# Uniffi based bindings for the Rust SDK crypto crate

This crate contains Uniffi based bindings for the `matrix-sdk-crypto` crate. The
README mainly describes how to build and integrate the bindings into a Kotlin
based Android project, but the Android specific bits can be skipped if you are
targeting an x86 Linux project.

To build and distribute bindings for iOS projects, see a
[dedicated page](../apple/README.md)

## 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](https://rustup.rs/).

### Android NDK

The Android NDK will be required as well, it can be installed either through
Android Studio or directly using an
[installer](https://developer.android.com/ndk/downloads).

### Configuring Rust for cross compilation

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

```text
# 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, by providing the Cargo setting of
[target.<triple>.linker](https://doc.rust-lang.org/cargo/reference/config.html#targettriplelinker)
with a value of the path to an appropriate linker in your NDK installation.

This may be set through an environment variable:

```text
$ export CARGO_TARGET_AARCH64_LINUX_ANDROID_LINKER="<path-to-ndk-installation>/toolchains/llvm/prebuilt/linux-x86_64/bin/aarch64-linux-android30-clang"
```

Alternatively, it may be set in the `.cargo/config.toml` file in the current
directory, any parent directory, or your home directory:

```text
[target.aarch64-linux-android]
linker = "<path-to-ndk-installation>/toolchains/llvm/prebuilt/linux-x86_64/bin/aarch64-linux-android30-clang"
```

## Building

To enable cross compilation for `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.

```text
$ export ANDROID_NDK=$HOME/Android/Sdk/ndk/<some-installed-version>
```

Also, include the NDK tools directory in your `PATH`:

```text
$ export PATH="$ANDROID_NDK/toolchains/llvm/prebuilt/linux-x86_64/bin:$PATH"
```

### Building for a target

The bindings can built for the `aarch64` target with:

```text
$ cargo build --target aarch64-linux-android
```

After that, a dynamic library can be found in the
`target/aarch64-linux-android/debug` directory, under the repository root
directory. The library will be called `libmatrix_sdk_crypto_ffi.so` and needs to
be renamed and copied into the `jniLibs` directory of your Android project, for
Element Android:

```text
$ cp ../../target/aarch64-linux-android/debug/libmatrix_sdk_crypto_ffi.so \
     /home/example/matrix-sdk-android/src/main/jniLibs/aarch64/libuniffi_olm.so
```

## License

[Apache-2.0](https://www.apache.org/licenses/LICENSE-2.0)

[Rust]: https://www.rust-lang.org/
[installer]: https://rustup.rs/
[targets]: https://doc.rust-lang.org/nightly/rustc/platform-support.html
[Cargo]: https://doc.rust-lang.org/cargo/