class YourActivity : AppCompatActivity(), OnCompleteListener<AuthResult>, View.OnClickListener {

    lateinit var mAuth: FirebaseAuth

    // register button on click listener
    override fun onClick(){
        mAuth.createUserWithEmailAndPassword(emailAddress, password).addOnCompleteListener(this)
    }

    override fun onComplete(task: Task<AuthResult>) {
        if (task.isSuccessful) {
            authUser(mAuth.currentUser)
        }
    }

    fun authUser(user: FirebaseUser?){
        // a user is returned upon successful registration
    }
}