ItemDecorator for sticky headers in Kotlin

StickyHeaderItemDecoration.kt

import android.graphics.*
import android.view.*
import android.view.MotionEvent.ACTION_DOWN
import androidx.recyclerview.widget.RecyclerView

class StickyHeaderItemDecoration(
  parent: RecyclerView,
  private val isHeader: (position: Int) -> Boolean
) : RecyclerView.ItemDecoration() {

  private var currentHeader: Pair<Int, RecyclerView.ViewHolder>? = null

  init {
    parent.adapter?.registerAdapterDataObserver(object : RecyclerView.AdapterDataObserver() {
      override fun onChanged() {
        currentHeader = null
      }
    })

    parent.doOnEachNextLayout {
      currentHeader = null
    }

    parent.addOnItemTouchListener(object : RecyclerView.SimpleOnItemTouchListener() {
      override fun onInterceptTouchEvent(
        recyclerView: RecyclerView,
        motionEvent: MotionEvent
      ): Boolean = if (motionEvent.action == ACTION_DOWN) {
        motionEvent.y <= currentHeader?.second?.itemView?.bottom ?: 0
      } else false
    })
  }

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

    val topChild = parent.getChildAt(0) ?: return
    val topChildPosition = parent.getChildAdapterPosition(topChild)
    if (topChildPosition == RecyclerView.NO_POSITION) return

    val headerView = getHeaderViewForItem(topChildPosition, parent) ?: return

    val contactPoint = headerView.bottom
    val childInContact = getChildInContact(parent, contactPoint) ?: return

    if (isHeader(parent.getChildAdapterPosition(childInContact))) {
      moveHeader(canvas, headerView, childInContact)
      return
    }

    drawHeader(canvas, headerView)
  }

  private fun getHeaderViewForItem(
    position: Int,
    parent: RecyclerView
  ): View? {
    if (parent.adapter == null) return null

    val headerPosition = getHeaderPositionForItem(position)
    val headerType = parent.adapter?.getItemViewType(headerPosition) ?: return null

    if (currentHeader?.first == headerPosition && currentHeader?.second?.itemViewType == headerType) return currentHeader?.second?.itemView

    val headerHolder = parent.adapter?.createViewHolder(parent, headerType)
    headerHolder?.let { safeHeaderHolder ->
      parent.adapter?.onBindViewHolder(safeHeaderHolder, headerPosition)
      fixLayoutSize(parent, safeHeaderHolder.itemView)

      currentHeader = headerPosition to safeHeaderHolder
    }

    return headerHolder?.itemView
  }

  private fun drawHeader(
    canvas: Canvas,
    header: View
  ) = with(canvas) {
    save()
    translate(0f, 0f)
    header.draw(this)
    restore()
  }

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

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

  private fun fixLayoutSize(
    parent: ViewGroup,
    view: View
  ) {
    // specs for parent (RecyclerView)
    val widthSpec = View.MeasureSpec.makeMeasureSpec(parent.width, View.MeasureSpec.EXACTLY)
    val heightSpec = View.MeasureSpec.makeMeasureSpec(parent.height, View.MeasureSpec.UNSPECIFIED)

    // specs for children (headers)
    val childWidthSpec = ViewGroup.getChildMeasureSpec(widthSpec, parent.paddingLeft + parent.paddingRight, view.layoutParams.width)
    val childHeightSpec = ViewGroup.getChildMeasureSpec(heightSpec, parent.paddingTop + parent.paddingBottom, view.layoutParams.height)

    view.apply {
      measure(childWidthSpec, childHeightSpec)
      layout(0, 0, view.measuredWidth, view.measuredHeight)
    }
  }

  private fun getHeaderPositionForItem(position: Int): Int {
    var headerPosition = 0
    var currentPosition = position
    do {
      if (isHeader(currentPosition)) {
        headerPosition = currentPosition
        break
      }
      currentPosition -= 1
    } while (currentPosition >= 0)
    return headerPosition
  }
}

inline fun View.doOnEachNextLayout(crossinline action: (view: View) -> Unit) {
  addOnLayoutChangeListener { view, _, _, _, _, _, _, _, _ -> action(view) }
}

Original (which I couldn't fork it 🤷‍♀️): https://gist.github.com/filipkowicz/1a769001fae407b8813ab4387c42fcbd by @filipkowicz

👍