Kotlin delegated property setValue cannot be kept

Question

I am reading this page about Delegated properties in Kotlin doc site.

import kotlin.reflect.KProperty

class Example {
    var p: String by Delegate()                                               // 1

    override fun toString() = "Example Class"
}

class Delegate() {
    operator fun getValue(thisRef: Any?, prop: KProperty<*>): String {        // 2     
        return "$thisRef, thank you for delegating '${prop.name}' to me!"
    }

    operator fun setValue(thisRef: Any?, prop: KProperty<*>, value: String) { // 2
        println("$value has been assigned to ${prop.name} in $thisRef")
    }
}

fun main() {
    val e = Example()
    println(e.p)
    e.p = "NEW"
}

The output is:

Example Class, thank you for delegating 'p' to me!
NEW has been assigned to p in Example Class

And I can understand the result.

But my question is, what if I print the e.p again after setting its value as NEW:

fun main() {
    val e = Example()
    println(e.p)
    e.p = "NEW"
    println(e.p) // print it again after setting new value on it
}

I expect it to print NEW. But the actual result is, it kept the same as the 1st println: Example Class, thank you for delegating 'p' to me!.

Kotlin playground at here.

Seems like the e.p = "NEW" cannot change the value correctly. What causes that? What do I do if I want to set the value as NEW?

Answer 1

Your delegate class instance completely takes over what the getter and setter of the property do. Since your delegate’s setValue function doesn’t actually store the passed-in value in any internal property, it’s getValue function has no way to retrieve and return it. And indeed, the implementation of getValue() in your code is only generating a String and returning that.

When you get the value of the delegated property, it returns whatever your delegate returns in getValue(), so the behavior is determined by how you program your delegate class.