[flutter_tool] [dap] Forward Flutter progress events to DAP client (#142524)

Builds can be slow and the legacy debug adapter would handle Flutter's `app.progress` events to update the toast notification during builds. This was lost in the new adapters - we should only a single "Launching.." notification for the whole progress.

This change listens to `app.progress` events and forwards those with `finished=false` to the client if the launch progress is still active.

Fixes https://github.com/Dart-Code/Dart-Code/issues/4938

https://github.com/flutter/flutter/assets/1078012/8c60cf08-e034-4a72-b31e-9c61dca388bf

packages/flutter_tools/lib/src/debug_adapters/flutter_adapter.dart

@@ -420,6 +420,19 @@ class FlutterDebugAdapter extends FlutterBaseDebugAdapter with VmServiceInfoFile
     );
   }
 
+  /// Handles any app.progress event from Flutter.
+  void _handleAppProgress(Map<String, Object?> params) {
+    // If this is a new progress starting (and we're still launching), update
+    // the progress notification.
+    //
+    // We ignore finished status because we have a limited API - the next
+    // item will replace it (or the launch progress will be completed by
+    // _handleAppStarted).
+    if (params case {'message': final String message, 'finished': false}) {
+      launchProgress?.update(message: message);
+    }
+  }
+
   /// Handles the app.started event from Flutter.
   Future<void> _handleAppStarted() async {
     launchProgress?.end();
@@ -481,6 +494,8 @@ class FlutterDebugAdapter extends FlutterBaseDebugAdapter with VmServiceInfoFile
         _handleDebugPort(params);
       case 'app.start':
         _handleAppStart(params);
+      case 'app.progress':
+        _handleAppProgress(params);
       case 'app.started':
         _handleAppStarted();
     }

packages/flutter_tools/test/general.shard/dap/flutter_adapter_test.dart

@@ -210,6 +210,45 @@ void main() {
         expect(adapter.dapToFlutterRequests, isNot(contains('app.restart')));
       });
 
+      test('includes build progress updates', () async {
+        final MockFlutterDebugAdapter adapter = MockFlutterDebugAdapter(
+          fileSystem: MemoryFileSystem.test(style: fsStyle),
+          platform: platform,
+        );
+        final Completer<void> responseCompleter = Completer<void>();
+
+        final FlutterLaunchRequestArguments args = FlutterLaunchRequestArguments(
+          cwd: '.',
+          program: 'foo.dart',
+        );
+
+        // Begin listening for progress events up until `progressEnd` (but don't await yet).
+        final Future<List<List<Object?>>> progressEventsFuture =
+            adapter.dapToClientProgressEvents
+              .takeWhile((Map<String, Object?> message) => message['event'] != 'progressEnd')
+              .map((Map<String, Object?> message) => <Object?>[message['event'], (message['body']! as Map<String, Object?>)['message']])
+              .toList();
+
+        // Initialize with progress support.
+        await adapter.initializeRequest(
+          MockRequest(),
+          InitializeRequestArguments(adapterID: 'test', supportsProgressReporting: true, ),
+          (_) {},
+        );
+        await adapter.configurationDoneRequest(MockRequest(), null, () {});
+        await adapter.launchRequest(MockRequest(), args, responseCompleter.complete);
+        await responseCompleter.future;
+
+        // Ensure we got the expected events prior to the progressEnd.
+        final List<List<Object?>> progressEvents = await progressEventsFuture;
+        expect(progressEvents, containsAllInOrder(<List<String?>>[
+          <String?>['progressStart', 'Launching…'],
+          <String?>['progressUpdate', 'Step 1…'],
+          <String?>['progressUpdate', 'Step 2…'],
+          // progressEnd isn't included because we used takeWhile to stop when it arrived above.
+        ]));
+      });
+
       test('includes Dart Debug extension progress update', () async {
         final MockFlutterDebugAdapter adapter = MockFlutterDebugAdapter(
           fileSystem: MemoryFileSystem.test(style: fsStyle),
@@ -242,7 +281,7 @@ void main() {
         await adapter.launchRequest(MockRequest(), args, responseCompleter.complete);
         await responseCompleter.future;
 
-        // Ensure we got the expected events prior to the
+        // Ensure we got the expected events prior to the progressEnd.
         final List<List<Object?>> progressEvents = await progressEventsFuture;
         expect(progressEvents, containsAllInOrder(<List<String>>[
           <String>['progressStart', 'Launching…'],

packages/flutter_tools/test/general.shard/dap/mocks.dart

@@ -104,9 +104,22 @@ class MockFlutterDebugAdapter extends FlutterDebugAdapter {
 
     await preAppStart?.call(this);
 
+    void sendLaunchProgress({required bool finished, String? message}) {
+      assert(finished == (message == null));
+      simulateStdoutMessage(<String, Object?>{
+        'event': 'app.progress',
+        'params': <String, Object?>{
+          'id': 'launch',
+          'message': message,
+          'finished': finished,
+        }
+      });
+    }
+
     // Simulate the app starting by triggering handling of events that Flutter
     // would usually write to stdout.
     if (simulateAppStarted) {
+      sendLaunchProgress(message: 'Step 1…', finished: false);
       simulateStdoutMessage(<String, Object?>{
         'event': 'app.start',
         'params': <String, Object?>{
@@ -116,6 +129,8 @@ class MockFlutterDebugAdapter extends FlutterDebugAdapter {
           'mode': 'debug',
         }
       });
+      sendLaunchProgress(message: 'Step 2…', finished: false);
+      sendLaunchProgress(finished: true);
       simulateStdoutMessage(<String, Object?>{
         'event': 'app.started',
       });