Callback-based Firebase auth code. This allows user to sign up and login and also monitors the state of user authentication

SubActivity.kt

import android.app.ProgressDialog
import android.databinding.DataBindingUtil
import android.os.Bundle
import android.support.v7.app.AppCompatActivity
import android.text.TextUtils
import android.view.View
import android.widget.Toast
import com.google.firebase.auth.FirebaseAuth
import com.nezspencer.testapp.databinding.ActivitySubBinding

class SubActivity : AppCompatActivity(), FirebaseAuth.AuthStateListener, View.OnClickListener{

    private lateinit var mAuth : FirebaseAuth
    private lateinit var progress : ProgressDialog
    private lateinit var binding : ActivitySubBinding
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        binding = DataBindingUtil.setContentView(this, R.layout.activity_sub)
        progress = ProgressDialog(this)
        progress.setCanceledOnTouchOutside(false)
        progress.setCancelable(false)

        binding.btnSignin.setOnClickListener(this)
        binding.btnSignup.setOnClickListener(this)
        mAuth = FirebaseAuth.getInstance()

    }

    override fun onStart() {
        super.onStart()
        mAuth.addAuthStateListener(this)
    }

    override fun onStop() {
        super.onStop()
        mAuth.removeAuthStateListener(this)
    }

    override fun onAuthStateChanged(p0: FirebaseAuth) {
        if (progress.isShowing)
            progress.hide()

        if (p0.currentUser != null) {
            //do some UI magic
            Toast.makeText(this, "Auth successful", Toast.LENGTH_SHORT).show()
        }
        else {
            // also do some UI unauthenticated magic
            Toast.makeText(this, "Auth failed", Toast.LENGTH_SHORT).show()
        }
    }

    override fun onClick(p0: View?) {
        when(p0!!.id){
            R.id.btn_signup -> {
                val email = binding.etEmail.text.toString()
                val password = binding.etPwd.text.toString()
                if (!TextUtils.isEmpty(email) && !TextUtils.isEmpty(password))
                    signupUser(email, password)
            }
            R.id.btn_signin ->{
                val email = binding.etEmail.text.toString()
                val password = binding.etPwd.text.toString()
                if (!TextUtils.isEmpty(email) && !TextUtils.isEmpty(password))
                    signInUser(email, password)
            }
        }
    }

    private fun signupUser(email : String, password : String){
        if (!progress.isShowing)
            progress.show()
        mAuth.createUserWithEmailAndPassword(email, password)
    }



    private fun signInUser(email : String, password: String) {
        if (!progress.isShowing)
            progress.show()
        mAuth.signInWithEmailAndPassword(email, password)
    }
}