Here is the code for RegistrationActivity:

class RegistrationActivity : AppCompatActivity() {

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

    }
}

Now add some logic to validate the username, password, and email id. Here is the code:

private fun validateName(): Boolean {
    if (username_input_reg.text.toString().trim().isEmpty()) {
        username_title_reg.error = getString(R.string.err_msg_name)
        requestFocus(username_input_reg)
        return false
    } else {
        username_title_reg.isErrorEnabled = false
    }

    return true
}

private fun validateEmail(): Boolean {
    if (email_input_reg.text.toString().trim().isEmpty() || !isValidEmail(email_input_reg.text.toString().trim())) {
        email_title_reg.error = getString(R.string.err_msg_email)
        requestFocus(email_input_reg)
        return false
    } else {
        email_title_reg.isErrorEnabled = false
    }

    return true
}

private fun validatePassword(): Boolean {
    if (password_input_reg.text.toString().trim().isEmpty()
    || con_password_input_reg.text.toString().trim().isEmpty()) {

        if (password_input_reg.text.toString().trim()
            == con_password_input_reg.text.toString().trim()){
            password_title_reg.error = getString(R.string.err_match_password)
            requestFocus(password_title_reg)
        }

        password_title_reg.error = getString(R.string.err_msg_password)
        requestFocus(password_title_reg)
        return false
    } else {
        password_title_reg.isErrorEnabled = false
    }

    return true
}

Add a TextWatcher inner class, which will send an alert if there is any invalid input:

private inner class MyTextWatcher (private val view: View) : TextWatcher {

    override fun beforeTextChanged(charSequence: CharSequence, i: Int, i1: Int, i2: Int) {}

    override fun onTextChanged(charSequence: CharSequence, i: Int, i1: Int, i2: Int) {}

    override fun afterTextChanged(editable: Editable) {
        when (view.id) {
            R.id.username_input_reg -> validateName()
            R.id.email_input_reg -> validateEmail()
            R.id.input_password -> validatePassword()
        }
    }
}

When the username, password, or the email id is invalid, it will show an alert.