LoginActivity.kt

private var emailField: AutoCompleteTextView? = null
private var passwordField: EditText? = null
private var signInButton: Button? = null

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

    emailField = findViewById(R.id.email) as AutoCompleteTextView
    passwordField = findViewById(R.id.password) as EditText
    signInButton = findViewById(R.id.email_sign_in_button) as Button

    val textWatcher = object:TextWatcher {
        override fun afterTextChanged(s: Editable?) { }
        override fun beforeTextChanged(s: CharSequence?, 
                                       start: Int, 
                                       count: Int,
                                       after: Int) { }
        override fun onTextChanged(s: CharSequence?, 
                                   start: Int, 
                                   before: Int,
                                   count: Int) {
            checkIsReadyToSignIn()
        }
    }
    emailField!!.addTextChangedListener(textWatcher)
    passwordField!!.addTextChangedListener(textWatcher)
    signInButton!!.setOnClickListener { performLogin() }
    signInButton!!.isEnabled = false
}

private fun performLogin() {
    val stringToShow  = emailField?.text.toString() 
                        + " " 
                        + passwordField?.text.toString()
    Toast.makeText(this, stringToShow, Toast.LENGTH_LONG).show()
    emailField?.setText("")
    passwordField?.setText("")
}

private fun checkIsReadyToSignIn() {
    val notEmptyStrings = TextHelper.isNotEmptyStrings(emailField!!.text.toString(),
                                                       passwordField!!.text.toString())
    signInButton!!.isEnabled = notEmptyStrings
}