使用-i和-a创建新项目，如下所示：

flutter create -i swift -a kotlin project_name

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另见：https://docs.flutter.dev/development/platform-integration/platform-channels#example-project

然后替换为旧项目中的lib文件夹。

更新2020.01.13

Swift和Kotlin现在是默认的。您可以使用这些命令来更新现有项目：

cd project
flutter create .

型
此命令将更新您的项目。然后你可以将旧代码合并到新项目中，并删除旧代码。

确保在运行项目之前备份它，并且您知道您到底在做什么

您也可以在应用程序文件夹中运行flutter create -i swift .，以重新生成ios文件夹。
我还没有测试过Android，但我想它也应该工作-flutter create -a kotlin .

我刚刚删除了默认的iOS和Android文件夹在我的flutter项目。现在，为了再次生成这些默认文件夹，您可以使用以下命令：

flutter create .

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注意：“.”也是命令的一部分，同时确保您的项目文件夹名称不应包含空格或特殊字符，这些字符对包名称无效。

生成iOS文件夹

仅在现有Flutter项目中创建iOS文件夹
第一个月
输出像这样

Creating project swift...
  swift/ios/Runner.xcworkspace/contents.xcworkspacedata (created)
  swift/ios/Runner.xcworkspace/xcshareddata/IDEWorkspaceChecks.plist (created)
  swift/ios/Runner.xcworkspace/xcshareddata/WorkspaceSettings.xcsettings (created)
  swift/ios/Runner/Info.plist (created)
  swift/ios/Runner/Assets.xcassets/LaunchImage.imageset/LaunchImage@2x.png (created)
  swift/ios/Runner/Assets.xcassets/LaunchImage.imageset/LaunchImage@3x.png (created)
  swift/ios/Runner/Assets.xcassets/LaunchImage.imageset/README.md (created)
  swift/ios/Runner/Assets.xcassets/LaunchImage.imageset/Contents.json (created)
  swift/ios/Runner/Assets.xcassets/LaunchImage.imageset/LaunchImage.png (created)
  swift/ios/Runner/Assets.xcassets/AppIcon.appiconset/Icon-App-76x76@2x.png (created)
  swift/ios/Runner/Assets.xcassets/AppIcon.appiconset/Icon-App-29x29@1x.png (created)
  swift/ios/Runner/Assets.xcassets/AppIcon.appiconset/Icon-App-40x40@1x.png (created)
  swift/ios/Runner/Assets.xcassets/AppIcon.appiconset/Icon-App-20x20@1x.png (created)
  swift/ios/Runner/Assets.xcassets/AppIcon.appiconset/Icon-App-1024x1024@1x.png (created)
  swift/ios/Runner/Assets.xcassets/AppIcon.appiconset/Icon-App-83.5x83.5@2x.png (created)
  swift/ios/Runner/Assets.xcassets/AppIcon.appiconset/Icon-App-20x20@3x.png (created)
  swift/ios/Runner/Assets.xcassets/AppIcon.appiconset/Contents.json (created)
  swift/ios/Runner/Assets.xcassets/AppIcon.appiconset/Icon-App-20x20@2x.png (created)
  swift/ios/Runner/Assets.xcassets/AppIcon.appiconset/Icon-App-29x29@3x.png (created)
  swift/ios/Runner/Assets.xcassets/AppIcon.appiconset/Icon-App-40x40@2x.png (created)
  swift/ios/Runner/Assets.xcassets/AppIcon.appiconset/Icon-App-60x60@3x.png (created)
  swift/ios/Runner/Assets.xcassets/AppIcon.appiconset/Icon-App-60x60@2x.png (created)
  swift/ios/Runner/Assets.xcassets/AppIcon.appiconset/Icon-App-76x76@1x.png (created)
  swift/ios/Runner/Assets.xcassets/AppIcon.appiconset/Icon-App-40x40@3x.png (created)
  swift/ios/Runner/Assets.xcassets/AppIcon.appiconset/Icon-App-29x29@2x.png (created)
  swift/ios/Runner/Base.lproj/LaunchScreen.storyboard (created)
  swift/ios/Runner/Base.lproj/Main.storyboard (created)
  swift/ios/Runner.xcodeproj/project.xcworkspace/contents.xcworkspacedata (created)
  swift/ios/Runner.xcodeproj/project.xcworkspace/xcshareddata/IDEWorkspaceChecks.plist (created)
  swift/ios/Runner.xcodeproj/project.xcworkspace/xcshareddata/WorkspaceSettings.xcsettings (created)
  swift/ios/Runner.xcodeproj/xcshareddata/xcschemes/Runner.xcscheme (created)
  swift/ios/Flutter/Debug.xcconfig (created)
  swift/ios/Flutter/Release.xcconfig (created)
  swift/ios/Flutter/AppFrameworkInfo.plist (created)
  swift/ios/.gitignore (created)
  swift/test/widget_test.dart (created)
  swift/swift.iml (created)
  swift/.gitignore (created)
  swift/.metadata (created)
  swift/pubspec.yaml (created)
  swift/README.md (created)
  swift/ios/Runner/Runner-Bridging-Header.h (created)
  swift/ios/Runner/AppDelegate.swift (created)
  swift/ios/Runner.xcodeproj/project.pbxproj (created)
  swift/lib/main.dart (created)
  swift/.idea/runConfigurations/main_dart.xml (created)
  swift/.idea/libraries/Dart_SDK.xml (created)
  swift/.idea/libraries/KotlinJavaRuntime.xml (created)
  swift/.idea/modules.xml (created)
  swift/.idea/workspace.xml (created)
Running "flutter pub get" in swift...                               4.9s
Wrote 50 files.

All done!
In order to run your application, type:

  $ cd swift
  $ flutter run

Your application code is in swift/lib/main.dart.

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