[macOS] Migrate @NSApplicationMain attribute to @main (flutter#146848)

This migrates Flutter to use the `@main` attribute introduced in Swift 5.3. The `@NSApplicationMain` attribute is deprecated and will be removed in Swift 6. See: https://github.com/apple/swift-evolution/blob/main/proposals/0383-deprecate-uiapplicationmain-and-nsapplicationmain.md

This change is split into two commits:

1. flutter@a508d3e - This updates the macOS app template and adds a migration to replace `@NSApplicationMain` uses with `@main`. 
2. flutter@f434827 - I ran `flutter run -d macos` on each Flutter macOS app in this repository to verify the app migrates and launches successfully.

Follow-up to flutter#146707
Fixes flutter#143044

dev/a11y_assessments/macos/Runner/AppDelegate.swift

@@ -5,7 +5,7 @@
 import Cocoa
 import FlutterMacOS
 
-@NSApplicationMain
+@main
 class AppDelegate: FlutterAppDelegate {
   override func applicationShouldTerminateAfterLastWindowClosed(_ sender: NSApplication) -> Bool {
     return true

dev/benchmarks/complex_layout/macos/Runner/AppDelegate.swift

@@ -5,7 +5,7 @@
 import Cocoa
 import FlutterMacOS
 
-@NSApplicationMain
+@main
 class AppDelegate: FlutterAppDelegate {
   override func applicationShouldTerminateAfterLastWindowClosed(_ sender: NSApplication) -> Bool {
     return true

dev/benchmarks/macrobenchmarks/macos/Runner/AppDelegate.swift

@@ -5,7 +5,7 @@
 import Cocoa
 import FlutterMacOS
 
-@NSApplicationMain
+@main
 class AppDelegate: FlutterAppDelegate {
   override func applicationShouldTerminateAfterLastWindowClosed(_ sender: NSApplication) -> Bool {
     return true

dev/integration_tests/channels/macos/Runner/AppDelegate.swift

@@ -5,7 +5,7 @@
 import Cocoa
 import FlutterMacOS
 
-@NSApplicationMain
+@main
 class AppDelegate: FlutterAppDelegate {
   override func applicationShouldTerminateAfterLastWindowClosed(_ sender: NSApplication) -> Bool {
     return true

dev/integration_tests/flavors/macos/Runner/AppDelegate.swift

@@ -5,7 +5,7 @@
 import Cocoa
 import FlutterMacOS
 
-@NSApplicationMain
+@main
 class AppDelegate: FlutterAppDelegate {
   override func applicationShouldTerminateAfterLastWindowClosed(_ sender: NSApplication) -> Bool {
     return true

dev/integration_tests/flutter_gallery/macos/Runner/AppDelegate.swift

@@ -5,7 +5,7 @@
 import Cocoa
 import FlutterMacOS
 
-@NSApplicationMain
+@main
 class AppDelegate: FlutterAppDelegate {
   override func applicationShouldTerminateAfterLastWindowClosed(_ sender: NSApplication) -> Bool {
     return true

dev/integration_tests/ui/macos/Runner/AppDelegate.swift

@@ -5,7 +5,7 @@
 import Cocoa
 import FlutterMacOS
 
-@NSApplicationMain
+@main
 class AppDelegate: FlutterAppDelegate {
   override func applicationShouldTerminateAfterLastWindowClosed(_ sender: NSApplication) -> Bool {
     return true

dev/manual_tests/macos/Runner/AppDelegate.swift

@@ -5,7 +5,7 @@
 import Cocoa
 import FlutterMacOS
 
-@NSApplicationMain
+@main
 class AppDelegate: FlutterAppDelegate {
   override func applicationShouldTerminateAfterLastWindowClosed(_ sender: NSApplication) -> Bool {
     return true

examples/api/macos/Runner/AppDelegate.swift

@@ -5,7 +5,7 @@
 import Cocoa
 import FlutterMacOS
 
-@NSApplicationMain
+@main
 class AppDelegate: FlutterAppDelegate {
   override func applicationShouldTerminateAfterLastWindowClosed(_ sender: NSApplication) -> Bool {
     return true

examples/flutter_view/macos/Runner/AppDelegate.swift

@@ -5,7 +5,7 @@
 import Cocoa
 import FlutterMacOS
 
-@NSApplicationMain
+@main
 class AppDelegate: FlutterAppDelegate {
   override func applicationShouldTerminateAfterLastWindowClosed(_ sender: NSApplication) -> Bool {
     return true

examples/hello_world/macos/Runner/AppDelegate.swift

@@ -5,7 +5,7 @@
 import Cocoa
 import FlutterMacOS
 
-@NSApplicationMain
+@main
 class AppDelegate: FlutterAppDelegate {
   override func applicationShouldTerminateAfterLastWindowClosed(_ sender: NSApplication) -> Bool {
     return true

examples/image_list/macos/Runner/AppDelegate.swift

@@ -5,7 +5,7 @@
 import Cocoa
 import FlutterMacOS
 
-@NSApplicationMain
+@main
 class AppDelegate: FlutterAppDelegate {
   override func applicationShouldTerminateAfterLastWindowClosed(_ sender: NSApplication) -> Bool {
     return true

examples/layers/macos/Runner/AppDelegate.swift

@@ -5,7 +5,7 @@
 import Cocoa
 import FlutterMacOS
 
-@NSApplicationMain
+@main
 class AppDelegate: FlutterAppDelegate {
   override func applicationShouldTerminateAfterLastWindowClosed(_ sender: NSApplication) -> Bool {
     return true

examples/platform_channel/macos/Runner/AppDelegate.swift

@@ -5,7 +5,7 @@
 import Cocoa
 import FlutterMacOS
 
-@NSApplicationMain
+@main
 class AppDelegate: FlutterAppDelegate {
   override func applicationShouldTerminateAfterLastWindowClosed(_ sender: NSApplication) -> Bool {
     return true

examples/platform_view/macos/Runner/AppDelegate.swift

@@ -5,7 +5,7 @@
 import Cocoa
 import FlutterMacOS
 
-@NSApplicationMain
+@main
 class AppDelegate: FlutterAppDelegate {
   override func applicationShouldTerminateAfterLastWindowClosed(_ sender: NSApplication) -> Bool {
     return true

packages/flutter_tools/lib/src/macos/build_macos.dart

@@ -24,6 +24,7 @@ import 'application_package.dart';
 import 'cocoapod_utils.dart';
 import 'migrations/flutter_application_migration.dart';
 import 'migrations/macos_deployment_target_migration.dart';
+import 'migrations/nsapplicationmain_deprecation_migration.dart';
 import 'migrations/remove_macos_framework_link_and_embedding_migration.dart';
 
 /// When run in -quiet mode, Xcode should only print from the underlying tasks to stdout.
@@ -83,6 +84,7 @@ Future<void> buildMacOS({
     XcodeScriptBuildPhaseMigration(flutterProject.macos, globals.logger),
     XcodeThinBinaryBuildPhaseInputPathsMigration(flutterProject.macos, globals.logger),
     FlutterApplicationMigration(flutterProject.macos, globals.logger),
+    NSApplicationMainDeprecationMigration(flutterProject.macos, globals.logger),
   ];
 
   final ProjectMigration migration = ProjectMigration(migrators);

packages/flutter_tools/lib/src/macos/migrations/nsapplicationmain_deprecation_migration.dart

@@ -0,0 +1,51 @@
+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import '../../base/file_system.dart';
+import '../../base/project_migrator.dart';
+import '../../xcode_project.dart';
+
+const String _appDelegateFileBefore = r'''
+@NSApplicationMain
+class AppDelegate''';
+
+const String _appDelegateFileAfter = r'''
+@main
+class AppDelegate''';
+
+/// Replace the deprecated `@NSApplicationMain` attribute with `@main`.
+///
+/// See:
+/// https://github.com/apple/swift-evolution/blob/main/proposals/0383-deprecate-uiapplicationmain-and-nsapplicationmain.md
+class NSApplicationMainDeprecationMigration extends ProjectMigrator {
+  NSApplicationMainDeprecationMigration(
+    MacOSProject project,
+    super.logger,
+  ) : _appDelegateSwift = project.appDelegateSwift;
+
+  final File _appDelegateSwift;
+
+  @override
+  Future<void> migrate() async {
+    // Skip this migration if the project uses Objective-C.
+    if (!_appDelegateSwift.existsSync()) {
+      logger.printTrace(
+        'macos/Runner/AppDelegate.swift not found, skipping @main migration.',
+      );
+      return;
+    }
+
+    // Migrate the macos/Runner/AppDelegate.swift file.
+    final String original = _appDelegateSwift.readAsStringSync();
+    final String migrated = original.replaceFirst(_appDelegateFileBefore, _appDelegateFileAfter);
+    if (original == migrated) {
+      return;
+    }
+
+    logger.printWarning(
+      'macos/Runner/AppDelegate.swift uses the deprecated @NSApplicationMain attribute, updating.',
+    );
+    _appDelegateSwift.writeAsStringSync(migrated);
+  }
+}

packages/flutter_tools/lib/src/xcode_project.dart

@@ -715,6 +715,9 @@ class MacOSProject extends XcodeBasedProject {
 
   File get pluginRegistrantImplementation => managedDirectory.childFile('GeneratedPluginRegistrant.swift');
 
+  /// The 'AppDelegate.swift' file of the host app. This file might not exist if the app project uses Objective-C.
+  File get appDelegateSwift => hostAppRoot.childDirectory('Runner').childFile('AppDelegate.swift');
+
   @override
   File xcodeConfigFor(String mode) => managedDirectory.childFile('Flutter-$mode.xcconfig');
 

packages/flutter_tools/templates/app_shared/macos.tmpl/Runner/AppDelegate.swift

@@ -1,7 +1,7 @@
 import Cocoa
 import FlutterMacOS
 
-@NSApplicationMain
+@main
 class AppDelegate: FlutterAppDelegate {
   override func applicationShouldTerminateAfterLastWindowClosed(_ sender: NSApplication) -> Bool {
     return true

packages/flutter_tools/test/general.shard/macos/macos_project_migration_test.dart

@@ -8,6 +8,7 @@ import 'package:flutter_tools/src/base/logger.dart';
 import 'package:flutter_tools/src/ios/plist_parser.dart';
 import 'package:flutter_tools/src/macos/migrations/flutter_application_migration.dart';
 import 'package:flutter_tools/src/macos/migrations/macos_deployment_target_migration.dart';
+import 'package:flutter_tools/src/macos/migrations/nsapplicationmain_deprecation_migration.dart';
 import 'package:flutter_tools/src/macos/migrations/remove_macos_framework_link_and_embedding_migration.dart';
 import 'package:flutter_tools/src/project.dart';
 import 'package:flutter_tools/src/reporting/reporting.dart';
@@ -400,6 +401,92 @@ platform :osx, '10.14'
       expect(testLogger.traceText, isEmpty);
     });
   });
+
+  group('migrate @NSApplicationMain attribute to @main', () {
+    late MemoryFileSystem memoryFileSystem;
+    late BufferLogger testLogger;
+    late FakeMacOSProject project;
+    late File appDelegateFile;
+
+    setUp(() {
+      memoryFileSystem = MemoryFileSystem();
+      testLogger = BufferLogger.test();
+      project = FakeMacOSProject();
+      appDelegateFile = memoryFileSystem.file('AppDelegate.swift');
+      project.appDelegateSwift = appDelegateFile;
+    });
+
+    testWithoutContext('skipped if files are missing', () async {
+      final NSApplicationMainDeprecationMigration migration = NSApplicationMainDeprecationMigration(
+        project,
+        testLogger,
+      );
+      await migration.migrate();
+      expect(appDelegateFile.existsSync(), isFalse);
+
+      expect(testLogger.statusText, isEmpty);
+    });
+
+    testWithoutContext('skipped if nothing to upgrade', () async {
+      const String appDelegateContents = '''
+import Cocoa
+import FlutterMacOS
+
+@main
+class AppDelegate: FlutterAppDelegate {
+  override func applicationShouldTerminateAfterLastWindowClosed(_ sender: NSApplication) -> Bool {
+    return true
+  }
+}
+''';
+      appDelegateFile.writeAsStringSync(appDelegateContents);
+      final DateTime lastModified = appDelegateFile.lastModifiedSync();
+
+      final NSApplicationMainDeprecationMigration migration = NSApplicationMainDeprecationMigration(
+        project,
+        testLogger,
+      );
+      await migration.migrate();
+
+      expect(appDelegateFile.lastModifiedSync(), lastModified);
+      expect(appDelegateFile.readAsStringSync(), appDelegateContents);
+
+      expect(testLogger.statusText, isEmpty);
+    });
+
+    testWithoutContext('updates AppDelegate.swift', () async {
+      appDelegateFile.writeAsStringSync('''
+import Cocoa
+import FlutterMacOS
+
+@NSApplicationMain
+class AppDelegate: FlutterAppDelegate {
+  override func applicationShouldTerminateAfterLastWindowClosed(_ sender: NSApplication) -> Bool {
+    return true
+  }
+}
+''');
+
+      final NSApplicationMainDeprecationMigration migration = NSApplicationMainDeprecationMigration(
+        project,
+        testLogger,
+      );
+      await migration.migrate();
+
+      expect(appDelegateFile.readAsStringSync(), '''
+import Cocoa
+import FlutterMacOS
+
+@main
+class AppDelegate: FlutterAppDelegate {
+  override func applicationShouldTerminateAfterLastWindowClosed(_ sender: NSApplication) -> Bool {
+    return true
+  }
+}
+''');
+      expect(testLogger.warningText, contains('uses the deprecated @NSApplicationMain attribute, updating'));
+    });
+  });
 }
 
 class FakeMacOSProject extends Fake implements MacOSProject {
@@ -411,4 +498,7 @@ class FakeMacOSProject extends Fake implements MacOSProject {
 
   @override
   File podfile = MemoryFileSystem.test().file('Podfile');
+
+  @override
+  File appDelegateSwift = MemoryFileSystem.test().file('AppDelegate.swift');
 }