Building an Android App with Rust Using UniFFI

Jan 22, 2024

Developing Android applications is usually associated with languages like Java or Kotlin. However, the rising popularity of Rust, a systems programming language celebrated for its safety and performance, combined with UniFFI, an interface generator for Rust, opens new avenues for Android app development. In this blog post, we delve into the process of building a simple Android app using Rust and UniFFI.

Prerequisites

Before starting, make sure you have the following tools installed:

Rust: Obtain it from the official Rust website.
Android Studio: Download from the official site.

Step 1: Setting Up Your Rust Library

Begin by creating a new Rust library:

cargo new app --lib

In your Cargo.toml file, add the uniffi dependency:

Cargo.toml

[lib]
crate_type = ["cdylib"]
name = "mobile"
 
[dependencies]
uniffi = { version = "0.25.3", features = [ "cli" ] }

Modify your lib.rs file. Here's an example:

src/lib.rs

uniffi::setup_scaffolding!();
 
#[uniffi::export]
fn say_hi() -> String {
    "Hello from Rust on Android!".to_string()
}

Include the uniffi-bindgen CLI tool:

src/bin/uniffi-bindgen.rs

fn main() {
    uniffi::uniffi_bindgen_main()
}

Build the library:

cargo build

Step 2: Preparing for Android Build

Set up cargo-ndk for cross-compiling:

cargo install cargo-ndk

Add targets for Android:

rustup target add \
        aarch64-linux-android \
        armv7-linux-androideabi \
        i686-linux-android \
        x86_64-linux-android

Step 3: Building Libraries for Android

Create your Android project in Android Studio.

Compile the dynamic libraries in ./app/src/main/jniLibs (next to java and res directories):

cargo ndk -o ./app/src/main/jniLibs \
        --manifest-path ./Cargo.toml \
        -t armeabi-v7a \
        -t arm64-v8a \
        -t x86 \
        -t x86_64 \
        build --release

Step 4: Generating Kotlin Bindings

Generate the bindings for Kotlin:

cargo run --bin uniffi-bindgen generate --library ./target/debug/libmobile.dylib --language kotlin --out-dir ./app/src/main/java/tech/forgen/todolist/rust

Step 5: Setting up Your Android Project

Create a new Android project in Android Studio.

In the project, import the Kotlin binding files and the compiled libraries from the jniLibs directory.

Step 6: Integrating Rust Functions in Android

Add these imports to your build.gradle file:

build.gradle

dependencies {
    // ...

    // Uniffi
    implementation("net.java.dev.jna:jna:5.7.0@aar")
    implementation("org.jetbrains.kotlinx:kotlinx-coroutines-core:1.6.4")
}

Edit your main activity or any Kotlin file to use the Rust functions. For example:

MainActivity.kt

import tech.forgen.todolist.rust.Mobile
 
class MainActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
 
        val message = Mobile.sayHi() // Calling the Rust function
        Toast.makeText(this, message, Toast.LENGTH_SHORT).show()
    }
}

Android App

Putting Everything Together

Create a script named build-android.sh to automate the Rust library building process for various Android targets.

build-android.sh

#!/bin/bash
 
# Set up cargo-ndk and add the Android targets
cargo install cargo-ndk
rustup target add aarch64-linux-android armv7-linux-androideabi i686-linux-android x86_64-linux-android
 
# Build the dylib
cargo build
 
# Build the Android libraries in jniLibs
cargo ndk -o ./app/src/main/jniLibs \
        --manifest-path ./Cargo.toml \
        -t armeabi-v7a \
        -t arm64-v8a \
        -t x86 \
        -t x86_64 \
        build --release
 
# Create Kotlin bindings
cargo run --bin uniffi-bindgen generate --library ./target/debug/libmobile.dylib --language kotlin --out-dir ./app/src/main/java/tech/forgen/todolist/rust

This approach simplifies the process of integrating Rust into your Android development, leveraging the advantages of Rust's safety and performance in your Android apps.

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