DimOnScrollObserver.kt

class DimOnScrollObserver : OnScrollChangeObserver() {

    private lateinit var target: AppCompatActivity

    fun attachTo(v: ScrollableViewCompat) {
        v.addOnScrollChangeListener(this)
        target = v.getAttachedActivity()
    }

    fun attachTo(v: RecyclerView, host: AppCompatActivity) {
        v.addOnScrollListener(this)
        target = host
    }

    override fun onScrollChange(v: View,
                                scrollX: Int,
                                scrollY: Int,
                                oldScrollX: Int,
                                oldScrollY: Int) {

        onScrollingInvoked(scrollY)
    }

    private fun onScrollingInvoked(trigger: Int) {
        if (trigger > 0) {
            invokeDimming()
        } else if (trigger == 0) {
            revokeDimming()
        }
    }

    override fun invokeDimming() {
        target.window.decorView.systemUiVisibility = View.SYSTEM_UI_FLAG_LOW_PROFILE
    }

    override fun revokeDimming() {
        target.window.decorView.systemUiVisibility = View.SYSTEM_UI_FLAG_VISIBLE
    }

    override fun onScrolled(recyclerView: RecyclerView?, dx: Int, dy: Int) {
        val pos = (recyclerView?.layoutManager as LinearLayoutManager)
                .findFirstCompletelyVisibleItemPosition()

        onScrollingInvoked(pos)
    }
}