Android - Kotlin - RetroFit, RecyclerView, Pull to load more, pull to refresh

data class ListItem (
    val title: String,
    val subtitle: String
)

class MainListAdapter (
    private val list: List<ListItem>
): RecyclerView.Adapter<MainListAdapter.ViewHolder>() {

    class ViewHolder(view: View): RecyclerView.ViewHolder(view)

    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): MainListAdapter.ViewHolder {
        return ViewHolder(
            LayoutInflater
                .from(parent.context)
                .inflate(R.layout.list_main, parent, false)
        )
    }

    override fun onBindViewHolder(holder: ViewHolder, position: Int) {
        holder.itemView.title.text = list[position].title
        holder.itemView.subtitle.text = list[position].subtitle
        Glide.with(holder.itemView)  // glide with this view
            .load(list[position].subtitle) // download using this url
            .override(250, 250) // set this size
            .placeholder(R.mipmap.ic_launcher) // default placeHolder image
            .error(R.mipmap.ic_launcher) // use this image if faile
            .into(holder.itemView.imageView) // set
    }

    override fun getItemCount(): Int {
        return list.size
    }
}

data class User(
    @SerializedName("login") val login: String?,
    @SerializedName("id") val id: Int?,
    @SerializedName("avatar_url") val avatarUrl: String?
)

class GetUsersService {

    interface GitHubService {
        @GET("users?")
        fun listUsers(@Query("since") since: Int?): Call<List<User?>?>
    }

    private val baseUrl = "https://api.github.com/"
    private val retrofit= Retrofit.Builder()
        .baseUrl(baseUrl)
        .addConverterFactory(GsonConverterFactory.create())
        .build()
    private val service = retrofit.create(GitHubService::class.java)
    private var lastId: Int? = null

    private fun fetchData(value: Int?, onResult: (List<User?>?) -> Unit) {
        val call = service.listUsers(value)
        call.enqueue(
            object : Callback<List<User?>?> {
                override fun onFailure(call: Call<List<User?>?>, t: Throwable) {
                    onResult(null)
                }
                override fun onResponse(call: Call<List<User?>?>, response: Response<List<User?>?>) {
                    println("List users are ${response.body()}")
                    lastId = response.body()?.last()?.id
                    onResult(response.body())
                }
            }
        )
    }

    fun users(onResult: (List<User?>?) -> Unit) {
        fetchData(null, onResult)
    }

    fun nextPage(onResult: (List<User?>?) -> Unit) {
        if (lastId == null) {
            onResult(null)
            return
        }
        fetchData(lastId, onResult)
    }
}

class MainActivity : AppCompatActivity() {
    val usersService = GetUsersService()
    var list = mutableListOf<ListItem>(ListItem("Sagar", "Kothari"))

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        listView.adapter = MainListAdapter(list)
        swipeRefresh.setOnRefreshListener {
            getUsers()
        }
        getUsers()
        listView.addOnScrollListener(object : RecyclerView.OnScrollListener() {
            override fun onScrollStateChanged(recyclerView: RecyclerView, newState: Int) {
                super.onScrollStateChanged(recyclerView, newState)
                if (!recyclerView.canScrollVertically(1) && newState == RecyclerView.SCROLL_STATE_IDLE) {
                    loadMoreUsers()
                }
            }
        })
    }

    private fun loadMoreUsers() {
        swipeRefresh.isRefreshing = true
        usersService.nextPage {
            if (it != null) {
                val newTransformed = it
                    .filter { it != null && it.login != null && it.avatarUrl != null }
                    .map { ListItem(it!!.login!!, it.avatarUrl!!) }
                list.addAll(newTransformed)
                (listView.adapter as MainListAdapter).notifyDataSetChanged()
            }
            swipeRefresh.isRefreshing = false
        }
    }

    private fun getUsers() {
        swipeRefresh.isRefreshing = true
        listView.adapter = MainListAdapter(listOf<ListItem>())
        usersService.users {
            swipeRefresh.isRefreshing = false
            if (it != null) {
                list = it
                    .filter { it != null && it.login != null && it.avatarUrl != null }
                    .map { ListItem(it!!.login!!, it.avatarUrl!!) } as MutableList<ListItem>
                listView.adapter = MainListAdapter(list)
            }
        }
    }
}