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Delegation in Kotlin
Ralf Wondratschek
@vRallev
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Delegation
Delegation is a design pattern … and often misunderstood.
Not part of the Gang of Four design pattern book.
Many languages don’t support object oriented delegation, e.g. Java
and C++.
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Delegation is powerful
One tool in a box with many other structural design patterns like the
proxy, adapter or composition patterns.
Plays nicely with dependency injection.
Helps breaking down big classes.
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Delegation pattern
public class Rectangle {
private final int width;
private final int height;
public Rectangle(int width, int height) {
this.width = width;
this.height = height;
}
public int area() {
return width * height;
}
}
https://en.wikipedia.org/wiki/Delegation_pattern
public class Square {
private final Rectangle rectangle;
public Square(int side) {
this.rectangle = new Rectangle(side, side);
}
public int area() {
return rectangle.area();
}
}
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Delegation
public class Area {
public int area(Square square) {
return square.side * square.side;
}
}
https://en.wikipedia.org/wiki/Delegation_(object-oriented_programming)
public static class Square {
private final int side;
private final Area area;
public Square(int side) {
this.side = side;
this.area = new Area();
}
public int area() {
return area.area(this);
}
}
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Delegation in Kotlin
Kotlin made delegation first class citizen like other design patterns,
e.g. Singleton.
Supports two types of delegation.
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Implementation by delegation
class Rectangle(
private val width: Int,
private val height: Int
) {
fun area(): Int = width * height
}
class Square(side: Int) {
private val rectangle = Rectangle(side, side)
fun area(): Int = rectangle.area()
}
https://kotlinlang.org/docs/reference/delegation.html
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Implementation by delegation
class Rectangle(
private val width: Int,
private val height: Int
) {
fun area(): Int = width * height
}
class Square(side: Int) {
private val rectangle = Rectangle(side, side)
fun area(): Int = rectangle.area()
}
interface Shape {
fun area(): Int
}
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Implementation by delegation
class Rectangle(
private val width: Int,
private val height: Int
) : Shape {
override fun area(): Int =
width * height
}
class Square(side: Int) {
private val rectangle = Rectangle(side, side)
fun area(): Int = rectangle.area()
}
interface Shape {
fun area(): Int
}
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Implementation by delegation
class Rectangle(
private val width: Int,
private val height: Int
) : Shape {
override fun area(): Int =
width * height
}
class Square(side: Int) : Shape {
private val rectangle = Rectangle(side, side)
override fun area(): Int = rectangle.area()
}
interface Shape {
fun area(): Int
}
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Implementation by delegation
class Rectangle(
private val width: Int,
private val height: Int
) : Shape {
override fun area(): Int =
width * height
}
class Square(side: Int) :
Shape by Rectangle(side, side)
interface Shape {
fun area(): Int
}
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Implementation by delegation
class Rectangle(
private val width: Int,
private val height: Int
) : Shape {
override fun area(): Int =
width * height
}
class Square(side: Int) :
Shape by Rectangle(side, side) {
override fun area(): Int = 0
}
interface Shape {
fun area(): Int
}
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Proxy class
interface Shape {
fun area(): Int
}
class Unimplemented : Shape {
override fun area(): Int {
TODO("not implemented")
}
}
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Proxy class
inline fun TODO(): T {
val clazz = T::class.java
return Proxy.newProxyInstance(clazz.classLoader, arrayOf(clazz)) { _, method, _ ->
kotlin.TODO("Method ${method.name}() not implemented")
} as T
}
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Proxy class
interface Shape {
fun area(): Int
}
class Unimplemented : Shape {
override fun area(): Int {
TODO("not implemented")
}
}
class Unimplemented : Shape by TODO()
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Property delegation
https://kotlinlang.org/docs/reference/delegated-properties.html
Object oriented delegation.
You hide implementation details and delegate the knowledge to
another object.
Built-in delegates.
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But why?
interface Lazy {
fun get(): T
}
class Square(side: Int) : Shape {
val lazyRectangle = object : Lazy {
override fun get(): Rectangle = Rectangle(side, side)
}
override fun area(): Int = lazyRectangle.get().area()
}
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Lazy
class Square(side: Int) : Shape {
val rectangle by lazy { Rectangle(side, side) }
override fun area(): Int = rectangle.area()
}
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Observable
class Square(side: Int) : Shape {
var side by Delegates.observable(side) { property, oldValue, newValue ->
println("Side changed from $oldValue to $newValue")
}
override fun area(): Int = side * side
}
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Interfaces
// Use for val
interface ReadOnlyProperty {
operator fun getValue(thisRef: R, property: KProperty<*>): T
}
// Use for var
interface ReadWriteProperty {
operator fun getValue(thisRef: R, property: KProperty<*>): T
operator fun setValue(thisRef: R, property: KProperty<*>, value: T)
}
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View delegate
class MainActivity : AppCompatActivity() {
private lateinit var button: Button
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
button = findViewById(R.id.button)
}
}
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View delegate
private class ViewDelegate(
@IdRes private val viewId: Int
) : ReadOnlyProperty {
private var view: T? = null
override fun getValue(thisRef: Activity, property: KProperty<*>): T {
return view ?: thisRef.findViewById(viewId).also { view = it }
}
}
fun view(
@IdRes viewId: Int
): ReadOnlyProperty = ViewDelegate(viewId)
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View delegate
class MainActivity : AppCompatActivity() {
private lateinit var button: Button
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
button = findViewById(R.id.button)
}
}
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View delegate
class MainActivity : AppCompatActivity() {
private val button by view(R.id.button)
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
}
}
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KProperty
Represents a property in Kotlin’s reflection API.
Part of the standard library.
Provides useful information like name, visibility and other attributes
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Saving instance state
private const val COUNT_KEY = "count_key"
class MainActivity : AppCompatActivity() {
private var clickCount = 0
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
clickCount = savedInstanceState?.getInt(COUNT_KEY, clickCount) ?: clickCount
}
override fun onSaveInstanceState(outState: Bundle) {
outState.putInt(COUNT_KEY, clickCount)
}
fun click(view: View) {
clickCount++
updateButtonText()
}
}
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Saving instance state
private sealed class BaseInstanceStateProperty(
private val defaultValue: T
) : ReadWriteProperty, InstanceStateProperty {
private var value = defaultValue
private var key: String? = null
private var savedInstanceState: Bundle? = null
}
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Saving instance state
private sealed class BaseInstanceStateProperty(
private val defaultValue: T
) : ReadWriteProperty, InstanceStateProperty {
private var key: String? = null
private fun KProperty<*>.toKey(): String = this.name
}
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Saving instance state
private sealed class BaseInstanceStateProperty(
private val defaultValue: T
) : ReadWriteProperty, InstanceStateProperty {
abstract fun fromBundle(bundle: Bundle, key: String, defaultValue: T): T
abstract fun saveInBundle(bundle: Bundle, key: String, value: T)
}
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Saving instance state
private sealed class BaseInstanceStateProperty(
private val defaultValue: T
) : ReadWriteProperty, InstanceStateProperty {
private var value = defaultValue
final override fun getValue(thisRef: Activity, property: KProperty<*>): T {
initializeValue(thisRef, property)
return value
}
final override fun setValue(thisRef: Activity, property: KProperty<*>, value: T) {
initializeValue(thisRef, property)
this.value = value
}
}
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Saving instance state
private sealed class BaseInstanceStateProperty(
private val defaultValue: T
) : ReadWriteProperty, InstanceStateProperty {
final override fun restore(savedInstanceState: Bundle) {
key?.let { key ->
if (savedInstanceState.containsKey(key) &&
savedInstanceState.getInt(key) != value &&
value != defaultValue) {
throw IllegalStateException("The value ...")
}
}
this.savedInstanceState = savedInstanceState
}
final override fun save(outState: Bundle) {
key?.let { saveInBundle(outState, it, value) }
}
}
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Saving instance state
private sealed class BaseInstanceStateProperty(
private val defaultValue: T
) : ReadWriteProperty, InstanceStateProperty {
private var value = defaultValue
private var key: String? = null
private var savedInstanceState: Bundle? = null
private fun initializeValue(thisRef: Activity, property: KProperty<*>) {
key = key ?: property.toKey()
InstanceStateController.register(thisRef, this)
val bundle = savedInstanceState
if (bundle != null) {
value = fromBundle(bundle, key!!, defaultValue)
savedInstanceState = null
}
}
}
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Saving instance state
private class IntInstanceStateProperty(
defaultValue: Int
) : BaseInstanceStateProperty(defaultValue) {
override fun fromBundle(bundle: Bundle, key: String, defaultValue: Int): Int =
bundle.getInt(key, defaultValue)
override fun saveInBundle(bundle: Bundle, key: String, value: Int) {
bundle.putInt(key, value)
}
}
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Saving instance state
private class StringInstanceStateProperty(
defaultValue: String
) : BaseInstanceStateProperty(defaultValue) {
override fun fromBundle(bundle: Bundle, key: String, defaultValue: String): String =
bundle.getString(key, defaultValue)
override fun saveInBundle(bundle: Bundle, key: String, value: String) {
bundle.putString(key, value)
}
}
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Saving instance state
fun instanceState(defaultValue: T): ReadWriteProperty {
return when (defaultValue) {
is Int -> IntInstanceStateProperty(defaultValue)
is String -> StringInstanceStateProperty(defaultValue)
else -> throw NotImplementedError("Missing implementation...")
} as ReadWriteProperty
}
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Saving instance state
private const val COUNT_KEY = "count_key"
class MainActivity : AppCompatActivity() {
private var clickCount = 0
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
clickCount = savedInstanceState?.getInt(COUNT_KEY, clickCount) ?: clickCount
}
override fun onSaveInstanceState(outState: Bundle) {
outState.putInt(COUNT_KEY, clickCount)
}
fun click(view: View) {
clickCount++
updateButtonText()
}
}
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Saving instance state
class MainActivity : AppCompatActivity() {
private var clickCount by instanceState(defaultValue = 0)
fun click(view: View) {
clickCount++
updateButtonText()
}
}
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Operator
// Use for val
interface ReadOnlyProperty {
operator fun getValue(thisRef: R, property: KProperty<*>): T
}
// Use for var
interface ReadWriteProperty {
operator fun getValue(thisRef: R, property: KProperty<*>): T
operator fun setValue(thisRef: R, property: KProperty<*>, value: T)
}
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Operator
class User(val map: Map) {
val name: String by map
val age: Int by map
}
val user = User(mapOf(
"name" to "John Doe",
"age" to 25
))
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Operator
operator fun Map.getValue(
thisRef: Any?,
property: KProperty<*>
): V1 = @Suppress("UNCHECKED_CAST") (getOrImplicitDefault(property.name) as V1)
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Operator
inline operator fun Bundle.getValue(
thisRef: Any?,
property: KProperty<*>
): V = when (V::class) {
String::class -> this.getString(property.name)
Int::class -> this.getInt(property.name)
else -> throw NotImplementedError("Missing for ${V::class}")
} as V
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Operator
class User(val bundle: Bundle) {
val name: String by bundle
val age: Int by bundle
}
val user = User(bundleOf(
"name" to "John Doe",
"age" to 25
))
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Source code
https://github.com/vRallev/delegate-meetup
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Summary
Kotlin supports true object oriented delegation.
Two types: implementation by delegation and property delegation.
Two very powerful mechanisms in our Kotlin toolbox.
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Resources
Shoulders of giants: Languages Kotlin learnt from (Andrey Breslav)
https://2018.geekout.ee/andrey-breslav/
Delegation pattern
https://en.wikipedia.org/wiki/Delegation_pattern
https://en.wikipedia.org/wiki/Delegation_(object-oriented_programming)
Using Prototypical Objects to Implement Shared Behavior in Object
Oriented Systems (Henry Lieberman)
http://web.media.mit.edu/~lieber/Lieberary/OOP/Delegation/Delegation.html
Documentation
https://kotlinlang.org/docs/reference/delegation.html
https://kotlinlang.org/docs/reference/delegated-properties.html
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Thank you!
Ralf Wondratschek
@vRallev