Simple End-less RecyclerView Adapter (Kotlin)

EndlessAdapter.kt

package mdzzohrabi.adapter

import android.support.v7.widget.RecyclerView
import java.util.ArrayList

/**
 * Endless Recycler view adapter
 * @author Masoud Zohrabi <[email protected]>
 */
abstract class EndlessAdapter<VH: RecyclerView.ViewHolder, T>: RecyclerView.Adapter<VH>() {

    // Adapter loaded items
    private val items: ArrayList<T> = ArrayList()
    // Get items loading threshold
    protected open fun getThreshold(): Int = 5

    override fun onBindViewHolder(holder: VH, position: Int) {
        if ( position >= ( items.size - getThreshold() ) ) {
            load( items.size )
        }
    }

    override fun onAttachedToRecyclerView(recyclerView: RecyclerView?) {
        super.onAttachedToRecyclerView(recyclerView)
        load(items.size)
    }

    override fun getItemCount(): Int = items.size

    // Insert item to loaded items
    protected fun insert(item: T) { items.add(item) }
    // Clear loaded items
    protected fun clearItems() { items.clear() }
    // Get loaded items
    protected fun getItems() = items
    // Get loaded item by position
    protected fun getItem(position: Int): T = items[position]
    
    // Load items called when needed to load more items
    abstract fun load(offset: Int, listener: OnLoadListener? = null)

    abstract class OnLoadListener {
        open fun onDataChanged() {}
        open fun onDataReceive() {}
    }

}

ExampleEndlessRecycler.kt

package mdzzohrabi.adapter

import android.content.Context
import android.os.Build
import android.support.v7.widget.RecyclerView
import android.transition.TransitionManager
import android.view.LayoutInflater
import android.view.ViewGroup
import com.android.volley.Request
import com.android.volley.RequestQueue
import com.android.volley.Response
import com.android.volley.toolbox.JsonObjectRequest
import com.android.volley.toolbox.Volley
import org.json.JSONObject

class ExampleEndlessRecycler() : EndlessAdapter<ContentViewHolder, JSONObject>() {

    private val limit = 10
    private val queue: RequestQueue = Volley.newRequestQueue(this.context)

    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): ContentViewHolder {
        val view = LayoutInflater.from(parent.context)
                .inflate(R.layout.fragment_content, parent, false)
        return ContentViewHolder(view as ViewGroup)
    }

    override fun onBindViewHolder(holder: ContentViewHolder, position: Int) {
        super.onBindViewHolder(holder, position)

        val content = getItem(position)
        // Bind item to view
        holder.bind(content)
      
    }

    fun refresh(callback: OnRefreshListener? = null) {
        load(0, object: OnLoadListener() {
            override fun onDataChanged() { callback?.onRefresh() }
            override fun onDataReceive() { clearItems() }
        })
    }

    override fun load(offset: Int, listener: OnLoadListener?) {

        val url = API.Builder(API.LATEST_CONTENTS)
                .appendQueryParameter("topic", filters.topicId)
                .appendQueryParameter("offset", offset.toString())
                .appendQueryParameter("limit", limit.toString())
                .build().toString()

        val request = JsonObjectRequest(
                Request.Method.GET,
                url,
                null,
                Response.Listener<JSONObject> { response ->

                    listener?.onDataReceive()

                    // Add received contents to cached contents
                    response.getJSONArray("contents").let {
                        (0 until it.length())
                                .forEach { i -> insert(  it.getJSONObject(i) ) }
                    }

                    notifyDataSetChanged()
                    listener?.onDataChanged()

                }, Response.ErrorListener {  })

        queue.add(request)
        queue.start()
    }

    interface OnRefreshListener {
        fun onRefresh()
    }

}