Issue With Deserialization Json Array with Ktor

Question

I am trying to use Ktor and Kotlinx Serialization to pull some dummy post data form jsonplaceholder.typicode.com (here) and deserialize the array. However, I get the following error:

Error:Expected class kotlinx.serialization.json.JsonObject (Kotlin reflection is not available) as the serialized body of kotlinx.serialization.Polymorphic<List>, but had class kotlinx.serialization.json.JsonArray (Kotlin reflection is not available)

Where in my code am I specifying that I am expecting the data as JsonObjects and not JsonArrays? This is likely the error, but I don't see where in the code I specify this.

Thanks in advance for the help.

Relevant code is posted here:

LoginRegisterFragment.kt

package com.example.groupupandroid

import android.content.Intent
import android.os.Bundle
import androidx.fragment.app.Fragment
import android.view.LayoutInflater
import android.view.View
import android.view.ViewGroup
import androidx.lifecycle.lifecycleScope
import androidx.navigation.Navigation
import androidx.navigation.fragment.findNavController
import com.example.groupupandroid.databinding.ActivityMainBinding
import com.example.groupupandroid.databinding.FragmentLoginRegisterBinding
import data.remote.PostResponse
import data.remote.PostsService
import kotlinx.coroutines.CoroutineScope
import kotlinx.coroutines.GlobalScope
import kotlinx.coroutines.coroutineScope
import kotlinx.coroutines.launch

class LoginRegisterFragment : Fragment(){
    // Getting xml objects
    private var binding: FragmentLoginRegisterBinding? = null
    // Creating service for networking
    private lateinit var service: PostsService

    private var posts: List<PostResponse> = emptyList()

    //@Composable
    override fun onCreateView(
        inflater: LayoutInflater, container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {
        binding = FragmentLoginRegisterBinding.inflate(layoutInflater)

        lifecycleScope.launch {getPosts()}

        // Inflate the layout for this fragment
        return binding?.root
    }

    private suspend fun getPosts() {
        service = PostsService.create()
        posts = service.getPosts()
        print(posts)
    }

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {

        // If register button is tapped make register visible
        binding?.registerToggleButton?.setOnClickListener {
            binding?.registerFields?.visibility = View.VISIBLE
            binding?.loginFields?.visibility = View.GONE
        }

        // If login button is tapped make login visible
        binding?.loginToggleButton?.setOnClickListener {
            binding?.registerFields?.visibility = View.GONE
            binding?.loginFields?.visibility = View.VISIBLE
        }

        // If login button is pushed swap to maps
        binding?.loginButton?.setOnClickListener {
            findNavController().navigate(R.id.loginToHomeScreen)
        }

        // If register button is pushed swap to maps
        binding?.registerButton?.setOnClickListener {
            findNavController().navigate(R.id.loginToHomeScreen)
        }

        binding?.materialButtonToggleGroup?.check(binding?.loginToggleButton!!.id)
    }

    override fun onDestroyView() {
        super.onDestroyView()
        binding = null
    }
}

PostResponse.kt

package data.remote

import kotlinx.serialization.Serializable

@Serializable
data class PostResponse (
    val body: String,
    val title: String,
    val id: Int,
    val userId: Int
)

PostsServiceImplementation.kt

package data.remote

import io.ktor.client.*
import io.ktor.client.call.*
import io.ktor.client.request.*
import io.ktor.client.statement.*
import io.ktor.http.*
import io.ktor.util.*
import java.lang.Exception

class PostsServiceImplementation(private val client: HttpClient): PostsService
 {
    override suspend fun getPosts(): List<PostResponse> {
        return try {
            client.get {url(HttpRoutes.POSTS)}.body()
        } catch(e: Exception){
            println("Error:${e.message}")
            emptyList()
        }
    }

    override suspend fun createPosts(postRequest: PostRequest): PostResponse? {
        return try {
            client.post {
                url(HttpRoutes.POSTS)
                contentType(ContentType.Application.Json)
                setBody(postRequest)
            }.body()
        } catch(e: Exception) {
            println("Error:${e.message}")
            null
        }
    }
}

---

Edit: More relevant code.

HttpRoutes.kt

package data.remote

object HttpRoutes {

    private const val BASE_URL = "https://jsonplaceholder.typicode.com"

    const val POSTS = "$BASE_URL/posts"
}

PostsService.kt

package data.remote

import io.ktor.client.*
import io.ktor.client.engine.android.*
import io.ktor.client.plugins.contentnegotiation.*
import io.ktor.client.plugins.kotlinx.serializer.*
import io.ktor.client.plugins.logging.*
import io.ktor.client.statement.*
import io.ktor.serialization.kotlinx.json.*

interface PostsService {

    suspend fun getPosts(): List<PostResponse>

    suspend fun createPosts(postRequest: PostRequest): PostResponse?

    companion object {
        fun create():PostsService {
            return PostsServiceImplementation (
                client = HttpClient(Android) {
                    install(Logging) {
                        level = LogLevel.ALL
                    }
                    install(ContentNegotiation) {
                        json()
                    }
                }
            )
        }
    }
}

PostRequest.kt

package data.remote

import kotlinx.serialization.Serializable

@Serializable
data class PostRequest (
    val body: String,
    val title: String,
    val userId: Int
)

Answer 1

I am going to close the question. Figured out that the following message was being displayed on my data class: "kotlinx.serialization compiler plugin is not applied to the module, so this annotation would not be processed. Make sure that you've setup your buildscript correctly and re-import project." The issue was that I did not include the following line in my build.gradle.kts (app)

    kotlin("plugin.serialization") version "1.7.10"

I also was using ProGuard, but did not include the relevant changes to the ProGuard rules (see here).