Kotlin Multiplatform on Apple Watch

TLDR

Figured out how to run shared Kotlin (KMM) code on an apple watch. There was a frustrating lack of information out there, so now its here.

But Why!?

I’ve been working on a little learning project called Location Alarm for a while now, and its up and running on ios and android, using a shared kotlin module for the main logic, and native ui (jetpack compose / swift ui). Its basically a location based alarm clock so i can fall asleep on busses and not miss my stop. Yes, i could go to bed earlier but… 🤷‍♂️

I wanted to add watch apps for both eco-systems, and since i actually have an apple watch i started there.

Step 1: Creating a wearOS target

I followed the official apple tutorial but i’ll provide a TLDR here:

• In XCode: File > New > Target.
• Select ‘watchOS’ tab then select the ‘App’ template and click Next.
• Fill out the ‘Product Name`
• Select ‘Watch App for Existing iOS App’ and ensure your app is selected
  • Sidenote, initially my app didn’t show. Restarting XCode fixed this.

You should now be able to run the sample watch app.

Step 2: Shared Gradle Configuration

If you try to import Shared in your watch app now, you’ll get a No such module 'Shared' error

1. Add the following to your shared build.gradle.kts file:

   watchosX64(),
   watchosArm64(),
   watchosSimulatorArm64()
   

For example:

    val iosTargets = listOf(
        iosX64(),
        iosArm64(),
        iosSimulatorArm64()
    )
    val appleWatchTargets = listOf(
        watchosX64(),
        watchosArm64(),
        watchosSimulatorArm64()
    )
    (iosTargets + appleWatchTargets).forEach { iosTarget ->
        iosTarget.binaries.framework {
            baseName = "Shared"
            isStatic = true
        }
    }

1. Sync Gradle project

Step 3: XCode Build Configuration

• Go into iosApp target -> Build Phases -> Compile Kotlin Framework, and copy the shell script from there. Mine is below, but yours may be different so its best to copy from your own project.

  •   if [ "YES" = "$OVERRIDE_KOTLIN_BUILD_IDE_SUPPORTED" ]; then
      echo "Skipping Gradle build task invocation due to OVERRIDE_KOTLIN_BUILD_IDE_SUPPORTED environment variable set to \"YES\""
      exit 0
      fi
      cd "$SRCROOT/.."
      ./gradlew :shared:embedAndSignAppleFrameworkForXcode
    

• Now switch to the {Project Name} Watch App target -> Build Phases -> Click the + icon -> Select New run script phase to add a new script build phase.
• Paste the script in, and rename the phase (double click the name) to something more sensible than Run script
• Now drag the new phase to the top of the custom phases (they’re the ones with the bin icon to the right)

This runs the embedAndSignAppleFrameworkForXcode gradle task as part of the Watch App’s build.

Step 4: Disable User Script Sandboxing

If you run now, you’ll likely see an error like You have sandboxing for user scripts enabled.

To fix this:

• Go into {Project name} Watch App target -> Build Settings, and search for Sandbox
• In ‘Build Options’, set User Script Sandboxing to No

Step 5: Xcode Build Settings

Note: I’m not entirely sure if this step is required. It seemed to be when i was originally figuring everything out. However, when i reverted everything, and followed this guide to check it, the watch app worked after step 4, so you may not need this.

Now we’re generating the framework as part of the build process, we now need to tell XCode where it lives.

• Go into {Project name} Watch App target -> Build Settings, and search for Framework Search Paths
• Double click the empty space where the values go
• Click the + icon on the popup
• Add the following two entries in this order
  • $(inherited)
  • $(SRCROOT)/../shared/build/xcode-frameworks/$(CONFIGURATION)/$(SDK_NAME)

Now XCode knows to look in the shared kotlin build directory for the frameworks, which are now generated as part of the watch build process.