Print events and views when first frame is taking awhile during tracing (#98957)

packages/flutter_tools/lib/src/tracing.dart

@@ -51,7 +51,10 @@ class Tracing {
           // It is safe to ignore this error because we expect an error to be
           // thrown if we're already subscribed.
         }
+        final StringBuffer bufferedEvents = StringBuffer();
+        void Function(String) handleBufferedEvent = bufferedEvents.writeln;
         vmService.service.onExtensionEvent.listen((vm_service.Event event) {
+          handleBufferedEvent('${event.extensionKind}: ${event.extensionData}');
           if (event.extensionKind == 'Flutter.FirstFrame') {
             whenFirstFrameRendered.complete();
           }
@@ -69,7 +72,19 @@ class Tracing {
           }
         }
         if (!done) {
+          final Timer timer = Timer(const Duration(seconds: 10), () {
+            _logger.printStatus('First frame is taking longer than expected...');
+            _logger.printTrace('Views:');
+            for (final FlutterView view in views) {
+              _logger.printTrace('id: ${view.id} isolate: ${view.uiIsolate?.id}');
+            }
+            _logger.printTrace('Received VM events:');
+            _logger.printTrace(bufferedEvents.toString());
+            // Swap to just printing new events instead of buffering.
+            handleBufferedEvent = _logger.printTrace;
+          });
           await whenFirstFrameRendered.future;
+          timer.cancel();
         }
       // The exception is rethrown, so don't catch only Exceptions.
       } catch (exception) { // ignore: avoid_catches_without_on_clauses

packages/flutter_tools/test/general.shard/tracing_test.dart

@@ -2,6 +2,9 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'dart:async';
+
+import 'package:fake_async/fake_async.dart';
 import 'package:file/memory.dart';
 import 'package:file_testing/file_testing.dart';
 import 'package:flutter_tools/src/base/file_system.dart';
@@ -181,6 +184,58 @@ void main() {
     ), throwsToolExit(message: 'Engine start event is missing in the timeline'));
   });
 
+  testWithoutContext('prints when first frame is taking a long time', () async {
+    final BufferLogger logger = BufferLogger.test();
+    final FileSystem fileSystem = MemoryFileSystem.test();
+    final Completer<void> completer = Completer<void>();
+    await FakeAsync().run((FakeAsync time) {
+      final Map<String, String> extensionData = <String, String>{
+        'test': 'data',
+        'renderedErrorText': 'error text',
+      };
+      final FakeVmServiceHost fakeVmServiceHost = FakeVmServiceHost(requests: <VmServiceExpectation>[
+        const FakeVmServiceRequest(
+          method: 'streamListen',
+          args: <String, Object>{
+            'streamId': vm_service.EventKind.kExtension,
+          }
+        ),
+        const FakeVmServiceRequest(
+          method: kListViewsMethod,
+          jsonResponse: <String, Object>{
+            'views': <Object>[
+              <String, Object?>{
+              'id': '1',
+              // No isolate, no views.
+              'isolate': null,
+              }
+            ],
+          },
+        ),
+        FakeVmServiceStreamResponse(
+          streamId: 'Extension',
+          event: vm_service.Event(
+            timestamp: 0,
+            extensionKind: 'Flutter.Error',
+            extensionData: vm_service.ExtensionData.parse(extensionData),
+            kind: vm_service.EventStreams.kExtension,
+          ),
+        ),
+      ]);
+      unawaited(downloadStartupTrace(fakeVmServiceHost.vmService,
+        output: fileSystem.currentDirectory,
+        logger: logger,
+      ));
+      time.elapse(const Duration(seconds: 11));
+      time.flushMicrotasks();
+      completer.complete();
+      return completer.future;
+    });
+    expect(logger.statusText, contains('First frame is taking longer than expected'));
+    expect(logger.traceText, contains('id: 1 isolate: null'));
+    expect(logger.traceText, contains('Flutter.Error: [ExtensionData {test: data, renderedErrorText: error text}]'));
+  });
+
   testWithoutContext('throws tool exit if first frame events are missing', () async {
     final BufferLogger logger = BufferLogger.test();
     final FileSystem fileSystem = MemoryFileSystem.test();