Kotlin Multiplatform: Building a “Fat” iOS Framework for iosArm64 and iosX64

If you are building a Kotlin Multiplatform library which will be consumed by an existing iOS application, using a framework is a great way to do this. Frameworks are typically compiled for a specific target architecture which can then be run on an iPhone, iPad or iOS Simulator on your Intel Macbook.

If you are going use your Kotlin Multiplatform library as a framework in an existing app, you will want to provide a “Fat” framework which will contain both Arm64 and X64 architectures. This article contains the configuration I used to build the “Fat” framework. This is not a full build.gradle.kts file, but just the additional parts needed for a Kotlin Multiplatform project to build a “Fat” framework.

⚠️Use an XCFramework instead ⚠️

This “Fat” framework method no longer works with XCode 12+. Use an XCFramework instead.

I ended up with this Error Message in XCode 12.4: “Building for iOS Simulator, but the linked and embedded framework ‘my_library.framework’ was built for iOS + iOS Simulator.”

This Stack Overflow post shows you how to create an XCFramework from your two frameworks, and I’ll follow up with a blog post on how to do it with an XCFramework a bit later. This solutions ends up combining the two frameworks into a single XCFramework.

xcrun xcodebuild -create-xcframework \
    -framework /path/to/ios.framework \
    -framework /path/to/sim.framework \
    -output combined.xcframework

Custom Gradle Task to Build “Fat” framework

import org.jetbrains.kotlin.gradle.tasks.FatFrameworkTask

kotlin {
    // Set a name for your framework in a single place and reuse the variable
    val libName = "my_library"

    // Configure your Kotlin Multiplatform lib to generate iOS binaries
    // NOTE: This will only work on Macs
    ios {

    // You can choose your output directory
    val frameworkDestinationDir = buildDir.resolve("cocoapods/framework")

    tasks {

        // Custom task to build the DEBUG framework
        // ./gradlew universalFrameworkDebug
        register("universalFrameworkDebug", FatFrameworkTask::class) {
            baseName = libName
                iosArm64().binaries.getFramework(libName, "Debug"),
                iosX64().binaries.getFramework(libName, "Debug")
            destinationDir = frameworkDestinationDir
            group = libName
            description = "Create the debug framework for iOS"

        // Custom task to build the RELEASE framework
        // ./gradlew universalFrameworkRelease
        register("universalFrameworkRelease", FatFrameworkTask::class) {
            baseName = libName
                iosArm64().binaries.getFramework(libName, "Release"),
                iosX64().binaries.getFramework(libName, "Release")
            destinationDir = frameworkDestinationDir
            group = libName
            description = "Create the release framework for iOS"

Here are two custom gradle tasks that build a “Fat” framework for debug or release. In this I have it outputting to the build/cocoapods/framework directory, but you can configure that as you like.

Gradle Task for “Fat” iOS framework

  • Build a “Fat” debug version of the framework
    • ./gradlew universalFrameworkDebug
  • Build a “Fat” release version of the framework
    • ./gradlew universalFrameworkRelease

Importing the iOS Framework into XCode

I previously wrote a blog post about how to do this which has a companion video along with it. 👇

Thanks and Related Resources

I didn’t figure this all out myself. I just got it to work for me and extracted out the bare minimum you need to make this work. Thanks to Swapnil Patil for letting me know that “Fat” frameworks are possible. Thanks so much to Marco Gomiero for his post Introducing Kotlin Multiplatform in an existing project.

How to Install a Specific Ruby Version for Cocoapods

After a few wrong turns, the way I was able to get the correct version of Ruby to install on my Macbook was using rvm. Use the steps below. If you run into issues, you may need to install homebrew as well.

  1. Install RVM
    • curl -sSL https://get.rvm.io | bash
  2. Install the Ruby version you need (x.x.x).
    • rvm installĀ 2.7.3
    • If you want to set that version as default, run rvm use 2.7.3 --default
    • More docs on rvm: https://rvm.io/rvm/install
  3. Install bundler
    • gem install bundler

That should be it! Now you should be able to run bundle install in your iOS project directory on the terminal, and you’re good to go from there!

Run Custom Gradle Task After “build”

After running ./gradlew build on my Kotlin Multiplatform project, I wanted to copy the JavaScript build artifacts (.js & .html) to publish a demo where someone could test my library via a web browser. A custom Gradle task is is a great way to write your own behavior to accomplish this using the Gradle build system.

My Custom Gradle Task

I found the Gradle documentation for how to copy files, and was able to write this custom task "myTaskName" to do it. You can then just run ./gradlew myTaskName and it’ll run the task independently. The problem was that I didn’t know how to get it to always run after ./gradlew build ran.

/** Copies files from "build/distributions" to "demo" directory */
tasks.register<Copy>("myTaskName") {
    println("Copying Build Artifacts!!!")

TL;DR – Use finalizedBy()

  • finalizedBy – Runs my task AFTER “build”. ✅
    • tasks.named("build") { finalizedBy("myTaskName") }
  • dependsOn – Runs “build” BEFORE my task, if my task is executed explicitly.
    • tasks.named("myTaskName") { dependsOn("build") }

To give you more details and to go into my process of how I figured it out, here are a few options I tried while figuring out how to run this custom task after every execution of the “build” Gradle task.

1. finalizedBy ✅

You can use finalizedBy() to say what task you should run after a named task. I think this reads nicely because it calls out the task dependency separately from it’s declaration. This will appropriately run after the build task executes as I needed.

tasks.named("build") { finalizedBy("myTaskName") }

2. shouldRunAfter 🤔

Another possible way to do this is with shouldRunAfter() which can just be added inside the block where you register your custom task. This will run after the build task executes. HOWEVER: I had to call .get() after registering my task to get it to work in order to have this actually run, which just feels wrong… Someone feel free to explain why, but I’m guessing this is some sort of lazy initialization happening if I don’t call “.get()”. Because of this, I don’t like this solution personally.

tasks.register<Copy>("myTaskName") {
    shouldRunAfter("build") // Tells Gradle to execute after "build" task
    // My Custom Task Code

3. dependsOn() 🙃

You would think dependsOn() may work, but it’s the opposite behavior than what I wanted. It’s saying that “myTaskName” needs “build” to have run before it executes. It’s the opposite task dependency relationship from finalizedBy(). This wasn’t the behavior I wanted for this use case because I wanted it to copy the artifacts after every time a ./gradlew build was run, but could be useful depending on your use case.

tasks.named("myTaskName") { dependsOn("build") }

Wrap Up

None of this is rocket science, but if you try to Google for how to do it, it may take you a while to figure out how to do it. Using Option 1, finalizedBy() was the solution that worked for my use case! I hope this saved you an hour or more!