ShadowScrollBehavior.kt

class ShadowScrollBehavior(context: Context, attrs: AttributeSet) : AppBarLayout.ScrollingViewBehavior(context, attrs) {

    @SuppressLint("PrivateResource")
    private val maxElevation = context.resources.getDimensionPixelSize(R.dimen.design_appbar_elevation).toFloat()

    override fun onDependentViewChanged(parent: CoordinatorLayout, child: View, dependency: View):
            Boolean {
        if (dependency is AppBarLayout) {
            when (child) {
                is NestedScrollView -> {
                    setElevation(child, dependency)
                    addScrollListener(child, dependency)
                }
                is RecyclerView -> {
                    setElevation(child, dependency)
                    addScrollListener(child, dependency)
                }
            }
        }

        return super.onDependentViewChanged(parent, child, dependency)
    }

    private fun addScrollListener(child: NestedScrollView, dependency: AppBarLayout) {
        child.setOnScrollChangeListener { _: NestedScrollView?, _: Int, _: Int, _: Int, _: Int ->
            setElevation(child, dependency)
        }
    }

    private fun addScrollListener(child: RecyclerView, dependency: AppBarLayout) {
        child.clearOnScrollListeners()
        child.addOnScrollListener(
            object : RecyclerView.OnScrollListener() {

                override fun onScrolled(recyclerView: RecyclerView, dx: Int, dy: Int) {
                    super.onScrolled(recyclerView, dx, dy)
                    setElevation(recyclerView, dependency)
                }
            }
        )
    }

    private fun setElevation(view: View, appBarLayout: AppBarLayout) {
        val elevation = if (view.canScrollVertically(SCROLL_DIRECTION_UP)) maxElevation else 0f

        ViewCompat.setElevation(appBarLayout, elevation)
    }

    companion object {
        private const val SCROLL_DIRECTION_UP = -1
    }
}