GrpcDecoratorCors.kt

import com.linecorp.armeria.common.HttpMethod
import com.linecorp.armeria.common.HttpService
import com.linecorp.armeria.server.SimpleDecoratingHttpService
import com.linecorp.armeria.server.Service
import com.linecorp.armeria.server.cors.CorsService
import com.linecorp.armeria.server.cors.CorsServiceBuilder
import org.kodein.di.Kodein
import org.kodein.di.direct
import org.kodein.di.generic.instance
import java.util.function.Function

interface GrpcDecorator<T> : Function<HttpService, SimpleDecoratingHttpService>

@Suppress("UNCHECKED_CAST")
class GrpcWebCors(kodein: Kodein) :
    GrpcDecorator<CorsService>,
    Function<HttpService, SimpleDecoratingHttpService>
    by
    CorsServiceBuilder
        .forOrigins(
            "https://example.com"
        )
        .allowCredentials()
        .allowNullOrigin()
        .allowRequestMethods(HttpMethod.POST, HttpMethod.GET)
        .allowRequestHeaders(
            "authorization",
            "content-type",
            "x-grpc-web"
        )
        .exposeHeaders(
            "grpc-message",
            "grpc-status",
            "x-server-version"
        )
        .newDecorator() as Function<HttpService, SimpleDecoratingHttpService>

Short description of what happens here:

1. The GrpcDecorator is so that DependencyInjection can pick up and add them to the GrpcServices.
2. the by keyword creates a delegate class and uses what comes to the right as the delegate. i.e. all (one) method of Function is delegated to the CorsServiceBuilder.newDecorator()

Used like:

val grpcDecorators: List<GrpcDecorator<*>> by kodein.allInstances()
...
val withDecorators = grpcDecorators
    .fold(grpcService) { service, decorator ->
        service.decorate(decorator)
    }

Revision #1 is generics-hell, but works fine.
Second revision throws:

java.lang.ClassCastException: class com.linecorp.armeria.server.FunctionalDecoratingService cannot be cast to class com.linecorp.armeria.server.HttpService (com.linecorp.armeria.server.FunctionalDecoratingService and com.linecorp.armeria.server.HttpService are in unnamed module of loader 'app')

Kodein is the DI framework