[Android ] RFIDReaderAppeared event not detect


I develop RFID Reader App for Android, and have created following code

class RFIDHandler(context: Context) : Readers.RFIDReaderEventHandler {
private val readers = Readers(context, ENUM_TRANSPORT.BLUETOOTH)
init {
override fun RFIDReaderAppeared(device: ReaderDevice?) {
// my processing here

override fun RFIDReaderDisappeared(device: ReaderDevice?) {
// TODO("not implemented") //To change body of created functions use File | Settings | File Templates.
According to API Reference, I can subscribe reader appear event through Readers.attach(Readers.RFIDReaderEventHandler).
But I can not subscribe.

Is there something wrong with above code?