PostPagingDataSource.kt

class PostPagingDataSource : PageKeyedDataSource<String, Post>(), KoinComponent {

    private val api: PostApiService by inject()
    private val httpClient: OkHttpClient by inject()
    private val gson: Gson by inject()

    override fun loadInitial(
        params: LoadInitialParams<String>,
        callback: LoadInitialCallback<String, Post>
    ) {
        val response = api.getFeed().execute()
        if (response.isSuccessful) {
            val nextPageUrl = parseNextPageUrl(response.headers())
            val postList = response.body().body()
            callback.onResult(postList, null, nextPageUrl)
        } else {
            // TODO: error handling
        }
    }

    override fun loadAfter(params: LoadParams<String>, callback: LoadCallback<String, Post>) {
        val url = params.key
        if (url.isNotEmpty()) {
            val response = httpClient.newCall(
                Request.Builder()
                    .url(url)
                    .build()
            ).execute()

            if (response.isSuccessful) {
                val nextPageUrl = parseNextPageUrl(response.headers())
                val listType = object : TypeToken<List<Post>>() {}.type
                val postList: List<Post> = gson.fromJson(response.body()?.string(), listType)
                callback.onResult(postList, nextPageUrl)
            }
        }
    }

    override fun loadBefore(params: LoadParams<String>, callback: LoadCallback<String, Post>) {
        // we don't need it, leave it empty
    }

    private fun parseNextPageUrl(headers: Headers): String? {
        // parse URL from link header
    }
}

class PostPagingDataSourceFactory : DataSource.Factory<String, Post>() {
    override fun create(): DataSource<String, Post> {
        return PostPagingDataSource()
    }
}