PostBoundaryCallback.kt

class PostBoundaryCallback : PagedList.BoundaryCallback<Post>(), KoinComponent {

    private val remoteDataSource: PostApiService by inject()
    private val localDataSource: PostDao by inject()
    private val httpClient: OkHttpClient by inject()
    private val gson: Gson by inject()

    private var nextPageUrl: String? = null

    override fun onZeroItemsLoaded() {
        super.onZeroItemsLoaded()
        val response = remoteDataSource.getFeed().execute()
        if (response.isSuccessful) {
            nextPageUrl = parseNextPageUrl(response.headers())
            val postList: List<Post> = response.body()
            upsertPostList(postList)
        }
    }

    override fun onItemAtEndLoaded(itemAtEnd: MessageModel) {
        super.onItemAtEndLoaded(itemAtEnd)
        nextPageUrl?.run {
            val response = httpClient.newCall(
                Request.Builder()
                    .url(this)
                    .build()
            ).execute()
            if (response.isSuccessful) {
                nextPageUrl = parseNextPageUrl(response.headers())
                val listType = object : TypeToken<List<Post>>() {}.type
                val postList: List<Post> = gson.fromJson(response.body()?.string(), listType)
                upsertPostList(postList)
            }
        }
    }
    
    private fun upsertPostList(postList: List<Post>) {
        postList.forEach { post ->
            localDataSource.upsert(post)
        }
    }
}