Kotlin bindings for the Rust SDK crypto layer.

Prerequisites

Rust

To build the bindings Rust will be needed it can be either installed using an OS specific package manager or directly with the provided installer.

Android NDK

The Android NDK will be required as well, it can be installed either through Android Studio or directly using an installer.

Uniffi

The bindings are using uniffi to generate the C translation layer between Rust and Kotlin. Uniffi is a Rust project and can be installed with our freshly installed Rust setup using:

$ cargo install uniffi_bindgen

Configuring Rust for cross compilation

First we'll need to install the Rust target for our desired Android architecture, for example:

# rustup target add aarch64-linux-android

This will add support to cross-compile for the aarch64-linux-android target, Rust supports many different targets, you'll have to make sure to pick the right one for your device or emulator.

After this is done, we'll have to configure Cargo to use the correct linker for our target. Cargo is configured using a TOML file that will be found in %USERPROFILE%\.cargo\config.toml on Windows or $HOME/.cargo/config on Unix platforms. More details and configuration options for Cargo can be found in the official docs over here.

[target.aarch64-linux-android]
ar = "NDK_HOME/toolchains/llvm/prebuilt/linux-x86_64/bin/ar"
linker = "NDK_HOME/toolchains/llvm/prebuilt/linux-x86_64/bin/aarch64-linux-android30-clang"

Building

A Makefile is provided that builds and installs the dynamic library into the appropriate target specific jniLibs directory. But before we can get started we'll need to tweak our Rust setup to allow cross compilation.

To enable cross compilation fro olm-sys which builds our libolm C library we'll need to set the ANDROID_NDK environment variable to the location of our Android NDK installation.

$ export ANDROID_NDK=$HOME/Android/Sdk/ndk/22.0.7026061/

Makefile build

After the prerequisites have been installed and the environment variable has been set a build for the aarch64 target can be build using:

make aarch64

Manual build

If the Makefile doesn't work on your system, the bindings can built for the aarch64 target with:

$ cargo build --target aarch64-linux-android

After that, a dynamic library can be found in the target/aarch64-linux-android/debug directory. The library will be called libmatrix_crypto.so and needs to be renamed and copied into the jniLibs directory:

$ cp target/aarch64-linux-android/debug/libmatrix_crypto.so \
     ../matrix-sdk-android/src/main/jniLibs/aarch64/libuniffi_olm.so

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