Introduction
As I commented in my previous article A complete migration of a multi-module app to Compose Kotlin Multiplatform (KMP), this last month I’ve been migrating my existing multi-module application to Compose Multiplatform, and besides that, I’ve been creating a new Compose Multiplatform multi-module project from scratch. In both I have encountered the same “problems” or “blockers”, so this article is aimed at both if you are migrating or starting a KMP project from scratch.
Tip 1: Previews
KMP does not support previews for the commonMain directory compose components, so I came up with the idea to create them in the androidMain directory and they work perfectly well.
e.g. commonMain/com/example/feature/component/FeatureScreen.kt
androidMain/com/example/feature/component/FeatureScreenPreview.kt
Tip 2: BackHandler
KMP does not support BackHandler action, so I created an expect function to the screen and added the BackHandler action on androidMain actual function, and leave the iosMain one empty (since I didn’t find a similar action in iOS).
e.g.
// commonMain/ com.example.feature.component.FeatureScreen.kt
@Composable
expect fun FeatureScreen(
viewModel: FeatureScreenViewModel,
)
@Composable
internal fun Content(
viewModel: FeatureScreenViewModel,
) {
…
}
// androidMain/ com.example.feature.component.FeatureScreenActual.kt (needs a name different from common)
@Composable
actual fun FeatureScreen(
viewModel: WorkScreenViewModel,
) {
BackHandler { viewModel.onIntent(WorkIntent.Back) }
Content(
viewModel = viewModel,
)
}
// extra: I have joined the preview in this same class to have it better organized.
// iosMain/ com.example.feature.component.FeatureScreenActual.kt (needs a name different from common)
@Composable
actual fun FeatureScreen(
viewModel: WorkScreenViewModel,
) {
Content(
viewModel = viewModel,
)
}
Tip 3: Test mocking
I like to use mockk library for mocking tests, and at the time of writing this article, it is not yet supported by KMP, so I decided to create the UnitTests in the androidUnitTest directory, adding the library dependencies to the androidUnitTest.dependencies {} block.
For the tests as such, I have used the kotlin-test jetbrains library, which supports KMP.
e.g.
mockk = { group = “io.mockk”, name = “mockk”, version.ref = “mockk-version” }
kotlin-test = { group = “org.jetbrains.kotlin”, name = “kotlin-test”, version.ref = “kotlin-version” }
// feature build.gradle.kts
kotlin {
…
sourceSets {
androidUnitTest.dependencies {
implementation(libs.mockk)
}
commonTest.dependencies {
implementation(libs.kotlin.test)
}
}
}
androidUnitTest/com/example/feature/usecase/UseCaseTest.kt
Tip 4: UI test
The official compose multiplatform UI test guide indicates that you have to use the commonTest directory for UI tests, but I prefer to use the androidInstrumentedTest directory, because with this approach, I separate the unit tests from the ui ones, and I can execute them directly from the same test class and run all the UI tests from the directory.
e.g.
mockk-android = { group = “io.mockk”, name = “mockk-android”, version.ref = “mockk-version” }
ui-test-junit4-android = { group = “androidx.compose.ui”, name = “ui-test-junit4-android”, version.ref = “uiTestJunit4AndroidVersion” }
ui-test-manifest = { group = “androidx.compose.ui”, name = “ui-test-manifest”, version.ref = “uiTestManifestVersion” }
kotlin-test = { group = “org.jetbrains.kotlin”, name = “kotlin-test”, version.ref = “kotlin-version” }
// feature build.gradle.kts
plugins {
…
alias(libs.plugins.compose.multiplatform)
alias(libs.plugins.compose)
}
kotlin {
…
sourceSets {
androidInstrumentedTest.dependencies {
implementation(libs.mockk.android)
implementation(libs.ui.test.junit4.android)
}
commonTest.dependencies {
implementation(libs.kotlin.test)
@OptIn(ExperimentalComposeLibrary::class)
implementation(compose.uiTest)
}
}
}
…
dependencies {
debugImplementation(libs.ui.test.manifest)
}
androidInstrumentedTest/com/example/feature/component/ScreenAndroidTest.kt
Conclusion
In this article we have seen some Kotlin Multiplatform tips, that I hope you find it useful. Thanks for reading.
