A RecyclerView that can display a 'masked' selection

Ext.kt

package com.himumsaiddad.example.util

import android.content.res.Resources

val Int.dp: Int
    get() = (this * Resources.getSystem().displayMetrics.density).toInt()

MaskedSelectorRecyclerView.kt

package com.himumsaiddad.example.ui.widget

import android.content.Context
import android.graphics.*
import android.util.AttributeSet
import androidx.core.content.ContextCompat
import androidx.recyclerview.widget.RecyclerView
import com.himumsaiddad.example.R
import com.himumsaiddad.example.util.dp

class MaskedSelectorRecyclerView @JvmOverloads constructor(
    context: Context, attrs: AttributeSet? = null, defStyleAttr: Int = 0
) : RecyclerView(context, attrs, defStyleAttr) {

    private val filter = LightingColorFilter(Color.WHITE, Color.WHITE)
    private val paint = Paint()
    private val rectf = RectF()
    private val rect by lazy {
        // Hard coded values to be replaced with correct vals
        Rect(width/2 - 44.dp, 0, width/2 + 44.dp, 32.dp)
    }

    override fun onDraw(c: Canvas?) {
        super.onDraw(c)
        rectf.set(rect)
        paint.apply {
            color = ContextCompat.getColor(context, R.color.purple)
            isAntiAlias = true
            colorFilter = null
        }
        // Hard coded values to be replaced with relevant dimens
        c?.drawRoundRect(rectf, 32.dp.toFloat(), 32.dp.toFloat(), paint)
    }

    override fun dispatchDraw(canvas: Canvas?) {
        val bitmap = Bitmap.createBitmap(width, height, Bitmap.Config.ARGB_8888)
        val canvasCopy = Canvas(bitmap)
        // draw them to that temporary bitmap
        super.dispatchDraw(canvasCopy)
        // copy the temporary bitmap to screen without the filter
        paint.colorFilter = null
        canvas?.drawBitmap(bitmap, 0f, 0f, paint)
        // copy the inverted rectangle
        paint.colorFilter = filter
        canvas?.drawBitmap(bitmap, rect, rect, paint)
    }

}

SliderLayoutManager.kt

package com.himumsaiddad.example.ui.widget

import android.content.Context
import androidx.recyclerview.widget.LinearLayoutManager
import androidx.recyclerview.widget.LinearSnapHelper
import androidx.recyclerview.widget.RecyclerView
import kotlin.math.abs
import kotlin.math.sqrt

// Adapted from @nbtk123 blog on sliding recyclerview layouts 
// Read it here - https://link.medium.com/wP4Dy4KxM0
class SliderLayoutManager(context: Context?, private val callback: (Int) -> Unit = {}) : LinearLayoutManager(context) {

    init {
        orientation = HORIZONTAL
        reverseLayout = false
    }

    private lateinit var recyclerView: RecyclerView

    var selectedIndex = -1

    override fun onAttachedToWindow(view: RecyclerView?) {
        super.onAttachedToWindow(view)
        recyclerView = view!!

        // Smart snapping (try PagerSnapHelper)
        LinearSnapHelper().attachToRecyclerView(recyclerView)
    }

    override fun onLayoutChildren(recycler: RecyclerView.Recycler?, state: RecyclerView.State) {
        super.onLayoutChildren(recycler, state)
        scaleDownView()
    }

    override fun scrollHorizontallyBy(dx: Int, recycler: RecyclerView.Recycler?, state: RecyclerView.State?): Int {
        return if (orientation == HORIZONTAL) {
            val scrolled = super.scrollHorizontallyBy(dx, recycler, state)
            scaleDownView()
            scrolled
        } else {
            0
        }
    }

    private fun scaleDownView() {
        val mid = width / 2.0f
        for (i in 0 until childCount) {

            // Calculating the distance of the child from the center
            val child = getChildAt(i) ?: continue
            val childMid = (getDecoratedLeft(child) + getDecoratedRight(child)) / 2.0f
            val distanceFromCenter = abs(mid - childMid)

            // The scaling formula
            val scale = 1- sqrt((distanceFromCenter/width).toDouble()).toFloat()*0.66f

            // Set scale to view
            child.scaleX = scale
            child.scaleY = scale
        }
    }

    override fun onScrollStateChanged(state: Int) {
        super.onScrollStateChanged(state)

        // When scroll stops we notify on the selected item
        if (state == RecyclerView.SCROLL_STATE_IDLE) {

            // Find the closest child to the recyclerView center --> this is the selected item.
            val recyclerViewCenterX = getRecyclerViewCenterX()
            var minDistance = recyclerView.width

            for (i in 0 until recyclerView.childCount) {
                val child = recyclerView.getChildAt(i)
                val childCenterX = getDecoratedLeft(child) + (getDecoratedRight(child) - getDecoratedLeft(child)) / 2
                val newDistance = abs(childCenterX - recyclerViewCenterX)
                if (newDistance < minDistance) {
                    minDistance = newDistance
                    selectedIndex = recyclerView.getChildLayoutPosition(child)
                }
            }

            // Notify on item selection
            callback.invoke(selectedIndex)
        }
    }

    private fun getRecyclerViewCenterX() : Int {
        return (recyclerView.right - recyclerView.left)/2 + recyclerView.left
    }

@ed-george How can I make first item selected at first time?