Umuzi Tech Department > Topics > Android-Kotlin > Delegation

Delegation

Hard Prerequisites

Class Delegation

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 inherit from an interface Base and delegate all of its public methods 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(args: Array<String>) {
    val b = BaseImpl(10)
    Derived(b).print() // prints 10
}

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.

Overriding a member of an interface implemented by delegation

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()
}

Target platform: JVMRunning on kotlin v. 1.3.72 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)
}

On the JVM: when an interface with default methods is used for delegation (including Kotlin interfaces with @JvmDefault), the default implementations are called even if the actual delegate type provides its own implementations. For details.