Sticky headers for RecyclerView. Supports only one sticky header view type.

StickyHeaders.kt

package kozyriatskyi.anton.sked.week.stickyheaders

import android.graphics.Canvas
import android.support.v4.view.ViewCompat
import android.support.v7.widget.RecyclerView
import android.util.SparseIntArray
import android.view.MotionEvent
import android.view.View
import android.view.ViewGroup


abstract class StickyHeaderAdapter<T : RecyclerView.ViewHolder> : RecyclerView.Adapter<T>() {

    abstract fun getHeaderPosition(itemPosition: Int): Int

    abstract fun getHeaderViewHolder(parent: ViewGroup): RecyclerView.ViewHolder

    abstract fun bindHeaderViewHolder(holder: RecyclerView.ViewHolder, headerPosition: Int)

    abstract fun isHeader(itemPosition: Int): Boolean
}


class StickyHeaderItemDecoration(recyclerView: RecyclerView) : RecyclerView.ItemDecoration() {

    private lateinit var headerHolder: RecyclerView.ViewHolder
    private var currentStickyHeaderPosition: Int = RecyclerView.NO_POSITION

    private val headerPositionsByItemPositions = SparseIntArray(7)

    private val adapter: StickyHeaderAdapter<RecyclerView.ViewHolder> = kotlin.run {
        recyclerView.adapter as? StickyHeaderAdapter<RecyclerView.ViewHolder>
                ?: throw IllegalArgumentException("adapter must not be null and must be a descendant of StickyHeaderAdapter")
    }

    init {
        recyclerView.addOnItemTouchListener(object : RecyclerView.SimpleOnItemTouchListener() {
            override fun onInterceptTouchEvent(recyclerView: RecyclerView, motionEvent: MotionEvent): Boolean {
                return motionEvent.y <= headerHolder.itemView.height
            }
        })

        val dataObserver = object : RecyclerView.AdapterDataObserver() {
            override fun onChanged() {
                currentStickyHeaderPosition = RecyclerView.NO_POSITION
                preprocessHeaders(recyclerView)
            }

            override fun onItemRangeRemoved(positionStart: Int, itemCount: Int) {
                onChanged()
            }

            override fun onItemRangeMoved(fromPosition: Int, toPosition: Int, itemCount: Int) {
                onChanged()
            }

            override fun onItemRangeInserted(positionStart: Int, itemCount: Int) {
                onChanged()
            }

            override fun onItemRangeChanged(positionStart: Int, itemCount: Int) {
                onChanged()
            }

            override fun onItemRangeChanged(positionStart: Int, itemCount: Int, payload: Any?) {
                onChanged()
            }
        }

        adapter.registerAdapterDataObserver(dataObserver)
    }

    override fun onDrawOver(c: Canvas, parent: RecyclerView, state: RecyclerView.State) {
        super.onDrawOver(c, parent, state)

        val topChild = parent.getChildAt(0) ?: return

        val topChildPosition = parent.getChildAdapterPosition(topChild)
        if (topChildPosition == RecyclerView.NO_POSITION) return

        val currentHeader = getHeaderViewForItemPosition(topChildPosition, parent) ?: return
        val contactPoint = currentHeader.bottom
        val childInContact = getChildInContact(parent, contactPoint)

        if (childInContact != null && adapter.isHeader(parent.getChildAdapterPosition(childInContact))) {
            moveHeader(c, currentHeader, childInContact)
            return
        }

        drawHeader(c, currentHeader)
    }

    private fun getHeaderViewForItemPosition(itemPosition: Int, parent: RecyclerView): View? {
        val headerPosition = headerPositionsByItemPositions[itemPosition]

        if (headerPosition != currentStickyHeaderPosition) {
            currentStickyHeaderPosition = headerPosition
            adapter.bindHeaderViewHolder(headerHolder, headerPosition)
            measureAndLayoutHeader(parent, headerHolder.itemView)
        }

        return headerHolder.itemView
    }

    private fun preprocessHeaders(parent: RecyclerView) {
        headerHolder = adapter.getHeaderViewHolder(parent)

        headerPositionsByItemPositions.clear()

        var itemPosition = adapter.itemCount - 1

        while (itemPosition > 0) {
            val headerPosition = adapter.getHeaderPosition(itemPosition)

            for (i in itemPosition downTo headerPosition - 1) {
                headerPositionsByItemPositions.put(i, headerPosition)
            }

            itemPosition = headerPosition - 1
        }
    }

    private fun drawHeader(c: Canvas, header: View) {
        header.draw(c)
    }

    private fun moveHeader(c: Canvas, currentHeader: View, nextHeader: View) {
        c.save()
        c.translate(0f, (nextHeader.top - currentHeader.height).toFloat())
        currentHeader.draw(c)
        c.restore()
    }

    private fun getChildInContact(parent: RecyclerView, contactPoint: Int): View? {
        for (i in 0 until parent.childCount) {
            val child = parent.getChildAt(i)
            if (child.bottom > contactPoint && child.top <= contactPoint) {
                // This child overlaps the contactPoint
                return child
            }
        }

        return null
    }

    private fun measureAndLayoutHeader(parent: RecyclerView, view: View) {
        val widthSpec = View.MeasureSpec.makeMeasureSpec(parent.width, View.MeasureSpec.EXACTLY)
        val heightSpec = View.MeasureSpec.makeMeasureSpec(parent.height, View.MeasureSpec.UNSPECIFIED)

        val paddingEnd = ViewCompat.getPaddingEnd(parent)
        val paddingStart = ViewCompat.getPaddingStart(parent)

        val horizontalPadding = paddingStart + paddingEnd
        val verticalPadding = parent.paddingTop + parent.paddingBottom

        val childWidthSpec = ViewGroup.getChildMeasureSpec(widthSpec, horizontalPadding, view.layoutParams.width)
        val childHeightSpec = ViewGroup.getChildMeasureSpec(heightSpec, verticalPadding, view.layoutParams.height)

        view.measure(childWidthSpec, childHeightSpec)

        view.layout(0, 0, view.measuredWidth, view.measuredHeight)
    }
}