How to do it...

1. Declare a new interface called EmailValidator:

interface EmailValidator {}

2. Add a string property responsible for holding the current text input:

interface EmailValidator {
    var input: String
}

3. Add the isEmailValid() function to the interface:

interface EmailValidator {
    var input: String
    fun isEmailValid(): Boolean = input.contains("@")
}

4. Add the getUserLogin() function:

interface EmailValidator {
    var input: String

    fun isEmailValid(): Boolean = input.contains("@")

    fun getUserLogin(): String =
        if (isEmailValid()) {
            input.substringBefore("@")
        } else {
            ""
        }
}