Stackoverflow question about selecting item in recyclerview

item_cell.xml

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:id="@+id/mainContainer"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:foreground="@drawable/item_selector"
    android:padding="16dp">


    <TextView
        android:id="@+id/left_text"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_toRightOf="@id/ivImage"
        android:layout_centerVertical="true"
        android:lines="1"
        android:text="@string/app_name"
        android:textColor="#000000"
        android:textSize="11sp" />

    <TextView
        android:id="@+id/right_text"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentRight="true"
        android:layout_centerVertical="true"
        android:lines="1"
        android:text="@string/app_name"
        android:textColor="#090909"
        android:textSize="11sp" />

    <ImageView
        android:id="@+id/ivImage"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_margin="10dp"
        android:layout_alignParentLeft="true"
        android:src="@drawable/ic_android_black_24dp" />

</RelativeLayout>

item_selector.xml

<selector xmlns:android="http://schemas.android.com/apk/res/android">
    <item android:drawable="@android:color/transparent" android:state_pressed="true" />
    <item android:drawable="@color/blue" android:state_selected="true" />
    <item android:drawable="@android:color/transparent" android:state_focused="true" />
    <item android:drawable="@android:color/transparent" /></selector>

MainActivity.kt

class MainActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        var arrayList = ArrayList<ItemModel>()
        for (index in 0 .. 50){
            arrayList.add(ItemModel("test $index",false))
        }
        recyclerview.adapter = ResAdapter(arrayList)
    }
}

ResAdapter.kt

package com.parthdave93.dummytestapp

import android.support.v7.widget.RecyclerView
import android.view.LayoutInflater
import android.view.View
import android.view.ViewGroup


/**
 * Created by parthdave on 07/03/18.
 */
class ResAdapter(private val moviesList: List<ItemModel>) : RecyclerView.Adapter<ResAdapter.MyViewHolder>() {


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

    inner class MyViewHolder(view: View) : RecyclerView.ViewHolder(view) {
        var mainContainer: View? = null

        init {
            mainContainer = view.findViewById(R.id.mainContainer)
        }
    }

    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): MyViewHolder {
        val itemView = LayoutInflater.from(parent.context)
                .inflate(R.layout.item_cell, parent, false)

        return MyViewHolder(itemView)
    }


    override fun onBindViewHolder(holder: MyViewHolder, position: Int) {
        val movie = moviesList[position]

        holder.mainContainer?.isSelected = movie.selected

        holder.itemView.setOnLongClickListener {
            movie.selected = !movie.selected
            notifyItemChanged(position)
            return@setOnLongClickListener true
        }
    }
}