Using Selfie with Kotest tldr  

• follow the regular JVM get started instructions
• add SelfieExtension(projectConfig) to your AbstractProjectConfig
• make the following swap:

  -import com.diffplug.selfie.Selfie.expectSelfie
  +import com.diffplug.selfie.coroutines.expectSelfie
  

And you're good to go!

Detailed setup  

Selfie has two artifacts which can run Kotest tests.

• com.diffplug.selfie:selfie-runner-junit5
  • Works with regular JUnit tests and also Kotest tests.
  • JVM only
• com.difplug.selfie:selfie-runner-kotest
  • Cannot run JUnit tests, only Kotest.
  • Written in Kotlin Multiplatform (only JVM implemented so far, PRs for other platforms welcome)

You must choose only one, you'll get an error if both are on the classpath. You'll need to add the SelfieExtension to your AbstractProjectConfig like so:

import com.diffplug.selfie.junit5.SelfieExtension // selfie-runner-junit5
import com.diffplug.selfie.kotest.SelfieExtension // selfie-runner-kotest
import io.kotest.core.config.AbstractProjectConfig

class ProjectConfig : AbstractProjectConfig() {
  override fun extensions() = listOf(SelfieExtension(this))
}

Selfie and coroutines  

In a regular JUnit 5 test, you call Selfie.expectSelfie(...), like so

import com.diffplug.selfie.Selfie.expectSelfie
class TestClass {
  @Test fun testMethod() {
    expectSelfie(something).toMatchDisk()
  }
}

Every Kotest test is a suspend fun, not a regular fun. You must use the functions in the coroutines package instead of Selfie.

import com.diffplug.selfie.coroutines.expectSelfie
class TestSpec : FunSpec({
  test("testMethod") {
    expectSelfie(something).toMatchDisk()
  }
})

If you use Selfie instead of coroutines you'll get a runtime error reminding you to use the coroutines function instead.

Threading implementation details  

Inline literal toBe snapshots don't need to know what test is currently running. But a toMatchDisk snapshot needs to know which test is running so that it can put the correct name into the snapshot file.

For a JUnit test, selfie-runner-junit5 uses a ThreadLocal to pass information about the currently running test. For a Kotest test using either runner, selfie-runner-junit5 or selfie-runner-kotest, the information is stored in the coroutine context, which can only be accessed from a suspend fun. This is why you must use coroutines.expectSelfie instead of Selfie.expectSelfie.

This issue only matters for disk snapshots. This creates a chance for confusion where you might be using inline literal toBe snapshots without any problem. Then you try toMatchDisk snapshots and get an error telling you to switch from Selfie.expectSelfie to coroutines.expectSelfie. This is expected, just make the switch and you'll be good to go.

Pull requests to improve the landing page and documentation are greatly appreciated, you can find the source code here.