Adding a new component to Application Services

Each component in the Application Services repository has three parts (the Rust code, the Kotlin wrapper, and the Swift wrapper) so there are quite a few moving parts involved in adding a new component. This is a rapid-fire list of all the things you'll need to do if adding a new component from scratch.

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, and please try to avoid using hyphens in the crate name.

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. By convention, put the interface definition file at ./components/<your_crate_name>/<your_crate_name>.udl. Use the builtin-bindgen feature of UniFFI to simplify the build process, by putting the following in your Cargo.toml:

[build-dependencies]
uniffi_build = { version = "<latest version here>", features=["builtin-bindgen"] }

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.

In order to be published to consumers, your crate must be included in the "megazord" crate for each target platform:

  • For Android, add it as a dependency in ./megazords/full/Cargo.toml and add a pub use <your_crate_name> to ./megazords/full/src/lib.rs.
  • For iOS, add it as a dependency in ./megazords/ios-rust/rust/Cargo.toml and add a pub use <your_crate_name> to ./megazords/ios-rust/src/lib.rs.

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 Kotlin Bindings

Make a ./components/<your_crate_name>/android subdirectory to contain Kotlin- and Android-specific code. This directory will contain a gradle project for building your Kotlin bindings.

Copy the build.gradle file from ./components/crashtest/android/ into your own component's directory, and edit it to replace the references to crashtest.udl with your own component's .udl file.

Create a file ./components/<your_crate_name>/uniffi.toml with the following contents:

[bindings.kotlin]
package_name = "mozilla.appservices.<your_crate_name>"
cdylib_name = "megazord"

Create a file ./components/<your_crate_name>/android/src/main/AndroidManifest.xml with the following contents:

<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="org.mozilla.appservices.<your_crate_name>" />

In the root of the repository, edit .buildconfig-android.ymlto add your component's metadata. This will cause it to be included in the gradle workspace and in our build and publish pipeline. Check whether it builds correctly by running:

  • ./gradlew <your_crate_name>:assembleDebug

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>/.

So you would 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
          • java/mozilla/appservices/<your_crate_name>/
            • Hand-written Kotlin code here.
        • test/java/mozilla/appservices/<your_crate_name>/
          • Kotlin test-cases here.

Run your component's Kotlin tests with ./gradlew <your_crate_name>:test to confirm that this is all working correctly.

The Swift Bindings

Creating the directory structure

Make a ./components/<your_crate_name>/ios subdirectory to contain Swift- and iOS-specific code. The UniFFI-generated swift bindings will be written to a subdirectory named Generated.

You can include hand-written Swift code alongside the automatically generated bindings, by placing .swift files in a directory named: ./ios/<your_crate_name>/.

So you would end up with a directory structure something like this:

  • components/<your_crate_name>/
    • Cargo.toml
    • uniffi.toml
    • src/
      • Rust code here.
    • ios/
      • <your_crate_name>/
        • Hand-written Swift code here.
      • Generated/
        • Generated Swift code will be written into this directory.

Adding your component to the Swift Package Manager Megazord

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

You will need to do the following steps to include the component in the megazord:

  1. Update its uniffi.toml to include the following settings:

    [bindings.swift]
ffi_module_name = "MozillaRustComponents"
ffi_module_filename = "<crate_name>FFI"

  2. Add the component as a dependency to the Cargo.toml in megazords/ios-rust/

  3. Add a pub use declaration for the component in megazords/ios-rust/src/lib.rs

  4. Add logic to the megazords/ios-rust/build-xcframework.sh to copy or generate its header file into the build

  5. Add an #import for its header file to megazords/ios-rust/MozillaRustComponents.h

  6. Add your component into the iOS "megazord" through the Xcode project, which can only really by done using the Xcode application, which can only really be done if you're on a Mac.

    1. Open megazords/ios-rust/MozillaTestServices/MozillaTestServices.xcodeproj in Xcode.

    2. In the Project navigator, add a new Group for your new component, pointing to the ./ios/ directory you created above. Add the following entries to the Group:

      • The .udl file for you component, from ../src/<your_crate_name>.udl.
      • Any hand-written .swift files for your component

Make sure that the "Copy items if needed" option is unchecked, and that nothing is checked in the "Add to targets" list.

The result should look something like this:

Screenshot of Xcode Project Navigator

Click on the top-level "MozillaTestServices" project in the navigator, then go to "Build Phases".

Double-check that <your_crate_name>.udl does not appear in the "Copy Bundle Resources" section.

Add <your_crate_name>.udl to the list of "Compile Sources". This will trigger an Xcode Build Rule that generates the Swift bindings automatically. Also include any hand-written .swift files in this list.

Finally, in the Project navigator, add a sub-group named "Generated", pointing to the ./Generated/ subdirectory, and containing entries for the files generated by UniFFI: * <your_crate_name>.swift * <your_crate_name>FFI.h Make sure that "Copy items if needed" is unchecked, and that nothing is checked in "Add to targets".

Double-check that <your_crate_name>.swift does not appear in the "Compile Sources" section.

The result should look something like this:

Screenshot of Xcode Compile Sources list

Build the project in Xcode to check whether that all worked correctly.

To add Swift tests for your component API, create them in a file under megazords/ios-rust/MozillaTestServicesTests/. Use this syntax to import your component's bindings from the compiled megazord:

@testable import MozillaTestServices

In Xcode, navigate to the MozillaTestServicesTests Group and add your new test file as an entry. Select the corresponding target, click on "Build Phases", and add your test file to the list of "Compile Sources". The result should look something like this:

Screenshot of Xcode Test Setup

Use the Xcode Test Navigator to run your tests and check whether they're passing.

Distribute your component with rust-components-swift

The Swift source code and generated UniFFI bindings are distributed to consumers (eg: Firefox iOS) through rust-components-swift.

A nightly taskcluster job prepares the rust-component-swift packages from the source code in the application-services repository. To distribute your component with rust-component-swift, add the following to the taskcluster script in taskcluster/scripts/build-and-test-swift.py:

  • Add the path to the <your_crate_name>.udl file to BINDINGS_UDL_PATHS
    • Optionally also to FOCUS_UDL_PATHS if your component is also targeting Firefox Focus
  • Add the path to the directory containing any hand-written swift code to SOURCE_TO_COPY
    • Optionally also to FOCUS_SOURCE_TO_COPY if your component is also targeting Firefox Focus

Your component should now automatically get included in the next rust-component-swift nightly release.