Foreign-language bindings

As stated in the Overview, this library and tutorial does not cover how to ship a Rust library on mobile, but how to generate bindings for it, so this section will only cover that.

Creating the bindgen binary

First, make sure you have installed all the prerequisites.

Ideally you would then run the uniffi-bindgen binary from the uniffi crate to generate your bindings. However, this is only available with Cargo nightly. To work around this, you need to create a binary in your project that does the same thing.

Add the following to your Cargo.toml:

[[bin]]
# This can be whatever name makes sense for your project, but the rest of this tutorial assumes uniffi-bindgen.
name = "uniffi-bindgen"
path = "uniffi-bindgen.rs"

Create uniffi-bindgen.rs:

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

You can now run uniffi-bindgen from your project using cargo run --features=uniffi/cli --bin uniffi-bindgen [args]

Multi-crate workspaces

If your project consists of multiple crates in a Cargo workspace, then the process outlined above would require you creating a binary for each crate that uses UniFFI. You can avoid this by creating a separate crate for running uniffi-bindgen:

  • Name the crate uniffi-bindgen
  • Add this dependency to Cargo.toml: uniffi = {version = "0.XX.0", features = ["cli"] }
  • Follow the steps from the previous section to add the uniffi-bindgen binary target

Then your can run uniffi-bindgen from any create in your project using cargo run -p uniffi-bindgen [args]

Use generate --library to generate foreign bindings by using a cdylib file built for your library. This flag was added in UniFFI 0.24 and can be more convenient than specifying the UDL file -- especially when multiple UniFFI-ed crates are built together in one library. The plan is to make library mode the default in a future UniFFI version, and it is highly recommended to specify the flag for now (because some features simply don't work otherwise).

Taking example/arithmetic as an example, you can generate the bindings with:

cargo build --release
cargo run --bin uniffi-bindgen generate --library target/release/libarithmetical.so --language kotlin --out-dir out

Then check out the out directory.

When using library mode, if multiple crates get built into the library that use UniFFI, all will have bindings generated for them.

Library mode comes with some extra requirements:

  • It must be run from within the cargo workspace of your project
  • Each crate must use exactly 1 UDL file when compiling the Rust library. However, crates can have multiple UDL files as long as they ensure only one is used for any particular build, e.g. by using feature flags.
  • Rust sources must use uniffi::include_scaffolding! to include the scaffolding code.

Running uniffi-bindgen with a single UDL file

Use the generate command to generate bindings by specifying a UDL file.

Kotlin

From the example/arithmetic directory, run:

cargo run --bin uniffi-bindgen generate src/arithmetic.udl --language kotlin

then have a look at src/uniffi/arithmetic/arithmetic.kt

Swift

Run

cargo run --bin uniffi-bindgen generate src/arithmetic.udl --language swift

then check out src/arithmetic.swift

Note that these commands could be integrated as part of your gradle/Xcode build process.

This is it, you have an MVP integration of UniFFI in your project.