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
  lateinit var tagForegroundColorChooser: ColorPanel
  lateinit var tagBackgroundColorChooser: ColorPanel

  var allowedChars: String
    get() = tagCharacters.text
    set(value) = tagCharacters.setText(value)

  var tagForegroundColor by tagForegroundColorChooser
  var tagBackgroundColor by tagBackgroundColorChooser
}

In order to get property delegation, I declare the following two extension functions:

  operator fun ColorPanel.getValue(a: SettingsPage, p: KProperty<*>) = selectedColor
  operator fun ColorPanel.setValue(a: SettingsPage, p: KProperty<*>, c: Color?) { selectedColor = c }

However, what I would like to write is something like the following:

class SettingsPage {
  lateinit var tagCharacters: JTextField
  lateinit var tagForegroundColorChooser: ColorPanel
  lateinit var tagBackgroundColorChooser: ColorPanel

  var allowedChars: String by Alias(tagCharacters.text)
  var tagForegroundColor by Alias(tagForegroundColorChooser.selectedColor)
  var tagBackgroundColor by Alias(tagBackgroundColorChooser.selectedColor)
}

Is this possible to do Kotlin? How do I write the class Alias?

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.