Building a Cross-Platform Mobile SDK in Rust.pdf
- 1. Ian Wagner, July 27, 2023 Building a Cross-Platform Mobile SDK in Rust Seoul Rust Meetup
- 2. About Me • iOS developer since 2008 • Shipping Rust in prod since 2018 • Currently role: Cofounder of Stadia Maps
- 3. Background • Building a mobile navigation SDK • Hexagonal architecture: • Everything communicates across clear API boundaries • Core business logic in Rust • Native Swift and Kotlin wrappers call into Rust via FFI
- 4. Interesting Problems • Idiomatic bindings for Swift and Kotlin • Compiling and linking for multiple targets • Shipping the library with a high-level native wrapper
- 5. UniFFI Idiomatic binding generator • Open-source idiomatic binding generator from Mozilla • Supports Kotlin, Swift, Python, and Ruby • Interface De fi nition Language (IDL) fi le speci fi es what’s exported
- 6. UniFFI Feels like magic • Generated APIs generally idiomatic as promised • Support for a wide variety of system types, incl. timestamps(!) • Generates protocols / interfaces as well as concrete types • Support for native callbacks and even full interface implementations(!!) • Bridge in platform-native APIs (ex: keychain) • Allow users to bring their own implementation (ex: custom o ffl ine routing)
- 7. uniffi-bindgen on iOS Currently using a build script (better automation TBD)
- 8. uniffi-bindgen on Android Gradle invokes cargo
- 9. Calling from Swift and Kotlin
- 10. UniFFI Challenges • Need to have all exported types visible from the top level(?) • Not very much documentation on the fi ner points of the UDL fi le • Unable to split into multiple modules / use multiple UDL fi les at once • Fairly useless errors when your UDL is wrong • Only generates bindings; doesn’t actually help you ship a lib (we’ll revisit this)
- 11. UniFFI Tips • Leverage generated protocol/interface types • UniFFI generates protocols/interfaces for you! • Rule of thumb: pass the protocol/interface; construct the concrete type • Key bene fi t: easier mocking • Third-party plugin for syntax highlighting UDL fi les available for CLion • Use interior mutability patterns (must be both Send and Sync!); you can’t use mutating struct methods; we currently use std : : sync : : Mutex<T>
- 12. Cross compilation Surprisingly easy on its face for iOS!
- 13. Cross-Platform Compilation Android quirks • In contrast to Apple, with deep support for C ABI libs, the Android NDK is a nightmare • Need to set up ~/.cargo/ config.toml with the exact linker paths for your NDK
- 14. Shipping the library • Cross-compilation is the start, but there’s more! • High-level wrapper (pretty straightforward but some quirks!) • Binary issues • Packaging • You have to deal with a number of quirks on each platform; we’ll only look at iOS today
- 15. Shipping the library Swift quirks • The Swift language has no (sanctioned) way to mark types for re-export • Common pattern: Rust core -> UniFFI lower-level wrapper -> high level Swift package that exposes a more ergonomic interface and hides FFI • Types returned by methods in your package are visible and inspectable… • But you can’t construct these types or even reference the type anywhere! • No variables • No return types • No function args • No enum variant unpacking(?!) • SPM has some strange rules, like needing to have Package.swift in the repo root
- 16. Shipping the library iOS quirks • Building an XCFramework • Apple’s native format for distributing multi-platform binary frameworks • Easiest distribution format for downstream consumers • Quirk: iOS Simulator is one platform but needs multiple archs (Intel and ARM) • Creating a fat simulator lib: lipo
- 17. Shipping the library SPM quirks • Local development hides challenges • .binaryTarget(name:path:) works great for local development, but… • If you want to add a local dependency on your package from another, SPM chokes since this isn’t meant to go more than one level deep (design fl aw) • So you have to push artifacts to GitHub or something • Silver lining: can still make Swift changes and reference master + your old release framework artifact and track main; just can’t touch the Rust lib
- 18. Bringing it all together Running in SwiftUI
- 19. Open Questions • Repo structure: monorepo seems best for dev • Build system: use native build systems or something like Bazel? • Android: haven’t explored the edges as far so probably some unknown unknowns
- 20. Resources • UniFFI User Guide - https://mozilla.github.io/uni ffi -rs/ • uni ffi -dl (Uno ffi cial IntelliJ plugin) - https://github.com/Lonami/uni ffi -dl • Compiling Rust libraries for Android apps: a deep dive - https:// gendignoux.com/blog/2022/10/24/rust-library-android.html • Ferrostar - https://github.com/stadiamaps/ferrostar