kotlin - 将属性委托(delegate)给另一个属性

Question

你可以将一个属性委托(delegate)给 Kotlin 中的另一个属性吗？我有以下代码:

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
}

为了获得属性委托(delegate)，我声明了以下两个扩展函数:

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

但是，我想写的是如下内容:

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)
}

这可以做 Kotlin 吗？ Alias类怎么写？

Answer 1

UPD: 从 Kotlin 1.4 开始，标准库包括必要的扩展，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

---

是的，有可能:您可以使用 bound callable reference对于您存储在别名中的属性，然后 Alias 实现将如下所示:

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) 
    }
}

及用法:

class Container(var x: Int)

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

要引用同一实例的属性，请使用 this::someProperty。