KMMBridge - XCFramework publishing for Kotlin Multiplatform

KMMBridge is a Gradle plugin that allows you to publish Xcode XCFrameworks from your Kotlin Multiplatform projects. Developers using Xcode can then directly consume these XCFrameworks using Swift Package Manager (SPM) or CocoaPods.

KMMBridge can be used to publish your iOS KMP builds to public or private repos, using various backend options to host the binary XCFramework files.

Publishing Overview

To make Kotlin code available to Xcode and/or Swift build targets, the Kotlin KMP compiler builds an Xcode Framework. Regardless of the method by which you include that Framework, that is how Kotlin code is made available to Xcode.

All out-of-the-box build configurations for KMP are locally integrated. That means, to call KMP from Xcode, you need to locally build your Kotlin code.

However, you can assemble and publish these same builds for others to use directly. That is what KMMBrdige does. The broad steps are:

• In CI, the Kotlin compiler builds an Xcode Framework from KMP source
• The Framework outputs are converted to XCFramework packages, then all architectures are zipped in one file
• The binary file is uploaded to some cloud host (GitHub Packages/Releases, AWS S3, Maven Repo, etc)
• The Package.swift file is updated in the KMP repo
• Xcode accesses the Kotlin build through SPM. It gets the Package.swift file from git, and the Kotlin XCFramework from the cloud host

Publishing for CocoaPods is possible with KMMBridge as well. The binary XCFramework is published in the same way. In fact, if you publish both SPM and CocoaPods, they’ll both use the same published binary. CocoaPods needs to publish it’s package metadata, a “podspec” file, to a separate repo whose only purpose is to host podspec files.

KMMBridge Setup Overview

To use KMMBridge, you’ll need the following:

1. A KMP module configured to build some Xcode Frameworks that you want to publish (this is in your Gradle config).
2. A DependencyManager implementation and it’s related configuration. The options are SPM and CocoaPods. You can use both at the same time if publishing to both.
3. An ArtifactManager. This interface actually “publishes” the zip file to a remote server.
4. Build and publication process. Generally a CI workflow. This is also your code. We provide starter templates, but CI generally winds up being custom over time.
5. Xcode config to access both the dependency config file and the zip file. Access to the config file and the binary zip is configured separately in Xocde.

Swift Package Manager (SPM)

kmmbridge {
    spm()
    // Other config, including the artifact manager
}

See the SPM Docs for details.

CocoaPods

CocoaPods has been around for much longer, and is in some ways more flexible than SPM. However, a CocoaPods publishing configuration requires multiple repos, and is generally more complex than SPM.

kmmbridge {
    cocoapods("git@github.com:yourorg/YourPodspecRepo.git")
    // Other config, including the artifact manager
}

See the CocoaPods Docs for more information.

Binary Artifacts

When publishing XCFrameworks, the Kotlin compiler creates a Framework for each target architecture. Each Framework is then converted to an XCFramework. All of these are then bundled into a single zip file.

That zip file needs to be hosted somewhere that is accessible by a stable URL.

Options provided by KMMBridge:

• GitHub Releases
• GitHub Packages (using Maven)
• General Maven repos
• Amazon S3 (only for public URLs without authentication)

You can implement a custom artifact location by implementing ArtifactManager.

Xcode Configuration

To access your frameworks from Xcode using SPM, if they are not public, you need to configure authentication in two places. This is a very common source of confusion. These are the places:

• Xcode GUI. Give it access to your git repo where your config is stored.
• ~/.netrc. This file is accessed by curl to actually download your zip binary. This file is the one that needs basic authentication to function.

See iOS Dev Setup for more info.

Reach Out

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