Kotlin Context Receivers allow nice Feature Toggles

FeatureFlag.kt

// client code

object NewFeature : Feature
object NewFeatureVariant : Feature

fun main() {

    // configuration
    val featureConfiguration = mapOf(
        NewFeature to true,
        NewFeatureVariant to true
    )
    val featureFlagContext = object : FeatureFlagContext {
        override fun isFeatureEnabled(feature: Feature): Boolean {
            return featureConfiguration[feature] ?: false
        }
    }

    // service bean creation (could be a @Bean method)
    val serviceA =
        with(featureFlagContext) { // provide the context
            createServiceA()
        }

    // invocation
    serviceA.doAStuff()
}

context(FeatureFlagContext)
fun createServiceA(): A {
    val newA = NewA()
    val oldA = OldA()

    return object : A {
        override fun doAStuff(): Boolean {
            return whenFeatureOnOrElse(
                NewFeature,
                { newA.doAStuff() },
                { oldA.doAStuff() }
            )
        }
    }
}

interface A {
    fun doAStuff(): Boolean
}

context(FeatureFlagContext)
class NewA : A {
    override fun doAStuff(): Boolean {
        if (NewFeatureVariant.isEnabled()) {
            println("Feature variant 2 enabled")
        } else {
            println("Feature variant 1 enabled")
        }
        return true
    }
}

class OldA : A {
    override fun doAStuff(): Boolean {
        println("Old impl")
        return false
    }
}


// infrastructure code

interface Feature {
    val name: String
        get() = this.javaClass.simpleName
}

interface FeatureFlagContext {
    fun isFeatureEnabled(feature: Feature): Boolean
}

// utility method to switch behavior on feature on/off
context(FeatureFlagContext)
fun <T> whenFeatureOnOrElse(feature: Feature, bodyEnabled: () -> T, bodyDisabled: () -> T): T {
    return when {
        isFeatureEnabled(feature) -> bodyEnabled()
        else -> bodyDisabled()
    }
}

// utility method to nicely get feature state
context(FeatureFlagContext)
fun <T : Feature> T.isEnabled(): Boolean {
    return isFeatureEnabled(this)
}