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skiptools / skip

Skip transpiler for creating SwiftUI apps for iOS and Android
https://skip.tools
GNU Lesser General Public License v3.0
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kotlin swift

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Skip

Skip is a technology for creating dual-platform mobile apps in Swift. Read this introduction to learn more about Skip.

This repository hosts the Skip development toolchain, a.k.a. SkipStone. It also hosts the Skip forums for general discussions as well as specific issues and bug reports.

Getting Started

System Requirements

Skip requires a macOS 13 development machine with Xcode 15, Android Studio 2023, 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. Note: If you don't already have a compatible JDK+ installed on your machine, you may need to enter an administrator password to complete the installation.

Ensure that the development prerequisites are satisfied by running:

skip checkup
Screenshot of terminal skip checkup command output

If the checkup passes, you're ready to start developing with Skip!

Creating an App {#app_development}

Create a new app project with the command:

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

For example:

skip init --open-xcode --appid=com.xyz.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.

Xcode will open the new project, but before you can build and launch the transpiled app, 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.

Screenshot of the Android Studio Device Manager

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 transpiled app will launch in the currently-running Android emulator.

Screenshot of Skip running in both the iOS Simulator and 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.

See the product documentation for further information developing with Skip. Happy Skipping!

Creating a Multi-Module App

Skip is designed to accommodate and encourage using multi-module projects. The default skip init command creates a single-module app for simplicity, but you can create a modularized project by specifying additional module names at the end of the chain. For example: 

skip init --open-xcode --appid=com.xyz.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 nascent SkipSQL or SkipXML libraries.

Creating a Dual-Platform Framework {#framework_development}

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

Each of the core Skip compatibility frameworks (skip-lib, skip-unit, skip-foundation, and skip-ui) are Skip framework projects. Other commonly-used projects include skip-sql, skip-script, and skip-zip.

A new framework project can be created and opened with:

skip init --build --test lib-name ModuleName

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

This project can be opened in Xcode.app, which you can use to build and run the unit tests. Running swift build and swift test from the Terminal can also be used for headless testing as part of a continuous integration process.

Skip Framework Structure

The structure of a Skip framework is exactly the same as any other SPM 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: "1.2.1"),
        .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")]),
    ]
)

skip init Reference

zap /tmp % skip init --help
OVERVIEW: Initialize a new Skip project

USAGE: skip init [<options>] <project-name> <module-names> ...

ARGUMENTS:
  <project-name>          Project folder name
  <module-names>          The module name(s) to create

OUTPUT OPTIONS:
  -o, --output <path>     Send output to the given file (stdout: -)
  -E, --message-errout    Emit messages to the output rather than stderr
  -v, --verbose           Whether to display verbose messages
  -q, --quiet             Quiet mode: suppress output
  -J, --json              Emit output as formatted JSON
  -j, --json-compact      Emit output as compact JSON
  -M, --message-plain     Show console messages as plain text rather than JSON
  -A, --json-array        Wrap and delimit JSON output as an array
  --plain/--no-plain      Show no colors or progress animations (default: --no-plain)

CREATE OPTIONS:
  --id <id>               Application identifier (default: net.example.MyApp)
  -d, --dir <directory>   Base folder for project creation
  -c, --configuration <c> Configuration debug/release (default: debug)
  -t, --template <id>     Template name/ID for new project (default: skipapp)
  -h, --template-host <host>
                          The host name for the template repository (default: https://github.com)
  -f, --template-file <zip>
                          A path to the template zip file to use
  --resource-path <resource-path>
                          Resource folder name (default: Resources)
  --chain/--no-chain      Create library dependencies between modules (default: --chain)
  --zero/--no-zero        Add SKIP_ZERO environment check to Package.swift (default: --zero)
  --git-repo/--no-git-repo
                          Create a local git repository for the app (default: --no-git-repo)
  --free                  Create package in free mode
  --show-tree/--no-show-tree
                          Display a file system tree summary of the new files (default: --no-show-tree)
  --module-tests/--no-module-tests
                          Whether to create test modules (default: --module-tests)
  --validate-package/--no-validate-package
                          Validate generated Package.swift files (default: --validate-package)

TOOL OPTIONS:
  --xcodebuild <path>     Xcode command path
  --swift <path>          Swift command path
  --gradle <path>         Gradle command path
  --adb <path>            ADB command path
  --emulator <path>       Android emulator path
  --android-home <path>   Path to the Android SDK (ANDROID_HOME)

BUILD OPTIONS:
  --build/--no-build      Run the project build (default: --build)
  --test/--no-test        Run the project tests (default: --no-test)
  --verify/--no-verify    Verify the project output (default: --verify)

OPTIONS:
  --appid <appid>         Embed the library as an app with the given bundle id
  --icon-color <RGB>      RGB hexadecimal color for icon background (default: 4994EC)
  --version <version>     Set the initial version to the given value
  --open-xcode            Open the resulting Xcode project
  --open-gradle           Open the resulting Gradle project
  -h, --help              Show help information.

Troubleshooting

Skip's architecture relies on recent advances in the plugin system used by Xcode 15 and Swift Package Manager 5.9. When unexpected issues arise, often the best first step is to clean your Xcode build (Product → Clean Build Folder) and reset packages (File → Packages → Reset Package Caches). Restarting Xcode is sometimes warranted, and trashing the local DerivedData/ folder might even be needed.

Specific known error conditions are listed below. Search the documentation, issues, and discussions for more information and to report problems.