test(l10n): pin the metadata editor's IME next-action in zh

test/features/songs/metadata_editor_ime_test.dart

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+// The metadata editor's IME "next" action must survive localization.
+//
+// `_buildField` used to decide the keyboard action from the *label text*:
+//
+//     textInputAction: label == 'Title' ? TextInputAction.next : null,
+//
+// Callers pass `l10n.title` in, so this worked in English and broke in every
+// other locale: the label became 标题, the comparison failed, and the Title
+// field silently lost its "next" key -- tabbing from Title to Artist stopped
+// working (fixed upstream in d6a69757, shipped as #297).
+//
+// This is the class A trap from source_guard_test.dart, and the source guard
+// cannot see it: the localized string enters `_buildField` through a parameter,
+// so the evidence is at the call site and no single-file data-flow rule reaches
+// it. Verified by restoring the line above and watching source_guard_test.dart
+// stay green. A widget test is the only cheap check that does catch it, so this
+// file pins the behaviour rather than the source text.
+
+import 'package:flutter/material.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:shared_preferences/shared_preferences.dart';
+
+import 'package:flick/features/songs/widgets/metadata_editor_bottom_sheet.dart';
+import 'package:flick/l10n/l10n.dart';
+import 'package:flick/models/song.dart';
+
+/// An editable song: `filePath != null`, no cue offsets, not external -- the
+/// three things `_isEditable` checks before it enables the fields.
+const Song _editableSong = Song(
+  id: 'song-1',
+  title: 'Roads',
+  artist: 'Portishead',
+  album: 'Dummy',
+  duration: Duration(minutes: 5),
+  fileType: 'flac',
+  filePath: '/storage/emulated/0/Music/Dummy/01 Roads.flac',
+);
+
+/// The app wiring `lib/app/app.dart` uses, so `l10n` follows the locale.
+Widget _app(Locale locale) {
+  return MaterialApp(
+    localizationsDelegates: AppLocalizations.localizationsDelegates,
+    supportedLocales: kSupportedLocales,
+    locale: locale,
+    home: Builder(
+      builder: (context) {
+        LocaleController.instance.bind(AppLocalizations.of(context));
+        return const ProviderScope(
+          child: Scaffold(
+            body: SingleChildScrollView(
+              child: MetadataEditorBottomSheet(song: _editableSong),
+            ),
+          ),
+        );
+      },
+    ),
+  );
+}
+
+/// The TextField that sits under the field labelled [label].
+///
+/// `_buildField` renders `Padding > Column > [Text(label), SizedBox, TextField]`,
+/// so the innermost Column ancestor of the label owns exactly one TextField.
+TextField _fieldUnder(WidgetTester tester, String label) {
+  final column = find.ancestor(
+    of: find.text(label),
+    matching: find.byType(Column),
+  );
+  expect(column, findsWidgets, reason: 'no field is labelled "$label"');
+  return tester.widget<TextField>(
+    find.descendant(of: column.first, matching: find.byType(TextField)).first,
+  );
+}
+
+/// Pump the editor in [locale] and hand back the localizations it rendered with.
+Future<AppLocalizations> _pump(WidgetTester tester, Locale locale) async {
+  await LocaleController.instance.select(locale);
+  await tester.pumpWidget(_app(locale));
+  await tester.pumpAndSettle();
+  return AppLocalizations.of(tester.element(find.byType(MetadataEditorBottomSheet)));
+}
+
+void main() {
+  setUp(() {
+    SharedPreferences.setMockInitialValues(<String, Object>{});
+  });
+
+  tearDown(() {
+    // Singleton: leave it following the device locale for the next test.
+    LocaleController.instance.value = null;
+  });
+
+  testWidgets('zh: the Title field keeps its IME next-action', (tester) async {
+    final l10n = await _pump(tester, const Locale('zh', 'CN'));
+
+    // We are really in Chinese -- otherwise the assertion below is vacuous,
+    // because the buggy `label == 'Title'` happens to be correct in English.
+    expect(l10n.title, isNot('Title'));
+    expect(find.text(l10n.title), findsOneWidget);
+
+    expect(
+      _fieldUnder(tester, l10n.title).textInputAction,
+      TextInputAction.next,
+      reason: 'The Title field lost its "next" action, so the IME key does '
+          'nothing and the user cannot advance from Title to Artist. That is '
+          'the d6a69757 regression: the action was chosen by comparing the '
+          'label against the English literal "Title".',
+    );
+  });
+
+  testWidgets('en: the Title field keeps its IME next-action', (tester) async {
+    final l10n = await _pump(tester, const Locale('en'));
+    expect(_fieldUnder(tester, l10n.title).textInputAction, TextInputAction.next);
+  });
+
+  testWidgets('zh: the other fields do not claim the next-action', (tester) async {
+    final l10n = await _pump(tester, const Locale('zh', 'CN'));
+
+    // Artist is where "next" leads; it must not advertise the same action, or
+    // the chain the Title field promises is a lie.
+    expect(
+      _fieldUnder(tester, l10n.artist).textInputAction,
+      isNot(TextInputAction.next),
+    );
+  });
+}