SkipCDemo

This is a Skip Swift/Kotlin library project demonstrating the use of C code from Swift on both Darwin (iOS & macOS) and Android. It utilizes Swift's built-in C integration on the Swift side, and takes advantage of gradle's support for building with the Android NDK using the cmake build tool on the Kotlin side.

The C code is called from the transpiled Kotlin using Java Native Access (JNA). A JNA direct mapping class SkipCDemo.swift contains the library's functions, marked with SKIP EXTERN to cause the Skip transpiler to annotate the functions so JNA lines them up with the equivalent C functions.

This project can used as a starting point for integrating C/C++ code into a dual-platform Skip app.

Project Structure

.
├── Package.swift
├── Sources
│   ├── LibCLibrary
│   │   ├── CMakeLists.txt
│   │   ├── include
│   │   │   └── demo.h
│   │   └── src
│   │       └── demo.c
│   └── SkipCDemo
│       ├── Skip
│       │   └── skip.yml
│       └── SkipCDemo.swift
└── Tests
    └── SkipCDemoTests
        ├── Skip
        │   └── skip.yml
        ├── SkipCDemoTests.swift
        └── XCSkipTests.swift

Implementation Details

Running swift test will run the Swift tests as well as the transpiled Kotlin tests against the native library's declared functions.

The LibCLibrary Swift target builds a SkipCDemo.framework for Xcode or libSkipCDemo.dylib for SwiftPM. These libraries are built for the host system (macOS, either ARM or Intel), and when testing the SkipCDemoTests target, the correct local shared library is linked, and so the Robolectric JVM tests use the same shared library that is used by the Swift tests.

When building and testing for Android (either the emulator or device), the generated gradle contains the externalNativeBuild clause, directing it to use the CMakeLists.txt to build the native libraries for each of the supported Android NDK targets. The resulting LibCLibrary.aar archive will look something like:

classes.dex
lib/arm64-v8a/libclibrary.so
lib/armeabi-v7a/libclibrary.so
lib/x86/libclibrary.so
lib/x86_64/libclibrary.so
AndroidManifest.xml
resources.arsc

Project Structure

The target definition for a native target in the Package.swift file must contain only .c and .cpp source files; it is not permitted to mix in .swift files. In addition, the target containing the native libraries must declare a publicHeadersPath and add the skipstone plugin in order for Skip to generate gradle build for the native module.

// swift-tools-version: 5.9
import PackageDescription

let package = Package(
    name: "skip-c-demo",
    defaultLocalization: "en",
    platforms: [.iOS(.v16), .macOS(.v13), .tvOS(.v16), .watchOS(.v9), .macCatalyst(.v16)],
    products: [
        .library(name: "SkipCDemo", type: .dynamic, targets: ["SkipCDemo"]),
    ],
    dependencies: [
        .package(url: "https://source.skip.tools/skip.git", from: "1.0.0"),
        .package(url: "https://source.skip.tools/skip-foundation.git", from: "1.0.0"),
        .package(url: "https://source.skip.tools/skip-ffi.git", from: "1.0.0")
    ],
    targets: [
        .target(name: "SkipCDemo", dependencies: [
            "LibCLibrary",
            .product(name: "SkipFoundation", package: "skip-foundation"),
            .product(name: "SkipFFI", package: "skip-ffi")
        ], plugins: [.plugin(name: "skipstone", package: "skip")]),

        .testTarget(name: "SkipCDemoTests", dependencies: [
            "SkipCDemo",
            .product(name: "SkipTest", package: "skip")
        ], plugins: [.plugin(name: "skipstone", package: "skip")]),

        .target(name: "LibCLibrary", sources: ["src"], publicHeadersPath: "include", plugins: [.plugin(name: "skipstone", package: "skip")]),
    ]
)

Building

Building the module requires that Skip be installed using Homebrew with brew install skiptools/skip/skip. This will also install the necessary build prerequisites: Kotlin, Gradle, and the Android build tools.

Testing

The module can be tested using the standard swift test command or by running the test target for the macOS destination in Xcode, which will run the Swift tests as well as the transpiled Kotlin JUnit tests in the Robolectric Android simulation environment.

Parity testing can be performed with skip test, which will output a table of the test results for both platforms.