Infinite scroll for Android recycler view

InfiniteRecyclerOnScrollListener.kt

import android.support.v7.widget.LinearLayoutManager
import android.support.v7.widget.RecyclerView
import rx.Observable
import rx.Subscriber

/**
 * Create infinite on scroll listener
 *
 * @param linearLayoutManager layout manager to be able to say when next loading is needed
 * @return observeable to inform about needed next page loading
 */
fun RecyclerView.onInfiniteLoading(linearLayoutManager: LinearLayoutManager): Observable<Int> {
    val subscribers: MutableList<Subscriber<in Int>> = mutableListOf()
    addOnScrollListener(InfiniteRecyclerOnScrollListener(linearLayoutManager, { page ->
        subscribers.forEach { s -> s.onNext(page) }
    }))
    return Observable.create({ p -> subscribers.add(p) })
}

/**
 * Listener informs when another portion of data should be loaded what
 * allows to provide infinite-scroll on UI lists
 *
 * @param linearLayoutManager layout manager to be able to say when next loading is needed
 * @param onLoadMore logic for loading next page
 * @param visibleThreshold the minimum number of items to have below current scroll position before loading more.
 */
class InfiniteRecyclerOnScrollListener(
        val linearLayoutManager: LinearLayoutManager,
        val onLoadMore: (Int) -> Unit,
        val visibleThreshold: Int = 5
) : RecyclerView.OnScrollListener() {

    private var previousTotal = 0
    private var loading = true
    private var currentPage = 1

    override fun onScrolled(recyclerView: RecyclerView?, dx: Int, dy: Int) {
        super.onScrolled(recyclerView, dx, dy)
        val visibleItemCount = recyclerView!!.childCount
        val totalItemCount = linearLayoutManager.itemCount
        val firstVisibleItem = linearLayoutManager.findFirstVisibleItemPosition()

        if (loading) {
            if (totalItemCount > previousTotal) {
                loading = false
                previousTotal = totalItemCount
            }
        }
        if (!loading && totalItemCount - visibleItemCount <= firstVisibleItem + visibleThreshold) {
            currentPage++
            onLoadMore(currentPage)
            loading = true
        }
    }

}

ListAdapter.kt

import android.content.Context
import android.support.v7.widget.RecyclerView
import android.view.View
import android.view.ViewGroup

/**
 * Adapter for UI list
 * @param viewFactory factory provide UI list items for view holder
 * @param display decorator for displaying data on UI list item
 * @param items current items in adapter (must be mutable due to android recycler view)
 */
class ListAdapter<T, TU : View>(
        val viewFactory: (Context) -> TU,
        val display: (T?, ViewHolder<TU>) -> Unit,
        val items: MutableList<T> = mutableListOf()
)
    : RecyclerView.Adapter<ListAdapter.ViewHolder<TU>>() {

    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): ViewHolder<TU> = ViewHolder(viewFactory(parent.context))

    override fun onBindViewHolder(holder: ViewHolder<TU>, position: Int) = display(items[position], holder)

    override fun getItemCount(): Int = items.size

    class ViewHolder<out TU : View>(val view: TU) : RecyclerView.ViewHolder(view)

    fun add(t: T) {
        items.add(t)
    }

}

SomeActivity.kt

import android.support.v7.widget.LinearLayoutManager
import android.support.v7.widget.RecyclerView

class ResourcesActivity : AppCompatActivity() {

  override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)

        val listView: RecyclerView = findViewById(R.id.resource_list) as RecyclerView
        val listAdapter = //create list adapter with proper factory and decorator
        listView.adapter = listAdapter
        val linearLayoutManager = LinearLayoutManager(this)
        listView.layoutManager = linearLayoutManager

        val generateItems: (Int) -> Unit = { page ->
            (1 until 20).forEach { i ->
                listAdapter.add(Resource("" + page + "-" + i + ".txt"))
            }
            listAdapter.notifyDataSetChanged()
        }

        listView.onInfiniteLoading(linearLayoutManager)
                .subscribeOn(Schedulers.io())
                .observeOn(AndroidSchedulers.mainThread())
                .subscribe { page ->
                    info("[LazyLoading] NextPage: " + page)
                    generateItems(page)
                }
        generateItems(0)

    }

}