Swift static library inside a Kotlin/Native lib

This Kotlin/Multiplatform Mobile demo library project demonstrates how to use a Swift static library into a Kotlin/Native KLib in order to use it as a dependency in another Kotlin/Multiplatform Mobile project.

This demoes the use of the cryptography algorithm Chacha20-Poly1305 that is only available in iOS as a Swift API.

Points of interest

• The SwiftCryptoKit directory contains an XCode project that generates a Swift static library:
  • All its Swift classes and methods are public and @objc annotated to be accessible from Objective-C & Kotlin.
  • An "Objective-C Bridging Header" is configured for Objective-C interoperability.
  • A "Run Script" build phase is added at the end of the build to export the Swift Objective-C header with the library.
  • A Gradle build script was added to enable the root Gradle project to compile the library with xcodebuild.
• The root directory contains a Gradle project that provides a Kotlin/Native wrapper around the Swift static library:
  • The Gradle build script declares a C-Interop that depends on the Swift static library
  • The interop def file defines the linker flags that needs to be used when linking with the library:
    • -ios_simulator_version_min 13.0.0 and -iphoneos_version_min 13.0.0 are needed because Swift interoperability needs iOS 13.0 or newer.
    • Swift library search path are also needed.
  • Note that any KMM appplication depending on the library must target iOS 13.0 or newer.
• You can run ./gradlew publishToMavenLocal and then have a look at the ~/.m2/repository/org/demo/crypto/ directory to see all the files that would be published.

Why, oh why?

Apple is pushing its Swift language more and more for iOS development. Some new native features that are added to the system are being made available only in Swift: this is the case for example of the new CryptoKit framework, which exposes some crypto algorithm that weren't available before, such as ChaCha20-Poly1305. Therefore, if we want to use a hardware-optimised implementation of such algorithms, we have no choice but use Swift.

While Kotlin/Native does not interface with Swift, both Swift & Kotlin/Native interop really well with Objective-C, so it is rather simple to create a small static library that exposes some Swift-only APIs (such as ChaChaPoly in CryptoKit) to Objective-C and therefore to Kotlin/Native.

This demos a Kotlin/Multiplatform Mobile library (KLib) that provides an API for the ChaCha20-Poly1305 algorithm and uses the system's most efficient implementation for each target. This library would then be used as a dependencies on multiple Kotlin/Multiplatfom Mobile application projects.

As Apple is releasing more and more features in Swift only, this use case is becoming more and more important.