PageDecorator.kt

class PageDotDecorator(
    private val itemCount: Int,
    private val context: Context?
) : RecyclerView.ItemDecoration() {

    companion object {
        /**
         * Draw Parameters
         */
        const val DOT_RADIUS = 3f
        const val DOT_PADDING = 6f
    }

    private var dotStroke: Paint = Paint().apply {
        style = Paint.Style.FILL
        color = context?.resources?.getColor(R.color.light_grey) ?: Color.GRAY
    }

    private val dotFill = Paint().apply {
        style = Paint.Style.FILL
        color = context?.resources?.getColor(R.color.almost_black) ?: Color.GRAY
    }
    private val dots = mutableListOf<Pair<Float, Float>>()

    private val dp = context?.resources?.displayMetrics?.density ?: 0f

    private var selectedDot = 0
    private var indicatorInitialized = false

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

        if (!indicatorInitialized) {
            setupIndicators(parent)
        }

        parent.adapter?.let {
            val visibleItem = (parent.layoutManager as? LinearLayoutManager ?: return).findFirstVisibleItemPosition()

            if (visibleItem >= 0) selectedDot = visibleItem

            for (i in 0 until itemCount) {
                drawDot(canvas, dots[i].first, dots[i].second, isSelected = selectedDot == i)
            }
        }
    }

    private fun drawDot(canvas: Canvas, x: Float, y: Float, isSelected: Boolean = false) {
        canvas.drawCircle(x, y, DOT_RADIUS * dp, if (isSelected) dotFill else dotStroke)
    }

    private fun setupIndicators(recyclerView: RecyclerView) {
        indicatorInitialized = true

        val indicatorTotalWidth = 2 * DOT_RADIUS + DOT_PADDING
        val indicatorPosX = ((recyclerView.width - (indicatorTotalWidth * itemCount * dp)) / 2f) + (DOT_PADDING * dp)
        val indicatorPosY = recyclerView.height - (DOT_RADIUS * 5 * dp)

        for (i in 0 until itemCount) dots.add((indicatorPosX + i * indicatorTotalWidth * dp) to indicatorPosY)
    }
}