Delegate property to another property

Question

Can you delegate a property to another property in Kotlin? I have the following code:

class SettingsPage {
  lateinit var tagCharacters: JTextField
  lateini         


        

Answer 1

UPD: Since Kotlin 1.4, the standard library includes the necessary extensions that allow this out of the box:

class MyClass(var memberInt: Int, val anotherClassInstance: ClassWithDelegate) {
    var delegatedToMember: Int by this::memberInt
    var delegatedToTopLevel: Int by ::topLevelInt
    
    val delegatedToAnotherClass: Int by anotherClassInstance::anotherClassInt
}
var MyClass.extDelegated: Int by ::topLevelInt

---

Yes, it's possible: you can use a bound callable reference for a property that you store in the alias, and then the Alias implementation will look like this:

class Alias<T>(val delegate: KMutableProperty0<T>) {
    operator fun getValue(thisRef: Any?, property: KProperty<*>): T = 
        delegate.get()

    operator fun setValue(thisRef: Any?, property: KProperty<*>, value: T) {
        delegate.set(value) 
    }
}

And the usage:

class Container(var x: Int)

class Foo {
    var container = Container(1)
    var x by Alias(container::x)
}

To reference a property of the same instance, use this::someProperty.

Answer 2

Follow-up, if you would like to see support for property references in Kotlin, please vote and track this issue for updates: https://youtrack.jetbrains.com/issue/KT-8658