rust_android_ios — An example of using a shared Rust lib for Android and iOS using rust-swig and cbindgen respectively.

Android / iOS app with shared Rust logic

This is an example that shows how to communicate with a shared Rust library from an Android and iOS app. The idea is to be able to share domain logic & most services (networking, database, bluetooth, etc.) using Rust and use the native SDKs for presentation and platform specific services.

Why?

This approach gives us the best of all worlds! We prevent code duplication by using a shared library. Rust, as a highly performant and safe language is a great fit for mobile. We keep a fully native UI experience and uncomplicated access to the latest APIs of the platforms.

Supported

✅ Functions

Calls Rust functions from Android / iOS, shows returned vale in UI.

✅ Callback

Passes callback to Rust, update UI from callback with result.

✅ Reactive

Subscribes in Android / iOS to events triggered by Rust and updates UI with them.

✅ Unidirectional flow

UI sends event to Rust. Rust emits to channel that is observed in Android / iOS. This allows to e.g. implement MVVM architecture writing the view models in Rust (with some glue to convert the observer-callbacks in observables/channels/SwiftUI/Jetpack compose/etc).

✅ JSON

Pass serialized Objects between Android / iOS and Rust using JSON

Android / iOS instructions

Ensure rustup is installed. This includes Cargo, so you should be able to compile & build Rust projects + install the required targets to build for Android and iOS.

List available targets:

rustup target list

Show currently installed targets:

rustup toolchain list

The Rust sources are here

Android instructions

These steps show how to build and run an Android app in debug mode for a 64 bits emulator.

See rust_swig documentation

NDK

Ensure the NDK is installed.

Environment variables

ANDROID_TOOLCHAINS=<Directory where targets should be installed> 
ANDROID_NDK=<NDK's root directory>

Add targets

rustup target add x86_64-linux-android

Add path to linker

• Update linker path in Cargo's config.

[target.x86_64-linux-android]
linker = "<Directory where targets were installed (provided in environment variable)>/android-29-x86_64-4.9/bin/clang"

Build

./gradlew assembleDebug

Install

./gradlew installDebug

Run

Ensure adb is installed.

then:

adb shell am start -n com.schuetz.rust_android_ios/com.schuetz.rust_android_ios.MainActivity

OR

Start the app in the emulator / device!

OR

Run the project in Android Studio. This will build, install and run.

Relevant configuration files

If you want to add targets or tweak the configuration, you have to edit one or more of these files:

• App's Gradle config: This contains the apk settings (like application id, sdk version) and build steps. It builds for the provided architectures using cargo and puts the generated shared libraries (.so files) in the expected directories. If you want to build for a new target, add it here. The target's key is the folder name where the shared library will be put, and the value is the toolchain's name used by rustup.

• Cargo config: Contains linker paths for targets.

• build.rs: This is a script invoked by Cargo before everything else. For Android, it's used to tell rust_swig to generate the glue files for Java interop. If you change the app's package structure / names, you have to update this file accordingly. It also sets the import to use for the NonNull annotation ( use_null_annotation_from_package). If you're using a recent Android SDK version, you don't need to change it.

Updating Rust

You edited something in Rust! Now you have to:

• Update java_glue.rs.in accordingly. This is a file rust_swig uses to generate the JNI glue. Consult rust_swig docs for syntax.

• Build, install, run, as described above.

Updating Kotlin/Java

The code of the Android app can be found here. This is a regular Android app which you can work with normally. Just don't modify the generated JNI files and remember to update build.rs as described in Relevant configuration files, if you change the package structure.

iOS instructions

App code

The iOS project is here.

Add targets

rustup target add x86_64-apple-ios

Build & run

From the project's root directory:

cargo build --target=x86_64-apple-ios

This will generate the required library files: <project root directory>/target/mobileapp-ios.h with the C headers and the (static) library for the target, <project root directory>/target/<target name>/libmobcore.a. At the moment you have to copy manually libmobcore.a into the iOS app's directory each time you update it.

With the header and library in place, you can run the iOS app.

Updating Rust

You edited something in Rust! Now you have to:

• Update the C glue Rust implementation file. Orient with the existing code. Differently to Android the glue has to be written manually, because there's no library like rust_swig.

• Build as described in "Build & run". Cargo will invoke build.rs, which uses cbindgen to generate the iOS library files.

• Copy manually the generated <project root directory>/target/<target name>/libmobcore.a to the iOS app project's root folder.

• Run!

Updating Swift/ObjC

The code of the iOS app can be found here. This is a regular iOS app which you can work with normally.

Wiki

• Debugging guide

Contribute

1. Fork
2. Commit changes to a branch in your fork
3. Push your code and make a pull request

Credits

Based on parts of https://github.com/Dushistov/rust_swig/tree/master/android-example and https://github.com/terhechte/rust-ios-android-example

TODO

• Pass / return struct pointers, casting / mapping to iOS structs and Kotlin classes (partly done already for iOS).
• Avoid using global variables in iOS app.
• Automate copying of libmobcore.a or reference properly & multiple targets.