Code Generation

Update Generator

A common pain point with Mobius.kt is wrapping Event types to their state change functions. In Mobius.kt this code is implemented in your Update function, taking an event and returning a Next instance.

Using KSP, mobiuskt-codegen provides code generation to reduce manual boilerplate when writing complex Update functions.

Given a sealed class Event declaration, an interface is generated defining update methods for each Event subclass and the exhaustive when block in the update method.

Example

To apply code generation, add the @GenerateUpdate annotation to your Update function class definition:

@GenerateUpdate
class TestUpdate : Update<TestModel, TestEvent, TestEffect>, TestGeneratedUpdate {
  // ...
}

Loop components (Click to expand)

data class TestModel(
    val counter: Int,
)

sealed class TestEvent {
    data object Increment : TestEvent()
    data object Decrement : TestEvent()
    data class SetValue(val newCounter: Int) : TestEvent()
}

sealed class TestEffect

Generated output

interface TestGeneratedUpdate : Update<TestModel, TestEvent, TestEffect> {
    override fun update(model: TestModel, event: TestEvent): Next<TestModel, TestEffect> {
        return when (event) {
            TestEvent.Increment -> increment(model)
            TestEvent.Decrement -> decrement(model)
            is TestEvent.SetValue -> setValue(model, event)
        }
    }

    fun increment(model: TestModel): Next<TestModel, TestEffect>

    fun decrement(model: TestModel): Next<TestModel, TestEffect>

    fun setValue(model: TestModel, event: TestEvent.SetValue): Next<TestModel, TestEffect>
}

Nested sealed classes

By default, nested sealed Events will produce a function for each subtype.

Child Sealed Class Default Behavior

sealed class Event {
    // ...
    sealed class Result : Event() {
        data class Success(val data: String) : Result()
        data class Error(val message: String) : Result()
    }
}

fun resultSuccess(event: Event.Result.Success): Next<Model, Effect> {
    // ...
}

fun resultError(event: Event.Result.Error): Next<Model, Effect> {
    // ...
}

This behavior can be changed with @DisableSubtypeSpec, causing the sealed class to be handled by one function.

Child Sealed Class Default Behavior

sealed class Event {
    @DisableSubtypeSpec
    sealed class Result : Event() {
        // ...

fun result(event: Event.Result): Next<Model, Effect> {
    // ...
}

Gradle Configuration

Use the following kts gradle configuration to apply the Update generator in your project:

Kotlin Gradle Script - JVM/Android (Click to expand)

plugins {
    kotlin("jvm") // or kotlin("android")
    id("com.google.devtools.ksp") version "<KSP-Version>"
}

kotlin {
    sourceSets.main {
        kotlin.srcDir("build/generated/ksp/$name/kotlin")
    }
}

dependencies {
    implementation("org.drewcarlson:mobiuskt-codegen-api:$mobiuskt_version")
    ksp("org.drewcarlson:mobiuskt-codegen:$mobiuskt_version")
}

Kotlin Gradle Script - Multiplatform

plugins {
    kotlin("multiplatform")
    id("com.google.devtools.ksp") version "<KSP-Version>"
}

kotlin {
    sourceSets {
        val commonMain by getting {
            kotlin.srcDir("build/generated/ksp/$name/kotlin")
            dependencies {
                implementation("org.drewcarlson:mobiuskt-codegen-api:$mobiuskt_version")
            }
        }
    }
}

// Note this must be in a top-level `dependencies` block, not `kotlin { sourceSets { .. } }`
dependencies {
    add("kspCommonMainMetadata", "org.drewcarlson:mobiuskt-codegen:$mobiuskt_version")
}

// This ensures that when compiling for any target, your `commonMain` sources are
// scanned and code is generated to `build/generated/ksp/commonMain` instead of a
// directory for the specific target. See https://github.com/google/ksp/issues/567
if (tasks.any { it.name == "kspCommonMainKotlinMetadata" }) {
    tasks.withType<org.jetbrains.kotlin.gradle.dsl.KotlinCompile<*>>().all {
        if (name != "kspCommonMainKotlinMetadata") {
            dependsOn("kspCommonMainKotlinMetadata")
        }
    }
}

For more details see the official KSP documentation.