Adding a new component to Application Services

This is a rapid-fire list for adding a component from scratch and generating Kotlin/Swift bindings.

The Rust Code

Your component should live under ./components in this repo. Use cargo new --lib ./components/<your_crate_name>to create a new library crate.

See the Guide to Building a Rust Component for general advice on designing and structuring the actual Rust code, and follow the Dependency Management Guidelines if your crate introduces any new dependencies.

Use UniFFI to define how your crate's API will get exposed to foreign-language bindings. Lookup the installed uniffi version on other packages (eg grep uniffi components/init_rust_components/Cargo.toml) and use the same version. Place the following in your Cargo.toml:

[dependencies]
uniffi = { version = "<current uniffi version>" }

New components should prefer using the proc-macro approach rather than a UDL file based approach. If you do use a UDL file, add this to Cargo.toml as well.

[build-dependencies]
uniffi = { version = "<current uniffi version>" }

Include your new crate in the application-services workspace, by adding it to the members and default-members lists in the Cargo.toml at the root of the repository.

Run cargo check -p <your_crate_name> in the repository root to confirm that things are configured properly. This will also have the side-effect of updating Cargo.lock to contain your new crate and its dependencies.

The Android Bindings

Run the cargo start-bindings android <your_crate_name> <component_description> command to auto-generate the initial code. Follow the directions in the output.

You will end up with a directory structure something like this:

• components/<your_crate_name>/
  • Cargo.toml
  • uniffi.toml
  • src/
    • Rust code here.
  • android/
    • build.gradle
    • src/
      • main/
        • AndroidManifest.xml

Dependent crates

If your crate uses types from another crate in it's public API, you need to include a dependency for the corresponding project in your android/build.gradle file.

For example, suppose use the remote_settings::RemoteSettingsServer type in your public API so that consumers can select which server they want. In that case, you need to a dependency on the remotesettings project:

dependencies {
    api project(":remotesettings")
}

Async dependencies

If your components exports async functions, add the following to your android/build.gradle file:

dependencies {
    implementation libs.kotlin.coroutines
}

Hand-written code

You can include hand-written Kotlin code alongside the automatically generated bindings, by placing `.kt`` files in a directory named:

• ./android/src/test/java/mozilla/appservices/<your_crate_name>/

You can write Kotlin-level tests that consume your component's API, by placing `.kt`` files in a directory named:

• ./android/src/test/java/mozilla/appservices/<your_crate_name>/.

You can run the tests with ./gradlew <your_crate_name>:test

The iOS Bindings

• Run the cargo start-bindings ios <your_crate_name> command to auto-generate the initial code
• Run cargo start-bindings ios-focus <your_crate_name> if you also want to expose your component to Focus.
• Follow the directions in the output.

You will end up with a directory structure something like this:

• components/<your_crate_name>/
  • Cargo.toml
  • uniffi.toml
  • src/
    • Rust code here.

Adding your component to the Swift Package Megazord

For more information on our how we ship components using the Swift Package Manager, check the ADR that introduced the Swift Package Manager

Add your component into the iOS "megazord" through the local Swift Package Manager (SPM) package MozillaRustComponentsWrapper. Note this SPM is for ease of testing APIs locally. The official SPM that is consumed by firefox-ios is a local package in their repo.

1. Place your Swift test code in:

   megazords/ios-rust/tests/MozillaRustComponentsWrapper/
   

Note: swift-specific tests are ideally suited in the consuming app as there is better integration coverage and ensures we're accurately testing how it's being consumed

That's it! At this point, if you don't intend on writing tests (are you sure?) you can skip this next section.

Writing and Running Tests

The current system combines all rust crates into one binary (megazord). To use your rust APIs simply import the local SPM into your tests:

@testable import MozillaRustComponentsWrapper

To test your component:

• Run the script:

./automation/build_ios_artifacts.sh

The script will:

1. Build the XCFramework (combines all rust binaries for SPM)
2. Generate UniFFi bindings (generated files will be found in megazords/ios-rust/sources/MozillaRustComponentsWrapper/Generated/)
3. Generate Glean metrics
4. Run any tests found in the test dir mentioned above

Distribution of the component

The UniFFi files will be generated & packaged via taskcluster/scripts/build-and-test-swift.py.

Consumers (eg: Firefox iOS) pull the latest artifacts via a nightly Gitub action. Once your changes are pulled via a nightly release you'll be able to use your new APIs!

Note: If you don't want to wait for a nightly, once the CI finishes your build -- you can request the action run in firefox-ios to get it sooner

Note pt2: If you want to locally test against firefox-ios, follow this guide