Annotation to transform query parameter

[christophehenry]

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there are cases when @Query is not enough and you would like to perform more sophisticated computations of query based on the method parameter. I have a use case in a feed reader implementing the defunct Google reader API. If you want to mark an article as read, you must hit the endpoint reader/api/0/edit-tag with query parameter a set to user/-/state/com.google/read. If you want to mark it as undread, though, you must use query parameter r.

Here is a rough implementation I made to solve that case using an interceptor:

@Target(AnnotationTarget.VALUE_PARAMETER)
@Retention(AnnotationRetention.RUNTIME)
internal annotation class QueryTransformer(val value: KClass<out QTransformer<out Any>>)

interface QTransformer<T> {
    fun transform(param: T): List<Pair<String, String>>
}

class APIInterceptor(): Interceptor {
    lateinit var service: API

    override fun intercept(chain: Interceptor.Chain): OkHttp3Response {
        val request = chain.request()
        val invocation = request.tag(Invocation::class.java)!!
        val method = invocation.method()
        val builder = request.newBuilder()
        var reqUrl = request.url()

        method.parameterAnnotations.forEachIndexed {index, annotations ->
            val annotation = annotations.find {it is QueryTransformer}
            if(annotation != null && annotation is QueryTransformer) {
                val parameter = invocation.arguments()[index]!!
                val parameterType = method.parameterTypes[index]!!
                val transformer = annotation.value.primaryConstructor!!.call()

                @Suppress("UNCHECKED_CAST")
                val queryParams = transformer.javaClass.getMethod("transform", parameterType)
                    .invoke(transformer, parameter) as List<Pair<String, String>>
                queryParams.forEach {
                    reqUrl = reqUrl.newBuilder().setQueryParameter(it.first, it.second).build()
                }
            }
        }

        return builder.url(reqUrl).build().let {chain.proceed(it)}
    }
}

It would be nice to have it natively in Retrofit because currenlty, this implementation forces me to use it in combination with @Query.

[JakeWharton]

This sounds awfully like #626.

If you want a quick workaround something like this might work:

sealed class ReadStatus : Map<String, String>
object Read : ReadStatus(), Map<String, String> by mapOf("a" to "user/-/state/com.google/read")
object Unread : ReadStatus(), Map<String, String> by mapOf("r" to "user/-/state/com.google/read")

interface Api {
  @Post("..")
  suspend fun updateReadStatus(.., @QueryMap readStatus: ReadStatus)
}

I can't remember if we allow subtypes of Map for @QueryMap or whether we require an exact match.

[christophehenry]

Nice! Thanks, I'll try.