If you use a ReadWriteProperty with the by operato...

# getting-started

[hfhbd]

If you use a ReadWriteProperty with the by operator, is there any possibility to access the underlying delegate? Ideally, I want to override the toString method but keep using idiomatic code

[hfhbd]

fun main() {
    var s: String by Print {
        "$this$this"
    }
    s = "foo"
    println(s)
}

class Print<T : Any>(private val format: T.() -> String) : ReadWriteProperty<Nothing?, T> {
    constructor(initial: T, formatter: T.() -> String) : this(formatter) {
        value = initial
    }

    private lateinit var value: T

    override fun getValue(thisRef: Nothing?, property: KProperty<*>): T = value

    override fun setValue(thisRef: Nothing?, property: KProperty<*>, value: T) {
        this.value = value
    }

    override fun toString(): String = value.format()
}

[Joffrey]

The door for a more idiomatic solution will be open by the "explicit backing fields" feature (the prototype is already out). In the meantime, you can declare the delegate in a separate variable, and use that variable in the by clause

[Joffrey]

But I'm not sure I understand your goal here. Do you want the toString of the property s to be overridden by that of the delegate?

[hfhbd]

Yeah, I am aware of using another delegate variable:

val sPrint = Print<String> { "$this$this" }
var s: String by sPrint
s = "foo"
println(sPrint)
println(s) // oops

But you could print the wrong variable

[hfhbd]

The explicit backing field alternative should interesting. I will take a look

[Joffrey]

I mean the backing field feature will not help with your goal here then. One use case is to help access the property delegate, but it won't change the fact that the value itself has a different toString from the delegate

[Joffrey]

The delegate is technically just a way to provide the value. After that, the value is like any other value. What you could do is make the delegate return a wrapper of your value with a different toString() , but that is not possible for every type, because you would need to return a custom subtype of the property type

[hfhbd]

Yeah, I noticed it too 😄 It is not possible because type of the variable would be Print<String>, ideally I want to have String. I think, I will use the first approach: Use a separate delegate variable and a custom detekt rule to prevent printing the primitive variable without the formatter.

[Joffrey]

It doesn't look like delegation is useful here to be frank, because nothing happens to s. You're just creating something on the side, so it should look like this - something on the side. For instance, something as simple as:

var s: String = "foo"
val formattedS: String get() = "$s$s"

[hfhbd]

In this simple example it is true. But the delegate should do more than only formatting 🙂

[hfhbd]

okay, with this keep it should be possible 😄 https://github.com/gradle/KEEP/blob/assign-operator/proposals/assign-operator-overload.md