[iOS] Migrate @UIApplicationMain attribute to @main (#146707)

This migrates Flutter to use the `@main` attribute introduced in Swift 5.3. The `@UIApplicationMain` 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. ad18797428 - This updates the iOS app template and adds a migration to replace `@UIApplicationMain` uses with `@main`. 
2. 8ecbb2f29f - I ran `flutter run` on each Flutter iOS app in this repository to verify the app migrates and launches successfully.

Part of https://github.com/flutter/flutter/issues/143044

dev/a11y_assessments/ios/Runner/AppDelegate.swift

@@ -5,7 +5,7 @@
 import Flutter
 import UIKit
 
-@UIApplicationMain
+@main
 @objc class AppDelegate: FlutterAppDelegate {
   override func application(
     _ application: UIApplication,

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

@@ -5,7 +5,7 @@
 import Flutter
 import UIKit
 
-@UIApplicationMain
+@main
 @objc class AppDelegate: FlutterAppDelegate {
   override func application(
     _ application: UIApplication,

dev/integration_tests/ios_app_with_extensions/ios/Runner/AppDelegate.swift

@@ -5,7 +5,7 @@
 import Flutter
 import UIKit
 
-@UIApplicationMain
+@main
 @objc class AppDelegate: FlutterAppDelegate {
   override func application(
     _ application: UIApplication,

dev/integration_tests/ios_host_app_swift/Host/AppDelegate.swift

@@ -4,7 +4,7 @@
 
 import UIKit
 
-@UIApplicationMain
+@main
 class AppDelegate: UIResponder, UIApplicationDelegate {
   func application(_ application: UIApplication, configurationForConnecting connectingSceneSession: UISceneSession, options: UIScene.ConnectionOptions) -> UISceneConfiguration {
     return UISceneConfiguration(name: "Default Configuration", sessionRole: connectingSceneSession.role)

dev/integration_tests/non_nullable/ios/Runner/AppDelegate.swift

@@ -5,7 +5,7 @@
 import Flutter
 import UIKit
 
-@UIApplicationMain
+@main
 @objc class AppDelegate: FlutterAppDelegate {
   override func application(
     _ application: UIApplication,

dev/manual_tests/ios/Runner/AppDelegate.swift

@@ -5,7 +5,7 @@
 import Flutter
 import UIKit
 
-@UIApplicationMain
+@main
 @objc class AppDelegate: FlutterAppDelegate {
   override func application(
     _ application: UIApplication,

examples/api/ios/Runner/AppDelegate.swift

@@ -5,7 +5,7 @@
 import UIKit
 import Flutter
 
-@UIApplicationMain
+@main
 @objc class AppDelegate: FlutterAppDelegate {
   override func application(
     _ application: UIApplication,

examples/platform_channel_swift/ios/Runner/AppDelegate.swift

@@ -19,7 +19,7 @@ enum MyFlutterErrorCode {
   static let unavailable = "UNAVAILABLE"
 }
 
-@UIApplicationMain
+@main
 @objc class AppDelegate: FlutterAppDelegate, FlutterStreamHandler {
   private var eventSink: FlutterEventSink?
 

packages/flutter_tools/lib/src/ios/mac.dart

@@ -35,6 +35,7 @@ import 'migrations/project_base_configuration_migration.dart';
 import 'migrations/project_build_location_migration.dart';
 import 'migrations/remove_bitcode_migration.dart';
 import 'migrations/remove_framework_link_and_embedding_migration.dart';
+import 'migrations/uiapplicationmain_deprecation_migration.dart';
 import 'migrations/xcode_build_system_migration.dart';
 import 'xcode_build_settings.dart';
 import 'xcodeproj.dart';
@@ -158,6 +159,7 @@ Future<XcodeBuildResult> buildXcodeProject({
     XcodeScriptBuildPhaseMigration(app.project, globals.logger),
     RemoveBitcodeMigration(app.project, globals.logger),
     XcodeThinBinaryBuildPhaseInputPathsMigration(app.project, globals.logger),
+    UIApplicationMainDeprecationMigration(app.project, globals.logger),
   ];
 
   final ProjectMigration migration = ProjectMigration(migrators);

packages/flutter_tools/lib/src/ios/migrations/remove_bitcode_migration.dart

@@ -16,14 +16,12 @@ class RemoveBitcodeMigration extends ProjectMigrator {
   final File _xcodeProjectInfoFile;
 
   @override
-  Future<bool> migrate() async {
+  Future<void> migrate() async {
     if (_xcodeProjectInfoFile.existsSync()) {
       processFileLines(_xcodeProjectInfoFile);
     } else {
       logger.printTrace('Xcode project not found, skipping removing bitcode migration.');
     }
-
-    return true;
   }
 
   @override

packages/flutter_tools/lib/src/ios/migrations/uiapplicationmain_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'''
+@UIApplicationMain
+@objc class AppDelegate''';
+
+const String _appDelegateFileAfter = r'''
+@main
+@objc class AppDelegate''';
+
+/// Replace the deprecated `@UIApplicationMain` attribute with `@main`.
+///
+/// See:
+/// https://github.com/apple/swift-evolution/blob/main/proposals/0383-deprecate-uiapplicationmain-and-nsapplicationmain.md
+class UIApplicationMainDeprecationMigration extends ProjectMigrator {
+  UIApplicationMainDeprecationMigration(
+    IosProject 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(
+        'ios/Runner/AppDelegate.swift not found, skipping @main migration.',
+      );
+      return;
+    }
+
+    // Migrate the ios/Runner/AppDelegate.swift file.
+    final String original = _appDelegateSwift.readAsStringSync();
+    final String migrated = original.replaceFirst(_appDelegateFileBefore, _appDelegateFileAfter);
+    if (original == migrated) {
+      return;
+    }
+
+    logger.printWarning(
+      'ios/Runner/AppDelegate.swift uses the deprecated @UIApplicationMain attribute, updating.',
+    );
+    _appDelegateSwift.writeAsStringSync(migrated);
+  }
+}

packages/flutter_tools/lib/src/xcode_project.dart

@@ -178,15 +178,15 @@ class IosProject extends XcodeBasedProject {
 
   File get appFrameworkInfoPlist => _flutterLibRoot.childDirectory('Flutter').childFile('AppFrameworkInfo.plist');
 
+  /// The 'AppDelegate.swift' file of the host app. This file might not exist if the app project uses Objective-C.
+  File get appDelegateSwift => _editableDirectory.childDirectory('Runner').childFile('AppDelegate.swift');
+
   File get infoPlist => _editableDirectory.childDirectory('Runner').childFile('Info.plist');
 
   Directory get symlinks => _flutterLibRoot.childDirectory('.symlinks');
 
-  /// True, if the app project is using swift.
+  /// True if the app project uses Swift.
-  bool get isSwift {
+  bool get isSwift => appDelegateSwift.existsSync();
-    final File appDelegateSwift = _editableDirectory.childDirectory('Runner').childFile('AppDelegate.swift');
-    return appDelegateSwift.existsSync();
-  }
 
   /// Do all plugins support arm64 simulators to run natively on an ARM Mac?
   Future<bool> pluginsSupportArmSimulator() async {

packages/flutter_tools/templates/app_shared/ios-swift.tmpl/Runner/AppDelegate.swift

@@ -1,7 +1,7 @@
 import Flutter
 import UIKit
 
-@UIApplicationMain
+@main
 @objc class AppDelegate: FlutterAppDelegate {
   override func application(
     _ application: UIApplication,

packages/flutter_tools/test/general.shard/ios/ios_project_migration_test.dart

@@ -13,6 +13,7 @@ import 'package:flutter_tools/src/ios/migrations/project_base_configuration_migr
 import 'package:flutter_tools/src/ios/migrations/project_build_location_migration.dart';
 import 'package:flutter_tools/src/ios/migrations/remove_bitcode_migration.dart';
 import 'package:flutter_tools/src/ios/migrations/remove_framework_link_and_embedding_migration.dart';
+import 'package:flutter_tools/src/ios/migrations/uiapplicationmain_deprecation_migration.dart';
 import 'package:flutter_tools/src/ios/migrations/xcode_build_system_migration.dart';
 import 'package:flutter_tools/src/ios/xcodeproj.dart';
 import 'package:flutter_tools/src/migrations/cocoapods_script_symlink.dart';
@@ -906,7 +907,7 @@ platform :ios, '12.0'
           project,
           testLogger,
         );
-        expect(await migration.migrate(), isTrue);
+        await migration.migrate();
         expect(xcodeProjectInfoFile.existsSync(), isFalse);
 
         expect(testLogger.traceText, contains('Xcode project not found, skipping removing bitcode migration'));
@@ -922,7 +923,7 @@ platform :ios, '12.0'
           project,
           testLogger,
         );
-        expect(await migration.migrate(), isTrue);
+        await migration.migrate();
 
         expect(xcodeProjectInfoFile.lastModifiedSync(), projectLastModified);
         expect(xcodeProjectInfoFile.readAsStringSync(), xcodeProjectInfoFileContents);
@@ -943,7 +944,7 @@ platform :ios, '12.0'
           project,
           testLogger,
         );
-        expect(await migration.migrate(), isTrue);
+        await migration.migrate();
 
         expect(xcodeProjectInfoFile.readAsStringSync(), '''
 				ASSETCATALOG_COMPILER_APPICON_NAME = AppIcon;
@@ -1408,6 +1409,104 @@ LD_RUNPATH_SEARCH_PATHS = $(inherited) /usr/lib/swift '@executable_path/../Frame
       expect(testLogger.statusText, contains('Adding input path to Thin Binary build phase.'));
     });
   });
+
+  group('migrate @UIApplicationMain attribute to @main', () {
+    late MemoryFileSystem memoryFileSystem;
+    late BufferLogger testLogger;
+    late FakeIosProject project;
+    late File appDelegateFile;
+
+    setUp(() {
+      memoryFileSystem = MemoryFileSystem();
+      testLogger = BufferLogger.test();
+      project = FakeIosProject();
+      appDelegateFile = memoryFileSystem.file('AppDelegate.swift');
+      project.appDelegateSwift = appDelegateFile;
+    });
+
+    testWithoutContext('skipped if files are missing', () async {
+      final UIApplicationMainDeprecationMigration migration = UIApplicationMainDeprecationMigration(
+        project,
+        testLogger,
+      );
+      await migration.migrate();
+      expect(appDelegateFile.existsSync(), isFalse);
+
+      expect(testLogger.statusText, isEmpty);
+    });
+
+    testWithoutContext('skipped if nothing to upgrade', () async {
+      const String appDelegateContents = '''
+import Flutter
+import UIKit
+
+@main
+@objc class AppDelegate: FlutterAppDelegate {
+  override func application(
+    _ application: UIApplication,
+    didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?
+  ) -> Bool {
+    GeneratedPluginRegistrant.register(with: self)
+    return super.application(application, didFinishLaunchingWithOptions: launchOptions)
+  }
+}
+''';
+      appDelegateFile.writeAsStringSync(appDelegateContents);
+      final DateTime lastModified = appDelegateFile.lastModifiedSync();
+
+      final UIApplicationMainDeprecationMigration migration = UIApplicationMainDeprecationMigration(
+        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 Flutter
+import UIKit
+
+@UIApplicationMain
+@objc class AppDelegate: FlutterAppDelegate {
+  override func application(
+    _ application: UIApplication,
+    didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?
+  ) -> Bool {
+    GeneratedPluginRegistrant.register(with: self)
+    return super.application(application, didFinishLaunchingWithOptions: launchOptions)
+  }
+}
+''');
+
+      final UIApplicationMainDeprecationMigration migration = UIApplicationMainDeprecationMigration(
+        project,
+        testLogger,
+      );
+      await migration.migrate();
+
+      expect(appDelegateFile.readAsStringSync(), '''
+import Flutter
+import UIKit
+
+@main
+@objc class AppDelegate: FlutterAppDelegate {
+  override func application(
+    _ application: UIApplication,
+    didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?
+  ) -> Bool {
+    GeneratedPluginRegistrant.register(with: self)
+    return super.application(application, didFinishLaunchingWithOptions: launchOptions)
+  }
+}
+''');
+      expect(testLogger.warningText, contains('uses the deprecated @UIApplicationMain attribute, updating'));
+    });
+  });
 }
 
 class FakeIosProject extends Fake implements IosProject {
@@ -1439,6 +1538,9 @@ class FakeIosProject extends Fake implements IosProject {
 
   @override
   Directory podRunnerTargetSupportFiles = MemoryFileSystem.test().directory('Pods-Runner');
+
+  @override
+  File appDelegateSwift = MemoryFileSystem.test().file('AppDelegate.swift');
 }
 
 class FakeIOSMigrator extends ProjectMigrator {