Subscribe_MainActivity.kt

mDatabase = FirebaseDatabase.getInstance()
mPersonTable = mDatabase.getReference("Person")
btn_subscribe.setOnClickListener {
            progressBarStyle.visibility = ProgressBar.VISIBLE
                val valueEventListener = object : ValueEventListener {

                    override fun onCancelled(databaseError: DatabaseError) {}

                    override fun onDataChange(dataSnapshot: DataSnapshot) {
                        progressBarStyle.visibility = ProgressBar.INVISIBLE
                        if (dataSnapshot.child(etPhone.text.toString()).exists()) {
                            "This phone number already exist.".toast(this@MainActivity)
                        } else {
                            val person = Person(
                                etName.text.toString(), etEmail.text.toString(), etPhone.text.toString()
                            )

                            mPersonTable.child(etPhone.text.toString()).setValue(person)
                            "Thanks for your subscription.".toast(this@MainActivity)
                            clearField()
                        }

                    }
                }
                mPersonTable.addValueEventListener(valueEventListener)
            }