login.kt

class Login : AppCompatActivity() {
    var auth = FirebaseAuth.getInstance()
    var username = ""
    var password = ""

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_login)
       // setActionButtonLogin()
        buttonLogin.setOnClickListener {
            username = fieldUsername.text.toString()
            password = fieldPassword.text.toString()
            execLogin(username, password)
        }

        buttonRegister.setOnClickListener {
            actionRegister()
        }
    }

    private fun actionRegister(){
        val intent = Intent(this, Register::class.java)
        startActivity(intent)
    }

    override fun onStart() {
        super.onStart()
        //Verifica se o usuário ja esta logado
        val currentUser = auth.currentUser
        updateUI(currentUser)
    }

    private fun setActionButtonLogin(){
        buttonLogin.setOnClickListener {
            username = fieldUsername.text.toString()
            password = fieldPassword.text.toString()
            execLogin(username, password)
        }

    }

    private fun execLogin(username: String, password: String){

        if (isEmailValid(username)) {
            if (isPasswordValid(password)){
                loginSession(username, password)
            }else{
                Toast.makeText(this, "Senha invalida", Toast.LENGTH_LONG).show()
            }
        }else {
            Toast.makeText(this, "Username invalido", Toast.LENGTH_LONG).show()
        }
    }


    private fun isEmailValid(email: String): Boolean {
        return android.util.Patterns.EMAIL_ADDRESS.matcher(email).matches()
    }

    private fun isPasswordValid(password: String): Boolean{
        return password.length > 8
    }

    private fun loginSession(username: String, password: String) {
        auth.signInWithEmailAndPassword(username, password)
            .addOnCompleteListener(this) { task ->
                if (task.isSuccessful) {
                    // Sign in success, update UI with the signed-in user's information

                    val user = auth.currentUser
                        updateUI(user)
                } else {
                    Toast.makeText(this, "Login não executado", Toast.LENGTH_LONG).show()
                    // If sign in fails, display a message to the user.

                }

                // ...
            }
    }

    private fun updateUI(currentUser: FirebaseUser?){
        if (currentUser != null) {
            Toast.makeText(this, "Usuario logado", Toast.LENGTH_LONG).show()
            val intent = Intent(this, MainActivity::class.java)
            startActivity(intent)
        }
    }

}