PropertyDelegation.kt

// PropertyDelegation/FileDelegate.kt
package propertydelegation

import kotlin.properties.ReadWriteProperty
import kotlin.reflect.KProperty
import checkinstructions.DataFile

class FileDelegate :
        ReadWriteProperty<Any?, String> {
    override fun getValue(
            thisRef: Any?,
            property: KProperty<*>
    ): String {
        val file =
                DataFile(property.name + ".txt")
        return if (file.exists())
            file.readText()
        else ""
    }

    override fun setValue(
            thisRef: Any?,
            property: KProperty<*>,
            value: String
    ) {
        DataFile(property.name + ".txt")
                .writeText(value)
    }
}

// PropertyDelegation/Configuration.kt
package propertydelegation
import checkinstructions.DataFile
import atomictest.eq

class Configuration {
    var user by FileDelegate()
    var id by FileDelegate()
    var project by FileDelegate()
}

fun main() {
    val config = Configuration()
    config.user = "Luciano"
    config.id = "Ramalho47"
    config.project = "MyLittlePython"
    DataFile("user.txt").readText() eq "Luciano"
    DataFile("id.txt").readText() eq "Ramalho47"
    DataFile("project.txt").readText() eq
            "MyLittlePython"
}