalizationsDelegate>[
+        delegates: <LocalizationsDelegate<dynamic>>[
           const FooMaterialLocalizationsDelegate(supportedLanguage: 'fr', backButtonTooltip: 'FR'),
           const FooMaterialLocalizationsDelegate(supportedLanguage: 'de', backButtonTooltip: 'DE'),
+          GlobalCupertinoLocalizations.delegate,
         ],
         supportedLocales: const <Locale>[
           Locale('en'),
@@ -211,8 +212,9 @@ void main() {
       buildFrame(
         // Accept whatever locale we're given
         localeResolutionCallback: (Locale? locale, Iterable<Locale> supportedLocales) => locale,
-        delegates: <FooMaterialLocalizationsDelegate>[
+        delegates: <LocalizationsDelegate<dynamic>>[
           const FooMaterialLocalizationsDelegate(supportedLanguage: 'allLanguages'),
+          GlobalCupertinoLocalizations.delegate,
         ],
         buildContent: (BuildContext context) {
           // Should always be 'foo', no matter what the locale is
@@ -240,8 +242,9 @@ void main() {
 
     await tester.pumpWidget(
       buildFrame(
-        delegates: <FooMaterialLocalizationsDelegate>[
+        delegates: <LocalizationsDelegate<dynamic>>[
           const FooMaterialLocalizationsDelegate(),
+          GlobalCupertinoLocalizations.delegate,
         ],
         // supportedLocales not specified, so all locales resolve to 'en'
         buildContent: (BuildContext context) {
@@ -297,6 +300,7 @@ void main() {
     // Yiddish was ji (ISO-639) is yi (ISO-639-1)
     await tester.binding.setLocale('ji', 'IL');
     await tester.pump();
+    expect(tester.takeException(), "Warning: This application's locale, yi_IL, is not supported by all of its localization delegates.");
     expect(tester.widget<Text>(find.byKey(textKey)).data, 'yi_IL');
 
     // Indonesian was in (ISO-639) is id (ISO-639-1)

packages/flutter_localizations/test/text_test.dart

@@ -22,9 +22,7 @@ void main() {
             return const Text('Next');
           },
         },
-        localizationsDelegates: const <LocalizationsDelegate<dynamic>>[
-          GlobalMaterialLocalizations.delegate,
-        ],
+        localizationsDelegates: GlobalMaterialLocalizations.delegates,
         supportedLocales: const <Locale>[
           Locale('en', 'US'),
           Locale('es', 'ES'),
@@ -108,9 +106,7 @@ void main() {
             return const Text('Next');
           },
         },
-        localizationsDelegates: const <LocalizationsDelegate<dynamic>>[
-          GlobalMaterialLocalizations.delegate,
-        ],
+        localizationsDelegates: GlobalMaterialLocalizations.delegates,
         supportedLocales: const <Locale>[
           Locale('en', 'US'),
           Locale('es', 'ES'),