refactoring

learning/tour-of-beam/frontend/lib/pages/tour/screen.dart

@@ -36,7 +36,11 @@ class TourScreen extends StatelessWidget {
   @override
   Widget build(BuildContext context) {
     return TobScaffold(
-      pageActions: [TobPipelineOptionsDropdown(tourNotifier: tourNotifier)],
+      pageActions: [
+        TobPipelineOptionsDropdown(
+          playgroundController: tourNotifier.playgroundController,
+        ),
+      ],
       child: MediaQuery.of(context).size.width > ScreenBreakpoints.twoColumns
           ? _WideTour(tourNotifier)
           : _NarrowTour(tourNotifier),

learning/tour-of-beam/frontend/lib/pages/tour/state.dart

@@ -145,6 +145,7 @@ class TourNotifier extends ChangeNotifier with PageStateMixin<void> {
 
   Future<void> _setPlaygroundSnippet(String? snippetId) async {
     if (snippetId == null) {
+      playgroundController.setEmpty();
       return;
     }
 

learning/tour-of-beam/frontend/lib/pages/tour/widgets/pipeline_options.dart

@@ -19,33 +19,27 @@
 import 'package:flutter/widgets.dart';
 import 'package:playground_components/playground_components.dart';
 
-import '../state.dart';
-
 class TobPipelineOptionsDropdown extends StatelessWidget {
-  final TourNotifier tourNotifier;
+  final PlaygroundController playgroundController;
 
-  const TobPipelineOptionsDropdown({required this.tourNotifier});
+  const TobPipelineOptionsDropdown({
+    required this.playgroundController,
+  });
 
   @override
   Widget build(BuildContext context) {
-    final controller = tourNotifier.playgroundController;
-
     return AnimatedBuilder(
-      animation: tourNotifier,
+      animation: playgroundController,
       builder: (_, __) {
-        return AnimatedBuilder(
-          animation: controller,
-          builder: (_, __) {
-            if (!tourNotifier.isUnitContainsSnippet) {
-              return const SizedBox.shrink();
-            }
+        if (playgroundController.isEmpty) {
+          return const SizedBox.shrink();
+        }
 
-            return PipelineOptionsDropdown(
-              pipelineOptions:
-                  controller.snippetEditingController?.pipelineOptions ?? '',
-              setPipelineOptions: controller.setPipelineOptions,
-            );
-          },
+        final pipelineOptions =
+            playgroundController.snippetEditingController?.pipelineOptions;
+        return PipelineOptionsDropdown(
+          pipelineOptions: pipelineOptions ?? '',
+          setPipelineOptions: playgroundController.setPipelineOptions,
         );
       },
     );

learning/tour-of-beam/frontend/lib/pages/tour/widgets/playground_demo.dart

@@ -76,6 +76,7 @@ class PlaygroundDemoWidget extends StatelessWidget {
   }
 
   void _handleError(BuildContext context, PlaygroundController controller) {
+    //TODO: https://github.com/apache/beam/issues/26319
     PlaygroundComponents.toastNotifier.add(
       Toast(
         description: controller.codeRunner.result?.errorMessage ?? '',

playground/frontend/lib/pages/standalone_playground/widgets/editor_textarea_wrapper.dart

@@ -82,6 +82,7 @@ class CodeTextAreaWrapper extends StatelessWidget {
   }
 
   void _handleError(BuildContext context, PlaygroundController controller) {
+    //TODO: https://github.com/apache/beam/issues/26319
     PlaygroundComponents.toastNotifier.add(
       Toast(
         description: controller.codeRunner.result?.errorMessage ?? '',

playground/frontend/playground_components/lib/src/constants/sizes.dart

@@ -42,7 +42,6 @@ class BeamSizes {
   static const double dividerHeight = 1;
   static const double elevation = 2;
   static const double headerButtonHeight = 46;
-  static const double labelFontSize = 16;
   static const double loadingIndicator = 40;
   static const double splitViewSeparator = BeamSizes.size8;
   static const double tabBarHeight = 50;

playground/frontend/playground_components/lib/src/controllers/example_loaders/examples_loader.dart

@@ -53,6 +53,10 @@ class ExamplesLoader {
     _playgroundController = value;
   }
 
+  void clearDescriptor() {
+    _descriptor = null;
+  }
+
   /// Loads examples from [descriptor]'s immediate list.
   ///
   /// Sets empty editor for SDKs of failed examples.

playground/frontend/playground_components/lib/src/controllers/playground_controller.dart

@@ -35,6 +35,7 @@ import '../models/example_loading_descriptors/user_shared_example_loading_descri
 import '../models/intents.dart';
 import '../models/sdk.dart';
 import '../models/shortcut.dart';
+import '../models/snippet_file.dart';
 import '../repositories/code_repository.dart';
 import '../services/symbols/loaders/map.dart';
 import '../services/symbols/symbols_notifier.dart';
@@ -128,6 +129,11 @@ class PlaygroundController with ChangeNotifier {
       selectedExample?.type != ExampleType.test &&
       [Sdk.java, Sdk.python].contains(sdk);
 
+  bool get isEmpty =>
+      selectedExample == null ||
+      selectedExample?.path == '' &&
+          (selectedExample?.files.contains(SnippetFile.empty) ?? false);
+
   /// If no SDK is selected, sets it to [sdk] and creates an empty state for it.
   void setEmptyIfNoSdk(Sdk sdk) {
     if (_sdk != null) {
@@ -141,6 +147,15 @@ class PlaygroundController with ChangeNotifier {
     );
   }
 
+  void setEmpty() {
+    examplesLoader.clearDescriptor();
+    setExample(
+      Example.empty(Sdk.java),
+      descriptor: const EmptyExampleLoadingDescriptor(sdk: Sdk.java),
+      setCurrentSdk: false,
+    );
+  }
+
   /// If the state for [sdk] does not exists, creates an empty state for it.
   void setEmptyIfNotExists(
     Sdk sdk, {