Add golden test: "renders empty create homework dialog as expected"

app/lib/pages/homework/homework_dialog.dart

@@ -105,7 +105,7 @@ class _HomeworkDialogState extends State<HomeworkDialog> {
         }
         return bloc_lib.BlocProvider(
           create: (context) => bloc,
-          child: __HomeworkDialog(
+          child: HomeworkDialogMain(
             isEditing: snapshot.data != null,
             bloc: bloc,
           ),
@@ -129,19 +129,20 @@ class HwDialogKeys {
   static const Key saveButton = Key("save-button");
 }
 
-class __HomeworkDialog extends StatefulWidget {
-  const __HomeworkDialog(
+@visibleForTesting
+class HomeworkDialogMain extends StatefulWidget {
+  const HomeworkDialogMain(
       {Key? key, required this.isEditing, required this.bloc})
       : super(key: key);
 
   final bool isEditing;
   final HomeworkDialogBloc bloc;
 
   @override
-  __HomeworkDialogState createState() => __HomeworkDialogState();
+  HomeworkDialogMainState createState() => HomeworkDialogMainState();
 }
 
-class __HomeworkDialogState extends State<__HomeworkDialog> {
+class HomeworkDialogMainState extends State<HomeworkDialogMain> {
   final titleNode = FocusNode();
 
   @override

app/test_goldens/homework/homework_dialog/homework_dialog_test.dart

@@ -0,0 +1,90 @@
+// Copyright (c) 2023 Sharezone UG (haftungsbeschränkt)
+// Licensed under the EUPL-1.2-or-later.
+//
+// You may obtain a copy of the Licence at:
+// https://joinup.ec.europa.eu/software/page/eupl
+//
+// SPDX-License-Identifier: EUPL-1.2
+
+import 'package:bloc_test/bloc_test.dart';
+import 'package:flutter/material.dart';
+import 'package:flutter_bloc/flutter_bloc.dart' as bloc_lib;
+import 'package:flutter_test/flutter_test.dart';
+import 'package:golden_toolkit/golden_toolkit.dart';
+import 'package:sharezone/blocs/homework/homework_dialog_bloc.dart';
+import 'package:sharezone/pages/homework/homework_dialog.dart';
+import 'package:sharezone_widgets/sharezone_widgets.dart';
+
+// ignore_for_file: invalid_use_of_visible_for_testing_member
+
+class MockHomeworkDialogBloc
+    extends MockBloc<HomeworkDialogEvent, HomeworkDialogState>
+    implements HomeworkDialogBloc {}
+
+void main() {
+  group('HomeworkDialog', () {
+    late MockHomeworkDialogBloc homeworkDialogBloc;
+
+    setUp(() {
+      homeworkDialogBloc = MockHomeworkDialogBloc();
+    });
+
+    Future<void> pumpAndSettleHomeworkDialog(WidgetTester tester,
+        {required ThemeData theme, required bool isEditing}) async {
+      await tester.pumpWidgetBuilder(
+        bloc_lib.BlocProvider<HomeworkDialogBloc>(
+          create: (context) => homeworkDialogBloc,
+          child: Scaffold(
+            body: HomeworkDialogMain(
+              isEditing: isEditing,
+              bloc: homeworkDialogBloc,
+            ),
+          ),
+        ),
+        wrapper: materialAppWrapper(theme: theme),
+      );
+
+      // We have a delay for displaying the keyboard (using a Timer).
+      // We have to wait until the timer is finished, otherwise this happens:
+      //  The following assertion was thrown running a test:
+      //  A Timer is still pending even after the widget tree was disposed.
+      // We use a very long timer to show that it doesn't actually
+      // make the test slower.
+      await tester.pumpAndSettle(const Duration(seconds: 25));
+    }
+
+    testGoldens('renders empty create homework dialog as expected',
+        (tester) async {
+      whenListen(
+        homeworkDialogBloc,
+        Stream.value(emptyCreateHomeworkDialogState),
+        initialState: emptyCreateHomeworkDialogState,
+      );
+
+      await pumpAndSettleHomeworkDialog(tester,
+          isEditing: false, theme: lightTheme);
+
+      await multiScreenGolden(
+        tester,
+        'homework_dialog_add_empty_light',
+      );
+
+      await pumpAndSettleHomeworkDialog(tester,
+          isEditing: false, theme: darkTheme);
+
+      await multiScreenGolden(
+        tester,
+        'homework_dialog_add_empty_dark',
+      );
+    });
+  });
+}
+
+// Used temporarily when testing so one can see what happens "on the screen" in
+// a widget test without having to use a real device / simulator to run these
+// tests.
+// --update-goldens
+Future<void> generateGolden(String name) async {
+  await expectLater(
+      find.byType(MaterialApp), matchesGoldenFile('goldens/golden_$name.png'));
+}