TestHttp.kt

package sample

import com.github.jeremyrempel.yaba.util.runBlockingTest
import io.ktor.client.HttpClient
import io.ktor.client.engine.mock.MockEngine
import io.ktor.client.engine.mock.respond
import io.ktor.client.features.json.JsonFeature
import io.ktor.client.features.json.serializer.KotlinxSerializer
import io.ktor.client.request.get
import kotlinx.serialization.Serializable
import org.junit.Test

class TestHttp {

    @Serializable
    data class UserId(
        val userId: String,
        val id: Int,
        val title: String,
        val completed: Boolean
    )


    @Test
    fun testFail() {

        runBlockingTest {

            // error
            val client = HttpClient(MockEngine) {
                install(JsonFeature) {
                    serializer = KotlinxSerializer().apply {
                        register(UserId.serializer())
                    }
                }
                engine {
                    addHandler {
                        respond(
                            """
                        {
                        "userId": 1,
                        "id": 3,
                        "title": "fugiat veniam minus",
                        "completed": false
                        }
                    """.trimIndent()
                        )
                    }
                }
            }

            // fails with error: No transformation found: class kotlinx.coroutines.io.ByteBufferChannel
            val response = client.get<UserId>("/")
            println(response)
        }
    }

    @Test
    fun testWorking() {

        runBlockingTest {
            val client = HttpClient {
                install(JsonFeature) {
                    serializer = KotlinxSerializer().apply {
                        register(UserId.serializer())
                    }
                }
            }

            // correctly parses
            val response = client.get<UserId>("https://jsonplaceholder.typicode.com/todos/3")
            println(response)
        }
    }
}