Setting up UniFFI for iOS, Simulators, and watchOS

Christian Kjær in a casual setting :)
Christian Kjær
6 min read


24. June 2024


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This is a part of the post Mobile: A different way using Rust?.

There are some great resources out there on UniFFI already such as this post, but it doesn’t cover watchOS, so let’s take a quick tour through what I’ve set up in the example repository

We've set up four crates:

  • appy: Our Leptos App, Capacitor, and the XCode project
  • capacitor-rs: Bridging code between the Capacitor JS library and our Rust code
  • shared: Our shared code that we might use in appy, and also want to expose in Swift to use in our Widgets or watchOS App
  • mobile: Where we will generate the Swift bindings from via UniFFI, reexporting everything from shared that’s made available to UniFFI via the macros

I won’t go over the details to get these to play nicely with Cargo and Workspaces, check out the repository for that. Let’s instead focus on a simplified version of what mobile does (the rest assumes you’re in the mobile/ directory).

If starting from scratch, create a new cargo project:

$ cargo init mobile --lib

Update your Cargo.toml:

2name = "mobile"
3version = "0.1.0"
4edition = "2021"
7crate-type = ["staticlib"]
8name = "mobile"
11name = "uniffi-bindgen"
12path = "src/bin/"
15opt-level = 'z' # Optimize for size.
16lto = true # Enable Link Time Optimization.
17strip = true # Automatically strip symbols from the binary.
18panic = "abort"
19debug = false
20# Optional:
21# codegen-units = 1 # Reduce Parallel Code Generation Units to Increase Optimization.
24# UniFFI dependencies for generating Swift bindings.
25uniffi = { version = "0.28.0", features = ["cli"] }

Update your src/

4pub enum Fruits {
5 Watermelon,
6 Cranberry,
7 Cherry,
11pub fn eat_fruit(fruit: Fruits) -> String {
12 match fruit {
13 Fruits::Watermelon => "Eating Watermelon".to_string(),
14 Fruits::Cranberry => "Eating Cranberry".to_string(),
15 Fruits::Cherry => "Eating Cherry".to_string(),
16 }

We’re using setup_scaffolding to avoid needing to manually construct headers, modulemaps, and the UDL files (check out the docs here).

And finally, create a new file src/bin/

1fn main() {
2 uniffi::uniffi_bindgen_main()

We’re now ready to build the binary for generating our bindings, and then use that to generate the actual bindings:

# Build the uniffi-bindgen binary and our initial library file.
$ cargo build
$ cargo run --bin uniffi-bindgen generate --library ./target/debug/libmobile.a --language swift --out-dir ./bindings

We also need to rename the FFI file to module.modulemap so that XCFramework will find it:

$ mv ./bindings/sharedFFI.modulemap ./bindings/module.modulemap

Now, let's add support for iOS, the Simulator and macOS via rustup:

$ rustup target add aarch64-apple-darwin
$ rustup target add aarch64-apple-ios
$ rustup target add aarch64-apple-ios-sim
$ rustup target add x86_64-apple-ios # iOS simulator, also needed on Arm Macs.

and then build the library for all of our targets:

$ carbo build --release --target=aarch64-apple-darwin
$ carbo build --release --target=aarch64-apple-ios
$ carbo build --release --target=aarch64-apple-ios-sim
$ carbo build --release --target=x86_64-apple-ios

We'll combine x86_64-apple-ios and aarch64-apple-ios-sim into a single binary later on, but for now we keep them separate.

If we want watchOS we need to handle things a bit differently, since these are Tier 3 targets (i.e. rustup won't have their stdlib):

$ cargo +nightly build -Zbuild-std=std,panic_abort --release --target=aarch64-apple-watchos-sim
$ cargo +nightly build -Zbuild-std=std,panic_abort --release --target=x86_64-apple-watchos-sim
$ cargo +nightly build -Zbuild-std=std,panic_abort --release --target=aarch64-apple-watchos
$ cargo +nightly build -Zbuild-std=std,panic_abort --release --target=armv7k-apple-watchos
$ cargo +nightly build -Zbuild-std=std,panic_abort --release --target=arm64_32-apple-watchos

That's a lot of targets, which represent all the various Watch models, as well as the simulators (we always need both ARM and x86).

xcodebuild won't be happy if we just drop them in individually, so we need to create a fat binary:

# Combine the watchOS simulator libraries into a single file using lipo.
$ mkdir -p target/watchOS-sim/release
$ lipo -create target/aarch64-apple-watchos-sim/release/libmobile.a \\
        target/x86_64-apple-watchos-sim/release/libmobile.a \\
        -o target/watchOS-sim/release/libmobile.a
# Confirm the architectures.
$ lipo -info target/watchOS-sim/release/libmobile.a

# Combine the watchOS libraries into a single file using lipo.
$ mkdir -p target/watchOS/release
$ lipo -create target/aarch64-apple-watchos/release/libmobile.a \\
        target/arm64_32-apple-watchos/release/libmobile.a \\
        target/armv7k-apple-watchos/release/libmobile.a \\
        -o target/watchOS/release/libmobile.a
# Confirm the architectures.
$ lipo -info target/watchOS/release/libmobile.a

We can then create our XCFramework:

$ xcodebuild -create-xcframework \\
    -library ./target/aarch64-apple-ios-sim/release/libmobile.a -headers ./bindings \\
    -library ./target/aarch64-apple-ios/release/libmobile.a -headers ./bindings \\
    -library ./target/aarch64-apple-darwin/release/libmobile.a -headers ./bindings \\
    -library ./target/watchOS-sim/release/libmobile.a -headers ./bindings \\
    -library ./target/watchOS/release/libmobile.a -headers ./bindings \\
    -output "ios/Shared.xcframework"

And finally, we'll combine x86_64-apple-ios and aarch64-apple-ios-sim into a single binary. If we included both of these in the XCFramework, xcodebuild would complain that these are the same, and not generate our XCFramework file. Oddly enough, it will not be able to build the project without both, so we let xcodebuild generate the XCFramework first, and then replace the binary with the fat binary:

# We need to combine the architectures for the iOS Simulator libraries after we've
# constructed the XCFramework, otherwise it will complain about them being the same,
# while also failing because of missing x86_64 if we omit it.
$ mkdir -p target/iOS-sim/release
$ lipo -create target/aarch64-apple-ios-sim/release/libmobile.a \\
        target/x86_64-apple-ios/release/libmobile.a \\
        -o target/iOS-sim/release/libmobile.a
# Confirm the architectures.
$ lipo -info target/iOS-sim/release/libmobile.a
# Move it into place.
$ rm ios/Shared.xcframework/ios-arm64-simulator/libmobile.a
$ cp target/iOS-sim/release/libmobile.a ios/Shared.xcframework/ios-arm64-simulator/libmobile.a


As the final step we drag-n-drop ./ios/Shared.xcframework and ./bindings/shared.swift into the XCode project whereever you want them. I personally like to create a new group (folder) called Generated for them (the script assumes that's the case).

👉 Let me know what you think over on Medium or in the comments below 👇