Explain unavailable debugging for unattached Flutter apps

packages/devtools_app/lib/src/shared/framework/screen.dart

@@ -9,6 +9,7 @@ library;
 
 import 'package:collection/collection.dart';
 import 'package:devtools_app_shared/service.dart';
+import 'package:devtools_app_shared/service_extensions.dart' as extensions;
 import 'package:devtools_app_shared/shared.dart';
 import 'package:devtools_app_shared/ui.dart';
 import 'package:flutter/foundation.dart';
@@ -558,6 +559,18 @@ abstract class Screen {
     return (show: false, disabledReason: ScreenDisabledReason.requiresFlutter);
   }
   if (screen.requiresDebugBuild && connectedApp.isProfileBuildNow == true) {
+    // Build mode is inferred from expression evaluation. A debug-only service
+    // extension identifies a debug app whose expressions could not be compiled,
+    // for example when connecting without Flutter Tools.
+    if (connectedApp.isFlutterAppNow == true &&
+        serviceManager.serviceExtensionManager.isServiceExtensionAvailable(
+          extensions.debugAllowBanner.extension,
+        )) {
+      return (
+        show: false,
+        disabledReason: ScreenDisabledReason.expressionEvaluationUnavailable,
+      );
+    }
     _log.finest('screen requires debug build: returning false');
     return (
       show: false,
@@ -598,6 +611,12 @@ enum ScreenDisabledReason {
   requiresDartLibrary(null),
   requiresDartVm('requires the Dart VM, but it is not available.'),
   requiresDebugBuild('only supports debug builds.'),
+
+  /// A Flutter debug app was detected, but expression evaluation failed.
+  expressionEvaluationUnavailable(
+    'could not evaluate expressions. Try running "flutter attach" and '
+    'reconnecting DevTools.',
+  ),
   requiresFlutter('only supports Flutter applications.'),
   requiresAdvancedDeveloperMode(
     'only works when Advanced Developer Mode is enabled',

packages/devtools_app/release_notes/NEXT_RELEASE_NOTES.md

@@ -15,6 +15,10 @@ To learn more about DevTools, check out the
 
 ## General updates
 
+* Added guidance to run "flutter attach" when a Flutter debug app cannot
+  evaluate expressions and a debug-only screen is unavailable.
+  [#4091](https://github.com/flutter/devtools/issues/4091)
+
 TODO: Remove this section if there are not any updates.
 
 * Improved the readability of ANSI-colored text in the console and logging

packages/devtools_app/test/shared/framework/visible_screens_test.dart

@@ -4,6 +4,7 @@
 
 import 'package:devtools_app/devtools_app.dart';
 import 'package:devtools_app/src/shared/framework/framework_controller.dart';
+import 'package:devtools_app_shared/service_extensions.dart' as extensions;
 import 'package:devtools_app_shared/utils.dart';
 import 'package:devtools_shared/devtools_shared.dart';
 import 'package:devtools_test/devtools_test.dart';
@@ -166,6 +167,37 @@ void main() {
       );
     });
 
+    testWidgets('explain unavailable evaluation in an unattached debug app', (
+      WidgetTester tester,
+    ) async {
+      // Failed expression evaluation makes the build-mode heuristic report
+      // profile mode, even though debug-only extensions are registered.
+      setupMockConnectedApp(flutter: true, debugMode: false);
+      final extensionManager =
+          fakeServiceConnection.serviceManager.serviceExtensionManager;
+      // The extension may still be pending its first frame.
+      await extensionManager.fakeAddServiceExtension(
+        extensions.debugAllowBanner.extension,
+      );
+
+      final result = shouldShowScreen(DebuggerScreen());
+      expect(result.show, isFalse);
+      expect(result.disabledReason?.message, contains('flutter attach'));
+      expect(
+        result.disabledReason?.message,
+        isNot(contains('only supports debug')),
+      );
+    });
+
+    testWidgets('keep the profile-build explanation for a real profile app', (
+      WidgetTester tester,
+    ) async {
+      setupMockConnectedApp(flutter: true, debugMode: false);
+      final result = shouldShowScreen(DebuggerScreen());
+      expect(result.show, isFalse);
+      expect(result.disabledReason?.message, 'only supports debug builds.');
+    });
+
     testWidgets('are correct for Flutter (non-web) profile app', (
       WidgetTester tester,
     ) async {