Step 2, finishing the ListenableBottomNavigationView

ListenableBottomNavigationView.kt

...
import com.google.android.material.bottomnavigation.BottomNavigationView
import com.google.android.material.bottomnavigation.BottomNavigationView.OnNavigationItemSelectedListener

class ListenableBottomNavigationView @JvmOverloads constructor(
...
) : BottomNavigationView(context, attrs, defStyleAttr), OnNavigationItemSelectedListener {

    private val onNavigationItemSelectedListeners = mutableListOf<OnNavigationItemSelectedListener>()

    init {
        super.setOnNavigationItemSelectedListener(this)
    }

    override fun setOnNavigationItemSelectedListener(listener: OnNavigationItemSelectedListener?) {
        if (listener != null) addOnNavigationItemSelectedListener(listener)
    }

    fun addOnNavigationItemSelectedListener(listener: OnNavigationItemSelectedListener) {
        onNavigationItemSelectedListeners.add(listener)
    }

    fun addOnNavigationItemSelectedListener(listener: (Int) -> Unit) {
        addOnNavigationItemSelectedListener(OnNavigationItemSelectedListener {
            for (i in 0 until menu.size()) if (menu.getItem(i) == it) listener(i)
            false
        })
    }

    override fun onNavigationItemSelected(item: MenuItem) = onNavigationItemSelectedListeners
        .map { it.onNavigationItemSelected(item) }
        .fold(false) { acc, it -> acc || it }

}