sticky item decoration

TopStickyItemDecoration.kt

class TopStickyItemDecoration : ItemDecoration() {

    // for logging purposes only (avoid multiple logs with same values).
    private var lastChildTop: Int = -1

    override fun onDraw(c: Canvas, parent: RecyclerView, state: RecyclerView.State) {
        if (state.itemCount <= 0) return
        val lm = requireNotNull(parent.layoutManager) as LinearLayoutManager
        require(lm.orientation == VERTICAL)

        val parentTopPadding = if (parent.clipToPadding) parent.paddingTop else 0
        val childCount = parent.childCount
        for (i in 0 until childCount) {
            val child = parent[i]
            val vhForChild = parent.getChildViewHolder(child)
            if (vhForChild !is StickyAdapter.Sticky) continue

            val position = parent.getChildAdapterPosition(child)
            if (position < 0) continue

            stickChildToParentTop(parent, child, parentTopPadding)
        }
    }

    /**
     * Translate child to keep at same level as parent top, to look like it is sticky
     * Explanation: if difference between childTop and parentPadding is negative, then we crossed parent top with our
     * child (it is getting out of viewport), so we translate down the child to maintain it's position.
     *
     *  ie. if parentPadding = 100, when:
     *      childTop = 120  ->  translationY = 0  (lower than parent top -> no translation needed)
     *      childTop = 100  ->  translationY = 0  (equal to parent top -> no translation needed)
     *      childTop = 90   ->  translationY = 10 (higher than parent top -> translate down)
     */
    private fun stickChildToParentTop(parent: RecyclerView, child: View, parentTopPadding: Int) {
        val childTop = child.top
        val negativeTopDiff = (childTop - parentTopPadding).coerceAtMost(0)
        val translationY = -negativeTopDiff // invert negative difference to translate towards bottom

        if (childTop != lastChildTop) {
            lastChildTop = childTop
            Timber.d("childTop:$childTop, parentPadding:$parentTopPadding, translationY:$translationY")
        }

        if (translationY > 0) {
            // in order to draw view above others we need a higher elevation
            raiseViewElevationAboveOtherChildren(parent, child)
        } else {
            resetViewElevation(child)
        }
        child.translationY = translationY.toFloat()
    }

    private fun raiseViewElevationAboveOtherChildren(recyclerView: RecyclerView, view: View) {
        var originalElevation: Float? = view.getTag(R.id.sticky_item_previous_elevation) as Float?
        if (originalElevation == null) {
            originalElevation = view.elevation
            view.elevation = 1f + findMaxElevation(recyclerView)
            view.setTag(R.id.sticky_item_previous_elevation, originalElevation)
        }
    }

    private fun findMaxElevation(recyclerView: RecyclerView): Float = recyclerView.children.maxOf { it.elevation }

    private fun resetViewElevation(view: View) {
        val previousElevationTag = view.getTag(R.id.sticky_item_previous_elevation) as Float? ?: return
        view.elevation = previousElevationTag
        view.setTag(R.id.sticky_item_previous_elevation, null)
    }
}