Getting Started

• System Requirements
• Installation
• Activation
• Creating an App
  • Creating a Dual-Platform App
  • Creating a Multi-Module App
  • Creating Separate iOS and Android Apps
• Creating a Dual-Platform Framework
  • Skip Framework Structure
• Migrating an Existing App to Skip
• Updating Skip
• Additional Resources

System Requirements

Skip requires a macOS 13 development machine with Xcode 16.1 or later, Android Studio 2023 or later, and Homebrew installed.

Installation

Install Skip by running the Terminal command:

brew install skiptools/skip/skip

This will download and install the skip tool itself, as well as the gradle and JDK dependencies that are necessary for building and testing the Kotlin/Android side of your apps.

Ensure that the basic development prerequisites are satisfied by running:

skip checkup

If the checkup fails, try running again with skip checkup --verbose to get more details, and check the FAQ for common solutions.

Once the checkup passes, install the native Swift Android toolchain:

skip android sdk install

If this command is successful, running swift sdk list will list the local SDKs, which should include an entry like: swift-6.0.2-RELEASE-android-24-0.1.

To verify that the toolchain is setup properly and able to build a test app with native Swift integration, run:

skip checkup --native

The output should end with:

[✓] Archive iOS ipa (28.01s)
[✓] Assemble HelloSkip-release.ipa 405 KB
[✓] Verify HelloSkip-release.ipa 405 KB
[✓] Assembling Android apk (133.23s)
[✓] Verify HelloSkip-release.apk 212 MB
[✓] Check Swift Package (0.97s)
[✓] Check Skip Updates: 1.1.24
[✓] Skip 1.1.24 checkup succeeded in 314.86s

Now you’re ready to start developing with Skip!

Activation

After a trial period, Skip will require a license key for closed-source or commercial development. See License Keys for instructions on how to obtain and install your license key when your free trial ends.

Creating an App

There are two primary ways to structure a Skip app. The most common structure is as a single, dual-platform app project. Creating a dual-platform app project does not prevent you from customizing your app for Android, even to the point of e.g. writing your entire Android UI in Kotlin and Compose. But it generally assumes that you’ll manage the iOS and Android versions of your app through Xcode as a single logical application.

The other common structure is to create separate iOS and Android apps that use a set of dual-platform frameworks for shared functionality. Using this option, it is up to you to create and manage the separate iOS and Android applications using Xcode and Android Studio (or your Android IDE of choice), respectively.

Creating a Dual-Platform App

Create a new dual-platform app project with the command:

skip init --appid=bundle.id project-name AppName

Your appid must contain at least two words, and each word must be separated by a .. It is conventional to use reverse-DNS naming, such as com.companyname.AppName. Also make sure that your project-name and AppName are different. It is conventional to use a lowercase, hyphenated name for your project (which Skip uses to create your app’s main SwiftPM package name), and UpperCamelCase for your app name.

Skip has two modes: native mode, in which your Swift is compiled, and transpiled mode, in which your Swift is translated to Kotlin. We discuss the tradeoffs and use cases for each mode in Native or Transpiled. If you want to use Skip’s native mode for your app’s model layer, use the following command instead:

skip init --native --appid=bundle.id project-name AppName AppNameModel

In the future, the --native option will use native SwiftUI as well a native model, but at this time native SwiftUI support is incomplete.

Pass the --open-xcode argument to immediately open the project in Xcode. For example:

skip init --open-xcode --appid=bundle.id.HelloSkip hello-skip HelloSkip

This will create a hello-skip/ folder with a new SwiftPM package containing a single module named HelloSkip, along with folders named Darwin and Android and the shared Skip.env app configuration file. The Darwin folder will contain a HelloSkip.xcodeproj project with a HelloSkip target, which can be opened in Xcode.

See the command line reference for a complete listing of skip init options.

If you include the --native flag and add a model module name to the end of the command, you instead get a multi-module project with a compiled model module and a transpiled UI module:

skip init --native --appid=bundle.id.HelloSkip hello-skip HelloSkip HelloSkipModel

hello-skip
├── Package.swift
└── Sources
    ├── HelloSkip
    │   ├── ContentView.swift
    │   └── Skip
    │       └── skip.yml
    └── HelloSkiptModel
        ├── Skip
        │   └── skip.yml
        └── ViewModel.swift

skip init creates a functional template app, but before you can build and launch it, an Android emulator needs to be running. Launch Android Studio.app and open the Virtual Device Manager from the ellipsis menu of the Welcome dialog. From there, Create Device (e.g., “Pixel 6”) and then Launch the emulator.

Once the Android emulator is running, select and run the HelloSkip target in Xcode. The first build will take some time to compile the Skip libraries, and you may be prompted with a dialog to affirm that you trust the Skip plugin. Once the build and run action completes, the SwiftUI app will open in the selected iOS simulator, and at the same time the Android app will launch in the currently-running Android emulator.

Browse to the ContentView.swift file and make a small change and re-run the target: the app will be re-built and re-run on both platforms simultaneously with your changes.

You’re now ready to continue working on your first Skip app! For more information - including common issues and workarounds - see the Development chapter. Consider browsing our other documentation as well. Happy Skipping!

Creating a Multi-Module App

Skip is designed to accommodate and encourage using multi-module projects. You can create a modularized project by specifying additional module names to skip init at the end of the chain. For example:

skip init --appid=bundle.id.HelloSkip multi-project HelloSkip HelloModel HelloCore

This command will create a SwiftPM project with three modules: HelloSkip, HelloModel, and HelloCore. The heuristics of such module creation is that the modules will all be dependent on their subsequent peer module, with the first module (HelloSkip) having an initial dependency on SkipUI, the second module depending on SkipModel, and the final module in the chain depending on SkipFoundation. The Package.swift file can be manually edited to shuffle around dependencies, or to add new dependencies on external Skip frameworks such as the SkipSQL or SkipFirebase libraries.

Creating Separate iOS and Android Apps

You might choose to share functionality using dual-platform frameworks, but create separate iOS and Android apps. Some development teams, for example, would like to share common model and business logic layers, but write the UI separately for each platform.

The Travel Posters sample app provides an example of this pattern. It has the following top-level entries:

• travel-posters-model: This SwiftPM package builds a dual-platform framework containing a common model layer for the iOS and Android apps. Skip ensures that the @Observable types you write in Swift can power not only a SwiftUI interface, but a Compose interface as well. See the Development documentation for details.
• iOS: Directory containing the TravelPosters iOS app and Xcode project, which has travel-posters-model as a package dependency.
• Android: Directory containing the Android version of the app. The Android/lib directory contains exported archives of travel-posters-model and the various Skip frameworks that it depends on.
• TravelPosters.xcworkspace: A workspace that includes both the iOS app and the travel-posters-model package.

Use TravelPosters.xcworkspace to iterate on the iOS app and/or shared model layer. To donate the latest travel-posters-model code to the Android app:

skip export --project travel-posters-model -d Android/lib/debug/ --debug
skip export --project travel-posters-model -d Android/lib/release/ --release

There are many ways to automate this process, from simple scripting to git submodules to publishing Skip’s transpiled Android travel-posters-model output to a local Maven repository. Use whatever system fits your team’s workflow best.

See the Deployment chapter for more information on skip export.

Additional notes:

• You must “Sync Project with Gradle Files” in Android Studio after updating the exported libraries.
• Using an exported library function which has transitive dependencies on additional Android libraries can cause a runtime error. You must ensure that all transitive dependencies are in your own app’s build.gradle.kts.

The TravelPosters sample linked above uses transpiled Swift. Read a case study on sharing a native Swift model between iOS and Android apps in this blog post.

Creating a Dual-Platform Framework

Skip framework projects are pure SwiftPM packages that encapsulate common functionality. Frameworks are simpler than app projects, as they do not need Darwin/ and Android/ folders.

Each of the core SkipStack frameworks (SkipLib, SkipUnit, SkipFoundation, and SkipUI) are Skip framework projects. Other commonly-used projects include SkipSQL and SkipFirebase. These existing libraries are rich sources of examples of various strategies for providing dual-platform functionality.

A new framework project can be created and opened with:

skip init lib-name ModuleName

This will create a new lib-name folder containing a Package.swift with targets of ModuleName and ModuleNameTests.

The generated package can be opened in Xcode.app, which you can use to build and run the unit tests. Or use swift build and swift test from the Terminal for headless testing as part of a continuous integration process.

Due to limitations on Xcode plugins, building your framework target only builds the iOS version. To build the Android version, you must run your unit tests. The Android build occurs as part of the testing process.

Just as before, include the --native option to create a native module. The module will be preconfigured to compile natively for Android, but there are a couple of additional steps you should take when sharing a model between iOS and Android apps:

1. In order for @Observables to work on Android, we need a dependency on skip-model. If you’ll be using @Observables in you module, edit the generated Package.swift to add the required dependency, as in the following example:

    ...
    let package = Package(
        name: "travel-posters-model",
        ...
        dependencies: [
            .package(url: "https://source.skip.tools/skip.git", from: "1.2.0"),
            .package(url: "https://source.skip.tools/skip-model.git", from: "1.0.0"), // <-- Insert
            .package(url: "https://source.skip.tools/skip-fuse.git", from: "1.0.0")
        ],
        targets: [
            .target(name: "TravelPostersModel",
                dependencies: [
                    .product(name: "SkipFuse", package: "skip-fuse"), 
                    .product(name: "SkipModel", package: "skip-model")                // <-- Insert
                ],
                plugins: [.plugin(name: "skipstone", package: "skip")]),
            ...
        ]
    )
   

2. The --native option we passed to skip init will configure Skip to automatically bridge our model’s public API from compiled Swift to Android’s ART Java runtime. This is done through the skip.yml configuration file included in every Skip module. By default, however, Skip assumes that you’ll be bridging to transpiled Swift and SwiftUI code. If you’ll be consuming the model from pure Kotlin, you’ll want to optimize the bridging for Kotlin compatibility. In this case, you can run skip init --native --kotlincompat to create a module with Kotlin compatiblity, which is the equivalent to editing skip.yml to look like this:

    skip:
      mode: 'native'
      bridging:
        auto: true
        options: 'kotlincompat'
   

We discuss bridging and skip.yml later.

Skip Framework Structure

The structure of a Skip framework is exactly the same as any other SwiftPM package:

lib-name
├── Package.resolved
├── Package.swift
├── README.md
├── Sources
│   └── ModuleName
│       ├── ModuleName.swift
│       ├── Resources
│       │   └── Localizable.xcstrings
│       └── Skip
│           └── skip.yml
└── Tests
    └── ModuleNameTests
        ├── ModuleNameTests.swift
        ├── Resources
        │   └── TestData.json
        ├── Skip
        │   └── skip.yml
        └── XCSkipTests.swift

Skip frameworks use a standard Package.swift file, with the exception of an added dependency on skip and use of the skipstone plugin for transpilation:

// swift-tools-version: 5.8
import PackageDescription

let package = Package(
    name: "lib-name",
    defaultLocalization: "en",
    platforms: [.iOS(.v16), .macOS(.v13), .tvOS(.v16), .watchOS(.v9), .macCatalyst(.v16)],
    products: [
        .library(name: "ModuleName", targets: ["ModuleName"]),
    ],
    dependencies: [
        .package(url: "https://source.skip.tools/skip.git", from: "0.7.31"),
        .package(url: "https://source.skip.tools/skip-foundation.git", from: "0.0.0"),
    ],
    targets: [
        .target(name: "ModuleName", plugins: [.plugin(name: "skipstone", package: "skip")]),
        .testTarget(name: "ModuleNameTests", dependencies: ["ModuleName"], plugins: [.plugin(name: "skipstone", package: "skip")]),
    ]
)

Migrating an Existing App to Skip

Migrating an existing app to Skip is not trivial. Most apps contain many iOS-only dependencies that make an Android port challenging.

Additionally, when you use skip init to create a new Skip app, it handles all the messy details involved in making an app that can build for both iOS and Android. The process is complex enough that we do not recommend trying to migrate an existing Xcode project. Instead, choose one of two options to create an Android version of your existing app:

1. Use skip init to create a new Skip app, then add your existing app’s dependencies and code.
2. Keep your existing Xcode app, and create a separate Android app using Android Studio or your IDE of choice. Manage the apps separately, but share code by creating dual-platform frameworks.

Regardless of which option you choose, your first steps are the same:

1. Modularize your app into Swift Package Manager packages, if it isn’t already.
2. Starting with your “base” module and working your way up the stack, attempt to get each module building for Android.

We recommend using Skip’s native mode where possible, as it offers greater compatibility with existing Swift code and dependencies.

Porting an app to an entirely new platform isn’t easy, even with Skip. Remember that we’re here to help.

Updating Skip

To update the skip command line tool:

% skip upgrade

To update your Xcode project to use the latest version of the Skip transpiler and libraries allowed by your Package.swift configuration, use the File -> Packages -> Update to Latest Package Versions Xcode menu option.

Additional Resources

• Use skip help for a complete list of skip tool commands.
• Check the general help page for troubleshooting and contact information.
• Continue browsing this documentation to learn more about developing with Skip.