Kotlin Tutorial · Advanced Kotlin · Lesson 25 of 31

Kotlin Delegation: by lazy and Delegated Properties

Delegate interface implementations with by, and properties with lazy, observable and map-backed delegates.

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Class delegation

class X(inner: Interface) : Interface by inner forwards every interface method to inner; you override only what you want to change. It's composition without boilerplate.

Class delegation with by

interface Repository {
    fun find(id: Int): String?
    fun count(): Int
}

class MemoryRepository : Repository {
    private val data = mapOf(1 to "Ana", 2 to "Ben")
    override fun find(id: Int) = data[id]
    override fun count() = data.size
}

class LoggingRepository(private val inner: Repository) : Repository by inner {
    override fun find(id: Int): String? {
        println("find($id)")
        return inner.find(id)
    }
}

fun main() {
    val repo = LoggingRepository(MemoryRepository())
    println(repo.find(2))
    println(repo.count())                 // forwarded without logging
}

Output

find(2)
Ben
2

Run this on your computer — see “Get Started” for setup.

Delegated properties

by lazy { } computes the value on first access and caches it (thread-safe by default).
Delegates.observable runs a callback on every change.
A property can be read from a Map (val name: String by map).

lazy, observable and map delegates

import kotlin.properties.Delegates

class Settings(values: Map<String, Any?>) {
    val theme: String by values
    val fontSize: Int by values

    val expensive: String by lazy {
        println("computing...")
        "ready"
    }

    var status: String by Delegates.observable("idle") { _, old, new ->
        println("status: $old -> $new")
    }
}

fun main() {
    val s = Settings(mapOf("theme" to "dark", "fontSize" to 14))
    println("${s.theme} ${s.fontSize}")
    println(s.expensive)
    println(s.expensive)                  // cached: no second "computing..."
    s.status = "saving"
    s.status = "saved"
}

Output

dark 14
computing...
ready
ready
status: idle -> saving
status: saving -> saved

Run this on your computer — see “Get Started” for setup.

Test yourself

When is a by lazy property's initializer run?

When the object is created
On first access, and the result is cached
Every time it's read
At compile time

Show answer

Answer: On first access, and the result is cached

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