Kotlin override member set and get

Does anybody know a better way to listen for member change within own base class?

class FOO {
    val t: String
}

class BOO: FOO {
    fun x(val t: String) {
        // notify when t was changed 
    }
}

In my opinion JavaRx with Observer would be to much. Kotlin Delegate does not work with inheritance (or i could not find a way yet). The best i came up with is to override setter and getter of "t" in "BOO". But this is a little bit awkward because of this "Why should i overwrite a member?" or "Why do i have to define set and get when i just need set?"

So my solution so far:

import kotlin.properties.Delegates

fun main(args: Array<String>) {
    var foo = FOO()
    foo.t = "foo"
    foo.printT()

    var boo = BOO()
    boo.t = "boo"
    boo.printT()
}

open class FOO () {
    open var t: String? = "origin"
    set(value) {
        println("\t origin setter is called")
        field = String() + value // create a copy (original it is not a String :D)
    }
    get() {
        println("\t origin getter is called")
        return field
    }

    fun printT() = println(t)
}


class BOO (): FOO () {
    override var t: String?
    set(value) {
        // thats all what i need
        println("\t overwritten setter is called")
        super.t = value // execute origin setter
    }
    get() {
        println("\t overwritten getter is called")
        return super.t
    }
}