Delegated properties are described on a separate page: Delegated Properties.
The Delegation pattern has proven to be a good alternative to implementation inheritance, and Kotlin supports it natively requiring zero boilerplate code. A class Derived can implement an interface Base by delegating all of its public members to a specified object:
interface Base {
fun print()
}
class BaseImpl(val x: Int) : Base {
override fun print() { print(x) }
}
class Derived(b: Base) : Base by b
fun main() {
val b = BaseImpl(10)
Derived(b).print()
}
The by-clause in the supertype list for Derived indicates that b will be stored internally in objects of Derived and the compiler will generate all the methods of Base that forward to b.
Overrides work as you might expect: the compiler will use your override implementations instead of those in the delegate object. If we were to add override fun printMessage() { print("abc") } to Derived, the program would print "abc" instead of "10" when printMessage is called:
interface Base {
fun printMessage()
fun printMessageLine()
}
class BaseImpl(val x: Int) : Base {
override fun printMessage() { print(x) }
override fun printMessageLine() { println(x) }
}
class Derived(b: Base) : Base by b {
override fun printMessage() { print("abc") }
}
fun main() {
val b = BaseImpl(10)
Derived(b).printMessage()
Derived(b).printMessageLine()
}
Note, however, that members overridden in this way do not get called from the members of the delegate object, which can only access its own implementations of the interface members:
interface Base {
val message: String
fun print()
}
class BaseImpl(val x: Int) : Base {
override val message = "BaseImpl: x = $x"
override fun print() { println(message) }
}
class Derived(b: Base) : Base by b {
// This property is not accessed from b's implementation of `print`
override val message = "Message of Derived"
}
fun main() {
val b = BaseImpl(10)
val derived = Derived(b)
derived.print()
println(derived.message)
}
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Licensed under the Apache License, Version 2.0.
https://kotlinlang.org/docs/reference/delegation.html