Exemplo Conexão Aplicativo

class MainActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        val database = FirebaseDatabase.getInstance()
        val myRef = database.getReference("luz-acesa")

        val self = this

        val button = findViewById<TextView>(R.id.lblAviso)

        myRef.addValueEventListener(object : ValueEventListener {
            override fun onDataChange(dataSnapshot: DataSnapshot) {

                val value = dataSnapshot.getValue(Boolean::class.java)

                if (value == true){
                    val notification = RingtoneManager.getDefaultUri(RingtoneManager.TYPE_NOTIFICATION)
                    val r = RingtoneManager.getRingtone(applicationContext, notification)
                    r.play()
                    button.setText("Luz Acesa! Cuidado!")
                } else {
                    button.setText("Luz Apagada")
                }
            }

            override fun onCancelled(error: DatabaseError) {
                Toast.makeText(self,  "Erro: ${error.message}", Toast.LENGTH_SHORT).show()
            }
        })


    }
}