val android: CommonExtension<*, *, *, *, *, *> by extensions
val androidComponents: AndroidComponentsExtension<*, *, *> by extensions

interface MyVariantExt : VariantExtension {
    val envApiKey: Property<String>
}
interface MyProjectExt {
    val envApiKey: Property<String>
}
interface MyBuildTypeExt {
    val envApiKey: Property<String>
}
interface MyProductFlavorExt {
    val envApiKey: Property<String>
}

androidComponents.registerExtension(
    DslExtension.Builder("myExt")
        .extendProjectWith(MyProjectExt::class.java)
        .extendBuildTypeWith(MyBuildTypeExt::class.java)
        .extendProductFlavorWith(MyProductFlavorExt::class.java)
        .build()
) {
    objects.newInstance<MyVariantExt>()
}

androidComponents.onVariants { variant ->
    val variantExt = variant.getExtension(MyVariantExt::class.java)!!
    val buildTypeExt = variant.buildType?.let { android.buildTypes.findByName(it) }?.extensions?.getByType<MyBuildTypeExt>()
    val flavorExt = variant.flavorName?.let { android.productFlavors.findByName(it) }?.extensions?.getByType<MyProductFlavorExt>()
    val projectExt = (android as ExtensionAware).extensions.getByType<MyProjectExt>()
    variantExt.envApiKey.convention(
        listOfNotNull(buildTypeExt?.envApiKey, flavorExt?.envApiKey, projectExt.envApiKey)
            .reduce(Provider<String>::orElse)
    )
}