SectionedRv.kt

package com.example.myapplication

import android.view.LayoutInflater
import android.view.View
import android.view.ViewGroup
import android.widget.TextView
import androidx.recyclerview.widget.RecyclerView
import java.lang.IllegalArgumentException
import java.lang.IllegalStateException

class TransactionAdapter(val data: ArrayList<Row>):
    RecyclerView.Adapter<TransactionAdapter.TransactionViewHolder>() {

    override fun getItemViewType(position: Int): Int {
        return data[position].rowType
    }
      
    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): TransactionViewHolder {
        val inflater = LayoutInflater.from(parent.context)
        return when(viewType) {
            ROW_TYPE_DATE -> DateViewHolder(inflater.inflate(R.layout.date_layout, parent, false))
            ROW_TYPE_DATA -> DataViewHolder(inflater.inflate(R.layout.txn_layout, parent, false))
            else -> throw IllegalArgumentException("Invalid rowType")
        }
    }

    override fun getItemCount() = data.size

    override fun onBindViewHolder(holder: TransactionViewHolder, position: Int) {
        val row = data[position]
        val txn = row.data
        when(row.rowType) {
            ROW_TYPE_DATA -> {}
            ROW_TYPE_DATE -> {
                txn?.let { (holder as DateViewHolder).dateTextView.text = txn.date }
            }
            else -> throw IllegalStateException("invalid/unknown row type")
        }
    }

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

    class DateViewHolder(view: View): TransactionViewHolder(view) {
        val dateTextView = view.findViewById<TextView>(R.id.dateTextVIew)
    }

    class DataViewHolder(view: View): TransactionViewHolder(view) //
}

// Model
package com.example.myapplication

import android.util.Log
import java.util.*
import kotlin.collections.ArrayList

val transactionDates = arrayOf(
    "today", "yesterday", "July 7, 2019",
    "June 7, 2019", "Jan 10, 2019", "Feb 21, 2019", "Mar 15, 2019",
    "Apr 13, 2019", "May 19, 2019", "Aug 17, 2019"
)

const val ROW_TYPE_DATA = 1
const val ROW_TYPE_DATE = 2

class Row(val rowType: Int = ROW_TYPE_DATA, val data: Transaction? = null)

class Transaction(val date: String = "")

fun fakeTransactionData(): ArrayList<Transaction> {
    return ArrayList<Transaction>().apply {
        val size = transactionDates.size - 1
        for (n in 0..30) {
            val rand = (0..size).random()
            add(Transaction(transactionDates[rand]))
        }
    }
}

// Activity
package com.example.myapplication

import android.os.Bundle
import android.util.Log
import androidx.appcompat.app.AppCompatActivity
import androidx.recyclerview.widget.LinearLayoutManager
import androidx.recyclerview.widget.RecyclerView
import kotlinx.android.synthetic.main.content_main.*

class TransactionListActivity: AppCompatActivity() {

    private val transactions = ArrayList<Row>()
    private val adapter by lazy { TransactionAdapter(transactions) }

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

        setupRecyclerView()
    }

    private fun setupRecyclerView() {
        rvTransactions.layoutManager = LinearLayoutManager(this,
                RecyclerView.VERTICAL, false)
        rvTransactions.adapter = adapter

        val txns = fakeTransactionData()
        val txnsCopy = txns
        txns.forEach { txn ->
            val date = txn.date
            addDate(date, txn)
            // scan txns one-by-one and add the ones with the same date
            // just below their date label
            var count = 0
            txnsCopy.forEach {
                if (it.date == date) {
                    count += 1
                    transactions.add(Row(ROW_TYPE_DATA, it))
                }
            }
        }
        adapter.notifyDataSetChanged()
    }

    private fun addDate(date: String, txn: Transaction?) {
        var exists = false
        for (it in transactions) {
            if (it.data != null && it.data.date == date) {
                exists = true
                break
            }
        }
        if (!exists) {
            transactions.add(Row(ROW_TYPE_DATE, txn))
        }
    }
}