Adapter extending BaseAdapter on Android to show a Spinner based on Enum. The Enum class should also implement Displayable.

EnumAdapter.kt

/**
 * If isOptional, then position 0 is a "No response" item.
 */
class EnumAdapter<DispEnum>(val context: Context, enumClass: Class<DispEnum>,
                            @IntegerRes var resource: Int = android.R.layout.simple_spinner_item,
                            @IntegerRes var dropDownResource: Int = android.R.layout.simple_spinner_dropdown_item,
                            val isOptional: Boolean = false) :
        BaseAdapter() where DispEnum : Displayable, DispEnum : Enum<DispEnum>
{
    val inflater: LayoutInflater = LayoutInflater.from(context)
    val konstants: Array<DispEnum> = enumClass.enumConstants

    override fun getItem(position: Int): Displayable {
        if (isOptional) {
            return if (position == 0) NoResponseDisplayable else konstants[position - 1]
        }
        return konstants[position]
    }

    override fun getItemId(position: Int): Long = position + 0L

    override fun getCount(): Int = konstants.size + if (isOptional) 1 else 0

    override fun getView(position: Int, convertView: View?, parent: ViewGroup): View {
        return createViewFromResource(position, convertView, parent, resource)
    }

    override fun getDropDownView(position: Int, convertView: View?, parent: ViewGroup): View {
        return createViewFromResource(position, convertView, parent, dropDownResource)
    }

    private fun createViewFromResource(position: Int, convertView: View?, parent: ViewGroup, resource: Int): View {
        val view = convertView ?: inflater.inflate(resource, parent, false)
        val textView: TextView
        try {
            textView = view as TextView
        } catch (e: ClassCastException) {
            Log.e("EnumAdapter", "You must supply a resource ID for a TextView")
            return view
        }

        val displayable = getItem(position)
        textView.text = displayable.displayString(context.resources)
        return view
    }
}

private object NoResponseDisplayable : Displayable {
    override fun displayString(res: Resources): String = res.getString(R.string.no_response)
}

interface Displayable {
    fun displayString(res: Resources): CharSequence = res.getString(resID)
    val resID: Int
}