Step 6, the init block.

BottomNavigationViewIndicator.kt

...

import ademar.bnindicator.R.styleable.*
import androidx.appcompat.content.res.AppCompatResources.getDrawable

class BottomNavigationViewIndicator 
...
    private val targetId: Int
    private val backgroundDrawable: Drawable

    init {
        if (attrs == null) {
            targetId = NO_ID
            backgroundDrawable = ColorDrawable(Color.TRANSPARENT)
        } else {
            with(context.obtainStyledAttributes(attrs, BottomNavigationViewIndicator)) {
                targetId = getResourceId(BottomNavigationViewIndicator_targetBottomNavigation, NO_ID)
                val clipableId = getResourceId(BottomNavigationViewIndicator_clipableBackground, NO_ID)
                backgroundDrawable = if (clipableId != NO_ID) {
                    getDrawable(context, clipableId) ?: ColorDrawable(Color.TRANSPARENT)
                } else {
                    ColorDrawable(getColor(BottomNavigationViewIndicator_clipableBackground, Color.TRANSPARENT))
                }
                recycle()
            }
        }
    }
...