A RecyclerView adapter that wraps an existing adapter to split data into sections.

ids.xml

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <item name="viewtype_header" type="id"/>
</resources>

item_section_header.xml

<?xml version="1.0" encoding="utf-8"?>
<TextView xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/section_label"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:gravity="center_vertical"
    android:minHeight="48dp"
    android:paddingEnd="16dp"
    android:paddingLeft="16dp"
    android:paddingRight="16dp"
    android:paddingStart="16dp"
    android:textAppearance="@style/TextAppearance.AppCompat.Medium"
    android:textColor="?attr/colorPrimary"
    android:textSize="14sp"
    tools:text="3 days ago" />

SectionedAdapter.kt

import android.support.annotation.IdRes
import android.support.annotation.LayoutRes
import android.support.v7.widget.RecyclerView
import android.util.SparseArray
import android.view.ViewGroup
import android.widget.TextView

open class SectionedAdapter(
    baseAdapter: RecyclerView.Adapter<RecyclerView.ViewHolder>,
    @LayoutRes private val headerLayoutResId: Int,
    @IdRes private val headerLabelId: Int
) : RecyclerView.Adapter<RecyclerView.ViewHolder>() {

    private val sections = SparseArray<SectionHeader>()
    private var isValid: Boolean = true

    private val baseAdapter: RecyclerView.Adapter<RecyclerView.ViewHolder> = baseAdapter.also {
        it.registerAdapterDataObserver(BaseAdapterObserver())
    }

    constructor(baseAdapter: RecyclerView.Adapter<RecyclerView.ViewHolder>) : this(
        baseAdapter,
        R.layout.item_section_header,
        R.id.section_label
    )

    override fun getItemCount() = if (isValid) baseAdapter.itemCount + sections.size() else 0

    override fun getItemViewType(position: Int): Int = if (isSectionPosition(position))
        R.id.viewtype_header else baseAdapter.getItemViewType(sectionedPositionToPosition(position))

    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): RecyclerView.ViewHolder =
        if (viewType == R.id.viewtype_header) {
            SectionViewHolder(parent, headerLayoutResId, headerLabelId)
        } else {
            baseAdapter.onCreateViewHolder(parent, viewType)
        }

    override fun onBindViewHolder(holder: RecyclerView.ViewHolder, sectionedPosition: Int) {
        if (holder.itemViewType == R.id.viewtype_header) {
            (holder as SectionViewHolder).label.text = sections[sectionedPosition].title
        } else {
            baseAdapter.onBindViewHolder(holder, sectionedPositionToPosition(sectionedPosition))
        }
    }

    override fun getItemId(position: Int): Long {
        return if (isSectionPosition(position)) Long.MAX_VALUE - sections.indexOfKey(position)
        else baseAdapter.getItemId(sectionedPositionToPosition(position))
    }

    fun setSections(sectionHeaders: List<SectionHeader>) {
        sections.clear()
        var offset = 0
        for (section in sectionHeaders.sorted()) {
            section.sectionedPosition = section.position + offset
            sections.append(section.sectionedPosition, section)
            offset++
        }

        notifyDataSetChanged()
    }

    protected fun isSectionPosition(position: Int): Boolean = sections[position] != null

    private fun positionToSectionedPosition(position: Int): Int {
        var offset = 0
        for (i in 0 until sections.size()) {
            if (sections.valueAt(i).position > position) break

            offset++
        }

        return position + offset
    }

    private fun sectionedPositionToPosition(sectionedPosition: Int): Int {
        if (isSectionPosition(sectionedPosition)) {
            return RecyclerView.NO_POSITION
        }

        var offset = 0
        for (i in 0 until sections.size()) {
            if (sections.valueAt(i).sectionedPosition > sectionedPosition) break
            offset--
        }

        return sectionedPosition + offset
    }

    internal class SectionViewHolder(
        parent: ViewGroup,
        @LayoutRes headerLayoutResId: Int,
        @IdRes headerLabelId: Int
    ) : RecyclerView.ViewHolder(parent.inflate(headerLayoutResId)) {
        val label: TextView = itemView.findViewById(headerLabelId)
    }

    private inner class BaseAdapterObserver : RecyclerView.AdapterDataObserver() {

        override fun onChanged() {
            isValid = baseAdapter.itemCount > 0
            notifyDataSetChanged()
        }

        override fun onItemRangeInserted(positionStart: Int, itemCount: Int) {
            isValid = baseAdapter.itemCount > 0
            notifyItemRangeInserted(positionStart, itemCount)
        }

        override fun onItemRangeChanged(positionStart: Int, itemCount: Int) {
            isValid = baseAdapter.itemCount > 0
            notifyItemRangeChanged(positionStart, itemCount)
        }

        override fun onItemRangeRemoved(positionStart: Int, itemCount: Int) {
            isValid = baseAdapter.itemCount > 0
            notifyItemRangeRemoved(positionStart, itemCount)
        }
    }
}

SectionHeader.kt

class SectionHeader(
        val position: Int,
        val title: String
) : Comparable<SectionHeader> {

    internal var sectionedPosition: Int = -1

    override fun compareTo(other: SectionHeader): Int {
        return position - other.position
    }
}